# Anatomy and Physiology of Animals/Urinary System

## Objectives

After completing this section, you should know:

-   Understand the parts of the urinary system.
-   The structure and function of a kidney.
-   The structure and function of a kidney tubule or nephron.
-   The processes of filtration, reabsorption, secretion and
    concentration that convert blood to urine in the kidney tubule.
-   The function of antidiuretic hormone in producing concentrated
    urine.
-   The composition, storage and voiding of normal urine.
-   Abnormal constituents of urine and their significance.
-   The functions of the kidney in excreting nitrogenous waste,
    controlling water levels and regulating salt concentrations and
    acid-base balance.
-   That birds do not have a bladder.

## Homeostasis

It is defined as the processes in which the animals or humans regulate
their internal temperature. Homeostasis is the maintenance of a stable
internal environment. Homeostasis is a term coined in 1959 to describe
the physical and chemical parameters that an organism must maintain to
allow proper functioning of its component cells, tissues, organs, and
organ systems.

Recall that enzymes function best when within a certain range of
temperature and pH, and that cells must strive to maintain a balance
between having too much or too little water in relation to their
external environment. Both situations demonstrate homeostasis. Just as
we have a certain temperature range (or comfort zone), so our body has a
range of environmental (internal as well as external) parameters within
which it works best. Multicellular organisms accomplish this by having
organs and organ systems that coordinate their homeostasis. In addition
to the other functions that life must perform (recall the discussion in
our Introduction chapter), unicellular creatures must accomplish their
homeostasis within but a single cell!

Single-celled organisms are surrounded by their external environment.
They move materials into and out of the cell by regulation of the cell
membrane and its functioning. Most multicellular organisms have most of
their cells protected from the external environment, having them
surrounded by an aqueous internal environment. This internal environment
must be maintained in such a state as to allow maximum efficiency. The
ultimate control of homeostasis is done by the nervous system. Often
this control is in the form of negative feedback loops. Heat control is
a major function of homeostatic conditions that involves the integration
of skin, muscular, nervous, and circulatory systems.

The difference between homeostasis as a single cell performs it and what
a multicelled creature does derives from their basic organizational
plan: a single cell can dump wastes outside the cell and just be done
with it. Cells in a multicelled creature, such as a human or cat, also
dump wastes outside those cells, but like the trash can or dumpster
outside my house/apartment, those wastes must be carted away. The
carting away of these wastes is accomplished in my body by the
circulatory system in conjunction with the excretory system. For my
house, I have the City of Phoenix sanitation department do that (and get
to pay each month for their service!).

The ultimate control of homeostasis is accomplished by the nervous
system (for rapid responses such as reflexes to avoid picking up a hot
pot off the stove) and the endocrine system (for longer-term responses,
such as maintaining the body levels of calcium, etc.). Often this
homeostatic control takes the form of negative feedback loops. There are
two types of biological feedback: positive and negative. Negative
feedback turns off the stimulus that caused it in the first place. Your
house's heater (or cooler for those of us in the Sun Belt) acts on the
principle of negative feedback. When your house cools off below the
temperature set by your thermostat, the heater is turned on to warm air
until the temperature is at or above what the thermostat is set at. The
thermostat detects this rise in temperature and sends a signal to shut
off the heater, allowing the house to cool of until the heater is turned
on yet again and the cycle (or loop) continues.

## Water In The Body

Water is essential for living things to survive because all the chemical
reactions within a body take place in a solution of water. An animal's
body consists of up to 80% water. The exact proportion depends on the
type of animal, its age, sex, health and whether or not it has had
sufficient to drink. Generally animals do not survive a loss of more
than 15% of their body water.

In vertebrates almost 2/3rd of this water is in the cells
(**intracellular fluid**). The rest is outside the cells
(**extracellular fluid**) where it is found in the spaces around the
cells (**tissue fluid**), as well as in the blood and lymph. Water is
considered to be the source of life. It is important for animal life
because of the following reasons:

\(i\) Water is vital body fluid which is essential for regulating the
processes such as , digestion , transport of nutrients and excretion.
Water dissolves ionic and large number of polar organic compounds. Thus,
it transports the products of digestion to the place of requirement of
the body.

\(ii\) Water regulates the body temperature by the process of sweating
and evaporation.

\(iii\) Water is a medium for all metabolic reactions in the body. All
metabolic reactions in the body take place in solution phase.

\(iv\) Water provides habitat for various animals in the form of ponds
and rivers, sea, etc.

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

Diagram 12.1 - Water in the body

## Maintaining Water Balance

Animals lose water through their skin and lungs, in the faeces and
urine. These losses must be made up by water in food and drink and from
the water that is a by-product of chemical reactions. If the animal does
not manage to compensate for water loss the dissolved substances in the
blood may become so concentrated they become lethal. To prevent this
happening various mechanisms come into play as soon as the concentration
of the blood increases. A part of the brain called the **hypothalamus**
is in charge of these homeostatic processes. The most important is the
feeling of thirst that is triggered by an increase in blood
concentration. This stimulates an animal to find water and drink it.

The kidneys are also involved in maintaining water balance as various
hormones instruct them to produce more concentrated urine and so retain
some of the water that would otherwise be lost (see later in this
Chapter and Chapter 16).

### Desert Animals

Coping with water loss is a particular problem for animals that live in
dry conditions. Some, like the camel, have developed great tolerance for
dehydration. For example, under some conditions, camels can withstand
the loss of one third of their body mass as water. They can also survive
wide daily changes in temperature. This means they do not have to use
large quantities of water in sweat to cool the body by evaporation.

Smaller animals are more able than large ones to avoid extremes of
temperature or dry conditions by resting in sheltered more humid
situations during the day and being active only at night.

The kangaroo rat is able to survive without access to any drinking water
at all because it does not sweat and produces extremely concentrated
urine. Water from its food and from chemical processes is sufficient to
supply all its requirements.

## Excretion

Animals need to excrete because they take in substances that are excess
to the body's requirements and many of the chemical reactions in the
body produce waste products. If these substances were not removed they
would poison cells or slow down metabolism. All animals therefore have
some means of getting rid of these wastes.

The major waste products in mammals are carbon dioxide that is removed
by the lungs, and urea that is produced when excess amino acids (from
proteins) are broken down. Urea is filtered from the blood by the
kidneys.

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

Diagram 12.2 - The position of the organs of the urinary system in a dog

BY GIZAW MEKONNEN

\'\'\'The Kidneys And Urinary System==

The urinary system, also known as the renal system or urinary tract,
consists of the kidneys, ureters, bladder, and the urethra. The purpose
of the urinary system is to eliminate waste from the body, regulate
blood volume and blood pressure, control levels of electrolytes and
metabolites, and regulate blood pH. The urinary tract is the body\'s
drainage system for the eventual removal of urine.\[1\] The kidneys have
an extensive blood supply via the renal arteries which leave the kidneys
via the renal vein. Each kidney consists of functional units called
nephrons. Following filtration of blood and further processing, wastes
(in the form of urine) exit the kidney via the ureters, tubes made of
smooth muscle fibres that propel urine towards the urinary bladder,
where it is stored and subsequently expelled from the body by urination
(voiding). The female and male urinary system are very similar,
differing only in the length of the urethra.

Urine is formed in the kidneys through a filtration of blood. The urine
is then passed through the ureters to the bladder, where it is stored.
During urination, the urine is passed from the bladder through the
urethra to the outside of the body.

800--2,000 milliliters (mL) of urine are normally produced every day in
a healthy human. This amount varies according to fluid intake and kidney
function.

The **kidneys** in mammals are bean-shaped organs that lie in the
abdominal cavity attached to the dorsal wall on either side of the spine
(see diagram 12.2). An artery from the dorsal aorta called the **renal
artery** supplies blood to them and the **renal vein** drains them.

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

Diagram 12.3 - The urinary system

To the naked eye kidneys seem simple enough organs. They are covered by
a fibrous coat or capsule and if cut in half lengthways (longitudinally)
two distinct regions can be seen - an inner region or **medulla** and
the outer **cortex**. A cavity within the kidney called the **pelvis**
collects the urine and carries it to the **ureter**, which connects with
the **bladder** where the urine is stored temporarily. Rings of muscle
(**sphincters**) control the release of urine from the bladder and the
urine leaves the body through the **urethra** (see diagrams 12.3 and
12.4).

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

Diagram 12.4 - The dissected kidney

## Kidney Tubules Or Nephrons

It is only when you examine kidneys under the microscope that you find
that their structure is not simple at all. The cortex and medulla are
seen to be composed of masses of tiny tubes. These are called **kidney
tubules** or **nephrons** (see diagrams 12.5 and 12.6). A human kidney
consists of over a million of them.

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

Diagram 12.5 - Several kidney tubules or nephrons

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

Diagram 12.6 - A kidney tubule or nephron

At one end of each nephron, in the cortex of the kidney, is a cup shaped
structure called the (**Bowman's** or **renal**) **capsule**. It
surrounds a tuft of capillaries called the **glomerulus** that carries
high-pressure blood. Together the glomerulus and capsule act as a
blood-filtering device (see diagram 12.7). The holes in the filter allow
most of the contents of the blood through except the _red and
white cells_ and _large protein
molecules_. The fluid flowing from the capsule into the rest
of the kidney tubule is therefore very similar to blood plasma and
contains many useful substances like water, glucose, salt and amino
acids. It also contains waste products like **urea**.

### Processes Occurring In The Nephron

After entering the glomerulus the filtered fluid flows along a coiled
part of the tubule (the **proximal convoluted tubule**) to a looped
portion (the **Loop of Henle**) and then to the **collecting tube** via
a second length of coiled tube (the **distal convoluted tubule**) (see
diagram 12.6). From the collecting ducts the urine flows into the
**renal pelvis** and enters the **ureter**.

Note that the glomerulus, capsule and both coiled parts of the tubule
are all situated in the cortex of the kidney while the loops of Henle
and collecting ducts make up the medulla (see diagram 12.5).

As the fluid flows along the proximal convoluted tubule useful
substances like glucose, water, salts, potassium ions, calcium ions and
amino acids are **reabsorbed** into the blood capillaries that form a
network around the tubules. Many of these substances are transported by
active transport and energy is required.

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

Diagram 12.7 - Filtration in the glomerulus and capsule

In a separate process, some substances, particularly potassium, ammonium
and hydrogen ions, and drugs like penicillin, are actively **secreted**
into the distal convoluted tubule.

By the time the fluid has reached the collecting ducts these processes
of absorption and secretion have changed the fluid originally filtered
into the Bowman's capsule into urine. The main function of the
collecting ducts is then to remove more water from the urine if
necessary. These processes are summarised in diagram 12.8.

**Normal urine** consists of water, in which waste products such as urea
and salts such as sodium chloride are dissolved. Pigments from the
breakdown of red blood cells give urine its yellow colour.

### The Production Of Concentrated Urine

Because of the high pressure of the blood in the glomerulus and the
large size of the pores in the glomerulus/capsule-filtering device, an
enormous volume of fluid passes into the kidney tubules. If this fluid
were left as it is, the animal's body would be drained dry in 30
minutes. In fact, as the fluid flows down the tubule, over 90% of the
water in it is reabsorbed. The main part of this reabsorption takes
place in the collecting tubes.

The amount of water removed from the collecting ducts is controlled by a
hormone called **antidiuretic hormone (ADH)** produced by the
**pituitary gland**, situated at the base of the brain. When the blood
becomes more concentrated, as happens when an animal is deprived of
water, ADH is secreted and causes more water to be absorbed from the
collecting ducts so that concentrated urine is produced. When the animal
has drunk plenty of water and the blood is dilute, no ADH is secreted
and no or little water is absorbed from the collecting ducts, so dilute
urine is produced. In this way the concentration of the blood is
controlled precisely.

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

Diagram 12.8 - Summary of the processes involved in the formation of
urine

## Water Balance In Fish And Marine Animals

### Fresh Water Fish

Although the skin of fish is more or less waterproof, the gills are very
porous. The body fluids of fish that live in fresh water have a higher
concentration of dissolved substances than the water in which they swim.
In other words the body fluids of fresh water fish are **hypertonic** to
the water (see chapter 3). Water therefore flows into the body by
**osmosis**. To stop the body fluids being constantly diluted fresh
water fish produce large quantities of dilute urine.

### Marine Fish

Marine fish like the sharks and dogfish have body fluids that have the
same concentration of dissolved substances as the water (**isotonic**)
have little problem with water balance. However, marine bony fish like
red cod, snapper and sole, have body fluids with a lower concentration
of dissolved substances than seawater (they are **hypotonic** to
seawater). This means that water tends to flow out of their bodies by
osmosis. To make up this fluid loss they drink seawater and get rid of
the excess salt by excreting it from the gills.

### Marine Birds

Marine birds that eat marine fish take in large quantities of salt and
some only have access to seawater for drinking. Bird's kidneys are
unable to produce very concentrated urine, so they have developed a salt
gland. This excretes a concentrated salt solution into the nose to get
rid of the excess salt.

## Diabetes And The Kidney

There are two types of diabetes. The most common is called sugar
diabetes or **diabetes mellitus** and is common in cats and dogs
especially if they are overweight. It is caused by the pancreas
secreting insufficient **insulin**, the hormone that controls the amount
of glucose in the blood. If insulin secretion is inadequate, the
concentration of glucose in the blood increases. Any increase in the
glucose in the blood automatically leads to an increase in glucose in
the fluid filtered into the kidney tubule. Normally the kidney removes
all the glucose filtered into it, but these high concentrations swamp
this removal mechanism and urine containing glucose is produced. The
main symptoms of this type of diabetes are the production of large
amounts of dilute urine containing glucose, and excessive thirst.

The second type of diabetes is called **diabetes insipidus**. The name
comes from the main symptom, which is the production of large amounts of
very dilute and "tasteless" urine. It occurs when the pituitary gland
produces insufficient ADH, the hormone that stimulates water
re-absorption from the kidney tubule. When this hormone is lacking,
water is not absorbed and large amounts of dilute urine are produced.
Because so much water is lost in the urine, animals with this form of
diabetes can die if deprived of water for only a day or so.

## Other Functions Of The Kidney

The excretion of urea from the body and the maintenance of water
balance, as described above, are the main functions of the kidney.
However, the kidneys have other roles in keeping conditions in the body
stable i.e. in maintaining homeostasis. These include:

:\* controlling the concentration of salt ions (Na+, K+, Cl-) in the
blood by adjusting how much is excreted or retained;

:\* maintaining the correct acidity of the blood. Excess acid is
constantly being produced by the normal chemical reactions in the body
and the kidney eliminates this.

## Normal Urine

Normal urine consists of water (95%), urea, salts (mostly sodium
chloride) and pigments (mostly from bile) that give it its
characteristic colour.

## Abnormal Ingredients Of Urine

If the body is not working properly, small amounts of substances not
normally present may be found in the urine or substances normally
present may appear in abnormal amounts.

:\* The presence of **glucose** may indicate diabetes (see above).

:\* Urine with red blood cells in it is called **haematuria**, and may
indicate inflammation of the kidney,or urinary tract, cancer or a blow
to the kidneys.

:\* Sometimes free **haemoglobin** is found in the urine. This indicates
that the red blood cells in the blood have **haemolysed** (the membrane
has broken down) and the haemoglobin has passed into the kidney tubules.

:\* The presence of **white blood cells** in the urine indicates there
is an infection in the kidney or urinary tract.

:\* **Protein molecules** are usually too large to pass into the kidney
tubule so no or only small amounts of proteins like **albumin** is
normally found in urine. Large quantities of albumin indicate that the
kidney tubules have been injured or the kidney has become diseased. High
blood pressure also pushes proteins from the blood into the tubules.

:\* **Casts** are tiny cylinders of material that have been shed from
the lining of the tubules and flushed out into the urine.

:\* **Mucus** is not usually found in the urine of healthy animals but
is a normal constituent of horses' urine, giving it a characteristic
cloudy appearance.

Tests can be carried out to identify any abnormal ingredients of urine.
These tests are normally done by "**stix**", which are small plastic
strips with absorbent ends impregnated with various chemicals. A colour
change occurs in the presence of an abnormal ingredient.

## Excretion In Birds

Birds' high body temperature and level of activity means that they need
to conserve water. Birds therefore do not have a bladder and instead of
excreting urea, which needs to be dissolved in large amounts of water,
birds produce uric acid that can be discharged as a thick paste along
with the feces. This is the white chalky part of the bird droppings that
land on you or your car.

CONCLUSION

-   The excretory system consists of paired **kidneys** and associated
    blood supply. **Ureters** transport urine from the kidneys to the
    bladder and the **urethra** with associated sphincter muscles
    controls the release of urine.
-   The kidneys have an important role in maintaining **homeostasis** in
    the body. They excrete the waste product urea, control the
    concentrations of water and salt in the body fluids, and regulate
    the acidity of the blood.
-   A kidney consists of an outer region or **cortex,** inner
    **medulla** and a cavity called the **pelvis** that collects the
    urine and carries it to the ureter.
-   The tissue of a kidney is composed of masses of tiny tubes called
    **kidney tubules** or **nephrons**. These are the structures that
    make the urine.
-   High-pressure blood is supplied to the nephron via a tuft of
    capillaries called the **glomerulus**. Most of the contents of the
    blood except the cells and large protein molecules filter from the
    glomerulus into the (**Bowmans) capsule.** This fluid flows down a
    coiled part of the tubule (**proximal convoluted tubule**) where
    useful substances like glucose, amino acids and various ions are
    reabsorbed. The fluid flows to a looped portion of the tubule called
    the **Loop of Henle** where water is reabsorbed and then to another
    coiled part of the tubule (**distal convoluted tubule**) where more
    reabsorbtion and secretion takes place. Finally the fluid passes
    down the **collecting duct** where water is reabsorbed to form
    concentrated urine.

`                                                                                BY GIZAW MEKONNEN`

## Worksheet

Use this Excretory System
Worksheet to
help you learn the parts of the urinary system, the kidney and kidney
tubule and their functions.

## Test Yourself

The Urinary System Test Yourself can then be used to see if you
understand this rather complex system.

1\. Add the following labels to the diagram of the excretory system
shown below. Bladder \| ureter \| urethra \| kidney \| dorsal aorta \|
vena cava \| renal artery \| vein

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

2\. Using the words/phrases in the list below fill in the blanks in the
following statements.

:

    :   \| cortex \| amino acids \| renal \| glucose \| water
        reabsorption \| large proteins \|
    :   \| bowman's capsule \| diabetes mellitus \| secreted \|
        antidiuretic hormone (ADH) \| blood cells \|
    :   \| glomerulus \| concentration of the urine \| medulla \|
        nephron \|

a\) Blood enters the kidney via the \...\...\...\...\...\...\...\....
artery.

b\) When cut across the kidney is seen to consist of two regions, the
outer\...\...\...\..... and the inner\...\...\...\.....

c\) Another word for the kidney tubule is
the\...\...\...\...\...\...\...\...\...\....

d\) Filtration of the blood occurs in
the\...\...\...\...\...\...\...\...\...\...

e\) The filtered fluid (filtrate) enters
the\...\...\...\...\...\...\...\...\.....

f\) The filtrate entering the e) above is similar to blood but does not
contain\...\...\...\...\...\... or\...\...\...\...\...\.....

g\) As the fluid passes along the first coiled part of the kidney
tubule\...\...\...\...\...\... and\...\...\...\...\...\..... are
removed.

h\) The main function of the loop of Henle
is\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\....

i\) Hydrogen and potassium ions
are\...\...\...\...\...\...\...\...\...\... into the second coiled part
of the tubule.

j\) The main function of the collecting tube
is\...\...\...\...\...\...\...\...\...\...\...\...\...\...\...\.....

k\) The hormone\...\...\...\...\...\...\...\...\...\...\...\..... is
responsible for controlling water reabsorption in the collecting tube.

**Write short answer for following question** l) When the pancreas
secretes inadequate amounts of the hormone insulin the condition known
as\...\...\...\...\...\...\...\...\...\.... results. This is most easily
diagnosed by testing for\...\...\...\...\...\...\...\...\...\..... in
the urine.What is Homeostasis?

2\. Give 2 examples of homeostasis.

3\. List 3 ways in which animals keep their body temperature constant
when the weather is hot.

4\. How does the kidney compensate when an animal is deprived of water
to drink?

6\. Describe how panting helps to reduce the acidity of the blood.

/Test Yourself Answers/

## Websites

-   <http://www.biologycorner.com/bio3/nephron.html> Biology Corner. A
    fabulous drawing of the kidney and nephron to print off, label and
    colour in with clear explanation of function.
-   <http://health.howstuffworks.com/adam-200032.htm> How Stuff Works.
    This animation traces the full process of urine formation and
    reabsorption in the kidneys, its path down the ureter to the
    bladder, and its excretion via the urethra. Needs Shockwave.
-   <http://en.wikipedia.org/wiki/Nephron> Wikipedia. A bit more detail
    than you need but still good clear explanations and lots of
    information.

## Glossary

-   Link to
    Glossary


# Anatomy and Physiology of Animals/Reproductive System

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

## Objectives

After completing this section, you should know:

-   the role of mitosis and meiosis in the production of gametes (sperm
    and ova)
-   that gametes are haploid cells
-   that fertilization forms a diploid zygote
-   the major parts of the male reproductive system and their functions
-   the route sperm travel along the male reproductive tract to reach
    the penis
-   the structure of a sperm and the difference between sperm and semen
-   the difference between infertility and impotence
-   the main parts of the female reproductive system and their functions
-   the ovarian cycle and the roles of FSH, LH, oestrogen and
    progesterone
-   the oestrous cycle and the signs of heat in rodents, dogs, cats and
    cattle
-   the process of fertilization and where it occurs in the female tract
-   what a morula and a blastocyst are
-   what the placenta is and its functions

## Reproductive System

In biological terms sexual reproduction involves the union of
**gametes** - the sperm and the ovum - produced by two parents. Each
gamete is formed by **meiosis** (see Chapter 3). This means each
contains only half the chromosomes of the body cells (**haploid**).
Fertilization results in the joining of the male and female gametes to
form a **zygote** which contains the full number of chromosomes
(**diploid**). The zygote then starts to divide by **mitosis** (see
Chapter 3) to form a new animal with all its body cells containing
chromosomes that are identical to those of the original zygote (see
diagram 13.1).

Diagram 13.1 - Sexual reproduction

The offspring formed by sexual reproduction contain genes from both
parents and show considerable variation. For example, kittens in a
litter are all different although they (usually) have the same mother
and father. In the wild this variation is important because it means
that when the environment changes some individuals may be better adapted
to survive than others. These survivors pass their "superior" genes on
to their offspring. In this way the characteristics of a group of
animals can gradually change over time to keep pace with the changing
environment. This "survival of the fittest" or "**natural selection**"
is the mechanism behind the theory of **evolution**.

## Fertilization

In most fish and amphibia (frogs and toads) fertilization of the egg
cells takes place outside the body. The female lays the eggs and then
the male deposits his sperm on or at least near them.

In reptiles and birds, eggs are fertilized inside the body when the male
deposits the sperm inside the **egg duct** of the female. The egg is
then surrounded by a resistant shell, "laid" by the female and the
embryo completes its development inside the egg.

In mammals the sperm are placed in the body of the female and the eggs
are fertilized internally. They then develop to quite an advanced stage
inside the body of the female. When they are born they are fed on milk
excreted from the mammary glands and protected by their parents until
they become independent.

## Sexual Reproduction In Mammals

The reproductive organs of mammals produce the **gametes** (sperm and
egg cells), help them fertilize and then support the developing embryo.

## The Male Reproductive System

The male reproductive system consists of a pair of testes that produce
**sperm** (or**spermatozoa**), ducts that transport the sperm to the
penis and glands that add secretions to the sperm to make **semen** (see
diagram 13.2).

The various parts of the male reproductive system with a summary of
their functions are shown in diagram 13.3.

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

Diagram 13.2. The reproductive organs of a male dog

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

Diagram 13.3 - Diagram summarizing the functions of the male
reproductive organs

### The Testicles

Sperm need temperatures between 2 and 10 degrees Centigrade lower than
the body temperature to develop. This is the reason why the testes are
located in a bag of skin called the **scrotal sacs** (or **scrotum**)
that hangs below the body and where the evaporation of secretions from
special glands can further reduce the temperature. In many animals
(including humans) the testes descend into the scrotal sacs at birth but
in some animals they do not descend until sexual maturity and in others
they only descend temporarily during the breeding season. A mature
animal in which one or both testes have not descended is called a
**cryptorchid** and is usually infertile if both testicles have not
descended.

The problem of keeping sperm at a low enough temperature is even greater
in birds that have a higher body temperature than mammals. For this
reason bird's sperm are usually produced at night when the body
temperature is lower and the sperm themselves are more resistant to
heat.

The testes consist of a mass of coiled tubes (the **seminiferous**
or**sperm producing tubules**) in which the sperm are formed by meiosis
(see diagram 13.4). Cells lying between the seminiferous tubules produce
the male sex hormone **testosterone**.

When the sperm are mature they accumulate in the **collecting ducts**
and then pass to the **epididymis**before moving to the **sperm duct**
or **vas deferens**. The two sperm ducts join the **urethra** just below
the bladder, which passes through the **penis** and transports both
sperm and urine.

**Ejaculation** discharges the semen from the erect penis. It is brought
about by the contraction of the epididymis, vas deferens, prostate gland
and urethra.

![](Anatomy_and_physiology_of_animals_The_testis_&_a_magnified_seminferous_tubule.jpg "Anatomy_and_physiology_of_animals_The_testis_&_a_magnified_seminferous_tubule.jpg")

Diagram 13.4 - The testis and a magnified seminiferous tubule

### Semen

Semen consists of 10% sperm and 90% fluid and as sperm pass down the
ducts from testis to penis, (accessory) glands add various secretion\...

### Accessory Glands

Three different glands may be involved in producing the secretions in
which sperm are suspended, although the number and type of glands varies
from species to species.

**Seminal vesicles** are important in rats, bulls, boars and stallions
but are absent in cats and dogs. When present they produce secretions
that make up much of the volume of the semen, and transport and provide
nutrients for the sperm.

The **prostate gland** is important in dogs and humans. It produces an
alkaline secretion that neutralizes the acidity of the male urethra and
female vagina.

**Cowper's glands** (bulbourethral glands) have various functions in
different species. The secretions may lubricate, flush out urine or form
a gelatinous plug that traps the semen in the female reproductive system
after copulation and prevents other males of the same species
fertilizing an already mated female. Cowper's glands are absent in bears
and aquatic mammals.

### The Penis

The penis consists of connective tissue with numerous small blood spaces
in it. These fill with blood during sexual excitement causing erection.

#### Penis Form And Shape

Dogs, bears, seals, bats and rodents have a special bone in the penis
which helps maintain the erection (see diagram 13.2). In some animals
(e.g. the bull, ram and boar) the penis has an "S" shaped bend that
allows it to fold up when not in use. In many animals the shape of the
penis is adapted to match that of the vagina. For example, the boar has
a corkscrew shaped penis, there is a pronounced twist in bulls' and it
is forked in marsupials like the opossum. Some have spines, warts or
hooks on them to help keep them in the vagina and copulation may be
extended to help retain the semen in the female system. Mating can last
up to three hours in minks, and dogs may "knot" or "tie" during mating
and can not separate until the erection has subsided.

### Sperm

Sperm are made up of three parts: a **head** consisting mainly a
prominent haploid nucleus which carries the genetic material and also an
acrosome, a **midpiece** containing many mitochondria to provide the
energy and a **tail** that provides propulsion (see diagram 13.5).

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

Diagram 13.5 - A sperm

A single ejaculation may contain 2-3 hundred million sperm but even in
normal semen as many as 10% of these sperm may be abnormal and
infertile. Some may be dead while others are inactive or deformed with
double, giant or small heads or tails that are coiled or absent
altogether.

When there are too many abnormal sperm or when the sperm concentration
is low, the semen may not be able to fertilize an egg and the animal is
infertile. Make sure you don\'t confuse infertility with impotence,
which is the inability to copulate successfully.

Sperm do not live forever. They have a definite life span that varies
from species to species. They survive for between 20 days (guinea pig)
to 60 days (bull) in the epididymis but once ejaculated into the female
tract they only live from 12 to 48 hours. When semen is used for
artificial insemination, storage under the right conditions can extend
the life span of some species.

### Artificial Insemination

In many species the male can be artificially stimulated to ejaculate and
the semen collected. It can then be diluted, stored and used to
**inseminate** females. For example bull semen can be diluted and stored
for up to 3 weeks at room temperature. If mixed with an antifreeze
solution and stored in "straws" in liquid nitrogen at minus 79^o^C it
will keep for much longer. Unfortunately the semen of chickens,
stallions and boars can only be stored for up to 2 days.

Dilution of the semen means that one male can be used to fertilise many
more females than would occur under natural conditions. There are also
advantages in the male and female not having to make physical contact.
It means that owners of females do not have to buy expensive males and
the possibility of transmitting sexually transmitted diseases is
reduced. Routine examination of the semen for sperm concentration,
quality and activity allows only the highest quality semen to be used so
a high success rate is ensured.

Since the lifespan of sperm in the female tract is so short and ova only
survive from 8 to 10 hours the timing of the artificial insemination is
critical. Successful conception depends upon detecting the time that the
animal is "on heat" and when ovulation occurs.

## The Female Reproductive Organs

The female reproductive system consists of a pair of **ovaries** that
produce egg cells or **ova** and **fallopian tubes** where fertilisation
occurs and which carry the fertilised ovum to the **uterus**. Growth of
the foetus takes place here. The **cervix** separates the uterus from
the **vagina** or birth canal, where the sperm are deposited (see
diagram 13.6).

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

Diagram 13.6. - The reproductive system of a female rabbit

Note that primates like humans have a uterus with a single compartment
but in most mammals the uterus is divided into two separate parts or
**horns** as shown in diagram 13.6.

### The Ovaries

Ovaries are small oval organs situated in the abdominal cavity just
ventral to the kidneys. Most animals have a pair of ovaries but in birds
only the left one is functional to reduce weight (see below).

The ovary consists of an inner region (**medulla**) and an outer region
(**cortex**) containing egg cells or ova. These are formed in large
numbers around the time of birth and start to develop after the animal
becomes sexually mature. A cluster of cells called the **follicle**
surrounds and nourishes each ovum.

### The Ovarian Cycle

The **ovarian cycle** refers to the series of changes in the ovary
during which the follicle matures, the ovum is shed and the **corpus
luteum** develops (see diagram 13.7).

Numerous undeveloped ovarian follicles are present at birth but they
start to mature after sexual maturity. In animals that normally have
only one baby at a time only one ovum will mature at once but in litter
animals several will. The mature follicle consists of outer cells that
provide nourishment. Inside this is a fluid-filled space that contains
the ovum.

A mature follicle can be quite large, ranging from a few millimetres in
small mammals to the size of a golf ball in large animals. It bulges out
from the surface of the ovary before eventually rupturing to release the
ovum into the abdominal cavity. Once the ovum has been shed, a blood
clot forms in the empty follicle. This develops into a tissue called the
**corpus luteum** that produces the hormone **progesterone** (see
diagram 13.9). If the animal becomes pregnant the corpus luteum
persists, but if there is no pregnancy it degenerates and a new ovarian
cycle usually.

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

Diagram 13.7 - The ovarian cycle showing from the top left clockwise:
the maturation of the ovum over time, followed by ovulation and the
development of the corpus luteum in the empty follicle

### The Ovum

When the ovum is shed the nucleus is in the final stages of meiosis
(cell division). It is surrounded by few layers of follicle cells and a
tough membrane called the **zona pelluc**ida (see diagram 13.8).

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

Diagram 13.8 - An ovum

### The Oestrous Cycle

The **oestrous cycle** is the sequence of hormonal changes that occurs
through the **ovarian cycle**. These changes influence the behaviour and
body changes of the female (see diagram 13.9).

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

Diagram 13.9 - The oestrous cycle

The first hormone involved in the oestrous cycle is **follicle
stimulating hormone (F.S.H.),** secreted by the **anterior pituitary
gland** (see chapter 16). It stimulates the follicle to develop. As the
follicle matures the outer cells begin to secrete the hormone
**oestrogen** and this stimulates the mammary glands to develop. It also
prepares the lining of the uterus to receive a fertilised egg. Ovulation
is initiated by a surge of another hormone from the anterior pituitary,
**luteinising hormone (L.H.).** This hormone also influences the
development of the corpus luteum, which produces **progesterone**, a
hormone that prepares the lining of the uterus for the fertilised ovum
and readies the mammary glands for milk production. If no pregnancy
takes place the corpus luteum shrinks and the production of progesterone
decreases. This causes FSH to be produced again and a new oestrous cycle
begins.

For fertilisation of the ovum by the sperm to occur, the female must be
receptive to the male at around the time of ovulation. This is when the
hormones turn on the signs of "**heat**", and she is "**in season**" or
"**in oestrous**". These signs are turned off again at the end of the
oestrous cycle.

During the oestrous cycle the lining of the uterus (**endometrium**)
thickens ready for the fertilised ovum to be implanted. If no pregnancy
occurs this thickened tissue is absorbed and the next cycle starts. In
humans and other higher primates, however, the endometrium is shed as a
flow of blood and instead of an oestrous cycle there is a **menstrual
cycle**.

The length of the oestrous cycle varies from species to species. In rats
the cycle only lasts 4--5 days and they are sexually receptive for about
14 hours. Dogs have a cycle that lasts 60--70 days and heat lasts 7--9
days and horses have a 21-day cycle and heat lasts an average of 6 days.

**Ovulation** is spontaneous in most animals but in some, e.g. the cat,
and the rabbit, ovulation is stimulated by mating. This is called
**induced ovulation**.

### Signs Of Oestrus Or Heat

:\*When on heat a bitch has a blood stained discharge from the **vulva**
that changes a little later to a straw coloured one that attracts all
the dogs in the neighbourhood.

:\* Female cats "call" at night, roll and tread the carpet and are
generally restless but will "stand" firm when pressure is placed on the
pelvic region (this is the lordosis response).

:\* A female rat shows the lordosis response when on heat. It will
"mount" other females and be more active than normal.

:\* A cow mounts other cows (bulling), bellows, is restless and has a
discharge from the vulva.

### Breeding Seasons And Breeding Cycles

Only a few animals breed throughout the year. This includes the higher
primates (humans, gorillas and chimpanzees etc.), pigs, mice and
rabbits. These are known as **continuous breeders**.

Most other animals restrict reproduction to one or two seasons in the
year-**seasonal breeders** (see diagram 13.10). There are several
reasons for this. It means the young can be born at the time (usually
spring) when feed is most abundant and temperatures are favourable. It
is also sensible to restrict the breeding season because courtship,
mating, gestation and the rearing of young can exhaust the energy
resources of an animal as well as make them more vulnerable to
predators.

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

Diagram 13.10 - Breeding cycles

The timing of the breeding cycle is often determined by day length. For
example the shortening day length in autumn will bring sheep and cows
into season so the foetus can gestate through the winter and be born in
spring. In cats the increasing day length after the winter solstice
(shortest day) stimulates breeding. The number of times an animal comes
into season during the year varies, as does the number of oestrous
cycles during each season. For example a dog usually has 2-3 seasons per
year, each usually consisting of just one oestrous cycle. In contrast
ewes usually restrict breeding to one season and can continue to cycle
as many as 20 times if they fail to become pregnant.

## Fertilisation and Implantation

### Fertilization

The opening of the fallopian tube lies close to the ovary and after
ovulation the ovum is swept into its funnel-like opening and is moved
along it by the action of cilia and wave-like contractions of the wall.

**Copulation** deposits several hundred million sperm in the vagina.
They swim through the cervix and uterus to the fallopian tubes moved
along by whip-like movements of their tails and contractions of the
uterus. During this journey the sperm undergo their final phase of
maturation so they are ready to fertilize the ovum by the time they
reach it in the upper fallopian tube.

High mortality means only a small proportion of those deposited actually
reach the ovum. The sperm attach to the outer **zona pellucida** and
enzymes secreted from a gland in the head of the sperm dissolve this
membrane so it can enter. Once one sperm has entered, changes in the
**zona pellucida** prevent further sperm from penetrating. The sperm
loses its tail and the two nuclei fuse to form a **zygote** with the
full set of paired chromosomes restored.

### Development Of The Morula And Blastocyst

As the fertilised egg travels down the fallopian tube it starts to
divide by mitosis. First two cells are formed and then four, eight,
sixteen, etc. until there is a solid ball of cells. This is called a
**morula**. As division continues a hollow ball of cells develops. This
is a **blastocyst** (see diagram 13.11).

### Implantation

Implantation involves the blastocyst attaching to, and in some species,
completely sinking into the wall of the uterus.

## Pregnancy

### The Placenta And Fetal Membranes

As the **embryo** increases in size, the **placenta**, **umbilical
cord** and **fetal membranes** (often known collectively as the
**placenta**) develop to provide it with nutrients and remove waste
products (see diagram 13.12). In later stages of development the embryo
becomes known as a **fetus**.

The placenta is the organ that attaches the fetus to the wall of the
uterus. In it the blood of the fetus and mother flow close to each other
but never mix (see diagram 13.13). The closeness of the maternal and
fetal blood systems allows diffusion between them. Oxygen and nutrients
diffuse from the mother's blood into that of the fetus and carbon
dioxide and excretory products diffuse in the other direction. Most
maternal hormones (except adrenaline), antibodies, almost all drugs
(including alcohol), lead and DDT also pass across the placenta.
However, it protects the fetus from infection with bacteria and most
viruses.

![](Anatomy_and_physiology_of_animals_Development_&_implantation_of_the_embryo.jpg "Anatomy_and_physiology_of_animals_Development_&_implantation_of_the_embryo.jpg")

Diagram 13.11 - Development and implantation of the embryo

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

Diagram 13.12. The fetus and placenta

The fetus is attached to the placenta by the **umbilical cord**. It
contains arteries that carry blood to the placenta and a vein that
returns blood to the fetus. The developing fetus becomes surrounded by
membranes. These enclose the amniotic fluid that protects the fetus from
knocks and other trauma (see diagram 13.12).

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

Diagram 13.13 - Maternal and fetal blood flow in the placenta

### Hormones During Pregnancy

The corpus luteum continues to secrete progesterone and oestrogen during
pregnancy. These maintain the lining of the uterus and prepare the
mammary glands for milk secretion. Later in the pregnancy the placenta
itself takes over the secretion of these hormones.

**Chorionic gonadotrophin** is another hormone secreted by the placenta
and placental membranes. It prevents uterine contractions before labour
and prepares the mammary glands for lactation. Towards the end of
pregnancy the placenta and ovaries secrete **relaxin**, a hormone that
eases the joint between the two parts of the pelvis and helps dilate the
cervix ready for birth.

### Pregnancy Testing

The easiest method of pregnancy detection is ultrasound which is
noninvasive and very reliable Later in gestation pregnancy can be
detected by taking x-rays.

In dogs and cats a blood test can be used to detect the hormone
**relaxin**.

In mares and cows palpation of the uterus via the rectum is the classic
way to determine pregnancy. It can also be done by detecting the
hormones **progesterone** or **equine chorionic gonagotrophin**
(**eCG**) in the urine. A new sensitive test measures the amount of the
hormone, **oestrone sulphate**, present in a sample of faeces. The
hormone is produced by the foal and placenta, and is only present when
there is a living foal.

In most animals, once pregnancy is advanced, there is a window of time
during which an experienced veterinarian can determine pregnancy by
feeling the abdomen.

### Gestation Period

The young of many animals (e.g. pigs, horses and elephants) are born at
an advanced state of development, able to stand and even run to escape
predators soon after they are born. These animals have a relatively long
gestation period that varies with their size e.g. from 114 days in the
pig to 640 days in the elephant.

In contrast, cats, dogs, mice, rabbits and higher primates are
relatively immature when born and totally dependent on their parents for
survival. Their gestation period is shorter and varies from 25 days in
the mouse to 31 days in rabbits and 258 days in the gorilla.

The babies of marsupials are born at an extremely immature stage and
migrate to the pouch where they attach to a teat to complete their
development. Kangaroo joeys, for example, are born 33 days after
conception and opossums after only 8 days.

## Birth

### Signs Of Imminent Birth

As the pregnancy continues, the mammary glands enlarge and may secrete a
milky substance a few days before birth occurs. The vulva may swell and
produce thick mucus and there is sometimes a visible change in the
position of the foetus. Just before birth the mother often becomes
restless, lying down and getting up frequently. Many animals seek a
secluded place where they may build a nest in which to give birth.

### Labour

Labour involves waves of uterine contractions that press the foetus
against the cervix causing it to dilate. The foetus is then pushed
through the cervix and along the vagina before being delivered. In the
final stage of labour the placenta or "afterbirth" is expelled.

### Adaptations Of The Fetus To Life Outside The Uterus

The fetus grows in the watery, protected environment of the uterus where
the mother supplies oxygen and nutrients, and waste products pass to her
blood circulation for excretion. Once the baby animal is born it must
start to breathe for itself, digest food and excrete its own waste. To
allow these functions to occur blood is re-routed to the lungs and the
glands associated with the gut start to secrete. Note that newborn
animals can not control their own body temperature. They need to be kept
warm by the mother, litter mates and insulating nest materials.

## Milk Production

Cows, manatees and primates have two mammary glands but animals like
pigs that give birth to large litters may have as many as 12 pairs.
Ducts from the gland lead to a nipple or teat and there may be a sinus
where the milk collects before being suckled (see diagram 13.14).

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

Diagram 13.14 - A mammary gland

The hormones **oestrogen** and **progesterone** stimulate the mammary
glands to develop and **prolactin** promotes the secretion of the milk.
**Oxytocin** from the pituitary gland releases the milk when the baby
suckles. The first milk is called **colostrum**. It is a rich in
nutrients and contains protective antibodies from the mother. Milk
contains fat, protein and milk sugar as well as vitamins and most
minerals although it contains little iron. Its actual composition varies
widely from species to species. For example whale's and seal's milk has
twelve times more fat and four times more protein than cow's milk. Cow's
milk has far less protein in it than cat's or dog's milk. This is why
orphan kittens and puppies cannot be fed cow's milk.

## Reproduction In Birds

Male birds have testes and sperm ducts and male swans, ducks, geese and
ostriches have a penis. However, most birds make do with a small amount
of erectile tissue known as a **papilla**. To reduce weight for flight
most female birds only have one ovary - usually the left, which produces
extremely yolky eggs. The eggs are fertilised in the upper part of the
oviduct (equivalent to the fallopian tube and uterus of mammals) and as
they pass down it **albumin** (the white of the egg), the membrane
beneath the shell and the shell are laid down over the yolk. Finally the
egg is covered in a layer of mucus to help the bird lay it (see diagram
13.15).

Most birds lay their eggs in a nest and the hen sits on them until they
hatch. Ducklings and chicks are relatively well developed when they
hatch and able to forage for their own food. Most other nestlings need
their parents to keep them warm, clean and fed. Young birds grow rapidly
and have voracious appetites that may involve the parents making up to
1000 trips a day to supply their need for food.

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

Diagram 13.15 - Female reproductive organs of a bird

## Summary

-   **Haploid** gametes (sperm and ova) are produced by meiosis in the
    **gonads** (testes and ovaries).
-   Fertilization involves the fusing of the gametes to form a diploid
    **zygote**.
-   The male reproductive system consists of a pair of **testes** that
    produce sperm (or **spermatozoa**), ducts that transport the sperm
    to the penis and glands that add secretions to the sperm to make
    semen.
-   Sperm are produced in the **seminiferous tubules**, are stored in
    the **epididymis** and travel via the **vas deferens** or **sperm
    duct** to the junction of the bladder and the **urethra** where
    various accessory glands add secretions. The fluid is now called
    **semen** and is ejaculated into the female system down the
    **urethra** that runs down the centre of the penis.
-   Sperm consist of a head, a midpiece and a tail.
-   **Infertility** is the inability of sperm to fertilize an egg while
    **impotence** is the inability to copulate successfully.
-   The female reproductive system consists of a pair of **ovaries**
    that produce **ova** and **fallopian tubes** where fertilization
    occurs and which carry the fertilized ovum to the **uterus**. Growth
    of the fetus takes place here. The **cervix** separates the uterus
    from the **vagina**, the birth canal and where the sperm are
    deposited.
-   The **ovarian cycle** refers to the series of changes in the ovary
    during which the follicle matures, the ovum is shed and the **corpus
    luteum** develops.
-   The **oestrous cycle** is the sequence of hormonal changes that
    occurs through the ovarian cycle. It is initiated by the secretion
    of **follicle stimulating hormone (F.S.H.),** by the **anterior
    pituitary gland** which stimulates the **follicle** to develop. The
    follicle secretes **oestrogen** which stimulates **mammary gland**
    development. **luteinising hormone (L.H.)** from the anterior
    pituitary initiates **ovulation** and stimulates the **corpus
    luteum** to develop. The corpus luteum produces **progesterone**
    that prepares the lining of the uterus for the fertilized ovum.
-   **Signs of oestrous** or heat differ. A bitch has a blood stained
    discharge, female cats and rats are restless and show the lordosis
    response, while cows mount other cows, bellow and have a discharge
    from the vulva.
-   After fertilization in the fallopian tube the **zygote** divides
    over and over by mitosis to become a ball of cells called a
    **morula**. Division continues to form a hollow ball of cells called
    the **blastocyst**. This is the stage that **implants** in the
    uterus.
-   The **placenta, umbilical cord** and **fetal membranes** (known as
    the **placenta**) protect and provide the developing fetus with
    nutrients and remove waste products.

## Worksheet

Reproductive System
Worksheet

## Test Yourself

1\. Add the following labels to the diagram of the male reproductive
organs below.

:

    :   testis \| epididymis \| vas deferens \| urethra \| penis \|
        scrotal sac \| prostate gland

Diagram of the Male Reproductive System

2\. Match the following descriptions with the choices given in the list
below.

:

    :   accessory glands \| vas deferens or sperm duct \| penis \|
        scrotum \| fallopian tube \| testes \| urethra \| vagina \|
        uterus \| ovary \| vulva

:   a\) Organ that delivers semen to the female vagina

:   b\) Where the sperm are produced

:   c\) Passage for sperm from the epididymis to the urethra

:   d\) Carries both sperm and urine down the penis

:   e\) Glands that produce secretions that make up most of the semen

:   f\) Bag of skin surrounding the testes

:   g\) Where the foetus develops

:   h\) This receives the penis during copulation

:   i\) Where fertilisation usually occurs

:   j\) Ova travel along this tube to reach the uterus

:   k\) Where the ova are produced

:   l\) The external opening of the vagina

3\. Which hormone is described in each statement below?

:   a\) This hormone stimulates the growth of the follicles in the ovary
:   b\) This hormone converts the empty follicle into the corpus luteum
    and stimulates it to produce progesterone
:   c\) This hormone is produced by the cells of the follicle
:   d\) This hormone is produced by the corpus luteum
:   e\) This hormone causes the mammary glands to develop
:   f\) This hormone prepares the lining of the uterus to receive a
    fertilised ovum

4\. State whether the following statements are true or false. If false
write in the correct answer.

:   a\) Fertilisation of the egg occurs in the uterus
:   b\) The fertilised egg cell contains half the normal number of
    chromosomes
:   c\) The morula is a hollow ball of cells
:   d\) The mixing of the blood of the mother and foetus allows
    nutrients and oxygen to transfer easily to the foetus
:   e\) The morula implants in the wall of the uterus
:   f\) The placenta is the organ that supplies the foetus with oxygen
    and nutrients
:   g\) Colostrum is the first milk
:   h\) Young animals often have to be given calcium supplements because
    milk contains very little calcium

/Test Yourself Answers/

## Websites

-   <http://www.anatomicaltravel.com/CB_site/Conception_to_birth3.htm>
    Anatomical travel. Images of fertilisation and the development of
    the (human) embryo through to birth.
-   <http://www.uchsc.edu/ltc/fert.swf> Fertilisation. A great animation
    of fertilisation, formation of the zygote and first mitotic
    division. A bit advanced but still worth watching.
-   <http://www.uclan.ac.uk/facs/health/nursing/sonic/scenarios/salfordanim/heart.swf>
    Sonic. An animation showing the foetal blood circulation through the
    placenta to the changes allowing circulation through the lungs after
    birth.
-   <http://en.wikipedia.org/wiki/Estrus> Wikipedia. As always, good
    interesting information although some terms and concepts are beyond
    the requirements of this level.

## Glossary

-   Link to
    Glossary


# Anatomy and Physiology of Animals/Nervous System

!Original image by Royalty-free image
collection.
Used under a CC-BY
licence.{width="400"}

## Objectives

After completing this section, you should know:

-   the role of the nervous system in coordinating an animal's response
    to the environment
-   that the nervous system gathers, sorts and stores information and
    initiates movement
-   the basic structure and functions of a neuron
-   the structure and function of a synapse and neurotransmitter
    chemicals
-   the nervous pathway known as a reflex with examples
-   that training can develop conditioned reflexes in animals
-   that the nervous system can be divided into the central and
    peripheral nervous systems
-   that the brain is surrounded by membranes called meninges
-   the basic parts of the brain and the function of the cerebral
    hemispheres, hypothalamus, pituitary, cerebellum and medulla
    oblongata
-   the structure and function of the spinal cord
-   that the peripheral nervous system consists of cranial and spinal
    nerves and the autonomic nervous system
-   that the autonomic nervous system consists of sympathetic and
    parasympathetic parts with different functions

## Coordination

Animals must be able to sense and respond to the environment in which
they live if they are to survive. They need to be able to sense the
temperature of their surroundings, for example, so they can avoid the
hot sun. They must also be able to identify food and escape predators.

The various systems and organs in the body must also be linked so they
work together. For example, once a predator has identified suitable prey
it has to catch it. This involves coordinating the contraction of the
muscle so the predator can run, there must then be an increased blood
supply to the muscles to provide them with oxygen and nutrients. At the
same time the respiration rate must increase to supply the oxygen and
remove the carbon dioxide produced as a result of this increased
activity. Once the prey has been caught and eaten, the digestive system
must be activated to digest it.

The adjustment of an animal's response to changes in the environment and
the complex linking of the various processes in the body that this
response involves are called **co-ordination**. Two systems are involved
in co-ordination in animals. These are the **nervous** and **endocrine
systems**. The first operates via electrical impulses along nerve fibres
and the second by releasing special chemicals or hormones into the
bloodstream from glands.

## Functions of the Nervous System

The nervous system has three basic functions:

:   1\. **Sensory function** - to sense changes (known as stimuli) both
    outside and within the body. For example the eyes sense changes in
    light and the ear responds to sound waves. Inside the body, stretch
    receptors in the stomach indicate when it is full and chemical
    receptors in the blood vessels monitor the acidity of the blood.

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:   2\. **Integrative function** - processing the information received
    from the sense organs. The impulses from these organs are analysed
    and stored as memory. The many different impulses from different
    sources are sorted, synchronised and co-ordinated and the
    appropriate response initiated. The power to integrate, remember and
    apply experience gives higher animals much of their superiority.

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:   3\. **Motor function** - The third function is the response to the
    stimuli that causes muscles to contract or glands to secrete.

All nervous tissue is made up of nerve cells or **neurons.** These
transmit high-speed signals called **nerve impulses**. Nerve impulses
can be thought of as being similar to an electric current.

## The Neuron

Neurons are cells that have been adapted to carry nerve impulses. A
typical neuron has a **cell body** containing a nucleus, one or more
branching filaments called **dendrites** which conduct nerve impulses
towards the cell body and one long fibre, an **axon**, that carries the
impulses away from it. Many axons have a sheath of fatty material called
**myelin** surrounding them. This speeds up the rate at which the nerve
impulses travel along the nerve (see diagram 14.1).

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

Diagram 14.1 - A motor neuron

The cell body of neurons is usually located in the brain or spinal cord
while the axon extends the whole distance to the organ that it supplies.
The neuron carrying impulses from the spinal cord to the hind leg or
tail of a horse, for example, can be several feet long. A **nerve** is a
bundle of axons.

A **sensory neuron** is a nerve cell that transmits impulses from a
sense receptor such as those in the eye or ear to the brain or spinal
cord. A **motor neuron** is a nerve cell that transmits impulses from
the brain or spinal cord to a muscle or gland. A **relay neuron**
connects sensory and motor neurons and is found in the brain or spinal
cord (see diagrams 14.1 and 14.2).

![](Anatomy_and_physiology_of_animals_Relation_btw_sensory,_relay_&_motor_neurons.jpg "Anatomy_and_physiology_of_animals_Relation_btw_sensory,_relay_&_motor_neurons.jpg")

Diagram 14.2 - The relationship between sensory, relay and motor neurons

### Connections Between Neurons

The connection between adjacent neurons is called a **synapse**. The two
nerve cells do not actually touch here for there is a microscopic space
between them. The electrical impulse in the neurone before the synapse
stimulates the production of chemicals called **neurotransmitters**
(such as **acetylcholine**), which are secreted into the gap.

The neurotransmitter chemicals diffuse across the gap and when they
contact the membrane of the next nerve cell they stimulate a new nervous
impulse (see diagram 14.3). After the impulse has passed the chemical is
destroyed and the synapse is ready to receive the next nerve impulse.

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

Diagram 14.3 - A nerve and magnification of a synapse

## Reflexes

A **reflex** is a rapid automatic response to a stimulus. When you
accidentally touch a hot object and automatically jerk your hand away,
this is a reflex action. It happens without you having to think about
it. Animals automatically blink when an object approaches the eye and
cats twist their bodies in the air when falling so they land on their
paws. (Please don't test this one at home with your pet cat!).

Swallowing, sneezing, and the constriction of the pupil of the eye in
bright light are also all reflex actions. Some other examples of reflex
actions in animals can be shivering with cold and the opening of the
mouth on hearing a sudden loud noise.

The path taken by the nerve impulses in a reflex is called a **reflex
arc**. Most reflex arcs involve only three neurons (see diagram 14.4).
The **stimulus** (a pin in the paw) stimulates the pain receptors of the
skin, which initiate an impulse in a sensory neuron. This travels to the
spinal cord where it passes, by means of a synapse, to a connecting
neuron called the relay neuron situated in the spinal cord. The relay
neuron in turn makes a synapse with one or more motor neurons that
transmit the impulse to the muscles of the limb causing them to contract
and remove the paw from the sharp object. Reflexes do not require
involvement of the brain although you are aware of what is happening and
can, in some instances, prevent them happening. Animals are born with
their reflexes. You can think of them as being wired in.

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

Diagram 14.4 - A reflex arc

### Conditioned Reflexes

In most reflexes the stimulus and response are related. For example the
presence of food in the mouth causes the salivary glands to release
saliva. However, it is possible to train animals (and humans) to respond
to different and often quite irrelevant stimuli. This is called a
**conditioned reflex**.

A Russian biologist called Pavlov carried out the classic experiment to
demonstrate such a reflex when he conditioned dogs to salivate at the
sound of a bell ringing. Almost every pet owner can identify reflexes
they have conditioned in their animals. Perhaps you have trained your
cat to associate food with the opening of the fridge door or accustomed
your dog to the routines you go through before taking them for a walk.

## Parts of the Nervous System

When we describe the nervous system of vertebrates we usually divide it
into two parts (see diagram 14.5).

:   1\. The **central nervous system** (**CNS**) which consists of the
    brain and spinal cord.
:   2\. The **peripheral nervous system** (**PNS**) which consists of
    the nerves that connect to the brain and spinal cord (cranial and
    spinal nerves) as well as the **autonomic** (or involuntary) nervous
    system.

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

Diagram 14.5 - The nervous system of a horse

### The Central Nervous System

The **central nervous system** consists of the brain and spinal cord. It
acts as a kind of 'telephone exchange' where a vast number of cross
connections are made.

When you look at the brain or spinal cord some regions appear creamy
white (**white matter**) and others appear grey (**grey matter**). White
matter consists of masses of nerve axons and the grey matter consists of
the cell bodies of the nerve cells. In the brain the grey matter is on
the outside and in the spinal cord it is on the inside (see diagram
14.2).

#### The Brain

The major part of the brain lies protected within the sturdy "box" of
skull called the **cranium**. Surrounding the fragile brain tissue (and
spinal cord) are protective membranes called the **meninges** (see
diagram 14.6), and a crystal-clear fluid called **cerebrospinal fluid**,
which protects and nourishes the brain tissue. This fluid also fills
four cavities or **ventricles** that lie within the brain.

Brain tissue is extremely active and, even when an animal is resting, it
uses up to 20% of the oxygen taken into the body by the lungs. The
**carotid artery**, a branch off the dorsal aorta, supplies it with the
oxygen and nutrients it requires. Brain damage occurs if brain tissue is
deprived of oxygen for only 4--8 minutes.

The brain consists of three major regions:

:   1\. the **fore brain** which includes the **cerebral hemispheres**,
    **hypothalamus** and **pituitary gland**;
:   2\. the **hind brain** or **brain stem**, contains the **medulla
    oblongata** and **pons** and
:   3\. the **cerebellum** or "little brain" (see diagram 14.6).

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

Diagram 14.6 - Longitudinal section through the brain of a dog

##### Mapping the brain

In humans and some animals the functions of the different regions of the
cerebral cortex have been mapped (see diagram 14.7).

Diagram 14.7 - The functions of the regions of the human cerebral cortex

##### The Forebrain

The **cerebral hemispheres** are the masses of brain tissue that sit on
the top of the brain. The surface is folded into ridges and furrows
called **sulci** (singular sulcus). They make this part of the brain
look rather like a very large walnut kernel. The two hemispheres are
separated by a deep groove although they are connected internally by a
thick bundle of nerve fibres. The outer layer of each hemisphere is
called the **cerebral cortex** and this is where the main functions of
the cerebral hemispheres are carried out.

The cerebral cortex is large and convoluted in mammals compared to other
vertebrates and largest of all in humans because this is where the
so-called "higher centres" concerned with memory, learning, reasoning
and intelligence are situated.

Nerves from the eyes, ears, nose and skin bring sensory impulses to the
cortex where they are interpreted. Appropriate voluntary movements are
initiated here in the light of the memories of past events.

Different regions of the cortex are responsible for particular sensory
and motor functions, e.g. vision, hearing, taste, smell, or moving the
fore-limbs, hind-limbs or tail. For example, when a dog sniffs a scent,
sensory impulses from the organ of smell in the nose pass via the
olfactory (smelling) nerve to the olfactory centres of the cerebral
hemispheres where the impulses are interpreted and co-ordinated.

In humans and some animals the functions of the different regions of the
cerebral cortex have been mapped (see diagram 14.8).

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

Diagram 14.8 - The functions of the regions of the cerebral cortex

The **hypothalamus** is situated at the base of the brain and is
connected by a "stalk" to the **pituitary gland**, the "master"
hormone-producing gland (see chapter 16). The hypothalamus can be
thought of as the bridge between the nervous and endocrine (hormone
producing) systems. It produces some of the hormones that are released
from the pituitary gland and controls the release of others from it.

It is also an important centre for controlling the internal environment
of the animal and therefore maintaining homeostasis. For example, it
helps regulate the movement of food through the gut and the temperature,
blood pressure and concentration of the blood. It is also responsible
for the feeling of being hungry or thirsty and it controls sleep
patterns and sex drive.

##### The Hindbrain

The **medulla oblongata** is at the base of the brain and is a
continuation of the spinal cord. It carries all signals between the
spinal cord and the brain and contains centres that control vital body
functions like the basic rhythm of breathing, the rate of the heartbeat
and the activities of the gut. The medulla oblongata also co-ordinates
swallowing, vomiting, coughing and sneezing.

##### The Cerebellum

The **cerebellum** (little brain) looks rather like a smaller version of
the cerebral hemispheres attached to the back of the brain. It receives
impulses from the organ of balance (vestibular organ) in the inner ear
and from stretch receptors in the muscles and tendons. By co-ordinating
these it regulates muscle contraction during walking and running and
helps maintain the posture and balance of the animal. When the
cerebellum malfunctions it causes a tremor and uncoordinated movement.

#### The Spinal Cord

The spinal cord is a cable of nerve tissue that passes down the channel
in the vertebrae from the hindbrain to the end of the tail. It becomes
progressively smaller as paired **spinal nerves** pass out of the cord
to parts of the body. Protective membranes or meninges cover the cord
and these enclose cerebral spinal fluid (see diagram 14.9).

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

Diagram 14.9 - The spinal cord

If you cut across the spinal cord you can see that it consists of white
matter on the outside and grey matter in the shape of an H or butterfly
on the inside.

### The Peripheral Nervous System

The **peripheral nervous system** consists of nerves that are connected
to the brain (**cranial nerves**), and nerves that are connected to the
spinal cord (**spinal nerves**). The **autonomic nervous system** is
also part of the peripheral nervous system.

#### Cranial Nerves

There are twelve pairs of cranial nerves that come from the brain. Each
passes through a hole in the cranium (brain case). The most important of
these are the olfactory, optic, acoustic and vagus nerves.

The **olfactory nerves** - (smell) carry impulses from the olfactory
organ of the nose to the brain.

The **optic nerves** - (sight) carry impulses from the retina of the eye
to the brain.

The **auditory (acoustic) nerves** - (hearing) carry impulses from the
cochlear of the inner ear to the brain.

The **vagus nerve** - controls the muscles that bring about swallowing.
It also controls the muscles of the heart, airways, lungs, stomach and
intestines (see diagram 14.5).

#### Spinal Nerves

**Spinal nerves** connect the spinal cord to sense organs, muscles and
glands in the body. Pairs of spinal nerves leave the spinal cord and
emerge between each pair of adjacent vertebrae (see diagram 14.9).

The **sciatic nerve** is the largest spinal nerve in the body (see
diagram 14.5). It leaves the spinal cord as several nerves that join to
form a flat band of nervous tissue. It passes down the thigh towards the
hind leg where it gives off branches to the various muscles of this
limb.

#### The Autonomic Nervous System

The **autonomic nervous system** controls internal body functions that
are not under conscious control. For example when a prey animal is
chased by a predator the autonomic nervous system automatically
increases the rate of breathing and the heartbeat. It dilates the blood
vessels that carry blood to the muscles, releases glucose from the
liver, and makes other adjustments to provide for the sudden increase in
activity. When the animal has escaped and is safe once again the nervous
system slows down all these processes and resumes all the normal body
activities like the digestion of food.

The nerves of the autonomic nervous system originate in the spinal cord
and pass out between the vertebrae to serve the various organs (see
diagram 14.10).There are two main parts to the autonomic nervous
system---the **sympathetic system** and the **parasympathetic system**.

The **sympathetic system** stimulates the "flight, fright, fight"
response that allows an animal to face up to an attacker or make a rapid
departure. It increases the heart and respiratory rates, as well as the
amount of blood flowing to the skeletal muscles while blood flow to less
critical regions like the gut and skin is reduced. It also causes the
pupils of the eyes to dilate. Note that the effects of the sympathetic
system are similar to the effects of the hormone adrenaline (see Chapter
16).

The **parasympathetic system** does the opposite to the sympathetic
system. It maintains the normal functions of the relaxed body. These are
sometimes known as the "housekeeping" functions. It promotes effective
digestion, stimulates defaecation and urination and maintains a regular
heartbeat and rate of breathing.

![](Anatomy_and_physiology_of_animals_Function_of_the_sympathetic_&_parasympathetic_nervous_systems.jpg "Anatomy_and_physiology_of_animals_Function_of_the_sympathetic_&_parasympathetic_nervous_systems.jpg")

Diagram 14.10 - The function of the sympathetic and parasympathetic
nervous systems

## Summary

-   The **neuron** is the basic unit of the nervous system. It consists
    of a **cell body** with a nucleus, filaments known as **dendrites**
    and a long fibre known as the **axon** often surrounded by a
    **myelin sheath**.
-   A **nerve** is a bundle of axons.
-   **Grey matter** in the brain and spinal cord consists mainly of
    brain cells while **white matte**r consists of masses of axons.
-   **Nerve Impulses** travel along axons.
-   Adjacent neurons connect with each other at **synapses**.
-   **Reflexes** are automatic responses to stimuli. The path taken by
    nerve impulses involved in reflexes is a **reflex arc**. Most reflex
    arcs involve 3 neurons - a **sensory neuron**, a **relay neuron**
    and a **motor neuron**. A stimulus, a pin in the paw for example,
    initiates an impulse in the sensory neuron that passes via a synapse
    to the relay neuron situated in the spinal cord and then via another
    synapse to the motor neurone. This transmits the impulse to the
    muscle causing it to contract and remove the paw from the pin.
-   The nervous system is divided into 2 parts: the **central nervous
    system**, consisting of the brain and spinal cord and the
    **peripheral nervous system** consisting of nerves connected to the
    brain and spinal cord. The **autonomic nervous system** is
    considered to be part of the peripheral nervous system.
-   The brain consists of three major regions: 1. the **fore brain**
    which includes the **cerebral hemispheres** (or **cerebrum**),
    **hypothalamus** and **pituitary gland**; 2. the **hindbrain** or
    **brain stem** containing the **medulla oblongata** and 3. the
    **cerebellum**.
-   Protective membranes known as the **meninges** surround the brain
    and spinal cord.
-   There are 12 pairs of cranial nerves that include the optic,
    olfactory, acoustic and **vagus** nerves.
-   The **spinal cord** is a cable of nerve tissue surrounded by
    meninges passing from the brain to the end of the tail. **Spinal
    nerves** emerge by a **ventral** and **dorsal root** between each
    vertebra and connect the spinal cord with organs and muscles.
-   The **autonomic nervous system** controls internal body functions
    not under conscious control. It is divided into 2 parts with 2
    different functions: the **sympathetic nervous system** that is
    involved in the flight and fight response including increased heart
    rate, bronchial dilation, dilation of the pupil and decreased gut
    activity. The **parasympathetic nervous system** is associated with
    decreased heart rate, pupil constriction and increased gut activity.

## Worksheet

Nervous System
Worksheet

## Test Yourself

1\. Add the following labels to this diagram of a motor neuron.

:   cell body \| nucleus \| axon \| dendrites \| myelin sheath \| muscle
    fibres

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

2\. What is a synapse?

3\. What is a reflex?

4\. Rearrange the parts of a reflex arc given below in the order in
which the nerve impulse travels from the sense organ to the muscle.

:   sense organ \| relay neuron \| motor neuron \| sensory neuron \|
    muscle fibres

5\. Add the following labels to the diagram of the dog's brain shown
below.

:   cerebellum \| cerebral hemisphere \| cerebral cortex \| pituitary
    gland \| medulla oblongata

![](Anatomy_and_physiology_unlabeled_LS_dog's_brain.jpg "Anatomy_and_physiology_unlabeled_LS_dog's_brain.jpg")

6\. What is the function of the meninges that cover the brain and spinal
cord

7\. Give 3 effects of the action of the sympathetic nervous system.

/Test Yourself Answers/

## Websites

-   <http://en.wikipedia.org/wiki/Neuron> Wikipedia. Lots of good
    information here but as usual a warning that there are terms and
    concepts that are beyond the scope of this course. Also try 'reflex
    action' ; 'autonomic nervous system' ;
-   <http://images.google.co.nz/imgres?imgurl=http://static.howstuffworks.com/gif/brain-neuron.gif&imgrefurl=http://science.howstuffworks.com/brain1.htm&h=296&w=394&sz=17&hl=en&start=5&tbnid=LWLRI9lW_5PZhM:&tbnh=93&tbnw=124&prev=/images%3Fq%3Dneuron%26svnum%3D10%26hl%3Den%26lr%3D%26sa%3DN>
    How Stuff Works. This site is for the neuron but try 'neuron types',
    'brain parts' and 'balancing act' too.
-   <http://web.archive.org/web/20060821134839/http://www.bbc.co.uk/schools/gcsebitesize/flash/bireflexarc.swf>
    Reflex Arc. Nice clear and simple animation of a reflex arc.

## Glossary

-   Link to
    Glossary