THE BLOOD AND HOW IT FLOWS THROUGH THE BODY
[Illustration: FIG. 68.--The cells in the blood. The two white ones
were drawn while crawling. Much enlarged.]
=The Blood keeps the Body Clean within and gives it Food.=--Every tiny
particle of the body, whether in the legs, arms, or head, must have
food to keep it alive and help it do its work. It must also have
oxygen, and it must be washed clean of its waste matter. All this is
done by the streams of blood, which bathe every cell to bring it food
and oxygen and to wash away its waste.
=Parts of the Blood.=--Blood consists of a clear, watery part called
_plasma_ and many little bodies named _cells_. The liquid found in a
blister is the clear part of the blood. The cells which float in the
watery part are so little and so close together that more than a
million are in each drop of blood.
A few of the cells are white, but most of them are red, and it is
their color that makes the blood look red. Your body contains about
one gallon of blood. It is carried through the body in branching tubes
called _blood vessels_ (Fig. 70).
[Illustration: FIG. 69.--Photograph of the heart from in front with
the lungs pinned aside. One fourth natural size.]
=The Blood Vessels.=--There are four kinds of blood vessels. They are
the _heart_, the _arteries_, the _capillaries_, and the _veins_. The
heart lies in the chest between the lungs. It squeezes the blood into
the arteries. These carry the blood to all parts of the body. It then
runs into the capillaries, which are tiny tubes connecting the
arteries with the veins. The veins return the blood to the heart.
The blood flows so fast that it goes from the heart down to the toes
and back again in a half minute.
=The Heart or Pump of Life.=--When the heart stops we die, because the
blood can no longer flow to carry food and oxygen to the hungry
tissues. The heart is a sac with thick walls of muscle. It is shaped
like a strawberry and is about as large as your fist. Its cavity is
divided into four parts. The two upper ones are called _auricles_ and
the lower ones are named _ventricles_. The blood enters the auricles
and then pours through an opening into each ventricle, from which it
passes out into the arteries.
=The Arteries or Sending Tubes.=--The blood is sent out from the heart
through the arteries leading to all parts of the body. The chief
artery is the _aorta_. It is larger than your thumb and extends from
the heart down through the body in front of the backbone. It has more
than twenty branches. All of these branch again and again like the
limbs of a tree until they are finer than hairs.
A large tube, the _lung artery_, takes blood directly from the heart
to the lungs. Here it branches into more than a thousand divisions, so
that the blood can take in oxygen and give off to the lungs its waste.
[Illustration: FIG. 70.--Arteries, the tubes carrying the blood from
the heart through the body. Only the chief vessels are shown on one
side.]
=The Capillaries or Feeding Tubes.=--These are the tiny tubes, finer
than hairs, which join the smallest end branches of the arteries with
the beginnings of the little veins. They are so thickly scattered in
the flesh that you cannot stick it with a pin without piercing one.
They are called feeding tubes because they have such very thin walls
that the food in the blood and the oxygen brought from the lungs can
pass through to feed the muscles and other organs. The dead parts of
the body and also the ashes of the food used up, pass from the organs
into the capillaries.
=The Veins or Returning Tubes.=--The veins, beginning in fine branches
formed by the capillaries, return the blood to the heart. The branches
unite into larger and larger vessels and finally flow into one main
vein, the _vena cava_. This extends along in front of the backbone and
opens into the heart.
=Why the Blood flows in only one Direction.=--The heart causes the
flow of the blood. It does this by squeezing together its walls so as
to make the blood go out into the arteries. When once in the arteries,
the blood must go forward because there are little doors at the mouths
of the arteries in the heart. These doors, called _valves_, open in
only one direction, so that the blood cannot flow backward (Fig. 71).
There are other valves between the upper and lower cavities of the
heart, preventing the blood from being pushed back into the veins.
[Illustration: FIG. 71.--The heart with the front part cut away to
show the four chambers and valves. The arrows show the direction in
which the blood flows.]
The movement of the walls of the heart in and out is called the _heart
beat_. This can be plainly felt by placing the hand on the left side
of the chest. The heart beats about seventy times each minute in grown
persons, but much oftener in children. At each beat a wave of blood
flows along the arteries. This is known as the _pulse_. It may be felt
at the base of the thumb, where an artery runs just under the skin.
=Why the Heart sometimes beats Faster.=--When we run or do hard work,
the heart may beat twice as fast as when we are lying down. This is
because the muscles need more oxygen to help them act. Work makes them
get hungry, and they send word by the nerves to the heart to hurry
along the blood to bring more oxygen from the lungs.
When germs make the body sick, the heart often beats faster because it
is affected by the poison made by the germs. The doctor then feels the
pulse to tell how much the body is poisoned.
=Use of Blood Cells.=--The red cells act like boats. They load up with
oxygen in the lungs and carry it to all parts of the body. Here they
trade it off for carbon dioxide, a waste substance. This they carry
back to the lungs to be cast out of the body.
There is one white blood cell to every four hundred red ones. The
white cells are the body-guards. They change their shape and are able
to crawl through the walls of the capillaries. Wherever the body is
hurt, they collect in large numbers and eat the germs which are always
trying to get into the body through sores. The white matter called
_pus_ in a sore is largely made of white blood cells which came there
to fight the germs and were killed in the battle.
The germs of boils and fevers often get into the blood, but the white
cells usually kill them before they have a chance to grow into large
numbers and make the body sick.
=How to stop Bleeding.=--Most of the larger arteries are deep in the
flesh and seldom get cut. There are many veins just under the skin. If
the blood comes out in spurts, it is from an artery; but if it flows
steadily, it is from a vein. If the blood does not run out in a
stream, it will stop without any special care. As soon as the blood
gets to the air it forms a jellylike mass called a _clot_. This helps
stop the flow. All hurt places in the skin should be tied up in a
clean cloth.
[Illustration: FIG. 72.--Stopping the flow of blood from an artery.]
If a large artery is cut, a bandage twisted tight with a stick around
the limb on the side of the wound next to the heart will stop the
bleeding. If a vein is cut, the bandage should be placed on the side
of the cut away from the heart.
=Alcoholic Drinks weaken the Blood.=--It has been noticed for some
years that when a user of beer or whisky is attacked with fever, the
disease is more severe than in one not using alcohol. The reason for
this has lately been explained by a well-known scientist working in
Paris. He put certain disease germs in rabbits, but they did not
become sick. When he gave them a little alcohol and put the same
amount of disease germs in them as before, they became sick and died.
By careful study he learned that the white blood cells had in the
first case killed the germs. In the second experiment the blood cells
were made so weak and lazy by the alcohol that they did not put up
such a strong fight against the germs.
=Tobacco and the Blood.=--Any one who chews or smokes tobacco
regularly gets much of the poison into the blood. The vessels in the
mouth and throat drink in some of the juice and also the poison from
the smoke. How much this poison affects the blood cells is not known,
but it is likely to do them some harm because it makes the growing
cells of the body less active.
=How Beer weakens the Heart.=--Whisky was at one time thought to
strengthen the heart, but doctors generally agree now that it weakens
the heart. It may make the heart beat a little stronger for a few
minutes, but after that the beating is weaker than usual.
Much use of beer is known to make fat collect around the heart and
also cause some of the heart muscle itself to change into fat. In this
way the heart becomes so weak that it can no longer do its work, and
death results. The reports from Germany show that hundreds of persons
die every year from weakened hearts made so by the use of much beer.
=Alcohol hurts the Blood Vessels.=--Careful examination of the blood
vessels of drunkards after death shows that in many cases the alcohol
has caused the walls of the vessels to become thick and sometimes
hard. The thickening of the wall makes the channel of the tube
smaller. The heart must then work much harder to get the blood through
to feed the tissues.
=Tobacco and the Heart.=--Many boys who use tobacco regularly do not
have a steady heart beat. This is specially true of those who smoke
several cigarettes daily. A few years ago, when our country was at war
with Spain, thousands of young men, wanted for soldiers, were examined
to find out whether their bodies were strong enough to endure the
hardships of war. Hundreds were refused admittance to the army because
of weak bodies, and many of them were reported by the physicians as
having hearts weakened by the use of tobacco.
The boys preparing for the army at the Military Academy at West Point
and for sea fighting at the Naval Academy at Annapolis are not allowed
to smoke cigarettes. Our country must have strong men for hard work.
Tobacco never gives strength, but often causes weakness.