# Anatomy and Physiology of Animals/Chemicals

!original image by
jurvetson
CC-BY{width="400"}

## Objectives

After completing this section, you should know the:

-   symbols used to represent elements;
-   names of molecules commonly found in animal cells;
-   characteristics of ions and electrolytes;
-   basic structure of carbohydrates with examples;
-   carbohydrates can be divided into mono- di- and poly-saccharides;
-   basic structure of fats or lipids with examples;
-   basic structure of proteins with examples;
-   function of carbohydrates, lipids and proteins in the cell and
    animals\' bodies;
-   foods which supply carbohydrates, lipids and proteins in animal
    diets.

## Elements And Atoms

The elements (simplest chemical substances) found in an animal's body
are all made of basic building blocks or atoms. The most common elements
found in cells are given in the table below with the symbol that is used
to represent them.

  Element       Symbol
  ------------- --------
  Calcium       Ca
  Carbon        C
  Chlorine      Cl
  Copper        Cu
  Iodine        I
  Hydrogen      H
  Iron          Fe
  Magnesium     Mg
  Nitrogen      N
  Oxygen        O
  Phosphorous   P
  Potassium     K
  Sodium        Na
  Sulphur       S
  Zinc          Zn

## Compounds And Molecules

A **molecule** is formed when two or more **atoms** join together. A
compound is formed when two or more different elements combine in a
fixed ratio by mass. Note that some atoms are never found alone. For
example **oxygen** is always found as molecules of 2 oxygen atoms
(represented as O~2~).

The table below gives some common compounds.

  Compound                           Symbol
  ---------------------------------- ---------------
  Calcium carbonate                  CaCO~3~
  Carbon dioxide                     CO~2~
  Copper sulphate                    CuSO~4~
  Glucose                            C~6~H~12~O~6~
  Hydrochloric acid                  HCl
  Sodium bicarbonate (baking soda)   NaHCO~3~
  Sodium chloride (table salt)       NaCl
  Sodium hydroxide                   NaOH
  Water                              H~2~O

## Chemical Reactions

Reactions occur when atoms combine or separate from other atoms. In the
process new products with different chemical properties are formed.

Chemical reactions can be represented by **chemical equations**. The
starting atoms or compounds are usually put on the left-hand side of the
equation and the products on the right-hand side.

For example

-   H~2~O + CO~2~ gives H~2~CO~3~
-   or H~2~O + CO~2~ = H~2~CO~3~
-   Water + Carbon dioxide gives Carbonic acid

## Ionization

When some atoms dissolve in water they become charged particles called
**ions**. Some become positively charged ions and others negatively
charged. Ions may have one, two or sometimes three charges.

The table below shows examples of positively and negatively charged ions
with the number of their charges.

  Positive Ions   Negative Ions
  --------------- ----------------
  H+              Hydrogen
  Ca^2^+          Calcium
  Na+             Sodium
  K+              Potassium
  Mg^2^+          Magnesium
  Fe^2^+          Iron (ferrous)
  Fe^3^+          Iron (ferric)

Positive and negative ions attract one another to hold compounds
together.

Ions are important in cells because they conduct electricity when
dissolved in water. Substances that ionise in this way are known as
**electrolytes**.

The molecules in an animal's body fall into two groups: **inorganic
compounds** and **organic compounds**. The difference between these is
that the first type does not contain **carbon** and the second type
does.

## Organic And Inorganic Compounds

Inorganic compounds include water, sodium chloride, potassium hydroxide
and calcium phosphate.

**Water** is the most abundant inorganic compound, making up over 60% of
the volume of cells and over 90% of body fluids like blood. Many
substances dissolve in water and all the chemical reactions that take
place in the body do so when dissolved in water. Other inorganic
molecules help keep the **acid/base balance (pH)** and concentration of
the blood and other body fluids stable (see Chapter 8).

Organic compounds include **carbohydrates, proteins** and **fats** or
**lipids**. All organic molecules contain carbon atoms and they tend to
be larger and more complex molecules than inorganic ones. This is
largely because each carbon atom can link with four other atoms. Organic
compounds can therefore consist of from one to many thousands of carbon
atoms joined to form chains, branched chains and rings (see diagram
below). All organic compounds also contain hydrogen and they may also
contain other elements.

![](Anatomy_and_physiology_of_animals-organic_compounds.jpg "Anatomy_and_physiology_of_animals-organic_compounds.jpg")

## Carbohydrates

The name "carbohydrate" tells you something about the composition of
these "hydrated carbon" compounds. They contain carbon, hydrogen and
oxygen and like water (H~2~O), there are always twice as many hydrogen
atoms as oxygen atoms in each molecule. Carbohydrates are a large and
diverse group that includes sugars, starches, glycogen and cellulose.
Carbohydrates in the diet supply an animal with much of its energy and
in the animal's body, they transport and store energy.

Carbohydrates are divided into three major groups based on size:
**monosaccharides** (single sugars), **disaccharides** (double sugars)
and **polysaccharides** (multi sugars).

**Monosaccharides** are the smallest carbohydrate molecules. The most
important monosaccharide is glucose which supplies much of the energy in
the cell. It consists of a ring of 6 carbon atoms with oxygen and
hydrogen atoms attached.

**Disaccharides** are formed when 2 monosaccharides join together.
Sucrose (table sugar), maltose, and lactose (milk sugar), are three
important disaccharides. They are broken down to monosaccharides by
digestive enzymes in the gut.

**Polysaccharides** like starch, glycogen and cellulose are formed by
tens or hundreds of monosaccharides linking together. Unlike mono- and
di-saccharides, polysaccharides are not sweet to taste and most do not
dissolve in water.

:\* **Starch** is the main molecule in which plants store the energy
gained from the sun. It is found in grains like barley and roots like
potatoes.

:\* **Glycogen**, the polysaccharide used by animals to store energy, is
found in the liver and the muscles that move the skeleton.

:\* **Cellulose** forms the rigid cell walls of plants. Its structure is
similar to glycogen, but it can't be digested by mammals. Cows and
horses can eat cellulose with the help of bacteria which live in
specialised parts of their gut.

![](Anatomy_and_physiology_of_animals-Polysaccharides.jpg "Anatomy_and_physiology_of_animals-Polysaccharides.jpg")

## Fats

**Fats** or **lipids** are important in the plasma membrane around cells
and form the insulating fat layer under the skin. They are also a highly
concentrated source of energy, and when eaten in the diet provide more
than twice as much energy per gram as either carbohydrates or proteins.

Like carbohydrates, fats contain carbon, hydrogen and oxygen, but unlike
them, there is no particular relationship between the number of hydrogen
and oxygen atoms.

The fats and oils animals eat in their diets are called
**triglycerides** or **neutral fats**. The building blocks of
triglycerides are 3 **fatty acids** attached to a backbone of
**glycerol** (**glycerine**). When fats are eaten the digestive enzymes
break down the molecules into separate fatty acids and glycerol again.

**Fatty acids** are divided into two kinds: **saturated** and
**unsaturated fatty acids** depending on if they contain much
(saturation) or little (unsaturation) hydrogen in their composition, and
whether any there is at least one double bond (saturation) between
carbons or not (unsaturation). The fat found in animals bodies and in
dairy products contains mainly saturated fatty acids and tends to be
solid at room temperature. Fish and poultry fats and plant oils contain
mostly unsaturated fatty acids and are more liquid at room temperature.

**Phospholipids** are lipids that contain a phosphate group. They are
important in the plasma membrane of the cell.

![](Triglyceride.JPG "Triglyceride.JPG")
![](Chain_of_amino_acids.jpg "Chain_of_amino_acids.jpg")

## Proteins

**Proteins** are the third main group of organic compounds in the cell -
in fact, if you dried out a cell, you would find that about 2/3 of the
dry dust you were left with would consist of protein. Like carbohydrates
and fats, proteins contain C, H and O, but all also **nitrogen**. Many
also contain **sulphur** and **phosphorus**.

In the cell, proteins are an important part of the plasma membrane of
the cell, but their most essential role is as **enzymes**. These are
molecules that act as biological catalysts and are necessary for
biochemical reactions to proceed. Protein is also found as **keratin**
in the skin, feathers and hair, in muscles, as well as in antibodies and
some hormones.

Proteins are built up of long chains of smaller molecules called **amino
acids**. There are 20 common types of amino acid and different numbers
of these arranged in different orders create the multitude of individual
proteins that exist in an animal's body.

Long chains of amino acids often link with other amino acid chains and
the resulting protein molecule may twist, spiral and fold up to make a
complex 3-dimensional shape. As an example, see the diagram of the
protein lysozyme below. Believe it or not, this is a small and
relatively simple protein.

![](Protein_conformation.jpg "Protein_conformation.jpg")

It is this shape that determines how proteins behave in cells,
particularly when they are acting as enzymes. If for any reason this
shape is altered, the protein stops working. This is what happens if
proteins are heated or put in a solution that is too acidic or alkaline.
Think what happens to the "white'" of an egg when it is cooked. The
"white" contains the protein albumin, which is changed or
"**denatured**" permanently by cooking. The catastrophic effect that
heat has on enzymes is one of the reasons animals die if exposed to high
temperatures.

In the animal's diet, proteins are found in meat (muscle), dairy
products, seeds, nuts and legumes like soya. When the enzymes in the gut
digest proteins they break them down into the separate amino acids,
which are small enough to be absorbed into the blood.

## Summary

-   **Ions** are charged particles, and **electrolytes** are solutions
    of ions in water.
-   **Carbohydrates** are made of carbon with hydrogen and oxygen (in
    the same ratio as water) linked together. The cell mainly uses
    carbohydrates for energy.
-   **Fats** are also made of carbon, hydrogen and oxygen. They are a
    powerful energy source, and are also used for insulation.
-   **Proteins** are the building materials of the body, and as
    **enzymes** make cell reactions happen. They contain nitrogen as
    well as carbon, hydrogen and oxygen.
-   Many also contain sulphur and phosphorous.

## Worksheet

Worksheet on Chemicals in the
Cell

## Test Yourself

1\. What is the difference between an atom and a molecule?

2\. What is the chemical name for baking soda?

:   And its formula?

3\. Write the equation for carbonic acid splitting into water and carbon
dioxide.

4\. A solution of table salt in water is an example of an electrolyte.

:   What ions are present in this solution?

5\. What element is always present in proteins but not usually in fats
or carbohydrates?

6\. List three differences between glucose and glycogen.

:   1\.
:   2\.
:   3\.

7\. Which will provide you with the most energy -- one gram of sugar or
one gram of butter?

8\. Why do organic compounds tend to be more complex and larger than
inorganic compounds ?

/Test Yourself Answers/

## Website

-   Survey of the living world organic
    molecules

A good summary of carbohydrates, fats and proteins.

## Glossary

-   Link to
    Glossary


# Anatomy and Physiology of Animals/Classification

!original image by
R\'Eyes cc
by{width="400"}

## Objectives

After completing this section, you should know:

-   how to write the scientific name of animals correctly
-   know that animals belong to the Animal kingdom and that this is
    divided into phyla, classes, orders, families
-   know the definition of a species
-   know the phylum and class of the more common animals dealt with in
    this course

**Classification** is the process used by scientists to make sense of
the 1.5 million or so different kinds of living organisms on the planet.
It does this by describing, naming and placing them in different groups.
As veterinary nurses you are mainly concerned with the Animal Kingdom
but don't forget that animals rely on the Plant Kingdom for food to
survive. Also many diseases that animals are affected by are members of
the other Kingdoms---fungi, bacteria and single celled animals.

## Naming And Classifying Animals

There are more than 1.5 million different kinds of living organism on
Earth ranging from small and simple bacteria to large, complex mammals.
From the earliest time that humans have studied the natural world they
have named these living organisms and placed them in different groups on
the basis of their similarities and differences.

## Naming Animals

Of course we know what a cat, a dog and a whale are but, in some
situations using the common names for animals can be confusing. Problems
arise because people in different countries, and even sometimes in the
same country, have different common names for the same animals. For
example a cat can be a chat, a Katze, gato, katt, or a moggie, depending
on which language you use. To add to the confusion sometimes the same
name is used for different animals. For example, the name 'gopher' is
used for ground squirrels, rodents (pocket gophers), for moles and in
the south-eastern United States for a turtle. This is the reason why all
animals have been given an official **scientific** or **binomial name**.
Unfortunately these names are always in Latin. For example:

-   Common rat: *Rattus rattus*
-   Human: *Homo sapiens*
-   Domestic cat: *Felis domesticus*
-   Domestic dog: *Canis familiaris*

As you can see from the above there are certain rules about writing
scientific names:

-   They always have **2 parts** to them.
-   The first part is the **genus** name and is always written with a
    **capital** first letter.
-   The second name is the **species** name and is always written in
    **lower case**.
-   The name is always **underlined** or printed in **italics**.

The first time you refer to an organism you should write the whole name
in full. If you need to keep referring to the same organism you can then
abbreviate the genus name to just the initial. Thus "*Canis familiaris*"
becomes "*C. familiaris*" the second and subsequent times you refer to
it.

## Classification Of Living Organisms

To make some sense of the multitude of living organisms they have been
placed in different groups. The method that has been agreed by
biologists for doing this is called the **classification system**. The
system is based on the assumption that the process of evolution has,
over the millennia, brought about slow changes that have converted
simple one-celled organisms to complex multi-celled ones and generated
the earth's incredible diversity of life forms. The classification
system attempts to reflect the evolutionary relationships between
organisms.

Initially this classification was based only on the appearance of the
organism. However, the development of new techniques has advanced our
scientific knowledge. The light microscope and later the electron
microscope have enabled us to view the smallest structures, and now
techniques for comparing DNA have begun to clarify still further the
relationships between organisms. In the light of the advances in
knowledge the classification has undergone numerous revisions over time.

At present most biologists divide the living world into 5 kingdoms,
namely:

-   bacteria
-   protists
-   fungi
-   plants
-   animal

We are concerned here almost entirely with the **Animal Kingdom**.
However, we must not forget that bacteria, protists, and fungi cause
many of the serious diseases that affect animals, and all animals rely
either directly or indirectly on the plant world for their nourishment.

## The Animal Kingdom

So what are animals? If we were suddenly confronted with an animal we
had never seen in our lives before, how would we know it was not a plant
or even a fungus? We all intuitively know part of the answer to this.

Animals:

-   eat organic material (plants or other animals)
-   move to find food
-   take the food into their bodies and then digest it
-   and most reproduce by fertilizing eggs by sperm

If you were tempted to add that animals are furry, run around on four
legs and give birth to young that they feed on milk you were thinking
only of mammals and forgetting temporarily that frogs, snakes and
crocodiles, birds as well as fish, are also animals.

These are all members of the group called the **vertebrates** (or
animals with a backbone) and mammals make up only about 8% of this
group. The diagram on the next page shows the percentage of the
different kinds of vertebrates.

![](Proportions_of_different_kinds_of_vertebrate.JPG "Proportions_of_different_kinds_of_vertebrate.JPG")

However, the term animal includes much more than just the Vertebrates.
In fact this group makes up only a very small portion of all animals.
Take a look at the diagram below, which shows the size of the different
groups of animals in the Animal Kingdom as proportions of the total
number of different animal species. Notice the small size of the segment
representing vertebrates! All the other animals in the Animal Kingdom
are animals with no backbone, or **invertebrates**. This includes the
worms, sea anemones, starfish, snails, crabs, spiders and insects. As
more than 90% of the invertebrates are insects, no wonder people worry
that insects may take over the world one day!

![](Fraction_of_vertebrates_within_the_animal_kingdom.jpg "Fraction_of_vertebrates_within_the_animal_kingdom.jpg")

## The Classification Of Vertebrates

As we have seen above the Vertebrates are divided into 5 groups or
classes namely:

-   Fish
-   Amphibia (frogs and toads)
-   Reptiles (snakes and crocodiles)
-   Birds
-   Mammals

These classes are all based on similarities. For instance all mammals
have a similar skeleton, hair on their bodies, are warm bodied and
suckle their young.

The class Mammalia (the mammals) contains 3 **subclasses**:

-   **Monotremes** (Duck billed platypus and the echidna)
-   **Marsupials** (animals like the kangaroo with pouches)
-   **True mammals** (with a placenta)

Within the subclass containing the true mammals, there are groupings
called **orders** that contain mammals that are more closely similar or
related, than others. Examples of six mammalian orders are given below:

-   Rodents (Rodentia) (rats and mice)
-   Carnivores (Carnivora) (cats, dogs, bears and seals)
-   Even-toed grazers (Artiodactyla) (pigs, sheep, cattle, antelopes)
-   Odd-toed grazers (Perissodactyla) (horses, donkeys, zebras)
-   Marine mammals (Cetacea) (whales, sea cows)
-   Primates (monkeys, apes, humans)

Within each order there are various **families**. For example within the
carnivore mammals are the families:

-   Canidae (dog-like carnivores)
-   Felidae (cat-like carnivores)

Even at this point it is possible to find groupings that are more
closely related than others. These groups are called **genera**
(singular genus). For instance within the cat family Felidae is the
genus Felis containing the cats, as well as genera containing panthers,
lynxes, and sabre toothed tigers!

The final groups within the system are the **species**. The definition
of a species is a **group of animals that can mate successfully and
produce fertile offspring**. This means that all domestic cats belong to
the species *Felis domesticus*, because all breeds of cat whether
Siamese, Manx or ordinary House hold cat can cross breed. However,
domestic cats can not mate successfully with lions, tigers or jaguars,
so these are placed in separate species, e.g. *Felis leo, Felis tigris
and Felis onca*.

Even within the same species, there can be animals with quite wide
variations in appearance that still breed successfully. We call these
different **breeds, races** or **varieties**. For example there are many
different breeds of dogs from Dalmatian to Chihuahua and of cats, from
Siamese to Manx and domestic short-hairs, but all can cross breed. Often
these breeds have been produced by**selective breeding** but varieties
can arise in the wild when groups of animals are separated by a mountain
range or sea and have developed different characteristics over long
periods of time.

To summarise, the classification system consists of:

The **A**nimal **K**ingdom which is divided into

**P**hyla which are divided into

**C**lasses which are divided into

**O**rders which are divided into

**F**amilies which are divided into

**G**enera which are divided into

**S**pecies.

"**Kings Play Cricket On Flat Green Surfaces**" OR "**Kindly Professors
Cannot Often Fail Good Students**" are just two of the phrases students
use to remind themselves of the order of these categories - on the other
hand you might like to invent your own.

## Summary

-   The **scientific name** of an animal has two parts, the **genus**
    and the **species**, and must be written in **italics** or
    **underlined**.
-   Animals are divided into **vertebrates** and **invertebrates**.
-   The classification system has groupings called **phyla**,
    **classes**, **orders**, **families**, **genera** and **species**.
-   Furry, milk-producing animals are all in the class **Mammalia.**
-   Members within a **species** can mate and produce fertile offspring.
-   Sub-groups within a species include **breeds, races** and
    **varieties**.

## Worksheet

Work through the exercises in this Classification
Worksheet to help
you learn how to write scientific names and classify different animals.

## Test Yourself

1a) True or False. Is this name written correctly? trichosurus
Vulpecula.

1b) What do you need to change?

2\. Rearrange these groups from the biggest to the smallest:

:   a\) cars \| diesel cars \| motor vehicles \| my diesel Toyota \|
    transportation

```{=html}
<!-- -->
```

:   b\) Class \| Species \| Phylum \| Genus \| Order \| Kingdom \|
    Family

## Websites

### Classification

-   <http://www.mcwdn.org/Animals/AnimalClassQuiz.html> Animal
    classification quiz

In fact much more than that. There is an elementary cell biology and
classification quiz but the best thing about this website are the links
to tables of characteristics of the different animal groups, for animals
both with and without backbones.

-   <http://animaldiversity.ummz.umich.edu/site/index.html> Animal
    diversity web

Careful! You could waste all day exploring this wonderful website. Chose
an animal or group of animals you want to know about and you will see
not only the classification but photos and details of distribution,
behaviour and conservation status etc.

-   <http://www.indianchild.com/animal_kingdom.htm> Indian child

Nice clear explanation of the different categories used in the
classification of animals.

## Glossary

-   Link to
    Glossary


# Anatomy and Physiology of Animals/The Cell

!original image by pong cc
by{width="400"}

## Objectives

After completing this section, you should know:

:\*that cells can be of different shapes and sizes

:\*the role and function of the plasma membrane; cytoplasm, ribosomes,
rough endoplasmic reticulum; smooth endoplasmic reticulum, mitochondria,
golgi bodies, lysosomes, centrioles and the nucleus

:\*the structure of the plasma membrane

:\*that substances move across the plasma membrane by passive and active
processes

:\*that passive processes include diffusion, osmosis and facilitated
diffusion and active processes include active transport, pinocytosis,
phagocytosis and exocytosis

:\*what the terms hypotonic, hypertonic isotonic and haemolysis mean

:\*that the nucleus contains the chromosomes formed from DNA

:\*that mitosis is the means by which ordinary cells divide

:\*the main stages of mitosis

:\*that meiosis is the process by which the chromosome number is halved
when ova and sperm are formed

## The Cell

!**Diagram 3.1**: A variety of animal
cells{width="400"}
The cell is the basic building block of living organisms. Bacteria and
the parasite that causes malaria consist of single cells, while plants
and animals are made up of trillions of cells. Most cells are spherical
or cube shaped but some are a range of different shapes (see diagram
3.1).

Most cells are so small that a microscope is needed to see them,
although a few cells, e.g. the ostrich's egg, are so large that they
could make a meal for several people.

A normal cell is about 0.02 of a millimetre (0.02mm) in diameter. (Small
distances like this are normally expressed in micrometres or microns
(μm). Note there are 1000 μms in every mm).
!**Diagram 3.2**: An animal
cell{width="400"}

When you look at a typical animal cell with a light microscope it seems
quite simple with only a few structures visible (see diagram 3.2).

Three main parts can be seen:

-   an outer cell membrane (plasma membrane),
-   an inner region called the cytoplasm and
-   the nucleus

!**Diagram 3.3**: An animal cell as seen with an electron
microscope{width="400"}

However, when you use an electron microscope to increase the
magnification many thousands of times you see that these seemingly
simple structures are incredibly complex, each with its own specialized
function. For example the plasma membrane is seen to be a double layer
and the cytoplasm contains many special structures called **organelles**
(meaning little organs) which are described below. A drawing of the cell
as seen with an electron microscope is shown in diagram 3.3.

## The Plasma Membrane

!**Diagram 3.4**: The structure of the plasma
membrane{width="400"}

The thin plasma membrane surrounds the cell, separating its contents
from the surroundings and controlling what enters and leaves the cell.
The plasma membrane is composed of two main
molecules,phospholipids(fats) and proteins. The phospholipids are
arranged in a double layer with the large protein molecules dotted about
in the membrane (see diagram 3.4). Some of the protein molecules form
tiny channels in the membrane while others help transport substances
from one side of the membrane to the other.

### How substances move across the Plasma Membrane

Substances need to pass through the membrane to enter or leave the cell
and they do so in a number of ways. Some of these processes require no
energy i.e. they are passive, while others require energy i.e. they are
**active**.

Passive processes include: a) diffusion and b) osmosis, while active
processes include: c) active transport, d) phagocytosis, e) pinocytosis
and f) exocytosis. These will be described below.

!**Diagram 3.5**: Diffusion in a
liquid{width="400"}
**a) Diffusion**

Although you may not know it, you are already familiar with the process
of diffusion. It is diffusion that causes a smell (expensive perfume or
smelly socks) in one part of the room to gradually move through the room
so it can be smelt on the other side. Diffusion occurs in the air and in
liquids.

Diagram 3.5 shows what happens when a few crystals of a dark purple dye
called potassium permanganate are dropped into a beaker of water. The
dye molecules diffuse into the water moving from high to low
concentrations so they become evenly distributed throughout the beaker.

In the body, diffusion causes molecules that are in a high concentration
on one side of the cell membrane to move across the membrane until they
are present in equal concentrations on both sides. It takes place
because all molecules have an in-built vibration that causes them to
move and collide until they are evenly distributed. It is an absolutely
natural process that requires no added energy.

Small molecules like oxygen, carbon dioxide, water and ammonia as well
as fats, diffuse directly through the double fat layer of the membrane.
The small molecules named above as well as a variety of charged
particles (ions) also diffuse through the protein-lined channels. Larger
molecules like glucose attach to a carrier molecule that aids their
diffusion through the membrane. This is called **facilitated
diffusion**.

In the animal's body diffusion is important for moving oxygen and carbon
dioxide between the lungs and the blood, for moving digested food
molecules from the gut into the blood and for the removal of waste
products from the cell.  !**Diagram 3.6**:
Osmosis{width="400"}
**b) Osmosis**

Although the word may be unfamiliar, you are almost certainly acquainted
with the effects of osmosis. It is osmosis that plumps out dried fruit
when you soak it before making a fruit cake or makes that wizened old
carrot look almost like new when you soak it in water. Osmosis is in
fact the diffusion of water across a membrane that allows water across
but not larger molecules. This kind of membrane is called a
**semi-permeable membrane**.

Take a look at side **A** of diagram 3.6. It shows a container divided
into two parts by an artificial semi-permeable membrane. Water is poured
into one part while a solution containing salt is poured into the other
part. Water can cross the membrane but the salt cannot. The water
crosses the semi-permeable membrane by diffusion until there is an equal
amount of water on both sides of the membrane. The effect of this would
be to make the salt solution more diluted and cause the level of the
liquid in the right-hand side of the container to rise so it looked like
side **B** of diagram 3.6. This movement of water across the
semi-permeable membrane is called osmosis. It is a completely natural
process that requires no outside energy.

Although it would be difficult to do in practice, imagine that you could
now take a plunger and push down on the fluid in the right-hand side of
container **B** so that it flowed back across the semi-permeable
membrane until the level of fluid on both sides was equal again. If you
could measure the pressure required to do this, this would be equal to
the **osmotic pressure** of the salt solution. (This is a rather
advanced concept at this stage but you will meet this term again when
you study fluid balance later in the course).
!**Diagram 3.7**: Osmosis in red cells placed in a hypotonic
solution{width="400"}

The plasma membrane of cells acts as a semi-permeable membrane. If red
blood cells, for example, are placed in water, the water crosses the
membrane to make the amount of water on both sides of it equal (see
diagram 3.7). This means that the water moves into the cell causing it
to swell. This can occur to such an extent that the cell actually bursts
to release its contents. This bursting of red blood cells is called
**haemolysis**. In a situation such as this when the solution on one
side of a semi-permeable membrane has a lower concentration than that on
the other side, the first solution is said to be **hypotonic** to the
second.  !**Diagram 3.8**: Osmosis in red cells
placed in a hypertonic
solution{width="400"}

Now think what would happen if red blood cells were placed in a salt
solution that has a higher salt concentration than the solution within
the cells (see diagram 3.8). Such a bathing solution is called a
**hypertonic** solution. In this situation the "concentration" of water
within the cells would be higher than that outside the cells. Osmosis
(diffusion of water) would then occur from the inside of the cells to
the outside solution, causing the cells to shrink.

!**Diagram 3.9**: Red cells placed in an isotonic
solution{width="400"}

A solution that contains 0.9% salt has the same concentration as body
fluids and the solution within red cells. Cells placed in such a
solution would neither swell nor shrink (see diagram 3.9). This solution
is called an **isotonic** solution. This strength of salt solution is
often called **normal saline** and is used when replacing an animal's
body fluids or when cells like red blood cells have to be suspended in
fluid.

**Remember** - osmosis is a special kind of diffusion. It is the
diffusion of water molecules across a semi-permeable membrane. It is a
completely passive process and requires no energy.

Sometimes it is difficult to remember which way the water molecules
move. Although it is not strictly true in a biological sense, many
students use the phrase **"SALT SUCKS"** to help them remember which way
water moves across the membrane when there are two solutions of
different salt concentrations on either side.

As we have seen water moves in and out of the cell by osmosis. All water
movement from the intestine into the blood system and between the blood
capillaries and the fluid around the cells (tissue or extra cellular
fluid) takes place by osmosis. Osmosis is also important in the
production of concentrated urine by the kidney.

**c) Active transport**

When a substance is transported from a low concentration to a high
concentration i.e. uphill against the concentration gradient, energy has
to be used. This is called **active transport**.

Active transport is important in maintaining different concentrations of
the ions sodium and potassium on either side of the nerve cell membrane.
It is also important for removing valuable molecules such as glucose,
amino acids and sodium ions from the urine.
!**Diagram 3.10**:
Phagocytosis{width="400"}

**d) Phagocytosis**

Phagocytosis is sometimes called "cell eating". It is a process that
requires energy and is used by cells to move solid particles like
bacteria across the plasma membrane. Finger-like projections from the
plasma membrane surround the bacteria and engulf them as shown in
diagram 3.10. Once within the cell, enzymes produced by the lysosomes of
the cell (described later) destroy the bacteria.

The destruction of bacteria and other foreign substance by white blood
cells by the process of phagocytosis is a vital part of the defense
mechanisms of the body.

**e) Pinocytosis**

Pinocytosis or "cell drinking" is a very similar process to phagocytosis
but is used by cells to move fluids across the plasma membrane. Most
cells carry out pinocytosis (note the pinocytotic vesicle in diagram
3.3).

**f) Exocytosis**

Exocytosis is the process by means of which substances formed in the
cell are moved through the plasma membrane into the fluid outside the
cell (or extra-cellular fluid). It occurs in all cells but is most
important in secretory cells (e.g. cells that produce digestive enzymes)
and nerve cells.

## The Cytoplasm

Within the plasma membrane is the **cytoplasm**. It consists of a clear
jelly-like fluid called the a) **cytosol** or **intracellular fluid** in
which b) **cell inclusions**, c) **organelles** and d)
**microfilaments** and **microtubules** are found.

### a) Cytosol

The cytosol consists mainly of water in which various molecules are
dissolved or suspended. These molecules include proteins, fats and
carbohydrates as well as sodium, potassium, calcium and chloride ions.
Many of the reactions that take place in the cell occur in the cytosol.

### b) Cell inclusions

These are large particles of fat, glycogen and melanin that have been
produced by the cell. They are often large enough to be seen with the
light microscope. For example the cells of adipose tissue (as in the
insulating fat layer under the skin) contain fat that takes up most of
the cell.

### c) Organelles

**Organelles** are the "little organs" of the cell - like the heart,
kidney and liver are the organs of the body. They are structures with
characteristic appearances and specific "jobs" in the cell. Most can not
be seen with the light microscope and so it was only when the electron
microscope was developed that they were discovered. The main organelles
in the cell are the **ribosomes, endoplasmic reticulum, mitochondrion,
Golgi complex** and **lysosomes**. A cell containing these organelles as
seen with the electron microscope is shown in diagram 3.3.

#### Ribosomes

!**Diagram 3.11**: Rough endoplasmic
reticulum{width="400"}
**Ribosomes** are tiny spherical organelles that make proteins by
joining amino acids together. Many ribosomes are found free in the
cytosol, while others are attached to the rough endoplasmic reticulum.

#### Endoplasmic reticulum

The **endoplasmic reticulum (ER)** is a network of membranes that form
channels throughout the cytoplasm from the nucleus to the plasma
membrane. Various molecules are made in the ER and transported around
the cell in its channels. There are two types of ER: smooth ER and rough
ER.

:

    :   **Smooth ER** is where the fats in the cell are made and in some
        cells, where chemicals like alcohol, pesticides and carcinogenic
        molecules are inactivated.

```{=html}
<!-- -->
```

:

    :   The **Rough ER** has ribosomes attached to its surface. The
        function of the Rough ER is therefore to make proteins that are
        modified stored and transported by the ER (Diagram 3.11).

#### Mitochondria

!**Diagram 3.12**: A
mitochondrion{width="400"}
**Mitochondria** (singular mitochondrion) are oval or rod shaped
organelles scattered throughout the cytoplasm. They consist of two
membranes, the inner one of which is folded to increase its surface
area. (Diagram 3.12)

Mitochondria are the "power stations" of the cell. They make energy by
"burning" food molecules like glucose. This process is called **cellular
respiration**. The reaction requires oxygen and produces carbon dioxide
which is a waste product. The process is very complex and takes place in
a large number of steps but the overall word equation for cellular
respiration is-

:

    :   **Glucose + oxygen = carbon dioxide + water + energy**
    :   **or C~6~H~12~O~6~ + 6O~2~** = **6CO~2~** + **6H~2~O** +
        **energy**

**Note** that cellular respiration is different from respiration or
breathing. Breathing is the means by which air is drawn into and
expelled from the lungs. Breathing is necessary to supply the cells with
the oxygen required by the mitochondria and to remove the carbon dioxide
produced as a waste product of cellular respiration.

Active cells like muscle, liver, kidney and sperm cells have large
numbers of mitochondria.

#### Golgi Apparatus

!**Diagram 3.13**: A Golgi
body{width="400"}

The **Golgi bodies** in a cell together make up the **Golgi apparatus**.
Golgi bodies are found near the nucleus and consist of flattened
membranes stacked on top of each other rather like a pile of plates (see
diagram 3.13). The Golgi apparatus modifies and sorts the proteins and
fats made by the ER, then surrounds them in a membrane as **vesicles**
so they can be moved to other parts of the cell.

#### Lysosomes

**Lysosomes** are large vesicles that contain digestive enzymes. These
break down bacteria and other substances that are brought into the cell
by phagocytosis or pinocytosis. They also digest worn-out or damaged
organelles, the components of which can then be recycled by the cell to
make new structures.

### d) Microfilaments And Microtubules

Some cells can move and change shape and organelles and chemicals are
moved around the cell. Threadlike structures called **microfilaments**
and **microtubules** that can contract are responsible for this
movement.

These structures also form the projections from the plasma membrane
known as **flagella** (singular flagellum) as in the sperm tail, and
**cilia** found lining the respiratory tract and used to remove mucus
that has trapped dust particles (see chapter 4).

Microtubules also form the pair of cylindrical structures called
**centrioles** found near the nucleus. These help organise the spindle
used in cell division.

## The Nucleus

!**Diagram 3.14**: A cell with an enlarged
chromosome{width="400"}
!**Diagram 3.15**: A full set of human
chromosomes

The **nucleus** is the largest structure in a cell and can be seen with
the light microscope. It is a spherical or oval body that contains the
**chromosomes**. The nucleus controls the development and activity of
the cell. Most cells contain a nucleus although mature red blood cells
have lost theirs during development and some muscle cells have several
nuclei.

A double membrane similar in structure to the plasma membrane surrounds
the nucleus (now called the nuclear envelope). Pores in this nuclear
membrane allow communication between the nucleus and the cytoplasm.

Within the nucleus one or more spherical bodies of darker material can
be seen, even with the light microscope. These are called **nucleoli**
and are made of RNA. Their role is to make new ribosomes.

### Chromosomes

Inside the nucleus are the chromosomes which:

-   contain DNA;
-   control the activity of the cell;
-   are transmitted from cell to cell when cells divide;
-   are passed to a new individual when sex cells fuse together in
    sexual reproduction.

In cells that are not dividing the chromosomes are very long and thin
and appear as dark grainy material. They become visible just before a
cell divides when they shorten and thicken and can then be counted (see
diagram 3.14).

The number of chromosomes in the cells of different species varies but
is constant in the cells of any one species (e.g. horses have 64
chromosomes, cats have 38 and humans 46). Chromosomes occur in pairs
(i.e. 32 pairs in the horse nucleus and 19 in that of the cat). Members
of each pair are identical in length and shape and if you look carefully
at diagram 3.15, you may be able to see some of the pairs in the human
set of chromosomes.

## Cell Division

!**Diagram 3.16**: Division by mitosis results in 2 new cells identical
to each other and to parent
cell{width="400"}
!**Diagram 3.17**: Division by meiosis results in 4 new cells that are
genetically different to each
other{width="400"}

Cells divide when an animal grows, when its body repairs an injury and
when it produces sperm and eggs (or ova). There are two types of cell
division: **Mitosis** and **meiosis**.

**Mitosis**. This is the cell division that occurs when an animal grows
and when tissues are repaired or replaced. It produces two new cells
(daughter cells) each with a full set of chromosomes that are identical
to each other and to the parent cell. All the cells of an animal's body
therefore contain identical DNA.

**Meiosis.** This is the cell division that produces the ova and sperm
necessary for sexual reproduction. It only occurs in the ovary and
testis.

The most important function of meiosis it to half the number of
chromosomes so that when the sperm fertilises the ovum the normal number
is regained. Body cells with the full set of chromosomes are called
**diploid** cells, while **gametes** (sperm and ova) with half the
chromosomes are called **haploid** cells.

Meiosis is a more complex process than mitosis as it involves two
divisions one after the other and the four cells produced are all
genetically different from each other and from the parent cell.

This fact that the cells formed by meiosis are all genetically different
from each other and from the parent cell can be seen in litters of
kittens where all the members of the litter are different from each
other as well as being different from the parents although they display
characteristics of both.

## The Cell As A Factory

To make the function of the parts of the cell easier to understand and
remember you can compare them to a factory. For example:

-   The nucleus (1) is the managing director of the factory consulting
    the blueprint (the chromosomes) (2);
-   The mitochondria (3) supply the power
-   The ribosomes (4) make the products;
-   The chloroplasts of plant cells (5) supply the fuel (food)
-   The Golgi apparatus (6) packages the products ready for dispatch;
-   The ER (7) modifies, stores and transports the products around the
    factory;
-   The plasma membrane is the factory wall and the gates (8);
-   The lysosomes dispose of the waste and worn-out machinery.

The cell compared to a
factory

## Summary

-   Cells consist of three parts: the **plasma membrane, cytoplasm** and
    **nucleus**.
-   Substances pass through the plasma membrane by **diffusion** (gases,
    lipids), **osmosis** (water), **active transport** (glucose, ions),
    **phagocytosis** (particles), **pinocytosis** (fluids) and
    **exocytosis** (particles and fluids).
-   **Osmosis** is the diffusion of **water** through a **semipermeable
    membrane**. Water diffuses from high water \"concentration\" to low
    water \"concentration\".
-   The cytoplasm consists of **cytosol** in which are suspended **cell
    inclusions** and **organelles**.
-   organelles include **ribosomes, endoplasmic reticulum, mitochondria,
    Golgi bodies** and **lysosomes**.
-   The **nucleus** controls the activity of the cell. It contains the
    **chromosomes** that are composed of **DNA**.
-   The cell divides by **mitosis** and **meiosis**

## Worksheets

There are several worksheets you can use to help you understand and
learn about the cell.

Plasma Membrane
Worksheet

Diffusion and Osmosis Worksheet
1

Diffusion and Osmosis Worksheet
2

Cell Division
Worksheet

## Test Yourself

You can then test yourself to see how much you remember.

1\. Complete the table below:

                     \|Requires energy   \|Requires a semi permeable membrane?   \|Is the movement of water molecules only?   \|Molecules move from high to low concentration?   \|Molecules move from low to high concentration?
  ------------------ ------------------- --------------------------------------- -------------------------------------------- -------------------------------------------------- --------------------------------------------------
  Diffusion          ?                   ?                                       ?                                            Yes                                                ?
  Osmosis            ?                   ?                                       ?                                            ?                                                  ?
  Active Transport   ?                   Yes                                     ?                                            ?                                                  Yes

2\. Red blood cells placed in a 5% salt solution would:

:   swell/stay the same/ shrink?

3\. Red blood cells placed in a 0.9% solution of salt would be in a:

:   hypotonic/isotonic/hypertonic solution?

4\. White blood cells remove foreign bodies like bacteria from the body
by engulfing them. This process is known as
..............................

5\. Match the organelle in the left hand column of the table below with
its function in the right hand column.

  Organelle                          Function
  ---------------------------------- ------------------------------------------------
  a\. Nucleus                        1\. Modifies proteins and fats
  b\. Mitochondrion                  2\. Makes, modifies and stores proteins
  c\. Golgi body                     3\. Digests worn out organelles
  d\. Rough endoplasmic reticulum    4\. Makes fats
  e\. Lysosome                       5\. Controls the activity of the cell proteins
  f\. Smooth endoplasmic reticulum   6\. Produces energy

6\. The cell division that causes an organism to grow and repairs
tissues is called:

7\. The cell division that produces sperm and ova is called:

8\. TWO important differences between the two types of cell division
named by you above are:

:   a\.
:   b\.

/Test Yourself Answers/

## Websites

-   <http://www.cellsalive.com/> Cells alive

    :   Cells Alive gives good animations of the animal cell.
-   Cell Wikipedia

    :   Wikipedia is good for almost anything you want to know about
        cells. Just watch as there is much more here than you need to
        know.
-   <http://personal.tmlp.com/Jimr57/textbook/chapter3/chapter3.htm>
    Virtual cell

    :   The Virtual Cell has beautiful pictures of lots of (virtual?)
        cell organelles.
-   <http://www.wisc-online.com/objects/index_tj.asp?objid=AP11403>
    Typical animal cell

    :   Great interactive animal cell.
-   <http://www.wiley.com/college/apcentral/anatomydrill/> Anatomy drill
    and practice

    :   Cell to test yourself on by dragging labels.
-   <http://www.maxanim.com/physiology/index.htm> Max Animations

    :   Great animations here of diffusion, osmosis, facilitated
        diffusion, endo- and exocytosis and the development and action
        of lysosomes. A bit higher level than you need but still not to
        be missed.
-   <http://www.stolaf.edu/people/giannini/flashanimat/transport/diffusion.swf>
    Diffusion

    :   Diffusion animation - good and clear.
-   <http://www.tvdsb.on.ca/westmin/science/sbi3a1/Cells/Osmosis.htm>
    Osmosis

    :   Nice simple osmosis animation.
-   <http://zoology.okstate.edu/zoo_lrc/biol1114/tutorials/Flash/Osmosis_Animation.htm>
    Osmosis

    :   Diffusion and osmosis. Watch what happens to the water and the
        solute molecules.
-   <http://www.wisc-online.com/objects/index_tj.asp?objid=NUR4004>
    Osmotic Pressure

    :   Do an online experiment to illustrate osmosis and osmotic
        pressure.
-   <http://www.stolaf.edu/people/giannini/flashanimat/transport/osmosis.swf>
    Osmosis

    :   Even better osmosis demonstration - you get to add the salt.

## Glossary

-   Link to Glossary


# Anatomy and Physiology of Animals/Body Organisation

!original image by
grrphoto cc
by{width="400"}

In this chapter, the way the cells of the body are organised into
different tissues is described. You will find out how these tissues are
arranged into organs, and how the organs form systems such as the
digestive system and the reproductive system. Also in this chapter, the
important concept of homeostasis is defined. You are also introduced to
those pesky things---directional terms.

## Objectives

After completing this section, you should know:

-   the "Mrs Gren" characteristics of living organisms
-   what a tissue is
-   four basic types of tissues, their general function and where they
    are found in the body
-   the basic organisation of the body of vertebrates including the main
    body cavities and the location of the following major organs:
    thorax, heart, lungs, thymus, abdomen, liver, stomach, spleen,
    intestines, kidneys, sperm ducts, ovaries, uterus, cervix, vagina,
    urinary bladder
-   the 11 body systems
-   what homeostasis is
-   directional terms including dorsal, ventral, caudal, cranial,
    medial, lateral, proximal, distal, rostral, palmar and plantar. Plus
    transverse and longitudinal sections

## The Organisation Of Animal Bodies

Living organisms move, feed, respire (burn food to make energy), grow,
sense their environment, excrete and reproduce. These seven
characteristics are sometimes summarized by the words "MRS GREN".
functions of:

**M**ovement

**R**espiration

**S**ensitivity

**G**rowth

**R**eproduction

**E**xcretion

**N**utrition

Living organisms are made from cells which are organised into tissues
and these are themselves combined to form organs and systems.

Skin cells, muscle cells, skeleton cells and nerve cells, for example.
These different types of cells are not just scattered around randomly
but similar cells that perform the same function are arranged in groups.
These collections of similar cells are known as **tissues**.

There are four main types of tissues in animals. These are:

-   **Epithelial** tissues that form linings, coverings and glands,
-   **Connective** tissues for transport and support
-   **Muscle** tissues for movement and
-   **Nervous** tissues for carrying messages.

### Epithelial Tissues

Epithelium (plural epithelia) is tissue that covers and lines. It covers
an organ or lines a tube or space in the body. There are several
different types of epithelium, distinguished by the different shapes of
the cells and whether they consist of only a single layer of cells or
several layers of cells.

#### Simple Epithelia - with a single layer of cells

!**Diagram 4.1**: Squamous
epithelium

##### Squamous epithelium

Squamous epithelium consists of a single layer of flattened cells that
are shaped rather like 'crazy paving'. It is found lining the heart,
blood vessels, lung alveoli and body cavities (see diagram 4.1). Its
thinness allows molecules to diffuse across readily.
 !**Diagram 4.2**: Cuboidal
epithelium

##### Cuboidal epithelium

Cuboidal epithelium consists of a single layer of cube shaped cells. It
is rare in the body but is found lining kidney tubules (see diagram
4.2). Molecules pass across it by diffusion, osmosis and active
transport.  !**Diagram 4.3**: Columnar
epithelium

##### Columnar epithelium

Columnar epithelium consists of column shaped cells. It is found lining
the gut from the stomach to the anus (see diagram 4.3). Digested food
products move across it into the blood stream.
!**Diagram 4.4**: Columnar epithelium with
cilia

##### Columnar epithelium with cilia

Columnar epithelium with cilia on the free surface (also known as the
apical side of the cell) lines the respiratory tract, fallopian tubes
and uterus (see diagram 4.4). The cilia beat rhythmically to transport
particles.  !**Diagram 4.5**: Transitional
epithelium

#### Transitional epithelium - with a variable number of layers

The cells in transitional epithelium can move over one another allowing
it to stretch. It is found in the wall of the bladder (see diagram 4.5).

#### Stratified epithelia - with several layers of cells

!**Diagram 4.6**: Stratified squamous
epithelium
Epithelia with several layers of cells are found where toughness and
resistance to abrasion are needed.

##### Stratified squamous epithelium

Stratified squamous epithelium has many layers of flattened cells. It is
found lining the mouth, cervix and vagina. Cells at the base divide and
push up the cells above them and cells at the top are worn or pushed off
the surface (see diagram 4.6). This type of epithelium protects
underlying layers and repairs itself rapidly if damaged.

##### Keratinised stratified squamous epithelium

Keratinised stratified squamous epithelium has a tough waterproof
protein called **keratin** deposited in the cells. It forms the skin
found covering the outer surface of mammals. (Skin will be described in
more detail in Chapter 5).

### Connective Tissues

Blood, bone, tendons, cartilage, fibrous connective tissue and fat
(adipose) tissue are all classed as connective tissues. They are tissues
that are used for supporting the body or transporting substances around
the body. They also consist of three parts: they all have cells
suspended in a ground substance or **matrix** and most have **fibres**
running through it.

#### Blood

Blood consists of a matrix - plasma, with several types of cells and
cell fragments suspended in it. The fibres are only evident in blood
that has clotted. Blood will be described in detail in chapter 8.

#### Lymph

Lymph is similar in composition to blood plasma with various types of
white blood cell floating in it. It flows in lymphatic vessels.

#### Connective tissue 'proper'

!**Diagram 4.7**: Loose connective
tissue
Connective tissue \'proper\' consists of a jelly-like matrix with a
dense network of collagen and elastic fibres and various cells embedded
in it. There are various different forms of 'proper' connective tissue
(see 1, 2 and 3 below).

##### Loose connective tissue

Loose connective tissue is a sticky whitish substance that fills the
spaces between organs. It is found in the dermis of the skin (see
diagram 4.7).

##### Dense connective tissue

Dense connective tissue contains lots of thick fibres and is very
strong. It forms tendons, ligaments and heart valves and covers bones
and organs like the kidney and liver.

#### Adipose tissue

Adipose tissue consists of cells filled with fat. It forms the fatty
layer under the dermis of the skin, around the kidneys and heart and the
yellow marrow of the bones.

!**Diagram 4.8**:
Cartilage

#### Cartilage

Cartilage is the 'gristle' of the meat. It consists of a tough
jelly-like matrix with cells suspended in it. It may contain collagen
and elastic fibres. It is a flexible but tough tissue and is found at
the ends of bones, in the nose, ear and trachea and between the
vertebrae (see diagram 4.8).

#### Bone

Bone consists of a solid matrix made of calcium salts that give it its
hardness. **Collagen** fibres running through it give it its strength.
Bone cells are found in spaces in the matrix. Two types of bone are
found in the skeleton namely **spongy** and **compact bone**. They
differ in the way the cells and matrix are arranged. (See Chapter 6 for
more details of bone).

### Muscle Tissues

Muscle tissue is composed of cells that contract and move the body.
There are three types of muscle tissue:

!**Diagram 4.9**: Smooth muscle
fibres

#### Smooth muscle

Smooth muscle consists of long and slender cells with a central nucleus
(see diagram 4.9). It is found in the walls of blood vessels, airways to
the lungs and the gut. It changes the size of the blood vessels and
helps move food and fluid along. Contraction of smooth muscle fibres
occurs without the conscious control of the animal.
 !**Diagram 4.10**: Skeletal muscle
fibres

#### Skeletal muscle

Skeletal muscle (sometimes called **striated**, **striped** or
**voluntary muscle**) has striped fibres with alternating light and dark
bands. It is attached to bones and is under the voluntary control of the
animal (see diagram 4.10).  !**Diagram 4.11**:
Cardiac muscle
fibres

#### Cardiac muscle

Cardiac muscle is found only in the walls of the heart where it produces
the 'heart beat'. Cardiac muscle cells are branched cylinders with
central nuclei and faint stripes (see diagram 4.11). Each fibre
contracts automatically but the heart beat as a whole is controlled by
the **pacemaker** and the involuntary **autonomic nervous system**.
 !**Diagram 4.12**: A motor
neuron

### Nervous Tissues

Nervous tissue forms the nerves, spinal cord and brain. Nerve cells or
**neurons** consist of a cell body and a long thread or axon that
carries the nerve impulse. An insulating sheath of fatty material
(**myelin**) usually surrounds the axon. Diagram 4.12 shows a typical
motor neuron that sends messages to muscles to contract.

## Vertebrate Bodies

We are so familiar with animals with backbones (i.e. vertebrates) that
it seems rather unnecessary to point out that the body is divided into
three sections. There is a well-defined **head** that contains the
brain, the major sense organs and the mouth, a **trunk** that contains
the other organs and a well-developed **tail**. Other features of
vertebrates may be less apparent. For instance, vertebrates that live on
the land have developed a flexible neck that is absent in fish where it
would be in the way of the gills and interfere with streamlining.
Mammals but not other vertebrates have a sheet of muscle called the
**diaphragm** that divides the trunk into the chest region or **thorax**
and the **abdomen**.

## Body Cavities

!**Diagram 4.13**: The body
cavities

In contrast to many primitive animals, vertebrates have spaces or **body
cavities** that contain the body organs. Most vertebrates have a single
body cavity but in mammals the diaphragm divides the main cavity into a
**thoracic** and an **abdominal cavity**. In the thoracic cavity the
heart and lungs are surrounded by their own membranes so that cavities
are created around the heart - the **pericardial cavity**, and around
the lungs -- the **pleural cavity** (see diagram 4.13).

## Organs

!**Diagram 4.14**: Cells, tissues and organs forming the digestive
system

Just as the various parts of the cell work together to perform the
cell's functions and a large number of similar cells make up a tissue,
so many different tissues can "cooperate" to form an organ that performs
a particular function. For example, connective tissues, epithelial
tissues, muscle tissue and nervous tissue combine to make the organ that
we call the stomach. In turn the stomach combines with other organs like
the intestines, liver and pancreas to form the digestive system (see
diagram 4.14).

## Generalised Plan Of The Mammalian Body

!**Diagram 4.15**: The main organs of the vertebrate
body

At this point it would be a good idea to make yourself familiar with the
major organs and their positions in the body of a mammal like the
rabbit. Diagram 4.15 shows the main body organs.

## Body Systems

Organs do not work in isolation but function in cooperation with other
organs and body structures to bring about the MRS GREN functions
necessary to keep an animal alive. For example the stomach can only work
in conjunction with the mouth and oesophagus (gullet). These provide it
with the food it breaks down and digests. It then needs to pass the food
on to the intestines etc. for further digestion and absorption. The
organs involved with the taking of food into the body, the digestion and
absorption of the food and elimination of waste products are
collectively known as the digestive system.

### The 11 body systems

1.  Skin

    :   The skin covering the body consists of two layers, the
        **epidermis** and **dermis**. Associated with these layers are
        hairs, feathers, claws, hoofs, glands and sense organs of the
        skin.
2.  Skeletal System

    :   This can be divided into the bones of the skeleton and the
        joints where the bones move over each other.
3.  Muscular System

    :   The muscles, in conjunction with the skeleton and joints, give
        the body the ability to move.
4.  Cardiovascular System

    :   This is also known as the circulatory system. It consists of the
        heart, the blood vessels and the blood. It transports substances
        around the body.
5.  Lymphatic System

    :   This system is responsible for collecting and "cleaning" the
        fluid that leaks out of the blood vessels. This fluid is then
        returned to the blood system. The lymphatic system also makes
        antibodies that protect the body from invasion by bacteria etc.
        It consists of lymphatic vessels, lymph nodes, the spleen and
        thymus glands.
6.  Respiratory System

    :   This is the system involved with bringing oxygen in the air into
        the body and getting rid of carbon dioxide, which is a waste
        product of processes that occur in the cell. It is made up of
        the trachea, bronchi, bronchioles, lungs, diaphragm, ribs and
        muscles that move the ribs in breathing.
7.  Digestive System

    :   This is also known as the **gastrointestinal system**,
        **alimentary system** or **gut**. It consists of the digestive
        tube and glands like the liver and pancreas that produce
        digestive secretions. It is concerned with breaking down the
        large molecules in foods into smaller ones that can be absorbed
        into the blood and lymph. Waste material is also eliminated by
        the digestive system.
8.  Urinary System

    :   This is also known as the **renal system**. It removes waste
        products from the blood and is made up of the kidneys, ureters
        and bladder.
9.  Reproductive System

    :   This is the system that keeps the species going by making new
        individuals. It is made up of the ovaries, uterus, vagina and
        fallopian tubes in the female and the testes with associated
        glands and ducts in the male.
10. Nervous System

    :   This system coordinates the activities of the body and responses
        to the environment. It consists of the sense organs (eye, ear,
        semicircular canals, and organs of taste and smell), the nerves,
        brain and spinal cord.
11. Endocrine System

    :   This is the system that produces chemical messengers or
        hormones. It consists of various **endocrine glands** (ductless
        glands) that include the pituitary, adrenal, thyroid and pineal
        glands as well as the testes and ovary.

## Homeostasis

All the body systems, except the reproductive system, are involved with
keeping the conditions inside the animal more or less stable. This is
called **homeostasis**. These constant conditions are essential for the
survival and proper functioning of the cells, tissues and organs of the
body. The skin, for example, has an important role in keeping the
temperature of the body constant. The kidneys keep the concentration of
salts in the blood within limits and the islets of Langerhans in the
pancreas maintain the correct level of glucose in the blood through the
hormone insulin. As long as the various body processes remain within
normal limits, the body functions properly and is healthy. Once
homeostasis is disturbed disease or death may result. (See Chapters 12
and16 for more on homeostasis).

## Directional Terms

!**Diagram 4.16**: The directional terms used with
animals{width="522"}
!**Diagram 4.17**: Transverse and longitudinal sections of a
mouse
In the following chapters the systems of the body in the list above will
be covered one by one. For each one the structure of the organs involved
will be described and the way they function will be explained.

In order to describe structures in the body of an animal it is necessary
to have a system for describing the position of parts of the body in
relation to other parts. For example it may be necessary to describe the
position of the liver in relation to the diaphragm, or the heart in
relation to the lungs. Certainly if you work further with animals, in a
veterinary clinic for example, it will be necessary to be able to
accurately describe the position of an injury. The terms used for this
are called **directional terms**.

The most common directional terms are **right** and **left**. However,
even these are not completely straightforward especially when looking at
diagrams of animals. The convention is to show the left side of the
animal or organ on the right side of the page. This is the view you
would get looking down on an animal lying on its back during surgery or
in a post-mortem. Sometimes it is useful to imagine 'getting inside' the
animal (so to speak) to check which side is which. The other common and
useful directional terms are listed below and shown in diagram 4.16.

  Term                    Definition                                                       Example
  ----------------------- ---------------------------------------------------------------- -----------------------------------------------------------
  Dorsal                  Nearer the back of the animal than                               The backbone is dorsal to the belly
  Ventral                 Nearer the belly of the animal than                              The breastbone is ventral to the heart
  Cranial (or anterior)   Nearer to the skull than                                         The diaphragm is cranial to the stomach
  Caudal (or posterior)   Nearer to the tail than                                          The ribs are caudal to the neck
  Proximal                Closer to the body than (only used for structures on limbs)      The shoulder is proximal to the elbow
  Distal                  Further from the body than (only used for structures on limbs)   The ankle is distal to the knee
  Medial                  Nearer to the midline than                                       The bladder is medial to the hips
  Lateral                 Further from the midline than                                    The ribs are lateral to the lungs
  Rostral                 Towards the muzzle                                               There are more grey hairs in the rostral part of the head
  Palmar                  The \"walking\" surface of the front paw                         There is a small cut on the left palmar surface
  Plantar                 The \"walking\" surface of the hind paw                          The pads are on the plantar side of the foot

Note that we don't use the terms **superior** and **inferior** for
animals. They are only used to describe the position of structures in
the human body (and possibly apes) where the upright posture means some
structures are above or superior to others.

In order to look at the structure of some of the parts or organs of the
body it may be necessary to cut them open or even make thin slices of
them that they can be examined under the microscope. The direction and
position of slices or sections through an animal's body have their own
terminology.

If an animal or organ is sliced lengthwise this section is called a
**longitudinal** or **sagittal section**. This is sometimes abbreviated
to LS.

If the section is sliced crosswise it is called a **transverse** or
**cross section**. This is sometimes abbreviated to TS or XS (see
diagram 4.17).

## Summary

-   The characteristics of living organisms can be summarised by the
    words "**MRS GREN**."
-   There are 4 main types of tissue namely: **epithelial, connective,
    muscle** and **nervous tissues**.
-   **Epithelial tissues** form the skin and line the gut, respiratory
    tract, bladder etc.
-   **Connective tissues** form tendons, ligaments, adipose tissue,
    blood, cartilage and bone, and are found in the dermis of the skin.
-   **Muscular tissues** contract and consist of 3 types: **smooth,
    skeletal and cardiac**.
-   Vertebrate bodies have a **head, trunk** and **tail**. Body organs
    are located in **body cavities**. 11 body systems perform essential
    body functions most of which maintain a stable environment or
    **homeostasis** within the animal.
-   **Directional terms** describe the location of parts of the body in
    relation to other parts.

## Worksheets

Students often find it hard learning how to use directional terms
correctly. There are two worksheets to help you with these and another
on tissues.

Directional Terms Worksheet
1

Directional Terms Worksheet
2

Tissues Worksheet

## Test Yourself

1\. Living organisms can be distinguished from non-living matter because
they usually move and grow. Name 5 other functions of living organisms:

:   1\.
:   2\.
:   3\.
:   4\.
:   5\.

2\. What tissue types would you find\...

:   a\) lining the intestine:
:   b\) covering the body:
:   c\) moving bones:
:   d\) flowing through blood vessels:
:   e\) linking the eye to the brain:
:   f\) lining the bladder:

3\. Name the body cavity in which the following organs are found:

:   a\) heart:
:   b\) bladder:
:   c\) stomach:
:   d\) lungs:

4\. Name the body system that\...

:   a\) includes the bones and joints:
:   b\) includes the ovaries and testes:
:   c\) produces hormones:
:   d\) includes the heart, blood vessels and blood:

5\. What is homeostasis?

6\. Circle which is correct:

:   a\) The head is cranial \| caudal to the neck
:   b\) The heart is medial \| lateral to the ribs
:   c\) The elbow is proximal \| distal to the fingers
:   d\) The spine is dorsal \| ventral to the heart

7\. Indicate whether or not these statements are true.

:   a\) The stomach is cranial to the diaphragm - true \| false
:   b\) The heart lies in the pelvic cavity - true \| false
:   c\) The spleen is roughly the same size as the stomach and lies near
    it - true \| false
:   d\) The small intestine is proximal to the kidneys - true \| false
:   e\) The bladder is medial to the hips - true \| false
:   f\) The liver is cranial to the heart - true \| false

/Test Yourself Answers/

## Websites

-   Animal organ systems and homeostasis

Overview of the different organ systems (in humans) and their functions
in maintaining homeostasis in the body.

<http://www.emc.maricopa.edu/faculty/farabee/biobk/BioBookANIMORGSYS.html>

-   Wikipedia

Directional terms for animals. A little more detail than required but
still great.

<http://en.wikipedia.org/wiki/Anatomical_terms_of_location>

## Glossary

-   Link to
    Glossary


# Anatomy and Physiology of Animals/The Skin

!original image by
Fran-cis-ca cc
by{width="400"}

The skin is the first of the eleven body systems to be described. Each
chapter from now on will cover one body system.

The skin, sometimes known as the **Integumentary System** is, in fact,
the largest organ of the body. It has a complex structure, being
composed of many different tissues. It performs many functions that are
important in maintaining homeostasis in the body. Probably the most
important of these functions is the control of body temperature. The
skin also protects the body from physical damage and bacterial invasion.
The skin has an array of sense organs which sense the external
environment, and also cells which can make **vitamin D** in sunlight.

The skin is one of the first systems affected when an animal becomes
sick so it is important for anyone working with animals to have a sound
knowledge of the structure and functioning of the skin so they can
quickly recognize signs of disease.

## Objectives

After completing this section, you should know:

-   the general structure of the skin
-   the function of the keratin deposited in the epidermis
-   the structure and function of keratin skin structures including
    calluses, scales, nails, claws, hoofs and horns
-   that antlers are not made either of keratin or in the epidermis
-   the structure of hairs
-   the structure of the different types of feathers and the function of
    preening
-   the general structure and function of sweat, scent, preen and
    mammary glands
-   the basic functions of the skin in sensing stimuli, temperature
    control and production of vitamin D
-   the mechanisms by which the skin regulates body temperature

## The Skin

The skin comes in all kinds of textures and forms. There is the dry
warty skin of toads and crocodiles, the wet slimy skin of fish and
frogs, the hard shell of tortoises and the soft supple skin of snakes
and humans. Mammalian skin is covered with hair, that of birds with
feathers, and fish and reptiles have scales. Pigment in the skin, hairs
or feathers can make the outer surface almost any color of the rainbow.

`   Skin is one of the largest organs of the body, making up 6-8% of the total body weight. It consists of two distinct layers. The top layer is called the `**`epidermis`**` and under that is the '''dermis'`

The epidermis is the layer that bubbles up when we have a blister and as
we know from this experience, it has no blood or nerves in it. The cells
at the base of the epidermis continually divide and push the cells above
them upwards. As these cells move up they die and become the dry flaky
scales that fall off the skin surface. The cells in the epidermis die
because a special protein called **keratin** is deposited in them.
Keratin is an extremely important substance for it makes the skin
waterproof. Without it, land vertebrates like reptiles, birds, and
mammals would, like frogs, be able to survive only in damp places.

## Skin Structures Made Of Keratin

### Claws, Nails and Hoofs

Reptiles, birds, and mammals all have nails or claws on the ends of
their toes. They protect the end of the toe and may be used for
grasping, grooming, digging or in defense. They are continually worn
away and grow continuously from a growth layer at their base (see
diagram 5.2).

![](_Anatomy_and_physiology_of_animals_Carnivores_claw.jpg "_Anatomy_and_physiology_of_animals_Carnivores_claw.jpg")

Diagram 5.2 - A carnivore's claw

**Hoofs** are found in sheep, cows, horses etc. otherwise known as
**ungulate mammals**. These are animals that have lost toes in the
process of evolution and walk on the "nails" of the remaining toes. The
hoof is a cylinder of horny material that surrounds and protects the tip
of the toe (see diagram 5.3).

![](_Anatomy_and_physiology_of_animal_Horses_hoof.jpg "_Anatomy_and_physiology_of_animal_Horses_hoof.jpg")

Diagram 5.3 - A horse's hoof

### Horns And Antlers

True horns are made of keratin and are found in sheep, goats, and
cattle. They are never branched and, once grown, are never shed. They
consist of a core of bone arising in the dermis of the skin and are
fused with the skull. The horn itself forms as a hollow cone-shaped
sheath around the bone (see diagram 5.4).

![](_Anatomy_and_physiology_of_animals_A_horn.jpg "_Anatomy_and_physiology_of_animals_A_horn.jpg")

Diagram 5.4 - A horn

The **antlers** of male deer have quite a different structure. They are
not formed in the epidermis and do not consist of keratin but are entire
of bone. They are shed each year and are often branched, especially in
older animals. When growing they are covered in the skin called
**velvet** that forms the bone. Later the velvet is shed to leave the
bony antler. The velvet is often removed artificially to be sold in Asia
as a traditional medicine (see diagram 5.5).

![](_Anatomy_and_physiology_of_animals_Deer_antler.jpg "_Anatomy_and_physiology_of_animals_Deer_antler.jpg")

Diagram 5.5 - A deer antler

Other animals have projections on their heads that are not true horns
either. The horns on the head of giraffes are made of bone covered with
skin and hair, and the 'horn' of a rhinoceros is made of modified and
fused hair-like structures.

### Hair

Hair is also made of keratin and develops in the epidermis. It covers
the body of most mammals where it acts as an insulator and helps to
regulate the temperature of the body (see below). The color in hairs is
formed from the same pigment, **melanin** that colors the skin. Coat
color may help camouflage animals and sometimes acts to attract the
opposite sex.

![](_Anatomy_and_physiology_of_animals_A_hair.jpg "_Anatomy_and_physiology_of_animals_A_hair.jpg")

Diagram 5.6 - A hair

Hairs lie in a **follicle** and grow from a **root** that is well
supplied with blood vessels. The hair itself consists of layers of dead
keratin-containing cells and usually lies at a slant in the skin. A
small bundle of smooth muscle fibers (the **hair erector muscle**) is
attached to the side of each hair and when this contracts the hair
stands on end. This increases the insulating power of the coat and is
also used by some animals to make them seem larger when confronted by a
foe or a competitor(see diagram 5.6).

The whiskers of cats and the spines of hedgehogs are examples of special
types of hairs.

### Feathers

The lightness and stiffness of keratin is also a key to bird flight. In
the form of feathers, it provides the large airfoils necessary for
flapping and gliding flight. In another form, the light fluffy down
feathers, also made of keratin, are some of the best natural insulators
known. This superior insulation is necessary to help maintain the high
body temperatures of birds.

![](_Anatomy_and_physiology_of_animals_Contour_feather.jpg "_Anatomy_and_physiology_of_animals_Contour_feather.jpg")

Diagram 5.7 - A Contour Feather

Contour feathers are large feathers that cover the body, wings, and
tail. They have an expanded **vane** that provides the smooth,
continuous surface that is required for effective flight. This surface
is formed by **barbs** that extend out from the central shaft. If you
look carefully at a feather you can see that on either side of each barb
are thousands of **barbules** that lock together by a complex system of
hooks and notches. if this arrangement becomes disrupted, the bird uses
its beak to draw the barbs and barbules together again in an action
known as **preening** (see diagram 5.7).

![](_Anatomy_and_physiology_of_animals_Down_feather.jpg "_Anatomy_and_physiology_of_animals_Down_feather.jpg")

Diagram 5.8 - A Down Feather

![](_Anatomy_and_physiology_of_animals_Pin_feather.jpg "_Anatomy_and_physiology_of_animals_Pin_feather.jpg")

Diagram 5.9 - A Pin Feather

Down feathers are the only feathers covering a chick and form the main
insulation layer under the contour feathers of the adult. They have no
shaft but consist of a spray of simple, slender branches (see diagram
5.8).

Pin feathers have a slender hair-like shaft often with a tiny tuft of
barbs on the end. They are found between the other feathers and help
tell a bird how its feathers are lying (see diagram 5.9).

## Skin Glands

Glands are organs that produce and secrete fluids. They are usually
divided into two groups depending upon whether or not they have channels
or ducts to carry their products away. Glands with ducts are called
**exocrine glands** and include the glands found in the skin as well as
the glands that produce digestive enzymes in the gut. **Endocrine
glands** have no ducts and release their products (hormones) directly
into the bloodstream. The pituitary and adrenal glands are examples of
endocrine glands.

Most vertebrates have exocrine glands in the skin that produce a variety
of secretions. The slime on the skin of fish and frogs is **mucus**
produced by skin glands and some fish and frogs also produce poison from
modified glands. In fact, the skin glands of some frogs produce the most
poisonous chemicals known. Reptiles and birds have a dry skin with few
glands. The **preen gland**, situated near the base of the bird's tail,
produces oil to help keep the feathers in good condition. Mammals have
an array of different skin glands. These include the wax producing,
sweat, sebaceous and mammary glands.

**Wax producing glands** are found in the ears.

**Sebaceous glands** secrete an oily secretion into the hair follicle.
This secretion, known as **sebum**, keeps the hair supple and helps
prevent the growth of bacteria (see diagram 5.6).

**Sweat glands** consist of a coiled tube and a duct leading onto the
skin surface. Their appearance when examined under the microscope
inspired one of the first scientists to observe them to call them
"fairies' intestines" (see diagram 5.1). Sweat contains salt and waste
products like urea and the evaporation of sweat on the skin surface is
one of the major mechanisms for cooling the body of many mammals. Horses
can sweat up to 30 liters of fluid a day during active exercise, but
cats and dogs have few sweat glands and must cool themselves by panting.
The scent in the sweat of many animals is used to mark territory or
attract the opposite sex.

**Mammary glands** are only present in mammals. They are thought to be
modified sweat glands and are present in both sexes but are rarely
active in males (see diagram 5.10). The number of glands varies from
species to species. They open to the surface in well-developed nipples.
Milk contains proteins, sugars, fats and salts, although the exact
composition varies from one species to another.

![](_Anatomy_and_physiology_of_animals_Mammary_gland.jpg "_Anatomy_and_physiology_of_animals_Mammary_gland.jpg")

Diagram 5.10 - A Mammary Gland

## The Skin And Sun

A moderate amount of UV in sunlight is necessary for the skin to form
**vitamin D**. This vitamin prevents bone disorders like rickets to
which animals reared indoors are susceptible. Excessive exposure to the
UV in sunlight can be damaging and the pigment **melanin**, deposited in
cells at the base of the epidermis, helps to protect the underlying
layers of the skin from this damage. Melanin also colors the skin and
variations in the amount of melanin produce colors from pale yellow to
black.

### Sunburn And Skin Cancer

Excess exposure to the sun can cause sunburn. This is common in humans,
but light skinned animals like cats and pigs can also be sunburned,
especially on the ears. Skin cancer can also result from excessive
exposure to the sun. As holes in the ozone layer increase exposure to
the sun's UV rays so too does the rate of skin cancer in humans and
animals.

## The Dermis

The underlying layer of the skin, known as the dermis, is much thicker
but much more uniform in structure than the epidermis (see diagram 5.1).
It is composed of loose connective tissue with a felted mass of
**collagen** and **elastic fibres**. It is this part of the skin of
cattle and pigs etc. that becomes commercial leather when treated, The
dermis is well supplied with blood vessels, so cuts and burns that
penetrate down into the dermis will bleed or cause serious fluid loss.
There are also numerous nerve endings and touch receptors in the dermis
because, of course, the skin is sensitive to touch pain and temperature.

When looking at a section of the skin under the microscope you can see
hair follicles, sweat, and sebaceous glands dipping down into the
dermis. However, these structures do not originate in the dermis but are
derived from the epidermis.

In the lower levels of the dermis is a layer of fat or **adipose
tissue** (see diagram 5.1). This acts as an energy store and is an
excellent insulator especially in mammals like whales with little hair.

## The Skin And Temperature Regulation

Vertebrates can be divided into two groups depending on whether or not
they control their internal temperature. Amphibia (frogs) and reptiles
are said to be"**cold blooded**" (**poikilothermic**) because their body
temperature approximately follows that of the environment. Birds and
mammals are said to be **warm blooded (homoiothermic**) because they can
maintain a roughly constant body temperature despite changes in the
temperature of the environment.

Heat is produced by the biochemical reactions of the body (especially in
the liver) and by muscle contraction. Most of the heat loss from the
body occurs via the skin. It is therefore not surprising that many of
the mechanisms for controlling the temperature of the body operate here.

### Reduction Of Heat Loss

When an animal is in a cold environment and needs to reduce heat loss
the erector muscles contract causing the hair or feathers to rise up and
increase the layer of insulating air trapped by them.

![](_Anatomy_and_physiology_of_animals_Hair_muscle.jpg "_Anatomy_and_physiology_of_animals_Hair_muscle.jpg")

Diagram 5.11a) Hair muscle relaxed\...\...\...\...\...Diagram 5.11b)
Hair muscle contracted

Heat loss from the skin surface can also be reduced by the contraction
of the abundant blood vessels that lie in the dermis. This takes blood
flow to deeper levels, so reducing heat loss and causing pale skin (see
diagram 5.12a).

![](_Anatomy_and_physiology_of_animals_Reduction_of_heat_loss_by_skin.jpg "_Anatomy_and_physiology_of_animals_Reduction_of_heat_loss_by_skin.jpg")

Diagram 5.12a) Reduction of heat loss by skin

Shivering caused by twitching muscles produces heat that also helps
raise the body temperature.

### Increase Of Heat Loss

There are two main mechanisms used by animals to increase the amount of
heat lost through the skin when they are in a hot environment or high
levels of activity are increasing internal heat production. The first is
the expansion of the blood vessels in the dermis so blood flows near the
skin surface and heat loss to the environment can take place. The second
is by the production of sweat from the sweat glands (see diagram 5.12b).
The evaporation of this liquid on the skin surface produces a cooling
effect.

The mechanisms for regulating body temperature are under the control of
a small region of the brain called the **hypothalamus**. This acts like
a thermostat.

![](_Anatomy_and_physiology_of_animals_Increase_heat_loss_by_skin.jpg "_Anatomy_and_physiology_of_animals_Increase_heat_loss_by_skin.jpg")

Diagram 5.12b) - Increase of heat loss by skin

### Heat Loss And Body Size

The amount of heat that can be lost from the surface of the body is
related to the area of skin an animal has in relation to the total
volume of its body.

Small animals like mice have a very large skin area compared to their
total volume. This means they tend to lose large amounts of heat and
have difficulty keeping warm in cold weather. They may need to keep
active just to maintain their body temperature or may hibernate to avoid
the problem.

Large animals like elephants have the opposite problem. They have only a
relatively small skin area in relation to their total volume and may
have trouble keeping cool. This is one reason that these large animals
tend to have sparse coverings of hair.

## Summary

-   Skin consists of two layers: the thin **epidermis** and under it the
    thicker **dermis.**
-   The **Epidermis** is formed by the division of base cells that push
    those above them towards the surface where they die and are shed.
-   **Keratin**, a protein, is deposited in the epidermal cells. It
    makes skin waterproof.
-   Various skin structures formed in the epidermis are made of keratin.
    These include claws, nails, hoofs, horn, hair, and feathers.
-   Various **Exocrine Glands** (with ducts) formed in the epidermis
    include sweat, sebaceous, and mammary glands.
-   **Melanin** deposited in cells at the base of the epidermis protects
    deeper cells from the harmful effects of the sun.
-   The **Dermis** is composed of loose connective tissue and is well
    supplied with blood.
-   Beneath the dermis is insulating **adipose tissue**.
-   Body Temperature is controlled by sweat, hair erection, dilation,
    and contraction of dermal capillaries and shivering.

## Worksheet

Use the Skin Worksheet to
help you learn all about the skin.

## Test Yourself

Now use this Skin Test Yourself to see how much you have learned and
remember.

```{=html}
<div class="collapsible">
```
```{=html}
<div class="title">
```
1\. The two layers that form the skin are the a)
_   _ and b) _   _

```{=html}
</div>
```
```{=html}
<div class="body">
```

:   a)epidermis
:   b)dermis

```{=html}
</div>
```
```{=html}
</div>
```

------------------------------------------------------------------------

```{=html}
<div class="collapsible">
```
```{=html}
<div class="title">
```
2\. The special protein deposited in epidermal cells to make them
waterproof is:

```{=html}
</div>
```
```{=html}
<div class="body">
```
keratin

```{=html}
</div>
```
```{=html}
</div>
```

------------------------------------------------------------------------

```{=html}
<div class="collapsible">
```
```{=html}
<div class="title">
```
3\. Many important skin structures are made of keratin. These include:

```{=html}
</div>
```
```{=html}
<div class="body">
```
hair,nails,foot pads,feathers,scales on reptiles

```{=html}
</div>
```
```{=html}
</div>
```

------------------------------------------------------------------------

```{=html}
<div class="collapsible">
```
```{=html}
<div class="title">
```
4\. Sweat, sebaceous and mammary glands all have ducts to the outside.
These kind of glands are known as:_    _

```{=html}
</div>
```
```{=html}
<div class="body">
```
exocrine

```{=html}
</div>
```
```{=html}
</div>
```

------------------------------------------------------------------------

```{=html}
<div class="collapsible">
```
```{=html}
<div class="title">
```
5\. What is the pigment deposited in skin cells that protects underlying
skin layers from the harmful effects of the sun?

```{=html}
</div>
```
```{=html}
<div class="body">
```
melanin

```{=html}
</div>
```
```{=html}
</div>
```

------------------------------------------------------------------------

```{=html}
<div class="collapsible">
```
```{=html}
<div class="title">
```
6\. How does the skin help cool an animal down when it is active or in a
hot environment?

```{=html}
</div>
```
```{=html}
<div class="body">
```
Panting and secretion from sweat glands.

```{=html}
</div>
```
```{=html}
</div>
```

------------------------------------------------------------------------

```{=html}
<div class="collapsible">
```
```{=html}
<div class="title">
```
7\. Name two mechanisms by means of which the skin helps prevent heat
loss when an animal is in a cold environment.

```{=html}
</div>
```
```{=html}
<div class="body">
```

:   a.shivering
:   b.contraction of blood vessels

```{=html}
</div>
```
```{=html}
</div>
```
/Test Yourself Answers/

## Websites

-   <http://www.auburn.edu/academic/classes/zy/0301/Topic6/Topic6.html>
    Comparative anatomy

Good on keratin skin structures - hairs, feathers, horns etc.

-   <http://www.olympusmicro.com/micd/galleries/brightfield/skinhairymammal.html>
    Hairy mammal skin

All about hairy mammalian skin.

-   <http://www.earthlife.net/birds/feathers.html> The wonder of bird
    feathers

Fantastic article on bird feathers with great pictures.

-   <http://en.wikipedia.org/wiki/Skin> Wikipedia

Wikipedia on (human) skin. Good as usual, but more information than you
need.

## Glossary

-   Link to
    Glossary