Chapter 2

body (Figure 2.1). Functions for motion and regulation include seeing, hearing, and moving joints.

How your bodies function

Organs inside and outside the body are involved in these functions.

(Mechanisms and functions of your body)1) Basic structure of the human body (cells, genes, tissues, and organs)

Learning objectives

The human body is made up of cells as minimum units. A cell is composed of a cell nucleus and the

You will be able to gain proper understanding and explain:cytoplasm surrounding it. The cell’s basic function is to produce proteins based on the information- The mechanisms and functions of a body that supports life.

possessed by the cell nucleus. Each cell nucleus contains 23 pairs of chromosomes (46 in total), and one- The regulatory mechanisms of the human body in supporting daily activities, in relation to yourchromosome contains hundreds to thousands of genes. Genes correspond to the blueprint of the humanbody and form a part (region) of deoxyribonucleic acids (DNAs), which holds specific genetic

own daily activities.

information such as “what kind of protein to make. DNA is a chain of small units called nucleotides
In this chapter, you will learn about the basic structure (structure and morphology of organs) andcomposed of sugar, a nitrogenous base, and a phosphate group. It usually has a double-helix structure infunctions (roles and functions of organs) of the human body; organs and tissues functioning while

which two strands of nucleotides running in opposite directions are bound to each other. Long-pitchinteracting with each other; and functions of the body that maintain a stable internal environment.

filamentous double-helix DNA is connected to histones (protein) to form chromosomes, and is locatedin the nucleus of a cell. There are four types of DNA bases, namely adenine (A), guanine (G), thymine

1. Structure and functions of the human body(T), and cytosine (C), where A binds to T and G binds to C (Figure 2.2). Sequences in which the fourtypes of bases are arranged (base sequence) provide encoded (symbolized) genetic information. There
are many areas in DNA whose functions have yet to be understood.

The human body carries out various functions. These functions are broadly grouped into those that

There are many types of cells in the body, such as heart, liver, and muscle cells. They vary in size

sustain life and those that actively use life for motion and regulation. Examples of functions that sustain

and shape by their functions. For example, red blood cells (erythrocytes) are disc-shaped and are able tolife include eating food to obtain nutrition, breathing to take in air, and circulating blood throughout thechange shape easily. This allows them to squeeze through narrow blood vessels and transport oxygen
throughout the body. Derived from a single fertilized egg, all cells essentially have the same DNA.
However, different genes are expressed in different cells (or at different times in the same cell), which is

Thyroid gland
Trachea
Superior vena cava
Lung
Liver
Inferior vena cava
Gallbladder
KidneyUreterVermiform appendix

Aorta
Heart
Esophagus
Diaphragm
Aorta

Figure 2.1 Anatomy of the human body (Organs)32

chromosome

chromatin

2nm

nucleosome

10nm

StomachhistonePancreas

Sugar
CH2base phosphate

Small intestine
Large intestineDNA

1400nmSpleenBladder
Urethrawhy cells are able to acquire diverse appearances and functions.

nucleotide

nm = nanometre, 10-9 (m)RectumFigure 2.2

DNA structure
Chapter 2

How your bodies function (Mechanisms and functions of your body)33

A collection of specifically differentiated cells is called a tissue. Tissues work together to create anorgan that performs a specific function. A group of organs with similar functions is called an organsystem, which can be classified as shown in Table 2.1:

Main functions

Motor system(Bone, and muscle)

Bones form the skeleton (strut) that supports the body.
The body is able to move simply by moving bones withjoints. Bones also protect internal organs and thehematopoietic function.

Main organs

Bones, cartilage, joints,
muscles

34

23, spinal column 26, thorax 25, upper limbs 64, lower limbs 62), with five types of bone shapes (long,for air.

CardiovascularsystemHeart, blood vessels(arteries, veins), blood,
lymph vessels

Respiratory system

The respiratory system takes in oxygen necessary for
activities into the body and discharges carbon dioxide. It
also functions to produce sound waves (pharynx) andresonate in the oral cavity to produce voice.

Nose, pharynx, trachea,
lungs
Brain, spinal cord(central nervous system)

Nervous system

The nervous system can be divided into the central
nervous system (which processes collected informationand sends out instructions) and the peripheral nervous
system (which conveys instructions from the central
nervous system to various parts of the body and conveys
systemic information to the central nervous system).
The digestive system functions to chew, swallow, digest,
and absorb the beneficial parts of food into the body.

Oral cavity, esophagus,
stomach, small intestine,large intestine, rectum,
liver, pancreas

Autonomic nerve,
somatic nerve (peripheral
nervous system)

Urinary system

The urinary system excretes unnecessary substances inthe blood as urine. It adjusts the amount and composition Kidney, urinary tract,of urine to maintain the constant composition and osmotic bladder, urethra
pressure of body fluids.

Endocrine system

The endocrine system secretes substances (hormones) that
Pituitary gland, thyroid
circulate throughout the body along with the blood flow
and work on specific organs. It regulates the functions of gland, adrenal gland,pancreas, gonads
those organs.

Sensory system

The sensory system mainly collects information outside
the body to regulate and control the body’s activity.

Eyes, ears, tongue, nose,
skin

Reproductive
systemThe reproductive system produces sperms and eggs andfunctions to preserve species (create future generations),from the binding (fertilization) of sperms and eggs todelivery.

[Male] Testis, seminal
duct

Immune system

The skeleton is the base tissue shaping the human body. It includes a total of about 200 bones (cranial
short, flat, irregular, and pneumatized bones). Pneumatized bones (hollow bones) are filled with space

The cardiovascular system consists of the heart (acting as
a pump to circulate blood) and blood vessels (acting as
pipes for supplying blood), and functions to deliver therequired nutrients and oxygen to cells throughout thebody, as well as carry carbon dioxide and other waste
products away for disposal.

The immune system functions to eliminate foreign
substances invading the body and abnormal cells in the
body. The body is affected if its function deteriorates orreacts excessively.

and other connective tissues. It functions to move and support the body.

(1) Skeleton

Skeletal muscles are able to contract and change theirlength to move each part of the body.

Digestive system

1) Motor system
The motor system consists of the bones (making up the skeleton), muscles, joints, ligaments, cartilage,

Table 2.1 Main functions and types of organ systems

Organ system

2. Structures and functions of organs[Female] Ovary, uterus
Lymphatic vessels, bonemarrow, spleen, thymus,
blood, skin, tonsils

The roles of bones are as follows: (1) supporting the head, internal organs, and trunk, (2) supporting
movement, (3) protecting key organs, (4) storing minerals (calcium, phosphorus), and (4) creating bloodin the bone marrow (red blood cells, white blood cells, and platelets).

(2) Muscle
Muscles can be grouped into skeletal, cardiac, and smooth (visceral) muscles based on their differencesin shape and function. Skeletal muscle can be consciously moved through the somatic nerve (voluntarymuscle). Its roles include (1) moving the body by contraction and relaxation of skeletal muscle, (2)maintaining posture, and (3) producing body heat. Through muscle contraction, the skeleton moves at
joints. The axis and range of joint movement are mainly determined by the shape of the joint. During
movement, the muscles that mainly work are called prime mover (agonist muscles), the muscles thatwork together to move the body are called synergist muscles, and the muscles that perform opposite
movements are called antagonist muscles. For example, both flexors and extensors are antagonisticmuscles (Figure 2.3).

Biceps brachii

Humerus

Triceps brachii

Ulna
RadiusFigure 2.3 Muscle structure (agonist and antagonist muscles)
Chapter 2

How your bodies function (Mechanisms and functions of your body)35

AortaThe myocardium is the muscle forming the heart wall. It is the same striated muscle as the skeletal
Superior vena cava

muscle, but unlike the skeletal muscle, it cannot be moved consciously and is regulated by the autonomic

Pulmonary trunk
Left pulmonary artery

nerves (involuntary muscle). It differs from the skeletal muscle in that it does not get fatigued no matter
how much it continues moving, and in that myocardial cells do not have the ability to divide, whichmeans that these cells will not be regenerated once they are damaged or undergo necrosis, for example,
due to events such as myocardial infarction.

Right pulmonary arteryLeft pulmonary veins

Right pulmonary veins

Smooth muscles are also involuntary muscles, which are controlled by the autonomic nerves in theLeft atrium

Right atrium

same way as myocardium. They serve as the internal organ walls and blood vessel walls of the digestive
tract, airways, etc. Their ability to contract is not as strong as skeletal or heart muscles, but they can

Right ventricle

continue to move without getting tired.

Left ventricle
Interventricular septum

Inferior vena cava

Aorta

1

Column: Why do elderly people fall easily? (Aging of bones and muscles)

Figure 2.4 Heart structure and blood flow

With aging come many changes in the skeleton and muscles. For example, sarcopenia, a phenomenon
in which muscle mass decreases with age, begins around 30 years of age and progresses throughout
life. During this process, the amount of muscle tissue and the number and size of muscle fibers(2) Blood pressure regulation

gradually decrease. As a result, muscle mass and strength gradually decrease. This mild weakness

The pressure of blood on the walls of blood vessels is called blood pressure. The blood pumped byincreases the load on some joints (e.g., knee joints), thereby increasing susceptibility to arthritis andcontraction of the ventricles generates blood pressure. Blood pressure decreases as distance from the left

falls. Although physical inactivity (lack of exercise) is assumed to be the main cause, its mechanism

ventricle increases or elasticity of the vascular wall (vascular resistance) changes while blood moves in

has yet to be fully understood.

the vasculature from arteries through arterioles, capillaries, and venules to veins (Figure 2.5).
Vascular resistance is defined by: (1) inner diameter of the blood vessels, (2) viscosity of blood, and2) Circulatory system(3) total length of the vessels. The smaller the inner diameter of the blood vessel, the greater the resistanceto blood flow. Generally, physiological changes in the vessel diameter of the arterioles affect bloodpressure. Blood pressure lowers during vasodilation and rises during vasoconstriction. Blood pressure

The circulatory system consists of the heart, blood vessels, blood, and lymphatic vessels. It sends

rises when the viscosity of blood, which is regulated by the amount of red blood cells and plasma,

oxygen and nutrients to cells throughout the body and takes back carbon dioxide and waste products that

increases, and falls when it falls. Blood pressure is maintained relatively constant at rest. In adults,

are no longer needed by the cells.

normal values are approximately 120 mmHg for systolic blood pressure and 80 mmHg for diastolic(1) Circulation
The heart acts like a pump and is slightly larger than the fist (weighs about 250–300 g in adults). It is
located slightly to the left of the center of the chest in the mediastinum space in the thoracic cavity,
surrounded on its right and left sides by the lungs, and connected to the diaphragm at the bottom.
The heart is composed of four chambers (left ventricle, left atrium, right ventricle, and rightatrium). The atriums receive blood to the heart, and the ventricles pump out blood from the heart(Figure 2.4). Blood from the left ventricle flows throughout the body to supply oxygen and nutrients toblood pressure.
bloodpressure(mm Hg)

140
120
100
80
60

somatic cells, receives waste from those cells, and returns to the right atrium. This circulation is called

40

systemic circulation. On the other hand, blood from the right ventricle passes through the lungs, where20

it undergoes gas exchange (exchange of oxygen and carbon dioxide), and then returns to the left atrium.

0This is called pulmonary circulation.

mean arterial pressure

aorta

large
arteries

arterioles
venules large
venae
venules cavae
capillaries

right
ventricle

Increasing distance from left ventricle

Figure 2.5 Changes in blood pressure along systemic circulation2

36

Chapter 2

How your bodies function (Mechanisms and functions of your body)373) Respiratory system

The nervous system can be divided into the central nervous system (brain and spinal cord, thecenter of functions of the nervous system) and the peripheral nervous system (somatic and autonomic

The respiratory system consists of the upper respiratory tract (nasal cavity, pharyngeal pharynx,

nervous systems). Peripheral nerves cover every corner of the body and play a key role in connecting

larynx), lower respiratory tract (trachea and bronchus), and lungs. It works to take into the bodyeach part of the body to the central nervous system (Figure 2.7).

necessary oxygen and excrete carbon dioxide outside of the body (Figure 2.6).
The function of taking oxygen (O2) from the air (outside air) into the blood and excreting the carbon
Brain

Cerebrum, Diencephalon (Thalamus,
Hypothalamus), Cerebellum, Brainstem (Midbrain, Pons, and Medulla
oblongata)

Spinal cord

Medulla oblongata, Thoracic spinal
cord, Lumbar spinal cord, andSacral cord

dioxide (CO2) generated in the body from the blood to outside the body is called external respiration.
The O2 and CO2 that enter and leave the body through breathing are collectively called respiratory

Central nervous
systemgases, and O2 and CO2 gas exchange is performed in the alveolis.
After emitting CO2 and taking in O2 by external respiration, the blood returns to the heart as arterial
blood, and is pumped out from the left ventricle to the entire body. This blood reaches the periphery,
enters the capillaries, and exchanges gas with body tissues. The direction of inflow and outflow of
respiratory gas at this stage is opposite to those in external respiration in the lungs, i.e. O2 is released
from the blood and taken in by tissue cells, while CO2 generated by cell metabolism is released from

Nervous systemcells and moves into blood. Such gas exchange at peripheral tissues is called internal respiration.

Somatic nervous
systemPeripheral nervous
systemNasal cavity
Pharynx
LarynxAutonomic nervous
systemTrachea

Right lungLeft lungSensory nerve (afferent nerve)
Conveys information from different
distal parts of the body to thecentral nervous system.
Motor nerve (efferent nerve)
Conveys information from thecentral nervous system to different
distal portions of the body.
Sympathetic nervous system
Parasympathetic nervous system

Figure 2.7 Structure of nervous system classification(1) Central nervous system3, 4
The central nervous system comprises the brain and spinal cord. Located in the cranial cavity, the brain
Diaphragm
Figure 2.6 Respiratory organs

4) Nervous systemweighs approximately 1,300–1,400 g in adults. It can be divided into four main parts (cerebrum,diencephalon, cerebellum, and brain stem). The cerebrum is the most developed part of the brain andoccupies 80% of the whole brain volume. Its functions include identifying information and instructing
movements accordingly (primary functions), as well as advanced mental functions such as memory,emotion, motivation, and cognition (higher functions). Under the cerebrum lies the cerebellum (motor
regulation). Between the left and right sides of the brain is the diencephalon (sensory information38

Neurons are the basic developmental, morphological, and functional units of the nervous system. They

processing, and integration of the autonomic nervous system). The brain stem is the center of theare excited by stimuli, and the excitement is transmitted to other neurons by synapses. Neuronal

functions supporting life, namely respiration, circulation, digestion, and thermoregulation. The medulla

excitement generated by the stimulation of receptors in various parts of the body is transmitted to the

oblongata at its lower rear is connected to the spinal cord. The spinal cord is located in the spinal canal

central nervous system by ascending neurons, while excitement of the central nervous system isand is approximately 40–45 cm long in adults. Each nerve of the peripheral nervous system runs fromtransmitted to peripheral effectors by descending neurons. The nervous system covers the entire bodythe central system and branches repeatedly toward the target organs under its control. If the spinal cord

like a mesh and controls the functions of organs and tissues through the conduction of excitement

loses its function, all sensations will be lost, and you will become numb. Furthermore, motor functions

generated by impulses of nerve fibers and conduction of excitement to effectors and other neurons.

are also lost, and you will not be able to move.
Chapter 2

How your bodies function (Mechanisms and functions of your body)39

(2) Peripheral nervous system (somatic nervous system and autonomic nervous system)

breathing becomes fast and deep, your palms get sweaty, and you get thirsty. These are due tohyperactivity of the sympathetic nervous system. On the other hand, the parasympathetic nervous

Somatic nerves control the skin and muscles. The sensory information can reach consciousness easily,

system works when relaxed to encourage you to rest for storing energy. The parasympathetic

and often motor commands are consciously sent from the cerebrum.

nervous system also plays an important role in regulating the function of the digestive system, which

The autonomic nervous system controls most of the complex communication networks thattogether makes its function referred to as “rest and digest.”

regulate body functions. It works without human consciousness, and you almost never notice that it isfunctioning. Substances called neurotransmitters transport messages in different parts of the nervous
system, or between the nervous system and other organs. There are two types of autonomic nervous

5) Digestive systemsystems: sympathetic nervous system and parasympathetic nervous system. They control processes
such as blood pressure, heart rate, body temperature, digestion, metabolism, sweating, excretion, andThe digestive system has four roles: food intake and digestion, nutrient absorption, and excretion ofsexual response. Many organs are regulated by both sympathetic and parasympathetic nerves. These two

unwanted substances. It is composed of a single gastrointestinal tract, which extends 9 m from the mouthtypes of nerves work antagonistically. For example, signal input to the sympathetic nerve increases theto the anus, and several organs attached to it (Figure 2.9). Specifically, these organs include the mouth

heart rate, while signal input to the parasympathetic nerve decreases it (Figure 2.8). They also play a(oral cavity) , pharynx, esophagus, stomach, small intestine, large intestine, and anus for moving

major role in regulating blood pressure via hormones.

and absorbing food, as well as the pancreas, liver, and gallbladder, which are responsible for making
digestive enzymes, removing toxins, and storing substances needed for digestion.

Sympathetic nerve
ExciteParasympathetic nerve
Brain

Relax

Widen pupil

Iris of eye

Constrict pupil

Reduce saliva

Salivary gland

Increase saliva

Promote sweating

Sweat gland

DilateTracheaConstrict

Increase heart rate
Constrict
Increase blood pressure

Heart

Decrease heart rate
DilateDecrease blood pressure

Blood vessel

Food taken from the mouth is digested by the actions of digestive enzymes in the digestive juice
secreted into the digestive tract while the food is transferred to the stomach and small intestine. The
digestive juice is secreted not only from the cells in the digestive tract such as the stomach and small
intestine, but also by the salivary glands, pancreas, and liver.
Digested nutrients, water, and various electrolytes are mainly absorbed from the wall of the small
intestine into the cell, then into the blood. Most of the absorbed nutrients are sent to the liver. There, they
are synthesized, decomposed and detoxified, and converted into a form that can be used by cells
throughout the body. What is not absorbed is subjected to water absorption in the large intestine and thenis excreted as stool. Gastrointestinal motility and secretory functions are regulated by autonomic nerves,
intestinal plexus, and gastrointestinal hormones secreted into the blood from the gastrointestinal wall.

Blood pressure
Increase glycogenolysis

Liver

Suppress the activity

Gastro-intestine

Decrease urination(urine retention)

Bladder

Increase glycogen synthesis
Promote the activity

Mouth

Pharynx
Esophagus

Increase urination

Diaphragm

Figure 2.8 Actions of sympathetic and parasympathetic nervous systems

StomachLiver

Pancreas

Gallbladder
4

Column: Autonomic nerves referred to as “fight or flight” and “rest and digest”

Generally, the sympathetic nervous system works when you are stressed or taken by surprise. For

Large intestine
Small intestine
Rectumthis reason, it is referred to as “fight or flight,” as sympathetic nerves work to either counteract (fight)Anusagainst or escape (flight) from the situation in order to adapt to the circumstances. This explains, forexample, why when you are taken by surprise, your heart beats intensely, your pupils dilate, your40

Figure 2.9 Digestive organs
Chapter 2

How your bodies function (Mechanisms and functions of your body)416) Urinary system

mainly affects the growth of bones andHypothalamus

skeletal muscles, determining the size andPineal gland

As you live, many waste products, excess salts, and non-volatile acids are produced in your bodies as aheight of the body. Growth hormone isresult of metabolism. The urinary system functions to produce urine in the kidneys so as to efficiently

secreted during exercise and sleep, that’s

remove these substances from the blood and excrete them from the body through the urine. By adjusting

why they say “children who sleep (andthe amount and concentration of urine produced, it maintains the amount of circulating blood and its

exercise) adequately grow well.” Reduced

chemical composition.

secretion from the pituitary gland during

The urinary system is comprised of the kidneys and urinary tracts (ureters, bladder, urethra)childhood causes short stature. The pancreas(Figure 2.10). The kidneys filter and concentrate blood to make urine. The kidneys are located on the left

secretes insulin and glucagon into the blood,and right sides of the spinal column. Each is about 10 cm in length, 6 cm in width, and 3 cm in thickness.

hormones involved in the regulation of blood

They are slightly larger than the fist, protrudes like a bow on the outside and indents on inside, giving

sugar levels (glucose levels in the blood).

them their bean-shape. By regulating urine

Insulin is the only hormone produced in the

production, the kidneys keep fluid volume, osmotic

body that can lower blood sugar levels.

pressure, and pH (acid-base balance) constant,

When insulin does not function properly,

excrete unnecessary metabolites and drugs, andblood sugar levels spike, leading to themaintain the homeostasis of the fluid environment.

possible onset of diabetes.

The kidneys also have the endocrine function of

Pituitary gland
Thyroid gland
Parathyroid glands
Thymus gland
Adrenal glands
Pancreas

TestesGonadsOvariessecreting renin, which is involved in blood pressure
regulation, and erythropoietin, which has ahematopoietic effect, as well as activating vitamin
D for calcium metabolism.

KidneyUreterFigure 2.11 Endocrine organs

The bladder is a stretchable sac-like organ that
temporarily collects urine. The amount of urine

Bladder

produced per day is about 1.5 L, but the capacity ofan adult bladder is 250– 600 mL. You feel a light

UrethraSensation is awareness and consciousness of sensory information input from the outside or inside of theurinary urge when about 150 mL is collected and astrong one with about 250 mL.

8) Sensory system4

Figure 2.10 Urinary organs

body. For example, when you say “Ouch!” it means that you have become aware of pain stimulus. For
humans to carry out actions and follow their will, they need to carry out a series of actions by swiftly7) Endocrine systemgrasping various information from the outside, making judgments based on the information, and working
on the surroundings. The sensory system detects internal and external information and conveys that
information to the central nervous system.

The endocrine system consists of endocrine glands that are found widely throughout the body (Figure

Receptors are parts of the body that are sensitive to physical and chemical stimuli such as light,

2.11). It produces chemical substances (transmitters) according to purpose. These substances are

sound, body posture and movement direction, temperature, taste, and smell of things. On the other hand,collectively called hormones. The endocrine system regulates various functions of the body by secreting

sensory organs have a series of structures that transmit these stimuli to the afferent nerves (Figure(releasing) hormones directly into the blood. Hormones work as messengers responding to instructions

2.12).

from the brain, and act on specific organs (target organs) in cooperation with the nervous system to

There are general senses and special senses. Receptors for the general sense, also called somatic

control the activities of each part of the body such as metabolism, growth/development, and reproduction.

sense, can be found all over the body, specifically in the skin, muscles, joints, and internal organs. The

Excessive secretion or deficiency of hormones leads to serious health consequences, with excessive

general senses include the senses of pain, touch, pressure, temperature, and proprioception (sensory

secretion possibly causing hyperfunction, and reduced secretion causing hypofunction.

function to feel the relative positional relationship of each part of the body, which forms the body-

Major endocrine glands include the pituitary gland, thyroid gland, pancreas, adrenal glands, andgonads. For example, growth hormone, one of the representative hormones from the pituitary gland,
42

image). Special senses include vision, hearing, taste, smell, and parallelism. Receptors for these senses
are located in specific organs such as eyes, ears and semicircular canals, tongue, nose, and skin.
Chapter 2

How your bodies function (Mechanisms and functions of your body)43

(1) Male reproductive system (structure and functions)
Sight

The male reproductive organs consist of (1) testes; (2) epididymises, sperm ducts, and ejaculatory ducts(for maturation of sperms, transport path, and ejection); (3) seminal vesicles and prostate (attached

Hearing andequilibrium

gland); and (4) penis and scrotum (extrinsic region) (Figure 2.13). Reproductive functions include (1)Centralnervous
SystemSmell

Afferent
neuronsperm formation in the testes, (2) sexual response (various physical and mental changes that occur whensexually stimulated), and (3) regulation of reproductive functions by hormones.

(2) Female reproductive system (structure and functions)
The female reproductive organs consist of (1) ovaries; (2) fallopian tubes, uterus, and vagina (forHear and cold,
pain, touchtransportation route); and (3) mons pubis, clitoris, labia majora, labia minora (Figure 2.13). Reproductive

Tastefunctions are (1) conception and preparation for pregnancy, (2) pregnancy, and, similar to male, (3)sexual response, and (4) regulation of reproductive functions by hormones.

Figure 2.12 Sensory organ system (exchange of signal information)

2

Column: Why can we see in the dark? (Mechanism allowing to see in the dark)

In “vision,” which is the mechanism by which you can see things, sensitivity to light constantly
changes according to the intensity (brightness) of light so that you can obtain appropriate light andcolor information. When you suddenly go to a bright place after being in a dark place for some time,
you feel it is very bright, but that glare disappears in a relatively short time (light adaptation). This isbecause photoreceptor cells rapidly reduce their photosensitivity, and miosis occurs due to the

Urinary bladder
Seminal vesicle

Vas deferens

Prostate gland

Urethra
Penis

Anus

contraction of the iris sphincter muscle (pupillary light reflex), which also reduces the inflow of light.
On the other hand, if you move from a bright place to a dark place suddenly, at first you will not

TestisScrotum

be able to see anything, after which you will gradually be able to see things (dark adaptation). In
dark adaptation, mydriasis is caused by the dilator muscle of the pupil. Vitamin A deficiency causes
failure to adapt to darkness and this is called “night blindness.” Vision is discussed in detail in

Body

Chapter 8.

9) Reproductive system

Fallopian tube

Ovary
Endometrium

Reproduction is the act or process by which male and female germ cells, namely eggs and sperms are
fertilized, to create new beings with the genetic information of each cell. The reproductive system has
two functions. The first is production, development, and transport of eggs and sperms, and the second is
the secretion of hormones. There are two types of reproductive organs, primary reproductive organs
and secondary reproductive organs. The primary reproductive organ, which is the gonad (ovary infemales and testes in males), has two functions: secretion of hormones and production of egg and sperm.

Myometrium
Perimetrium

Cervix

UterusVaginaDuring growth, the functions of the reproduction system change significantly, the details of which are
discussed in Chapter 11.
44

Figure 2.13 Reproductive system (top: male reproductive organs, bottom: female)
Chapter 2

How your bodies function (Mechanisms and functions of your body)45

First line of defense

10) Immune system5

Chemical barriers

Immunity is a self-defense mechanism in which immune cells in the body prevent the entry of foreign

Mechanical barriers
Tears

invaders or pathogens, such as bacteria and viruses, and eliminate harmful substances produced in the
body. Many types of immune cells work together to play the roles of discovering, transmitting
information about, and attacking foreign invaders. There are two main types of immune functions that

Reflexes

Skin and mucous
membranes

Acid

Sneezing

eliminate foreign substances entering the body: innate (natural) immunity and adaptive (acquired)immunity.
Innate immunity is a mechanism that identifies a wide range of characteristics common to pathogens
and acts swiftly. It includes phagocytosis (action of taking foreign substances into cells and digesting

Second line of defense

Nonspecific
Immunitythem) by neutrophils, monocytes, dendritic cells, and macrophages to eliminate foreign substances,

Natural killer (NK)Cells

inflammatory reactions, and attack by natural killer (NK) cells. On the other hand, adaptive (acquired)immunity works against foreign substances that cannot be eliminated by innate immunity. In this case,lymphocytes act specifically on foreign substances. It can be divided into humoral immunity, in which

Phagocytosis

B cells produce antibodies from the actions of lymphocytes, and cellular immunity, in which killer T
cells directly attack cancer cells and infected cells (Table 2.2).

Inflammation

The immune system works to identify what is in the body as its “self ” or “nonself,” and to eliminate
the latter. To do this, the immune system coordinates innate immunity and adaptive (acquired) immunity

Fever

Protective proteinsto deal with disease-causing microorganisms (pathogens). On the other hand, because the immune

Copyright © 2017, Elsevier KK. All rights reserved. Translated and modified with permission.

system may overreact to your “self ” and cause diseases (autoimmune diseases), there is a system that

Figure 2.14 Immunity functions (innate immunity)4

adjusts the immune system to maintain the immune homeostasis of the living body in a balanced way.
There are many types of organs that contribute to immune cells and immunity (Figure 2.14). For
example, almost all immune cells are produced in the bone marrow. Immature T cells produced in the

3. Mechanisms that support human activities in daily life

bone marrow gather and mature in the thymus. The thymus, consisting of left and right parts, is locatedin such a way that it covers the heart. During puberty, the thymus reaches a maximum weight of 30–40

Mechanisms maintaining defense functions and regulatory mechanisms for sustaining life and theg. After puberty, the thymus begins to atrophy. The details of immunity are discussed in Chapter 7.

homeostasis of the human body

Table 2.2 Types and functions of human immunity and defense

Type

Main functions(1) Physical/chemical defense

Phagocytotic elimination, inflammatory
reactions, NK cell attack(2) Innate immunity(3) Adaptive immunity(Acquired immunity)

Skin, mucous membranes, saliva, sweat, tears,
runny nose, gastric acid

HumoralImmunity
Cellular
Immunity

TargetVarious foreign
substances(Nonspecific)

Antibodies produced by B cells react specificallywith the antigens to eliminate themSpecific foreign
matter(Specific)
Killer T cells and B cells eliminate cancer andinfected cells

1) Homeostasis of internal environment and its regulatory mechanism
What happens to your bodies in your daily lives? Living things have functions that help them staybalanced and maintain a similar state with their surrounding environment. This is called homeostasis(see also Chapter 6). To maintain homeostasis, the mechanism of information transmission in the living
body is crucial, and two systems serve this purpose: the nervous system that transmits information
by electrical signals and the endocrine system that transmits information via chemical substances.

2) Thermoregulation
The system that keeps the body temperature constant is very important for the brain and for sustaining
life. For example, hyperthermia such as heat stroke causes brain dysfunction, and can be life-threatening.

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Chapter 2

How your bodies function (Mechanisms and functions of your body)47

This reduces the amount of blood flowing to prevent heat dissipation, and inadvertent muscle contraction(1) Body heat production and dissipation

causes heat production. Exceeding the limit of the thermoregulation range may lead to consciousness

Your body temperature differs by location. The temperature at the core of the body is called core

disturbance or death.

temperature and cannot be measured directly. For this reason, rectal temperature, oral temperature, and(3) Diurnal fluctuation (see also Chapter 6)

axillary temperature are used. The rectal temperature is the closest to the core temperature. Even if theenvironmental temperature fluctuates, it remains more or less constant at about 37°C, which is 0.4–0.7°C

Body temperature drops to the lowest level during sleep in the early morning, starts rising as you wakehigher than axillary temperature. The temperature of the skin and muscles is called outer shellup, and sharply rises after breakfast. After that, it continues to rise gradually and peaks in the evening,temperature, and it differs according to the body part and outside temperature.

after which it starts dropping. This change in body temperature in the cycle of one day is called diurnal
fluctuation (circadian rhythm). Circadian rhythm is present in many vital activities such as body(2) Thermoregulation mechanism

temperature and blood pressure and affects sleep and dietary patterns. If disturbed, sleep rhythm will be

The temperature regulatory center in the hypothalamus (diencephalon) works on cutaneous blood

disrupted, for example, resulting in sleep disorders. It is said that a good way to regulate this rhythm

vessels, sweat glands, and arrector pili muscle to regulate temperature. Thermoregulation in humans isnormally is to receive sunlight in the morning.

controlled by the sensitivity of skin temperature receptors and temperature receptors in the hypothalamus.
(Figure 2.15).

Column: Body temperature rhythm of women during and after puberty

When the body temperature rises, the sympathetic nerves work less, the blood vessels passively

6

dilate, and sweating dissipates heat from the skin to regulate the body temperature within the normal

In women during and after puberty, the body temperature fluctuates due to the effects of hormone

range. Meanwhile, when the body temperature decreases, the sympathetic nerves constrict blood vessels.

secretion according to the menstrual cycle. From menstruation to preovulation, body temperature islow (normal body temperature). After ovulation, the body temperature rises. The difference between

Thermostat

the low and high temperature periods is about 0.5°C. This fluctuation is due to the progesterone(secreted from the corpus luteum formed in the ovaries during the secretory period) acting on the

Hypothalamus

hypothalamus in the temperature regulatory center.

Exercises for further thought and research
Increased Temperature:

Blood vessels dilate, giving flushed
appearance.
Sweat glands become more active.

[2-1] Use information about the body: Challenge the quiz!

Decreased Temperature:

Blood vessels constrict, heat istrapped in deeper tissues.
Sweat glands become less active.
Skeletal muscles contract, causing shivering.

(1) Does the number of times you blink change when using a smartphone?
		(i) Increases(ii) Same(iii) Decreases(2) How much do you breathe in one day? (In terms of 500 mL plastic bottles)(3)	How much blood is sent from the heart to the entire body in one minute? Measure your pulse
for one minute and make a prediction based on it (see Chapter 15 for how to measure yourpulse).

References
1

Ministry of Health, Labor and Welfare: e-Health Net.
https://www.e-healthnet.mhlw.go.jp/information/dictionary/exercise/ys-087.html

Copyright © 2017, Elsevier KK. All rights reserved. Translated and modified with permission.

Figure 2.15 Mechanism of body temperature regulation

48

4

2

Atsushi Fujimoto (sp.), Mamoru Fujita, Yoshiaki Doi (ed.): Visual Anatomical Physiology. Nouvelle Hirokawa, Tokyo. 2007.

3

Anatomical Physiology: Structure and functions of the human body (1). Igaku-Shoin, Tokyo. 2018.

4

Human Body Original 5th Edition: Anatomical Physiology for Understanding the Body. Elsevier Japan, Tokyo. 2017.

5

Tadashi Ueda, Katsumi Maenaka (ed.): Pharmaceutical Immunology. Revised 3rd Edition. Nankodo, Tokyo. 2018.

6

Mechanisms and Functions of the Human Body, Asakura Publishing Co. Ltd., Tokyo. 2015.
Chapter 2

How your bodies function (Mechanisms and functions of your body)49

Supplemental materials (Suppl.)
Answers[2-1] Use information about the body: Challenge the quiz!

Cranium(1) Does the number of times you blink change when using a smartphone?
A. (iii). You blink less when concentrating on something such as when looking at the screen ofyour smartphone. When you blink less, tears do not spread on the surface of the eyes, causing theClavicle

Mandible

Scapulaeyes to dry. It can also cause “dry eye which leads to subjective symptoms such as dryness anduncomfortable sensation and damage (scratches) on the surface of the eyes.

Sternum(2) How much do you breathe in one day? (In terms of 500 mL plastic bottles)

HumerusA. About 11,520 L (taking the amount you breathe each time to be about 0.5 L)

Ribs

Vertebral column(3) How much blood is sent from the heart to the entire body in one minute?
A. Approximately 5 L. For adult, approximately 60–70 mL of blood is pumped out by one

Ulnacontraction of the heart. The heart beats 60–80 times per min. Normally, your heart rate is equal

RadiusCoxal (hip) bone

to your pulse rate. For example, 60 mL × 80 times = approx. 5 L volume of blood is sent to theentire body in 1 min. The volume of blood pumped out per minute varies depending on theindividual’s age, body size, gender, health condition, and exercise experience.

Carpals
Metacarpals

SacrumPhalanges

Femur

PatellaTibia
FibulaTarsals
Metatarsals
Phalanges

Calcaneus

Suppl 2.1 Skelton. Axial skeleton (pink) and appendicular skeleton (tan).

50

Chapter 2

How your bodies function (Mechanisms and functions of your body)51

Frontalis
Orbicularis oculi

Trapezius

Buccinator
Orbicularis oris

MasseterSternocleidomastoid

Trapezius

Pectoralis major

Deltoid

Deltoid
Biceps brachii
Brachialis

Serratus anterior
Triceps brachii

Rectus abdominis
Latissimus dorsiInternal oblique
External oblique
Gluteus medius

Brachioradialis

Iliopsoas
Quadriceps femoris
Rectus femoris

Sartorius

Vastus lateralis

Tibialis anterior

Biceps femoris
Semitendinosus
Semimembranosus

Vastus medialis
Vastus intermedius(not illustrated)

Gluteus maximus

Quadriceps ligament

Hamstring group
Gastrocnemius

SoleusAchilles tendon(calcaneal tendon)

Suppl 2.2 Major muscles of the body. Anterior view.

52

Suppl 2.3 Major muscles of the body. Posterior view.

Chapter 2

How your bodies function (Mechanisms and functions of your body)53