The circulatory
and respiratory systems.
All parts of the body must have nourishment and oxygen in order to function
and grow, and their waste products must be removed before they accumulate
and poison the body. The circulatory system distributes needed materials and
removes unneeded ones. It is made up of the heart, blood vessels, and blood.
The blood is also part of the body's defense system. It has antibodies and
white blood cells that protect the body against foreign invaders.
The heart is a muscle
that is divided into two nearly identical halves: one half receives blood
from the lungs and sends it to the rest of the body, the other half sends
blood that has traveled through the body back to the lungs. When the heart
muscle contracts, the blood is forced out into arteries and enters small capillaries.
Blood returns to the heart through veins.
Materials enter and leave the blood across the thin walls of capillaries,
which are located near every cell of the body. In almost every case, blood
leaving a group of capillaries travels to the heart and then to the lungs
for more oxygen before it returns to the capillaries. The one exception is
blood that has traveled through capillaries in the digestive system. The vein
from this system, called the portal vein, carries blood directly to the liver,
where nutrients are stored before the blood returns to the heart.
Some of the fluid that surrounds cells does not reenter the blood vessels
directly. This fluid, called lymph, returns to the heart by way of another
system of channels the lymph vessels. Lymph nodes along these vessels filter
the fluid before it reenters the blood. The spleen is a large lymphatic organ,
located under the stomach, that filters the blood.
The respiratory system takes in oxygen from the air and expels carbon dioxide
and water vapor. Air enters the nose and mouth and travels through the larynx,
or voice box, and trachea, or windpipe. The trachea divides to enter each
of the two lungs and then divides more than 20 times to form a very large
number of small air spaces. Oxygen from the air enters the blood through capillaries
in the walls of these air spaces, and the blood releases carbon dioxide into
the air spaces to be exhaled.
The digestive system consists
of a tube extending from the mouth to the anus. In it, food and fluids are
taken in, moved through the body, and broken down into small molecules that
are absorbed into the circulatory system. This breakdown, known as digestion,
is both a mechanical and a chemical process.
Food enters through the mouth, where chewing and saliva start to break it
up and make it easier to swallow. Next, the food travels down through the
esophagus to the stomach. Contractions of the stomach's muscular wall continue
to break down the food mechanically, and chemical digestion continues when
acid and enzymes are secreted into the stomach cavity.
The liquified food gradually passes into the small intestine. In the first
part of the small intestine, called the duodenum, enzymes from the pancreas
are added. These enzymes complete the chemical breakdown of the food. The
digestion of fat is aided by bile, which is made in the liver and stored in
the gall bladder. The small intestine of an adult is about 21 feet (6.4 meters)
long. Most of its length is devoted to absorbing the nutrients released during
these digestive activities.
The liquid remainder of the food enters the large intestine, or colon, which
is about 12 feet (3.7 meters) long. It is more than twice as wide as the small
intestine. In the large intestine most of the fluid is absorbed, and the relatively
dry residues are expelled.
The urinary system
maintains normal levels of water and of certain small molecules such
as sodium and potassium in the body. It does this by passing blood through
the kidneys, two efficient filtering organs that get rid of any excess of
various molecules and conserve those molecules that are in short supply.
The fluid that leaves the kidneys, known as urine, travels through a tube
called the ureter to the bladder. The bladder holds the urine until it is
voided from the body through another tube, the urethra.
The glandular system. The two
systems that control body activities are the glandular system and the nervous
system. The glandular system exerts its control by means of chemical messengers
called hormones. Hormones are produced by a variety of endocrine glands, which
release the hormones directly into the blood stream.
A major gland is the pituitary, which is located under the brain in the middle
of the head. It produces at least eight hormones, which affect growth, kidney
function, and development of the gonads, or sex organs. Because some of the
pituitary's hormones stimulate other glands to produce their own hormones,
the pituitary is called the master gland.
Another gland, the thyroid, is located between the collar bones. Its hormone
controls the rate of the body's metabolism. The sex organs (ovaries and testes)
make the sex cells and also make hormones that control certain characteristics
of males and females. Located on top of each kidney is the adrenal gland,
which produces cortisone and epinephrine (also called adrenaline). The pancreas
produces not only digestive enzymes but also insulin and glucagon, which control
the body's use of sugar and starches.
--------------------------------------------------
Excerpted from Compton's Interactive Encyclopedia
Copyright (c) 1994, 1995 Compton's NewMedia, Inc. All Rights Reserved