The human body can perform amazing acts of strength and grace. A
highly trained athlete can push the body to the limits of its powers.
The skeletal system includes more than 200 bones and makes up about 18
per cent of the body's weight It provides the body with a sturdy framework.
Bone is strong, yet light and flexible.
The muscular system consists of more than 600 muscles and makes up about
40 per cent of the body's weight. Muscles can contract
(shorten). By contracting, the muscles enable the body to move.
he human eye, like many parts of the body, rapidly adjusts to
changes. In a darkened room, the pupil of the eye opens wide and so lets in
more light. If the room lights are turned up, the pupil
automatically shrinks within seconds.
The heart is a powerful pump. It beats about 100,000 times each
day as it sends blood throughout the body.
A microscopic view of the skin shows bacteria as tiny g balls. Countless bacteria
live on the skin. These bacteria an harmless unless they enter the body through
a break in the
The brain is
one of the body's most complicated parts. The outer surface of the brain is
made up of more than 8 billion cells. A few of these cells are shown in the
circle.
Ligaments and tendons Ligaments and tendons consist of tough, elastic connective tissue.
Ligaments connect one bone to
another. They hold the bones in place but
still allow some movement. Tendons connect a muscle to a bone. When the muscle
contracts, the strong, cablelike tendon pulls the bone to which it is attached.
The large Achilles' tendon links the calf muscle to the heel bone.
A magnified cross section of the small
intestine shows the tiny, fingerlike structures that
line this organ. These structures, called villi, increase the surface area of the small intestine.
They enable the organ to absorb large amounts of useful substances from
digested food.
Carbon dioxide and oxygen are exchanged in the lungs, which lie close to the heart. One lung is shown with the
heart on the upper left. Each lung contains millions of alveoli, or air sacs. Blood
vessels, shown here only on the bottom sac, surround each alveolus. As blood
flows through these vessels, it releases carbon dioxide, a waste picked up from
the body tissues, into the alveoli. It then receives fresh oxygen from the
alveoli.
Blood consists of a liquid and three kinds of solid particles. Plasma, the liquid portion, brings
food to the body cells, and carries away wastes. Red blood cells transport oxygen, and white blood cells defend against
disease. Platelets help
prevent bleeding from damaged blood vessels.
One egg cell is released from an ovary about every 28 days during a woman's
childbearing years. The egg is surrounded by a covering, which appears as a
ring in the photograph above. If a sex cell from a man penetrates the covering
and unites with the egg, a new human being begins to develop.
Human body
Human body. People sometimes call the human body a machine—the
most wonderful one ever built Of course, the human body is not a machine. But
it can be compared to one in many ways. Like a machine, the body is made up of
many parts. Each part of the body, like each part of a machine, does special
jobs. But all the parts work together and so make the body or the machine run
smoothly. Also like a machine, the body needs energy to work. In such a machine
as a car, the energy comes from petrol. In the body, it comes from food and
oxygen.
Although the human body can be compared to
a machine, it is far more amazing than any machine. It can do things that no
machine can do. For example, the body can grow. The body starts out as one
cell. In time, this tiny cell develops into a body consisting of trillions of
cells. The human body can also replace certain worn-out parts. "Each day,
about 2 billion of the body's cells wear out and are replaced. Thus, the body
is always rebuilding itself. Every 15 to 30 days, for instance, the human body
replaces the outermost layer of skin.
The human body can defend itself against
hundreds of diseases. The body can also repair itself after most small
injuries. Many body parts, such as the heart and kidneys, work continuously.
The heart of a 70-year-old person, for example, has pumped at least 174 million
litres of blood during that person's life. In addition, the person's kidneys
have removed wastes from more than 3.8 million litres of blood.
By using its senses, the body can detect
changes in its surroundings, such as changes in temperature, light, or sounds.
It can adjust to these changes immediately. The body's senses are truly
incredible. For instance, people can learn to identify thousands of odours, yet
smell is one of the least developed senses in human beings. The human body can
also detect changes that occur within itself, such as changes in body
temperature. The various parts of the body continuously adjust their activities
to keep the "inside" environment normal. Such adjustments rely on a
system of nerves that carries messages from one part of the body to another.
The messages travel at speeds of up to 90 metres per second.
The most remarkable part of the human body
is the brain. The human brain is so highly developed that it makes people
different from all other living things. Their magnificent
brain makes people able to think. They can compose silly rhymes or beautiful
poetry. They can imagine a dream world or study the mysteries of the atom. No
animal—no matter how cunning—and no computer—no matter how powerful—can think
like a human being.
The brain and the exquisitely complex
nervous system, which ramifies throughout the body, work in close
collaboration with the blood-borne hormones. These are the body's inbuilt
control systems, orchestrating all that we are and we do.
What the body is made of
The human body has many parts. This
section of the article describes the organization of the body, from its basic
unit—the cell—to its integrated systems.
Chemical elements and molecules. Like all things— living and nonliving—the human body
consists of atoms of chemical elements. The most common chemical elements in
the body are carbon, hydrogen, nitrogen, and oxygen. The body also contains
smaller amounts of many other elements, including calcium, iron, phosphorus,
potassium, and sodium.
Atoms of chemical elements combine and
form microscopic structures called molecules. The most common molecule in the human body is water.
A molecule
of water consists of two atoms of hydrogen
and on* atom of oxygen. Water makes up about 65 per cent the body. Most of the
chemical reactions that occur the body require water.
Except for water, all of the chief
molecules in the body contain the element carbon. The most important
carbon-containing molecules are large, complicated structures called macromolecules. There are four main kinds of macromolecules in the
body: carbohydrates, lipids, proteins, and nucleic acids.
Carbohydrates provide energy that powers all the body's activities. Lipids have
several jobs. Some lipids, particularly the fats, store extra fuel. Other
lipids serve as one of the building materials for the cells that make up the
body. Protein also have various duties. Many proteins serve as building blocks
for cells. Other proteins, called enzymes, speed up the
chemical reactions within the body. Nucleic acids carry instructions that tell
each cell how to perform its particular jobs. For more information on
macromolecules, see the article Life (The chemical basic of life).
Cells and tissues. The cell is the basic unit of all living things. The
cells of the human body consist chiefly of molecules of water, proteins, and
nucleic acids. The molecules that make up the cells are not alive, but the
cells themselves are living things. Each of the body's cells is able to take in
food, get rid of wastes, and grow. Most of the cells can also reproduce. A thin
covering consisting of lipid molecules encloses each cell. This lipid envelope
permits only certain substances to enter or leave the cell.
Nearly all the cells in the body are too
tiny to see without a microscope. Yet packed within each cell is the machinery
that the cell needs to carry out its many activities. For a detailed
discussion of a cell's machinery and how it works, see the article Cell (Inside
a living cell; The work of a cell).
The body has many basic kinds of cells,
such as blood cells, muscle cells, and nerve cells. Each kind of cell has
special features and jobs. Cells of the same type form tissues. The body has
four chief kinds of tissues. (1) Connective tissue
helps support and join together various parts of the body. Most connective
tissue is strong and elastic. (2) Epithelial tissue
covers the body surface and so forms the skin. It also lines such body openings
as the mouth and throat Epithelial tissue prevents harmful substances from
entering the body. (3) Muscle tissue consists
of threadlike fibres that can contract (shorten). Muscle
tissue makes it possible for the body to move. (4) Nervous tissue carries signals. Its system of nerve cells permits
various parts of the body to Communicate with one another.
Organs and organ systems. An organ consists of two or more kinds of tissues
joined into one structure that has a certain task. The heart, for example, is
an organ whose job is to pump blood throughout the body. Connective tissue,
muscle tissue, and nervous tissue make up the heart
Groups of organs form organ systems. Each
organ system carries out a major activity in the body. For example, the
digestive system consists of various organs that enable the body to use food.
Similarly, the nervous system is made up of organs that carry messages from
one part of the body to another. The remainder of this article discusses the
main organ systems of the human body. For more detailed descriptions of the
major organs and organ systems, see the articles listed in the Related articles at the end of this article.
The skin
The skin, which is sometimes called the integumentary system, is the largest organ of the body. If the skin of a
68-kilogram person was spread out flat, it would cover about 2 square metres.
Skin has three layers: (1) the epidermis, (2) the dermis, and (3) the
subcutaneous tissues.
The epidermis forms the outermost layer of the skin. It serves as a
barrier between the outside world and the inner tissues of the body. The outer
portion of the epidermis consists of tough, dead ceils that prevent bacteria,
chemicals, and other harmful substances from entering the body. It also
protects the body's inner tissues from the harsh rays of the sun and prevents
the loss of water from these tissues.
The dermis is the middle layer of the skin. The dermis helps
keep the temperature of the body within its normal range. The body produces
tremendous amounts of heat as it uses food. Some of this heat escapes from the
body through the blood vessels in the dermis. When the body needs to retain
heat, these blood vessels narrow and so limit heat loss. When the body needs to
give off heat, the blood vessels in the dermis expand and so increase heat
loss. The sweat glands, which are part of the epidermis, also help control body
temperature. These glands produce sweat, which is released through pores on the
skin surface. As the sweat evaporates from the surface, it cools the body.
The dermis also serves as an important
sense organ. Nerve endings within the dermis respond to cold, heat, pain,
pressure, and touch.
Subcutaneous tissues form the innermost layer of the skin. This layer
provides extra fuel for the body. The fuel is stored in fat cells. The
subcutaneous layer also helps retain body heat, and it cushions the inner
tissues against blows to the body.
The skeletal system
The skeleton of an adult consists of more
than 200 bones. The skeleton forms a strong framework that supports the body.
It also helps protect the internal organs. For example, the brain is shielded
by the skull, the spinal cord by the spinal column, and the heart and lungs by
the ribs.
The skeleton works together with the
muscles in enabling the body to move. The bones of the shoulders and arms, for
instance, serve as levers against which the muscles that move the arm can pull.
The place where bones meet is called a joint There are two basic kinds of
joints. (1) Freely movable
joints, such as the elbow, knee, and shoulder
joints, permit varying degrees of motion. The bones of a movable joint are held
together by bundles of tough, flexible connective tissue called ligaments. (2) Immovable joints
do not permit any movement of the bones. The bones of the skull, except for
the jawbones, meet in fixed joints.
The skeleton serves as more than a
framework for the body and a system of levers to help move the body.
Bone tissue contains various kinds of
cells that play a major role in keeping the blood healthy. The cells of bone marrow—
the soft, fatty core of many bones—produce new blood cells and release them
into the bloodstream. Two kinds of bone cells regulate the mineral content of
the blood. One kind removes calcium, phosphorus, and other minerals from the
blood and deposits them in the bone. The other kind dissolves old mineral
deposits and releases the minerals back into the bloodstream as needed.
The muscular system
The muscular system moves the body. The
body has more than 600 muscles, each of which consists of special fibres that
can contract When a muscle contracts, it pulls the tissue to which it is
attached. This pulling results in movement
The muscles of the human body can be
divided into two main types: (1) skeletal muscles and (2) smooth muscles. A
third kind of muscle, cardiac muscle,
is found only in the heart It has features of both skeletal muscle and smooth
muscle.
Skeletal muscles are attached to the bones. They move the bones of the
arms, legs, fingers, and other parts of the skeleton. We can consciously
control the skeletal muscles, and so they are sometimes called voluntary muscles. The fibres that make up a
skeletal muscle have alternate light and dark
crossbands called striations.
One end of each skeletal muscle is attached
to a bone that does not move when the muscle contracts. In most cases, the
other end of the muscle is attached to another bone, either directly or by
means of cordlike bundles of connective tissue called tendons.
This second bone moves when the muscle contracts.
Muscles move the body only by pulling.
They cannot push the tissues to which they are attached. Two sets of muscles
therefore control most skeletal movements, such as the raising and then
lowering of the forearm. One set of muscles pulls the bones in one direction,
and the other set pulls the bones in the opposite direction. For example, one
set of muscles pulls the forearm up, but it cannot push the forearm down. To
lower the forearm, a second set of muscles must contract and pull it down.
Smooth muscles are found in most of the body's internal organs.
Unlike skeletal muscles, smooth muscles do not have striations. Smooth muscles
in the walls of the stomach and intestines move food through the digestive
system. Smooth muscles also control the width of the blood vessels and the size
of the breathing passages. In all these cases, the smooth muscles contract and
relax automatically—that is, we do not consciously control them. For this
reason, they are often called involuntary muscles.
Smooth muscles cannot contract as rapidly
as skeletal muscles. However, smooth muscles can contract more completely than
skeletal muscles, and they do not tire as quickly. Smooth muscles can thus
produce powerful, rhythmic contractions over long periods.
Cardiac muscle has striations like skeletal muscle. But like smooth
muscle, it contracts automatically and rhythmically without tiring. Cardiac
muscle enables the heart to beat an average of 70 times a minute without rest
throughout a person's lifetime.
The digestive system
The digestive system breaks down food into
simple substances that the cells can use. It then absorbs these substances into
the bloodstream and eliminates any leftover waste matter. The main part of the
digestive system is a long tube called the alimentary canal. This tube consists of (1) the mouth, oesophagus, and
stomach; and (2) the small intestine and large intestine. Other parts of the
digestive system include the gall bladder, liver, pancreas, salivary glands,
and teeth.
The mouth, oesophagus, and stomach. Digestion begins in the mouth, where the teeth tear
and grind food into small pieces. Small pieces of food are more easily broken
down during the digestive process than large ones. Therefore, thorough chewing
is important. As food is chewed, three pairs of large salivary glands pour
saliva into the mouth. Saliva moistens the food, making it easier to swallow.
Saliva also contains the first of the system's several digestive enzymes. The
digestive enzymes break food down into chemicals the body can use.
After the food is
swallowed, it enters the oesophagus The oesophagus is a long, muscular tube
that leads to the stomach. Contractions of smooth muscles move the food down
the oesophagus and into the stomach. The stomach is the widest part of the
alimentary canal. It serves as a sort of 'holding tank' in which food remains for
several hours. During this time, the stomach produces an acid and an enzyme
that further break down much of the food. Muscle contractions mix the partly digested
food into a thick liquid called chyme.
The small intestine and large intestine. Chyme passes from the stomach into the small intestine
at a steady rate. Various digestive enzymes complete the breakdown of the food
within the first section of the small intestine. The small intestine produces
some of these enzymes. The rest are made by the pancreas. The pancreatic
enzymes empty into the small intestine through a duct
(tube). Bile, a liquid made by the liver and stored in the gall
bladder, also enters the small intestine through a duct Bile does not contain
digestive enzymes, but it aids digestion by breaking up large molecules of
fatty foods.
By the time the food leaves the first
section of the small intestine, it has been completely digested. Special cells
line the walls of the remainder of the small intestine. These cells absorb
useful substances from the digested food. The absorbed substances enter the
blood. Some of the substances are carried directly to cells throughout the
body. The rest are transported to the liver. The liver stores some of the
substances, releasing them as the body requires. It chemically alters the other
substances, changing them into forms needed by the body.
The substances not absorbed by the small
intestine pass to the large intestine. These substances consist of
water, minerals, and wastes. The large
intestine absorbs most of the water and minerals, which then enter the
bloodstream. The wastes move down toward the rectum,
the end of the large intestine, and leave the body as faeces.
The respiratory system
The respiratory system consists of the
organs of breathing. These organs include the nose, the trachea (windpipe),
and a pair of lungs. The respiratory system has two main jobs. (1) It provides
the body with oxygen. (2) It rids the body of carbon dioxide. The cells of the
body need oxygen to break down and so release the energy in food. During this
process, carbon dioxide forms as a waste product
Breathing involves the acts of inhaling
and exhaling. Inhaling occurs as the chest cavity expands. As the chest
expands, so do the lungs. Air from the atmosphere rushes in and fills the
enlarged lungs. Exhaling occurs as the chest cavity shrinks, which pushes air
out of the lungs. Inhaling and exhaling result chiefly from contractions of
the diaphragm, a large muscle that forms the floor of the chest
cavity. As the diaphragm contracts, the cavity expands. As it relaxes, the
cavity shrinks. The muscles that move the ribs also play a part in the
breathing process.
The air passages. When we inhale, air enters the body through the nose.
The air flows from the nostrils to the nasal passages. The nasal-passages are
lined with tiny hair like structures and a sticky substance called mucus.
These structures and the mucus filter dust and dirt from the air. In addition,
cold air is warmed and moistened as it moves through the nasal passages. From
the nose, the air passes through the pharynx
(the cavity behind the nose and mouth) and the larynx
(the voice box). The air then enters the trachea.
The trachea carries the air toward the
lungs. Before reaching the lungs, the trachea splits into two tubes called the primary bronchi. Each tube enters one lung. Within the lungs, the
primary bronchi divide into smaller and smaller tubes, finally branching into extremely
tiny tubes called bronchioles.
The bronchioles end in hundreds of millions of thin-walled structures called alveoli
or air sacs. The alveoli give the lungs a tremendously extended
surface area. If the air sacs were flattened out, the lungs would cover from 55
to 90 square metres.
The exchange of carbon dioxide and oxygen occurs in the alveoli. Each alveolus is surrounded by
a network of small blood vessels. Like the alveoli, these small blood vessels
have extremely thin walls. Blood that enters the vessels has a high level of
carbon dioxide, which it picked up from the body tissues. It contains little
oxygen. The carbon dioxide leaves the blood and moves through the walls of the
blood vessels and alveoli into the lungs. Oxygen from the air in the lungs
then passes through the walls of the alveoli and blood vessels and into the
blood. The blood, now rich in oxygen, leaves the lungs and travels to the
heart. The heart then pumps it to cells throughout the body. The carbon dioxide
is finally expelled from the lungs when we exhale.
The circulatory system
The circulatory system moves blood
throughout the body. Blood transports food and oxygen to the cells and carries
away carbon dioxide and other wastes. The cells cannot live without a
continuous supply of fresh blood. The circulatory system also carries
disease-fighting substances that help protect the body. In addition, it transports
chemical messengers called hormones. Hormones are
discussed in the section of this article called The endocrine system.
The circulatory system consists chiefly of
(1) the heart, (2) the blood vessels, (3) the blood, and (4) the lymphatic
system.
The heart is a hollow muscle that pumps blood through the
circulatory system by contracting and relaxing rhythmically. The heart
actually consists of two pumps that lie side by side. The left side of the
heart makes up the stronger pump. It receives oxygen-rich blood from the lungs
and sends it to cells throughout the body. The blood, which picks up carbon
dioxide and other wastes from the cells, returns to the right side of the
heart. This weaker pump moves the blood to the lungs and then back to the left
side of the heart. In the lungs, the carbon dioxide is removed from the blood,
and oxygen is added.
The blood vessels form a branching network of about 97,000 kilometres.
They can be divided into three types W Arteries carry blood from the heart. (2) Veins carry
blood to the heart. (3) Capillaries
connect the arteries and veins.
Blood leaves the left side of the heart
through the aorta. This vessel is the largest artery in the body. Several
major arteries branch off the aorta. These arteries, in turn, divide into
smaller and smaller vessels. Finally, the smallest arteries empty into the tiny
capillaries. Through the thin walls of the capillaries, food and oxygen in the
blood are exchanged for carbon dioxide and other wastes from individual cells.
From the capillaries, the blood enters
small veins, which join larger and larger veins. Finally, the blood enters the
right side of the heart through the superior vena cava and inferior vena cava,
the body's two largest veins. The right side of the heart then pumps the blood
through the pulmonary arteries
to the capillaries surrounding the air sacs
in the lungs. The blood returns from the lungs to the left side of the heart through
four pulmonary veins. The left side of the heart then pumps the blood out
through the aorta, and the blood's journey begins once more.
The blood consists of a liquid and three kinds of solid
particles called formed elements. The liquid, which makes up 55 to 65 per cent of the
total volume of blood, is known as plasma. It carries many
important substances. The food that enters the blood from the intestines and
liver dissolves in the plasma, much as sugar dissolves in water. The plasma
transports the dissolved food throughout the body. Many of the wastes that the
blood picks up from the body tissues are carried in the plasma. These wastes
include ammonia, urea, and much of the carbon dioxide.
The formed elements in blood consist of red blood cells, white blood cells, and platelets.
Red blood cells carry oxygen from the lungs to the body tissues. They also carry
some of the carbon dioxide from the tissues. White blood cells help protect the
body from disease. These cells attack bacteria, viruses, poisons, and other
harmful substances. Platelets are disclike structures that help prevent
bleeding from damaged blood vessels. Together with various proteins in the
plasma, platelets seal broken vessels by forming a clot.
The lymphatic system consists of a network of tubes that carries a clear,
watery fluid called lymph. Lymph comes from
the blood and eventually returns to
it. Water, proteins, and dissolved food
leave the blood through the capillary walls. This fluid, which is known as interstitial fluid, bathes and nourishes the cells of the body tissues.
The fluid then drains into tiny, open-ended tubes called lymphatic vessels. At this point, the fluid is known as lymph.
The lymph flows through the small tubes
into larger and larger lymphatic vessels. Lymph nodes occur at various points along the lymphatic vessels.
These beadlike structures produce many white blood cells, which filter harmful
substances out of the lymph. Eventually, all the lymph flows into either the thoracic duct or the right lymphatic duct.
The lymph drains from these ducts into veins near the neck and so rejoins the
blood.
The urinary system
The urinary system removes various wastes
from the blood and flushes them from the body. The chief organs of this system
are the two kidneys. Each kidney has about a million microscopic filtering
units called nephrons. As blood passes through a nephron, a complicated
network of capillaries and tubes filters out a small amount of water together
with urea, sodium chloride, and certain other wastes. This filtered-out
material forms a yellowish fluid called urine.
Two tubes called ureters carry urine from the kidneys to the urinary bladder, a hollow storage organ. Urine eventually is squeezed out
of the bladder by muscular contractions. It then leaves the body through a tube
known as the urethra.
The reproductive system
The organs of the reproductive system
enable men and women to have children. Human beings reproduce sexually. Sexual
reproduction involves the union of sex cells. A new human being begins to
develop after a sex cell produced by the father unites with a sex cell produced
by the mother. The father's sex cells are called sperm,
and the mother's are called eggs. The union of a
sperm and an egg results in fertilization.
The fertilized egg has all the information necessary for the development of a
new human being.
The male reproductive system includes two testicles, which hang between the legs in a pouch called the scrotum.
The testicles are glands that produce sperm. The sperm travel through tubes to
the penis, an organ in front of the scrotum. Sperm leave a man's
body through the penis.
Most of the female reproductive system
lies inside the woman's body. Deep within the body are two glands called ovaries,
each of which contains about 400,000 eggs. Only about 400 eggs will mature
during a woman's childbearing years. About once a month, one of the ovaries
releases an egg. The egg travels down a narrow duct called the Fallopian tube. The female body has two Fallopian tubes, one leading
from each ovary. The Fallopian tubes open into the top of the uterus,
a hollow, muscular organ. The other end of the uterus leads to a canal called
the vagina. The vagina extends to the outside of the body,
opening between the legs.
During sexual intercourse, sperm from the
penis enters the vagina. Each sperm has a tiny tail and can swim. The sperm
swim from the vagina to the uterus and into the Fallopian tubes, if an egg is
present in one of the tubes, a sperm may fertilize it.
The fertilized egg cell continues its
journey to the uterus, where it becomes attached to the wall of the organ. The
cell divides over and over, forming the beginning of a developing baby. Soon,
a complex organ called the placenta forms. The
placenta enables the developing baby to obtain food and oxygen from the
mother's bloodstream.
After about nine months, the baby is ready
to be born. Powerful contractions of the uterus push the baby out through the
mother's vagina, which widens to allow the baby to pass through.
The endocrine system
The endocrine system consists of glands
that regulate various body functions. The system plays a major role in
regulating growth, the reproductive process, and the way the body uses food. It
also helps prepare the body to deal with stress and emergencies.
The endocrine glands control body
functions by producing hormones. These chemicals are released into the blood,
which carries them throughout the body. Hormones act as chemical messengers.
After a hormone reaches the organs or tissues it affects, it triggers certain
actions. Many hormones have widespread effects. For example, the hormone insulin
causes cells throughout the body to take in and use sugar from the bloodstream.
The chief endocrine glands include the
adrenal glands, the pituitary gland, the parathyroid glands,
the sex glands, and the thyroid gland. The brain, the kidneys, the stomach,
and the pancreas also have endocrine tissues and produce hormones. The
pituitary gland, which lies near the base of the brain, is often called the master gland. It releases a number of hormones, which, in turn,
regulate other endocrine glands. However, the pituitary itself is controlled by
hormones produced by the hypothalamus,
a part of the brain. The hypothalamus links the nervous and endocrine control
systems.
The body also has glands that do not
produce hormones. These exocrine glands
make chemicals that perform specific jobs in the area where they are released.
Major exocrine products include the digestive juices, mucus, sweat, and tears.
The nervous system
The nervous system regulates and
coordinates the activities of all the other systems of the body. It enables
the body to adjust to changes that occur within itself and in its surroundings.
The nervous system is made up of countless nerve cells, or neurons.
The neurons form a communications network that extends to every part of the
body. The nervous system has three main divisions. They are (1) the central
nervous system; (2) the peripheral nervous system, which includes the eyes,
ears, nose, and other sense organs; and (3) the autonomic nervous system.
The central nervous system consists of the brain and spinal cord. It functions as
the control centre of the nervous system. The central nervous system receives information
from the senses. It analyses this information and decides how the body should
respond. It then sends instructions that trigger the required actions.
The central nervous system makes some
simple decisions, such as directing the hand to pull away from a hot object,
within the spinal cord. Such simple decisions are called spinal reflexes. Most decisions, however, involve the brain. The brain
is an enormously complicated collection of billions of neurons. These neurons
are linked together in precise patterns that enable the brain to think and
remember. Much brain activity occurs at the conscious level. We are aware of
decisions made at this level and can voluntarily control them. Other activity
occurs at the subconscious level. This activity regulates the smooth muscles
and is beyond voluntary control.
The peripheral nervous system is made up of the nerves that connect the central
nervous system with every part of the body. These nerves include both sensory neurons, which carry information to the central nervous
system, and motor neurons, which relay instructions from the central nervous
system.
Sensory neurons run between the sense
organs and the central nervous system. The sense organs have special sensory
neurons called receptors. Receptors translate information about the internal
or external environment into nerve impulses. These impulses are electrical
signals that nerves can carry.
The body has many kinds of sense
receptors. Vision receptors in the eyes change light waves into nerve impulses.
Similarly, hearing receptors in the ears convert sound waves into nerve
impulses. Smell receptors in the nose and taste receptors on the tongue convert
chemical information into nerve impulses. Receptors in the skin respond to
heat, cold, pressure, and pain. Sense receptors deep within the body provide
information on the chemical and physical conditions of the inner body tissues.
Nerve impulses from the sense receptors
travel along sensory neurons to the central nervous system. The central
nervous system analyses the information and decides what actions, if any, are
necessary. If a response is needed, the central nervous system sends out
instructions. The motor neurons of the peripheral nervous system carry the
instructions from the central nervous system to the appropriate tissues.
The autonomic nervous system is a special part of the peripheral nervous system. It
carries messages from the subconscious level of the brain to the internal organs.
The autonomic nervous system regulates the automatic functions of the body, such
as the beating of the heart and the movement of food through the digestive
system.
Outline
What the body is made of
Chemical elements and molecules
Cells and tissues
Organs and organ systems
The skin
The epidermis
The dermis
Subcutaneous tissues
The skeletal system
The muscular system
Skeletal muscles
Smooth muscles C Cardiac muscle
The digestive system
The mouth, oesophagus, and stomach
The small intestine and large intestine
The respiratory system
The air passages
The exchange of carbon dioxide and oxygen
The circulatory system
The heart
The blood vessels C The blood
D. The lymphatic system
The urinary system
The reproductive system
The endocrine system XI. The nervous
system
The central nervous system
The peripheral nervous system
The
autonomic nervous system
Questions
How much of the human body consists of
water?
What are the two main jobs of the
respiratory system?
How do the salivary glands aid in
digestion?
What are receptors? What
role do they play in the nervous system?
In what ways can the human body be
compared to a machine?
How does it differ from one?
Why are capillaries important in the
circulatory system?
How does the skin help keep the
temperature of the body within its normal range?
Why is the pituitary gland called the master
gland?
What are the four chief kinds of tissues
in the human body?
What are the most common chemical elements in the human body?
What are the most common chemical elements in the human body?
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