This section is from the book "Health", by W. H. Coefield.
We have seen roughly now what the apparatus is, and how it works.
An adult in breathing air in, inspires on an average from 20 to 30 cubic inches of air, that is to say about three-quarters of a pint; and he breathes out the same quantity.
This air is called tidal air, because it is continually flowing in and out. He can breathe out, if he expires as hard as he is able, about 100 cubic inches more, and besides that there still remain in the lungs another 100 cubic inches, which he can never breathe out, so that the lungs always contain some air, and this air that remains in the lungs, and is never got rid of, goes by the name of the residual air. The whole quantity, from 220 to 230 cubic inches, goes by the name of the vital capacity of the lungs, and varies in different persons, and in the same persons at different times of life, and in disease.
Besides that, a person can draw in, by a deep inspiration, about another 100 cubic inches of air.
Let us consider what happens to the air that is drawn in, and what happens to the blood that is in the lungs.
If we examine the air that goes into the lungs, we find that it consists of three gases, with a certain variable quantity of watery vapour dissolved in it (because air will dissolve water very much in the same way as water will dissolve sugar or salt). The air that goes in consists of three gases chiefly. It contains nitrogen, about 79 parts in 100; it contains a gas which we call oxygen, very nearly 21 parts in 100. But you say that makes up the 100 parts-remember I said "nearly 21 parts of oxygen." And it contains another gas called carbonic acid, to the extent of about 4 parts in 10,000. I will give you the exact composition in a future Lecture.
So the air consists of nearly 4-5ths nitrogen, more than l-5th oxygen, and a trace of carbonic acid.
This is the air that is drawn into the lungs. Now what air comes out of the lungs, and what does it contain ?
We find that, however much or little water the air contains when drawn into the lungs, the air that comes out of the lungs is always saturated with moisture, so that we see that a great deal of moisture is continually got rid of from the lungs. We find further, that whether the air that we take in is cold or hot, the air we breathe out is always very nearly as hot as the blood; so that a considerable quantity of heat is lost from the body by means of the lungs. Then we find that the air that we breathe out contains just about as much nitrogen as we breathe in, so that in the lungs very little change takes place with regard to the nitrogen that is breathed. We find, on the other hand, that about 5 per cent has been lost, and that that loss has been oxygen. Of every 100 parts of air that we breathe in very nearly 5 per cent is lost, and that 5 per cent consists of oxygen. Whereas the air we breathe in contains nearly 21 parts, the air we breathe out contains only about 16 parts, or a little more, and the place of that is supplied by carbonic acid, so that the air we breathe out contains, at any rate, more than 4 per cent of carbonic acid.
You will see I have mentioned 5 per cent in the case of oxygen as lost, and 4 per cent in the case of carbonic acid, which takes its place; I have done this in order to fix in your minds the fact that rather more oxygen is absorbed in the lungs than carbonic acid given out. So that while the air contained only 4 parts in 10,000 of carbonic acid, the air that we breathe out contains at least 4 parts in 100, or at least 100 times as lnuch carbonic acid as the air that we breathe in. Besides that, the air that we breathe out contains a certain quantity of foul organic matter, and this it is that renders the air of rooms which has been breathed over and over again injurious; this it is that chiefly tends to render the air of crowded rooms disagreeable, and this stuffiness of the air is a certain sign, and has been shown to be a very accurate index, of the state of the impurity of the air.
Now before I go on to consider the changes in the blood, I must point out that air that has been breathed once is not fit to breathe again, and this for several reasons.
I suppose if I were to ask you what it was that caused this unfitness, your answer would be because it contains a large quantity of carbonic acid. That is not the first reason-the first reason is because it contains a large quantity of foul organic matter; other reasons are because the amount of oxygen in the air has been seriously diminished, and because its carbonic acid has been increased. The quantity of carbonic acid is not the most important thing, but we shall say more on that farther on.
Now, then, what happens to the blood as it goes through the lungs? The mixture of blood which comes into the right auricle of the heart is pumped by the right ventricle through the pulmonary artery into the lungs; that blood is what we call the venous blood; it is of a dark purple colour, so dark that it is commonly called black blood. We call it venous blood, because it is the kind of blood that is contained in the large veins in the body. You will remember the blood of the right side of the heart comes from the body by the large veins (venae cavae), that is why it is called venous blood. This black blood is pumped through the pulmonary artery and its branches into the capillaries of the lungs. Through these little capillary vessels, situated upon the walls of the air-sacs, the blood passes into the pulmonary veins, and runs through the pulmonary veins into the left auricle. When it has passed through these little capillary vessels it is no longer venous blood; it has become bright red blood, which we call arterial blood; this comes back to the left auricle of the heart, and from that into the left ventricle, and is pumped by means of that left ventricle through the arteries of the body, and that is why we call it arterial blood.
I told you, in the last Lecture, that blood contained substances in suspension, as well as in solution, and I mentioned some of these to you; but I did not tell you that blood has a property of dissolving the gases of the air, and blood contains dissolved in it the gases of the air, not, however, in the proportions in which they exist in the air, but in the proportions in which they are capable of dissolving in blood, and combining with certain substances in the blood. It contains a large quantity of carbonic acid, a considerable quantity of oxygen, and some nitrogen dissolved. We have already said, that in the lungs the blood gains oxygen and loses carbonic acid. The oxygen, which has been gained by the blood, gets into it from the air in the air-sacs, and the carbonic acid and moisture, which the expired air contains, come from the venous blood in the capillary vessels into the air in those air-sacs. It is a fact that moist membranes have the property of allowing substances in solution to pass through them, whether they are solids in solution or gases, and these substances in solution pass through moist membranes according to definite laws and at definite rates.