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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.

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.

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.

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.

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.


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.