THE BLOOD
=81. Blood.=--After food becomes blood, it goes to every part of the
body to feed the cells. Even a pin prick anywhere in the body draws
blood. The blood makes the skin pink. There are five or six quarts of
blood in a man's body. This is about 1/13 of his body.
[Illustration: =Blood corpuscles (x400).=
_a_ a pile of red blood cells.
_b_ red blood cells seen flatwise.
_c_ red blood cells seen edgewise.
_d_ white blood cells.]
Blood looks like a red liquid. But if you look at it through a strong
microscope, it looks like water, and millions of little red cells.
These cells carry air through the body. They make the blood look red.
There are also a smaller number of white cells. Blood is made of red
cells, white cells, and a liquid.
=82. The liquid in blood.=--The liquid part of the blood is albumin,
and water, with a little fat, sugar, and minerals. It is food and
drink for the cells of the body. When blood is drawn from the body it
soon becomes like jelly. We call the jelly a _clot_. When you cut your
finger, a clot forms in the cut and plugs up the bleeding place. If
it did not, the blood would all run out of the body and we should die.
[Illustration: =Diagram of the heart while it is beating.=
_a_ vein entering the auricle.
_b_ auricle.
_c_ closed valve to keep blood from flowing back into the auricle.
_d_ ventricle.
_e_ artery.
_f_ valve to keep blood from returning to the ventricle.]
=83. The heart.=--The blood is held in tubes. A pump inside the body
keeps it always moving. This pump is called the _heart_. The heart is
a bag of muscle with thick sides. It is about as large as your fist.
When it is full, it has the power to make itself smaller, and so it
squeezes the blood out through a tube. We can feel each squeeze as a
heart-beat. You can find the heart-beat just to the left of the middle
of the body about two hand-breadths below the neck.
=84. The heart-beat.=--A man's heart beats about seventy times each
minute. Boys' and girls' hearts beat much faster. Running or hard work
of any kind makes the heart beat faster yet. Your heart will keep on
beating until you die. It does not seem to rest at all, yet it works
only while you feel it beat. Between each beat it rests while the
blood is filling it again. So it really rests one half of the time.
=85. Arteries.=--The heart pumps the blood through a single tube. This
tube opens into smaller tubes. These open into still smaller ones. You
must use a strong microscope to see the finest blood tubes. The tubes
reach every part of the body, and carry blood to its cells. They are
called _arteries_. At each heart-beat a wave of blood can be felt in
an artery. This wave is the _pulse_. It can be felt in the wrist,
temples, and other places. By the pulse we can tell how often and how
strongly the heart is beating.
[Illustration: =Arrangement of capillaries.=
_a_ smallest artery.
_b_ smallest vein.
_c_ network of capillaries.]
=86. Capillaries.=--The smallest arteries divide into a fine network
of small tubes. These tubes are the _capillaries_. They lie around
every cell of the body. Their sides are very thin. As the blood flows
through them, some of it soaks through the sides of the tubes. Blood
contains all kinds of food for the cells. Each cell is always wet with
food and can eat it at any time. The cells are like the tiny animal,
the ameba, and can take in the food by any part of their bodies. The
cells are better off than the ameba, for their food is brought to
them. They pay the body for their food by working for it.
=87. Veins.=--The capillaries come together again to form large tubes.
These tubes are called _veins_. Only a little of the blood goes through
the sides of a capillary. The rest flows on into the veins. The veins
unite to form two large tubes. These two tubes open into the heart.
=88. How the blood flows.=--The blood is pumped out of the heart,
through the arteries to the capillaries. There some goes out to the
cells. The rest flows into the veins and goes back to the heart. All
the blood in the body passes through the heart every two minutes. It
takes only twenty seconds for a drop of blood to go from the heart to
the toes and back again. The arteries are deep in the flesh, but some
of the large veins can be seen upon the back of the hands.
=89. Bleeding.=--If a large artery or vein is cut there is a great
deal of bleeding. You can always stop a cut from bleeding by holding
it fast between the hands. Do not be afraid of the blood when you see
any one bleeding, but hold the sides of the cut tightly with both of
your hands. This will stop any bleeding until help comes. You may keep
a person from bleeding to death by doing this when other persons are
afraid of the blood.
=90. Healing cuts.=--When your flesh is cut it soon grows together
again. The work of the little white cells in the blood is to help heal
cuts and wounds and bruises. These cells are like little amebas in the
blood. They keep moving around with the blood, and now and then burrow
outside the capillaries to see if all is well. If they find a cut,
hundreds and thousands rush to the spot at once. Some eat up any
specks of dirt on the cut. Others fit themselves into the sides of the
cut and grow long and slender, like strings, and so bind the two edges
of the cut together. In this way all cuts are healed.
[Illustration: =Bacteria growing in a kidney and producing an abscess
(x300).=
_a_ kidney tube.
_b_ white blood cell attacking bacteria.
_c_ bacteria.
_d_ blood vessel of the kidney.]
=91. The white blood cells kill disease germs.=--There are tiny living
beings everywhere in the air, and soil, and water. Some of them can grow
inside a man and make him sick. These tiny things are called _disease
germs_. One kind gives a man typhoid fever, and another diphtheria.
Another kind grows on cuts, and sometimes makes them very sore. The
white cells of the blood are always watching for these enemies, like a
cat hunting mice, and when they find them they at once try to kill them.
But sometimes the white blood cells get killed. Then they look like
cream in the cut. We call this creamy liquid _matter_ or _pus_, and say
"We have caught cold in the cut." In most pricks and cuts the white
cells of the blood can kill all these enemies and also heal the cut.
=92. Catching cold.=--Sometimes the cold air blows on our head and
hurts the cells of the nose. If there are disease germs in the air,
they may grow in the injured part of the nose and make us have a "cold
in the head." Then the white blood cells gather at the spot so as to
kill the disease germs. Also the arteries bring a great deal of blood
to the nose so as to heal the injured parts. Some of the white blood
cells and the liquid from the blood run out, and we have to blow the
nose. The white blood cells help to make us well whenever we catch a
cold or other kind of sickness.
=93. Red blood cells.=--The red blood cells are like tiny flat plates.
They float in the liquid part of the blood and make the blood look
red. They carry air from the lungs to the cells of every part of the
body, and thus help all the cells to breathe.
=94. Why the heart beats hard when we run.=--When we work hard, the
cells of our bodies need a great deal of food. So the heart beats much
harder, and sends them much more blood. We can feel our heart beat
when we run hard.
When the cells work they get more blood in another way. The arteries
become larger and hold more blood. Then the part looks red and feels
warm. Thus your face gets red when you run hard. This is because your
heart and arteries bring more blood to feed the working cells.
=95. Need of a strong heart.=--The heart must keep sending blood to
feed the cells. If it should stop for only a little while, the cells
would starve to death and we should die. We need strong hearts. When
we work very hard for a long time, the heart gets tired. Then the
cells do not get enough food and we feel weak all over. Boys ought not
to run and lift till they are tired out, for this hurts their hearts.
=96. What alcohol does to the blood.=--Alcohol hinders the digestion
of food. Then too little food will reach the blood, and so the cells
of the body will get too little. Alcohol does not add strength to the
body, but it takes it away. It seems to make men stronger, for it
takes away their tired feelings. But it makes them really weaker, for
it harms the blood.
=97. How alcohol affects the heart.=--Alcohol at first makes the heart
beat more strongly and quickly, but it tires it out and makes it
weaker. Then the heart pumps too little blood to the rest of the body,
and a man is weaker all over.
If a drinker tries to run or work hard, his heart may not pump enough
food to the working cells of his arms and legs. Strong drink takes
away a man's strength and makes him less able to endure a long strain.
=98. How alcohol harms the arteries.=--Alcohol causes the arteries to
become larger and to carry more blood. Then the face will be red and
the skin will become warm. This makes a person feel well, and he seems
to be helped. His blood seems to be flowing faster because his face is
red. But really it is flowing slower.
When the arteries have been made large very often, they stay large all
the time. A drinker's nose is often red from this cause.
Alcohol sometimes causes the arteries to become hard, and even to
change to a kind of bone. Then they cannot change their size to carry
just so much blood as each part needs.
=99. How tobacco affects the heart.=--Tobacco weakens all the body,
but it harms the heart more than the rest. It often makes the heart
beat slowly at one time and fast at another. It weakens the heart and
keeps it from working harder when the working cells need more food. A
smoker gets out of breath quickly. He cannot run far or work very
hard. Chewing is a still more harmful form of using tobacco. When men
train for a game or a race they never use tobacco.
Boys are not so strong as men, and so tobacco is more hurtful to them.
Boys are harmed by tobacco far more than men. Cigarette smoke harms
their stomachs and keeps food from their blood. If boys smoke, they
become pale and weak. The poisonous smoke weakens the heart, and they
cannot run or work so hard as they should. Even if a father uses
tobacco, he should not allow his boys to use it.
WHAT WE HAVE LEARNED
1. Blood is a liquid. It contains many round red cells and a few
white cells.
2. Blood contains all kinds of food for the cells of the body.
3. The blood is kept moving by the heart.
4. The heart pumps or beats about seventy times a minute.
5. The blood flows through arteries to all parts of the body.
6. The arteries open into the capillaries. Capillaries make a
network around each cell of the body.
7. Some of the liquid parts of the blood go out through the sides
of the capillaries and become food for the cells of the
body.
8. From the capillaries the blood flows into the veins and back
to the heart.
9. Bleeding can be stopped by holding the cut tightly between the
hands.
10. The white blood cells grow into the sides of cuts, and so
heal them. They also guard the body against the seeds of
many diseases.
11. The red blood cells carry air to the cells of the body.
12. Alcohol weakens the heart and arteries.
13. Tobacco harms the heart.