Physiology

by Sir M. Foster

For at the same time that the right ventricle pumped a quantity of blood into the pulmonary artery and safely lodged it there, the left ventricle pumped a like quantity into the aorta, safely lodged it there, and was left empty itself. But just at that moment the left auricle began to contract and to squeeze the blood that was in it.

Where could that blood go? It could not go back into the pulmonary veins, for they were already full, and the blood in them was being pressed behind by the over-full pulmonary arteries. But it could pass easily into the empty ventricle—and in it tumbled, the mitral flaps readily flying back and opening up a wide way. And so the auricle emptied itself into the ventricle. But now the auricle ceases to contract—its walls no longer squeeze—it is empty and wants filling, and so comes the moment when the pulmonary veins can pour into it the blood which has been driven into them by the over-full pulmonary artery.

Thus the right ventricle drives the blood into the over-full pulmonary artery, the pulmonary artery overflows into the pulmonary veins, the pulmonary veins carry the surplus to the empty left auricle, the left auricle presses it into the empty left ventricle, the left ventricle pumps it into the aorta—(the stretching of the aorta and of its branches is what we call the pulse)—the over-full aorta overflows just as did the pulmonary artery, through the capillaries of the body into the great venæ cavæ—through these the blood falls into the empty right auricle, the right auricle drives it into the empty right ventricle, and the full right ventricle is the point at which we began.

Thus the alternate contractions of auricles and ventricles, thanks to the valves in the heart and in the veins, pump the blood, stroke by stroke, through the wide system of tubes; and thus in every capillary all over the body we find blood pressed upon behind by over-full arteries, with a way open to it in front, thanks to the auricles, which are, once a second or oftener, empty and ready to take up a fresh supply from the veins. Thus it comes to pass that every little fragment of your body is bathed by blood, which a few moments ago was in your heart, and a few moments before that was in some other part of your frame. Thus it is that no part of your body can keep itself to itself; the blood makes all things common as it flies from spot to spot. The red corpuscle that a minute ago was in your brain, is now perhaps in your liver, and in another minute may be in a muscle of your arm or in a bone of your leg: wherever it goes it has something to bring, and something to fetch. A restless heart is for ever driving a busy blood, which wherever it goes buys and sells, making perhaps an occasional bargain as it shoots along the great arteries and great veins, but busiest of all as it lingers in the narrow pathways of the capillaries.

[35.] When you look down upon a great city from a high place, as upon London from St. Paul’s, you see stretching below you a network of streets, the meshes of which are filled with blocks of houses. You can watch the crowds of men and carts jostling through the streets, but the work within the houses is hidden from your view. Yet you know that, busy as seems the street, the turmoil and press which you see there are but tokens of the real business which is being carried on in the house.

So is it with any piece of the body upon which you look through the microscope. You can watch the red blood jostling through the network of capillary streets. But each mesh bounded by red lines is filled with living flesh, is a block of tiny houses, built of muscle, or of skin, or of brain, as the case may be. You cannot see much going on there, however strong your microscope; yet that is where the chief work goes on. In the city the raw material is carried through the street to the factory, and the manufactured article may be brought out again into the street, but the din of the labour is within the factory gates. In the body the blood within the capillary is a stream of raw material about to be made muscle, or bone, or brain, and of stuff which, having been muscle, or bone, or brain, is no longer of any use, and is on its way to be cast out. The actual making of muscle, or of bone, or of brain, is carried on, and the work of each is done, outside the blood, in the little plots of tissue into which no red corpuscle comes.

The capillaries are closed tubes; they keep the red corpuscles in their place. But their walls are so thin and delicate that they let the watery plasma of the blood, the colourless fluid in which the corpuscles float, soak through them into the parts inside the mesh. You probably know that many things will pass through thin skins and membranes in which no holes can be found even after the most careful search. If you put peas into a bladder and tie the neck, the peas will not get out until the bladder is untied or torn. But if you were to put a solution of sugar or of salt into the bladder, and place the bladder with its neck tied ever so tightly in a basin of pure water, you would find that very soon the water in the basin would begin to taste of sugar or salt—and that without your being able to discover any hole, however small, in the bladder. By putting various substances in the bladder, you will find that solid particles and things which will not dissolve in water keep inside the bladder, whereas sugar and salt, and many other things which dissolve in water, will make their way through the bladder into the water outside, and will keep on passing until the water in the basin is as strong of sugar or salt as the water in the bladder. This property which membranes such as a bladder have of letting certain substances pass through them is called osmosis. You will at once see how important a part it plays in your own body. It is by osmosis chiefly that the raw nourishing material in the blood gets into the little islets of flesh lying, as we have seen, in the meshwork of the capillaries. It is by osmosis chiefly that the worn-out stuff from the same islets gets back into the blood. It is by osmosis chiefly that food gets out of the stomach into the blood. It is by osmosis chiefly that the waste, worn-out matters are drained away from the blood, and so cast out of the body altogether. By osmosis the blood nourishes and purifies the flesh. By osmosis the blood is itself nourished and kept pure.

There are two chief things by which the blood, and through the blood the body, is nourished. These are food and air. The air we have always with us, we have no need to buy it or toil for it; hence we take it as we want it, a little at a time, and often. We gather up no store of it; and cannot bear the lack of it for more than a few moments.

For our food we have to labour; we store it up in our bodies from time to time, at intervals of hours, in what we call meals, and can go hours or even days without a fresh supply.

Let us first of all see how the blood, and, through the blood, the body, is nourished by air.