WHY THE HEART BEATS.
~1.~ If you place your hand on the left side of your chest, you will
feel something beating. If you cannot feel the beats easily, you may run
up and down stairs two or three times, and then you can feel them very
distinctly. How many of you know the name of this curious machine inside
the chest, that beats so steadily? You say at once that it is the heart.
[Illustration: THE HEART.]
~2.~ The Heart.--The heart may be called a live pump, which keeps
pumping away during our whole lives. If it should stop, even for a
minute or two, we would die. If you will place your hand over your heart
and count the beats for exactly one minute, you will find that it beats
about seventy-five or eighty times. When you are older, your heart will
beat a little more slowly. If you count the beats while you are lying
down, you will find that the heart beats more slowly than when you are
sitting or standing. When we run or jump, the heart beats much harder
and faster.
~3. Why the Heart Beats.~--We have learned in preceding lessons that the
digested food is taken into the blood. We have also learned that both
water and oxygen are taken into the blood. Thus the blood contains all
the materials that are needed by the various parts of the body to make
good the wastes that are constantly taking place. But if the blood were
all in one place it could do little good, as the new materials are
needed in every part of the body. There has been provided a wonderful
system of tubes running through every part of the body. By means of
these tubes the blood is carried into every part where it is required.
These tubes are connected with the heart. When the heart beats, it
forces the blood through the tubes just as water is forced through a
pipe by a pump or by a fire-engine.
~4. The Heart Chambers.~--The heart has four chambers, two upper and
two lower chambers. The blood is received into the upper chambers, and
is then passed down into the lower chambers. From the lower chambers it
is sent out to various parts of the body.
[Illustration: THE INSIDE OF THE HEART.]
~5. The Blood-Vessels.~--The tubes through which the blood is carried
are called _blood-vessels_. There are three kinds of blood-vessels. One
set carry the blood away from the heart, and are called _arteries_
(ar´-te-ries). Another set return the blood to the heart, and are called
_veins_. The arteries and veins are connected at the ends farthest from
the heart by many very small vessels. These minute, hairlike vessels are
called _capillaries_ (cap´-il-la-ries).
~6. The Arteries.~--An artery leads out from the lower chamber of each
side of the heart. The one from the right side of the heart carries the
blood only to the lungs. The one from the left side of the heart carries
blood to every part of the body. It is the largest artery in the body,
and is called the _aorta_. Soon after it leaves the heart the aorta
begins to send out branches to various organs. These divide in the
tissues again and again until they become so small that only one
corpuscle can pass through at a time, as shown in the colored plate.
(Frontispiece.)
~7. The Veins.~--These very small vessels now begin to unite and form
larger ones, the veins. The small veins join to form larger ones, until
finally all are gathered into two large veins which empty into the upper
chamber of the right side of the heart. The veins which carry blood from
the lungs to the heart empty into the upper chamber of the left side of
the heart.
~8. What is Done in the Blood-Vessels.~--While the blood is passing
through the small blood-vessels in the various parts of the body, each
part takes out just what it needs to build up its own tissues. At the
same time, the tissues give in exchange their worn-out or waste matters.
The red blood corpuscles in the capillaries give up their oxygen, and
the blood receives in its stead a poisonous substance called
carbonic-acid gas.
~9. Red and Blue Blood.~--While in the arteries the blood is of a bright
red color; but while it is passing through the capillaries the color
changes to a bluish red or purple color. The red blood is called
_arterial blood_, because it is found in the arteries. The purple blood
is called _venous blood_, because it is found in the veins. The loss of
oxygen in the corpuscles causes the change of color.
~10. Change of Blood in the Lungs.~--Exactly the opposite change occurs
in the blood when it passes through the lungs. The blood which has been
gathered up from the various parts of the body is dark, impure blood. In
the lungs this dark blood is spread out in very minute capillaries and
exposed to the air. While passing through the capillaries of the lungs,
the blood gives up some of its impurities in exchange for oxygen from
the air. The red corpuscles absorb the oxygen and the color of the blood
changes from dark purple to bright red again. The purified blood is then
carried back to the upper chamber of the left side of the heart through
four large veins. The blood is now ready to begin another journey around
the body.
~11. The Pulse.~--If you place your finger on your wrist at just the
right spot, you can feel a slight beating. This beating is called the
_pulse_. It is caused by the movement of the blood in the artery of the
wrist at each beat of the heart. The pulse can be felt at the neck and
in other parts of the body where an artery comes near to the surface.
~12. How much Work the Heart Does.~--The heart is a small organ, only
about as large as your fist, and yet it does an amount of work which is
almost beyond belief. Each time it beats, it does as much work as your
arm would do in lifting a large apple from the ground to your mouth. It
beats when we are asleep as well as when we are awake. When we run we
know by the way in which it beats that it is working very fast. Do you
know how much a ton is? Well, in twenty-four hours the heart does as
much work as a man would do in lifting stones enough to weigh more than
one hundred and twenty tons.
~13. The Lymphatics.~--While the blood is passing through the
capillaries, some of the white corpuscles escape from the blood-vessels.
What do you suppose becomes of these runaway corpuscles? Nature has
provided a way by which they can get back to the heart. In the little
spaces among the tissues outside of the blood-vessels very minute
channels called _lymph channels_ or _lymphatics_ (lym--phat´-ics) begin.
The whole body is filled with these small channels, which run together
much like the meshes of a net. In the centre of the body the small
lymphatics run into large ones, which empty into the veins near the
heart. This is the way the stray white blood corpuscles get back into
the blood.
~14. The Lymph.~--In the lymph channels the white corpuscles float in a
colorless fluid called _lymph_. The lymph is composed of the fluid
portion of the blood which has soaked through the walls of the small
vessels. The chief purpose of the lymphatics is to carry the lymph from
the tissues back to the heart.
~15. Lymphatic Glands.~--Here and there, scattered through the body, are
oval structures into each of which many lymphatic vessels are found to
run, as shown in the illustration. These are called _lymphatic glands_.
[Illustration: LYMPH GLAND AND VESSELS.]
~16.~ The heart and blood-vessels are among the most wonderful
structures in the body. It is no wonder, then, that alcohol, tobacco,
and other narcotics and stimulants produce their most deadly effects
upon these delicate organs. What these effects are we shall learn more
fully in the next chapter.
SUMMARY.
1. The heart beats to circulate the blood.
2. The heart has four chambers, two upper and two lower.
3. There are tubes called blood-vessels which carry the blood to all
parts of the body.
4. These tubes are connected with the heart.
5. The vessels which carry blood away from the heart are called
arteries, and those which carry blood back to the heart are called
veins.
6. The arteries and veins are connected by small tubes called
capillaries.
7. The blood found in the arteries is red; that in the veins is dark
blue or purple.
8. The color of the blood changes from red to blue in going through the
capillaries. The change is due to the loss of oxygen.
9. In the circulation of the lungs, the blood in the arteries is blue,
that in the veins, red.
10. The change from blue to red takes place while the blood is passing
through the capillaries of the lungs. The change is due to the oxygen
which the corpuscles of the blood take up in the lungs.
11. The pulse is caused by the beating of the heart.
12. The heart does a great deal of work every day in forcing the blood
into different parts of the body.
13. Some of the white blood corpuscles escape from the blood-vessels
through the thin walls of the capillaries.
14. These corpuscles return to the heart through small vessels called
lymph channels or lymphatics.
15. The lymphatics in many parts of the body run into small roundish
bodies called lymphatic glands.
16. The object of the lymphatics is to remove from the tissues and
return to the general circulation the lymph and white blood corpuscles
which escape through the walls of the capillaries.