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The Heart





Category: THE CARE OF THE HEART-PUMP AND ITS PIPE-LINES
Source: A Handbook Of Health

Structure and Action of the Heart. Now what is it that keeps the blood
whirling round and round the body in this wonderful way? It is done by a
central pump (or more correctly, a little explosive engine), with thick
muscular walls, called the heart, which every one knows how to find by
putting the hand upon the left side of the chest and feeling it beat.
The heart is really a bulb, or pouch, which has ballooned out from the
central feed pipe of the blood supply system, somewhat in the same way
that the stomach has ballooned out from the food tube.

The walls of this pouch, or bulb, are formed of a thick layer of very
elastic and powerful muscles almost as thick as the palm of your hand.
When the great vein trunk has poured blood into this pouch until it is
swollen full and tight, these muscles in its walls shut down sharply and
squirt or squeeze the blood in the heart-pouch into the great
artery-pipe, the aorta. In fact, you can get a very fair, but rough,
idea of the way in which the heart acts by putting your half-closed hand
down into a bowl of water and then suddenly squeezing it till it is shut
tight, driving the water out of the hollow of your hand in a jet, or
squirt.

But, some of you will ask at once, what is to prevent the blood in
the heart, when the muscle wall squeezes down upon it, from shooting
backward into the vena cava, instead of forward into the aorta?

Nature thought of that long ago, and ingeniously but very simply
guarded against it by causing two little folds of the lining of the
blood pipes to stick up both where the vena cava enters the heart and
where the aorta leaves it, so as to form little flaps which act as
valves. These valves allow the blood to flow forward, but snap together
and close the opening as soon as it tries to flow backward. While
largest and best developed in the heart, these valves are found at
intervals of an inch or two all through the veins in most parts of the
body, allowing the blood to flow freely toward the heart, but preventing
it from flowing back.

As the heart has to pump all the blood in the body twice,--once around
and through the lungs, and once around and through the whole of the
body,--it has become divided into two halves, a right half, which pumps
the blood through the lungs and is slightly the smaller and the thinner
walled of the two; and a left half, which pumps the purified blood,
after it has come back from the lungs, all over the rest of the body.


Each half, or side, of the heart has again divided itself into a
receiving cavity, or pouch, known as the auricle; and a pumping or
delivering pouch, known as the ventricle. And another set of valves
has grown up between the auricle and the ventricle on each side of the
heart. These valves have become very strong and tough, and are tied back
in a curious and ingenious manner by tough little guy ropes of tendon,
or fibrous tissues, such as you can see quite plainly in the heart of an
ox. It is important for you to remember this much about them, because,
as we shall see in the next chapter, these valves are one of the parts
of the heart most likely to wear out, or become diseased.


Heart Beat and Pulse. The heart fills and empties itself about eighty
times a minute, varying from one hundred and twenty times for a baby,
and ninety for a child of seven, to eighty for a woman, and seventy-two
for a full-grown man.

When the walls of the ventricles squeeze down to drive out their blood
into the lungs and around the body, like all other muscles they harden
as they contract and thump the pointed lower end, or apex, of the
heart against the wall of the chest, thus making what is known as the
beat of the heart, which you can readily feel by laying your hand upon
the left side of your chest, especially after you have been running or
going quickly upstairs. As each time the heart beats, it throws out half
a teacupful of blood into the aorta, this jet sends a wave of swelling
down the arteries all over the body, which can be felt clearly as far
away as the small arteries of the wrist and the ankle. This wave of
swelling, which, of course, occurs as often as the heart beats, is
called the pulse; and we take it, or count and feel its force and
fullness, to estimate how fast the heart is beating and how well it is
doing its work. We generally use an artery in the wrist (radial) for
this purpose because it is one of the largest arteries in the body which
run close to the surface and can be easily reached.

Summary of the Circulation of the Blood. We will now sum up, and put
together in their order, the different things we have learned about the
circulation of the blood through the body.


Starting from the great vein trunk, the vena cava, it pours into the
receiving chamber, or auricle, of the right side of the heart, passes
between the valves of the opening into the lower chamber, the right
ventricle. When this is full, the muscles in the wall of the ventricle
contract, the valve flaps fly up, and the blood is squirted out through
the pulmonary artery to the lungs. Here it passes through the
capillaries round the air cells, loses its carbon dioxid, takes in
oxygen, and is gathered up and returned through great return pipes to
the receiving chamber, or auricle, of the left side of the heart. Here
it collects while the ventricle below is emptying itself, then pours
down between the valve flaps through the opening to the left ventricle.
When this is full, it contracts; the valves fly up and close the
orifice; and the blood is squirted out through another valve-guarded
opening, into the great main artery, the aorta. This carries it, through
its different branches, all over the body, where the tissues suck out
their food and oxygen through the walls of the capillaries, and return
it through the small veins into the large vein pipes, which again
deliver it into the vena cava, and so to the right side of the heart
from which we started to trace it.

Although the two sides of the heart are doing different work, they
contract and empty themselves, and relax and fill themselves, at the
same time, so that we feel only one beat of the whole heart.

One of the most wonderful things about the entire system of blood tubes
is the way in which each particular part and organ of the body is
supplied with exactly the amount of blood it needs. If the whole body is
put to work, so that a quicker circulation of blood, with its millions
of little baskets of oxygen, is needed to enable the tissues to breathe
faster, the heart meets the situation by beating faster and harder.
This, as you all know, you can readily cause by running, or jumping, or
wrestling.





Next: The Care Of The Heart-pump

Previous: The Blood Vessels



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