The human body, with all its powers of endurance, its life preserving
systems for combating disease and taking in nourishment, and its
ability through the senses to interpret what is happening in the
world outside it, is like some marvelously complex machine.
But unlike a machine, it also has the capacity for pleasure and
sensitivity to pain. And no machine, however futuristic, could match
the body's ability to grow and to repair broken bones and damaged
tissues, or its even more remarkable ability to maintain or multiply
the human population by generating new life.
All these powers and capacities, all the strengths and intricacies
of the body, could be reduced to a few handful of chemical elements
such as oxygen, hydrogen, proteins, minerals, fats, trace elements
and water which are all contained in the body's cells - microscopic
structures, only a few hundredths of millimeter in diameter, but
with the ability to absorb nourishment, grow, excrete wastes and
increase in numbers by dividing in two.
SYSTEMS OF THE BODY
Various collections of cells make up body tissues such as skin,
muscle and bone; and tissues are grouped to form organs, such as
the heart, lungs and stomach. A set of organs make up a system,
and the ultimate physical aim of the systems, working together,
is to convert food into energy to keep the body working.
Foodstuffs are eaten; prepared in the mouth into a form suitable
for digestion; broken down in the digestive system into smaller
units by the action of chemical substances called enzymes; and absorbed
into the body; where they are partly used as fuel.
For the burning of fuel in the body, as for any form of combustion,
a supply of oxygen is needed. This is taken in from the lungs -
a major function of the respiratory system - and distributed to
the tissues by the blood, carried in the blood vessels and pumped
by the heart.
The blood vessels and the heart make up the circulatory system,
which also conveys foodstuffs and waste products round the body.
The body needs to get rid of waste products formed by the release
of energy, the process known as excretion. Carbon dioxide and some
water are excreted from the lungs in the air breathed out; a little
water and salts are lost from the skin in sweat, and water and salts,
together with complex waste products such as urea, and uric acid,
are excreted from the kidneys.
These form part of the urinary system. This also includes connecting
tubes, known as the Ureters (from the kidneys to the bladder) and
the urethra (from the bladder to outside the body). The bladder
itself is merely a reservoir of urine. Fibrous wastes and indigestible
food residues pass out of the body in the faeces.
Some of the energy produced by the body keeps the various systems
working and the rest is used for movement. This is carried out by
the locomotor system, consisting of muscles, which act on the bony
skeleton. The bones have an important role, not only in providing
a frame work for the whole body, but also in protecting the vital
organs, such as the lungs and the brain, from injury.
Acting on information provided by the sense organs, such as the
eyes and ears, the brain and the rest of the nervous system can
control a variety of bodily processes, either directly or by causing
various glands to release hormones- chemical messengers, which in
turn act on the tissues.
The release of the many types of hormones into the blood stream
is controlled by the endocrine system, a series of glands in different
parts of the body that regulate growth and the ability to reproduce.
The body also needs to be maintained. Treatment with medicines or
surgery may be required to repair the damage by injury or disease,
but often the body can cope with the problem by itself.
The body?s repair system consists of the normal continual process
of replacing worn-out and damaged tissues. The ability to repair
itself without outside help and the ability to grow, which is particularly
obvious during childhood are two of the physical properties distinguishing
the living organisms from a non-living organisms. A third is the
body?s ability to perpetuate the species by reproduction - the role
of the reproductive system. This involves the creation of new life
by the joining of two sex cells - sperm and an egg from the parents,
and the subsequent development while protected inside the mother?s