THE HEART-PUMP AND ITS PIPE-LINE SYSTEM
THE BLOOD VESSELS
Where the Body Does its Real Eating. When once the food has been
dissolved in the food-tube and absorbed by the cells of its walls, the
next problem is how it shall be sent all over the body to supply the
different parts that are hungry for it; for we must remember that the
real eating of the food is done by the billions upon billions of tiny
living cells of which the body is made up.
The Pipe Lines of the Body. What do we do when we want to carry water,
or oil, or sewage, quickly and surely from one place to another? We put
down a pipe line. We are wonderfully proud of our systems of water and
gas supply, and of the great pipe lines that carry oil from wells in
Ohio and Indiana clear to the Atlantic coast. But the very first man
that ever laid a pipe to carry water was simply imitating nature--only
about ten or fifteen million years behind her. No sooner has our food
passed through the cells in the wall of the food-tube, than it goes
straight into a set of tiny tubes--the blood-pipes, or _blood
vessels_--which carry it to the heart; and the heart pumps it all over
the body.
Veins and Arteries. These blood-tubes running from the walls of the
food-tube to the heart are called _veins_; and the other tubes through
which the heart pumps the blood all over the body are called _arteries_.
If you will spell this last word "air-teries," it may help you to
remember why the name was given to these tubes ages ago. When the body
was examined after death, they were found to be empty and hence were
not unnaturally supposed to carry air throughout the body, and
"air-teries" they have remained ever since. While absurd in one way, the
name is not so far amiss in another, for an important part of their work
is to carry all over the body swarms of tiny baskets, or sponges, of
oxygen taken from the air.
Why the Blood is Red. The first and main purpose of the blood-pipes
and the heart is to carry the dissolved food from the stomach and
intestines to the cells all over the body. But the cells need air as
well as food; and, to carry this, there are little basket-cells--the
_red corpuscles_. Take a drop of blood and put it under a microscope,
and you will see what they look like. The field will be simply crowded
with tiny, rounded lozenges--the red cells of the blood, which give it
its well-known color.
[Illustration: BLOOD CORPUSCLES (Greatly magnified)
_A_, red blood; _B_, white blood.]
The White Corpuscles or Scavengers of the Blood. As the blood-tubes
are not only supply-pipes but sewers and drainage canals as well, it is
a good thing to have some kind of tiny animals living and moving about
in them, which can act as scavengers and eat up some of the waste and
scraps; and hence your microscope will show you another kind of little
blood corpuscle, known, from the fact that it is not colored, as the
_white corpuscle_. These corpuscles are little cells of the body, which
in shape and behavior are almost exactly like an _ameba_--a tiny "bug,"
seen only under the microscope, that lives in ditch-water. Under the
microscope the white corpuscles look like little round disks, about
one-third larger than the red corpuscles, and with a large kernel, or
_nucleus_, in their centre. They have the same power of changing their
shape, of surrounding and swallowing scraps of food, as has the ameba,
and are a combination of scavengers and sanitary police. When disease
germs get into the blood, they attack and endeavor to eat and digest
them; and whenever inflammation, or trouble of any sort, begins in any
part of the body, they hurry to the scene in thousands, clog the
blood-tubes and squeeze their way out through the walls of the smallest
blood-tubes to attack the invaders or repair the damage. This causes the
well-known swelling and reddening which accompanies inflammation.
Blood, then, is a sticky red fluid, two-thirds of which is food-soup,
and the other third, corpuscles. How tiny the blood-corpuscles are, may
be guessed from the fact that there are about 5,000,000 red cells and
10,000 white cells in every _cubic centimetre_ (fifteen drops) of our
blood.
How the Blood Circulates through the Body. Now let us see how some
portion of the body, say the right thumb, gets its share of food and of
oxygen through the blood. We will start at the very beginning. The food,
of course, is put into the mouth, chewed by the teeth, and softened and
digested in the stomach and intestines. It is then taken up by the cells
of the mucous coat of the intestines and passed into the network of tiny
blood-pipes surrounding them, between the lining of the bowels and their
muscular coat. These tiny blood-pipes, called _capillaries_, run
together to form larger pipes--the small veins; and the small veins from
the walls of the intestine and stomach finally run together into one
large pipe, or trunk-line (called the _portal vein_), which carries them
to the liver.
[Illustration: DIAGRAM OF THE CIRCULATORY SYSTEM
All details are omitted. The connection between arteries and veins is
shown only in the brain. Both heart and blood vessels are considerably
enlarged to show clearly the course of the blood.]
In passing through the liver, the blood is purified of some irritating
substances picked up from the food-tube, and the melted food which it
contains is further prepared for the use of the cells of the body. The
portal vein of the liver breaks up into a network of veins, and these
again break up into a number of tiny capillaries, in which the blood is
acted upon by the cells of the liver. These capillaries gather together
again to form veins, and finally unite into two large veins at the back
of the liver, which run directly into the great trunk-pipe of all the
veins of the body--the _vena cava_ (or "empty vein," so called because
it is always found empty after death), about an inch from where this
opens into the right side of the heart.
In the vena cava the blood from the food-tube, rich in food, but poor in
oxygen, mixes with the impure, or used-up, blood brought back by the
veins from all over the body and, passing into the right side of the
heart, is pumped by the heart through a large blood-pipe to the lungs.
This large blood-pipe divides into two branches, one for each lung; and
these again break up into smaller branches, and finally into tiny
capillaries, which are looped about in fine meshes, or networks, around
the air-cells of the lung. Here, through the thin and delicate walls of
the capillaries the blood cells give off, or breathe out, their carbon
dioxid and other waste gases (which are passed out with our outgoing
breath), and at the same time they breathe in oxygen which our incoming
breath has drawn into the lungs.
This oxygen is picked up by, and combines with, the red coloring matter
of the millions of little oxygen sponges, or baskets--the red
corpuscles--and turns them a light red color, causing the blood to
become bright red, such as runs in the arteries and is known as
_arterial blood_.
The loops of tiny capillaries around the air cells of the lungs run
together again to form larger pipes; and these unite, at the point of
each lung nearest the heart, to form two large blood pipes--one from
each lung--which pour the rich, pure blood, loaded with both food and
oxygen into the left side of the heart. The left side of the heart pumps
this blood out into the great main delivery-pipe for pure blood, known
as the _aorta_, and this begins to give off branches to the different
parts of the body, within a few inches of where it leaves the heart.
[Illustration: SURFACE VEINS AND DEEP-LYING ARTERIES OF INNER SIDE OF
RIGHT ARM AND HAND
The deep-lying veins that run parallel to the arteries have been
omitted; so have the veins of three of the fingers.]
One of the first of these branches to be given off by the aorta is a
large blood pipe, or artery, to supply the shoulder and arm; this artery
runs across the chest, thence across the armpit, and down the arm to the
elbow. Here it divides into two branches, one to supply the right, and
the other the left, side of the forearm and hand. These branches have by
this time got down to about the size of a wheat straw; the one supplying
the right side is the artery which we feel throbbing in the wrist, and
which we use in counting the pulse. From it run off smaller branches to
supply the thumb and fingers. These branches break up again into still
smaller branches, and they into a multitude of tiny capillaries, which
run in every direction among all the muscle cells, delivering the food
and oxygen at their very doors, as it were. The muscle cells eagerly
suck out the food-stuffs, and breathe in the oxygen of the blood; at the
same time, they pour into it their waste stuffs of all sorts, including
carbon dioxid. These rob the blood of its bright red oxygen color and
turn it a dirty purplish, or bluish, tint.
The loops of capillaries again run together, as they did in the liver
and in the lung, to form tiny veins; and these run together at the base
of the thumb and in the wrist, to form larger ones through which the now
poor and dirty blood is carried back up the arm over much the same
course as it took in coming down it. Indeed, the veins usually run
parallel with, and often directly alongside of, the arteries. The blood
passes through the armpit, across the chest, into the great main pipe
for impure blood, the vena cava, and through this into the right side of
the heart, where it again meets the rich, but waste-laden blood from the
food tube and liver, and starts on its circuit through the lungs and
around the body again.
The blood reaches every portion of our body in precisely this same
manner, only taking a different branch of the great pure-blood delivery
pipe, the aorta, according to the part of the body which it is to reach,
and coming back by a different vein-pipe.
Why the Arteries are more deeply Placed than the Veins. In the limbs
and over the surface of the body generally, the arteries are more deeply
placed than the veins, so as to protect them from injury, because the
blood in the arteries is driven at much higher pressure than in the
veins and spurts out with dangerous rapidity, if they are cut. Some of
the veins, indeed, run quite a little distance away from any artery and
quite close to the surface of the body, so that you can see them as
bluish streaks showing through the skin, particularly upon the front and
inner side of the arms.
The Capillaries. Of course, the blood pipes into which the food is
sucked through the walls of the food tube, and those in the lung,
through which the oxygen is breathed, as well as those in the thumb
through which food is taken to the muscle-cells, have the tiniest and
thinnest walls imaginable. For once, the name given them by the wise
men--capillaries (from the Latin _capilla_, a little hair)--fits them
beautifully, except that the hairs in this case are hollow, and about
one-twentieth of the size of the finest hair you can see with the naked
eye. So tiny are they that they compare with the big veins near the
heart into which they finally empty much as the smallest and slenderest
twigs of an elm do with its trunk. What they lack in size, however, they
more than make up in numbers; and a network of them as fine and close as
the most delicate gauze goes completely around the food tube between its
mucous lining and muscular coat.
Though thickest and most abundant on the inner and outer surfaces of the
body, every particle of the body substance is shot through and through
with a network of these tiny tubes. So close and fine is this network in
the skin, for instance, that, as you can readily prove, it is impossible
to thrust the point of the finest needle through the skin without
piercing one of them and "drawing blood," as we say, or making it bleed.
From this network of tiny, thin-walled tubes, the body-cells draw their
food from the blood.
[Illustration: DIAGRAM OF ARTERY, CAPILLARIES, AND VEIN]
The Meaning of Good Color. It is the red blood in this spongy network
of tiny vessels that gives a pink coloring to our lips and the flush of
health to our cheeks. Whenever for any reason the blood is less richly
supplied with food or oxygen, or more loaded with "smoke" and other body
dirt than it should be, we lose this good color and become pale or
sallow. If we will remember that our hearts, our livers, our brains, and
our stomachs, are at the same time often equally "pale" and sallow--that
is, badly supplied with blood--as our complexions, we can readily
understand why it is that we are likely to have poor appetites, poor
memories, bad tastes in our mouths, and are easily tired whenever, as we
say, our "blood is out of order." The blood is the life. Starve or
poison that, and you starve or poison every bit of living stuff in the
body.
THE HEART
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.
[Illustration: THE EXTERIOR OF THE HEART
Showing the strands of muscle that compose it, the arteries and veins
that feed and drain the muscle coat, and fat protecting these.]
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.
[Illustration: DIAGRAM OF VALVES IN THE VEINS AND HEART
In _A_ the blood flows forward naturally. In _B_ and _C_ is shown what
would happen were the blood to reverse its course, as it does when it
meets an obstruction: the pockets would fill until they met and closed
the passageway.]
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.
[Illustration: THE BLOOD-ROUTE THROUGH THE HEART
_R.A._, right auricle; _L.A._, left auricle; _R.V._, right ventricle;
_L.V._, left ventricle; _A_, aorta; _P.A._, pulmonary artery; _P.V._,
pulmonary veins; _V.C.s._, Vena cava superior; _V.C.i._, Vena cava
inferior. At the entrance to the pulmonary artery are shown two of the
pockets of the valve, the third pocket having been cut away with the
front side of the artery. The other blood-tubes have similar valves, not
shown in the diagram.]
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.