Physiology

by Sir M. Foster

HOW THE BLOOD IS CHANGED BY AIR: BREATHING. § VI.

[36.] I have already said, perhaps more than once, that our muscles burn, burn in a wet way without giving light. And when I say our muscles, I might say our whole body, some parts burning more fiercely than others.

You have learnt from your Chemistry Primer (Art. 2, p. 2) what happens when a candle is placed in a closed jar of pure air. The oxygen gets less, carbonic acid comes in its place, and after a while the candle goes out for want of oxygen to carry on that oxidation which is the essence of burning. You also know that exactly the same thing would happen if you were (only you need not do it) to put a bird or a mouse in the jar instead of a candle. The oxygen would go, carbonic acid would come, and the little flame of life in the mouse would flicker and go out, and after a while its body would be cold.

But suppose you were to put a fish or a snail in a jar of pure fresh water, and cork the jar tight. There seems at first sight to be no air in the jar. But there is. If you were to take that fresh water, and put it under an air-pump, you could pump bubbles of air out of it; and if you were to examine these bubbles you would find them to contain oxygen and nitrogen, with very little carbonic acid. The water contains dissolved air. After the fish or the snail had been some time in the jar, you would see its flame of life flicker and die out, just like that of the bird in air; and if you then pumped the air out of the water you would find that the oxygen was nearly gone and that carbonic acid had come in its place.

You see, then, that air can be breathed, as we call it, even when it is dissolved in water.

Now to return to our muscle. When you were watching the circulation in the frog’s foot, you could tell the artery from the vein, because in the artery the blood was flowing to the capillaries, and in the vein from them. Both artery and vein were rather red, and of about the same tint of colour. But if you could see in your own body a large artery going to your biceps muscle, and a large vein coming away from it, you would be struck at once with the difference of colour between them. The artery would look bright scarlet, the vein a dark purple; and if you were to prick both, the blood would gush from the artery in a bright scarlet jet, and bubble from the vein in a dark purple stream. And wherever you found an artery and a vein (with a great exception of which I shall have to speak directly), the blood in the artery would be bright scarlet, and that in the vein dark purple. Hence we call the bright scarlet blood which is found in the arteries arterial blood, and the dark purple blood which is found in the veins venous blood.

What is the difference between the two? If you were to pump away at some arterial blood, as you did at the water in which you put your fish, you would be able to obtain from it some air, or, more correctly, some gas; a great deal more gas, in fact, than you did from the water. A pint of blood would yield you half a pint of gas. This gas you would find on examination not to be air, i.e. not made up of a great deal of nitrogen and the rest oxygen. (Chemical Primer, Art. 9.) There would be very little nitrogen, but a good deal of oxygen, and still more carbonic acid.

If you were to pump away at some venous blood you would get about as much gas, but it would be very different in composition. The little nitrogen would remain about the same, but the oxygen would be about half gone, while the carbonic acid would be much increased.

This, then, is one great difference (for there are others) between venous and arterial blood, that while both contain, dissolved in them, oxygen, nitrogen, and carbonic acid, venous blood contains less oxygen and more carbonic acid than arterial blood.

[37.] In passing through the capillaries on its way to the vein, the blood in the artery has lost oxygen and gained carbonic acid. Where has the oxygen gone to? Whence comes the carbonic acid? To and from the islets of flesh between the capillaries, to the bloodless muscular fibre or bit of nerve or skin which the blood-holding capillaries wrap round. The oxygen has passed from the blood within the capillaries to the flesh outside; from the flesh outside the carbonic acid has passed to the blood within the capillaries. And this goes on all over the body. Everywhere the flesh is breathing blood, is breathing gas dissolved in the blood, just as a fish breathes water, i.e. breathes the air dissolved in the water.