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 {vena; cav‘), that is why it is called venous blood. This black blood is pumped through the pulmonary artery and its tranches 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.

Now, we say, then, that by passing through these capillary vessels, the blood has gained oxygen from the air, and has given up carbonic acid and water into the air; by these changes the blood becomes bright red - that is a thing ascertained by experiment. If you take dark venous blood, and put it in a bottle and shake it up with oxygen gas, it becomes bright scarlet, or arterial blood; and, on the other hand, if you take the bright scarlet blood, and shake it up in a bottle with carbonic acid, it becomes dark venous blood. There is no doubt that the change in the colour which takes place in the lungs is a change caused by getting rid of carbonic acid and gaining oxygen. It goes through these capillary vessels of the lungs which lie upon the air-sacs, and these are moist membranes, so you see that by this contrivance the black blood in the branches of the pulmonary artery is brought into contact with the largest possible quantity of air in an immense number of small sacs of moist membrane at the ends of tubes, which are in communication with the external air. This blood has too much carbonic acid and too much water. Some of the water passes through the membranes into the air in these air-sacs; some of the carbonic acid goes out from the blood into the air in these air-sacs; and some of the oxygen of the air in these air-sacs goes in to take the place of the carbonic acid, and so the blood goes on with less carbonic acid, less water, and more oxygen. This extra carbonic acid, and the extra water that are got rid of in the lungs, where do they come from? The arterial blood which has been pumped by the left ventricle of the heart into the different tissues of the body, runs through the capillary vessels of those tissues, so that there you have this arterial blood running in an immense number of fine streams through these fine capillary vessels, made of moist membrane, in the different tissues of the body; an interchange takes place between the contents of those capillary vessels and the juices in the tissues of the body; the blood gives up substances which the tissues require to nourish them, and some substances go back from the juices of the tissues into the blood, substances which the tissues have done with. The surplus of all this is picked up by the lymphatic vessels.

The result is that, when the blood has gone through any tissue in the capillary vessels, it comes out through these capillary vessels into the little veins as black venous blood, and it is this change of the blood from the bright arterial blood which is supplied to the tissues of the body into black venous blood that is reversed in the lungs.

What becomes of the oxygen that is absorbed from the air into the blood?

This oxygen gas is found to be dissolved in the blood to a much greater extent than it would be dissolved in water. Blood is capable of holding more oxygen than water, and it is found that this oxygen gas attaches itself to the little red corpuscles in the blood. One of the uses, at any rate, of these little red bodies in the blood would appear, then, to be to carry the oxygen about. The oxygen finds in the blood certain substances with which it can combine, substances partly derived from the food and partly waste substances that have got into the blood; it combines with those substances, and two of the results of that combination are two substances that I have already mentioned as being got rid of from the lungs, water, and carbonic acid. It is by means of this combination of the oxygen with the substances that it finds in the blood that our animal heat is kept up. Whenever things combine together to form other substances, heat is given out, and it is by this combination of oxygen that we breathe in with substances that it finds in the blood that the heat which we call animal heat is produced, and that the temperature of our bodies is kept up. The temperature of the blood must be kept up to within a few degrees of the ordinary standard, or else we could not exist.

Respiration takes place at various rates during different periods of life. The young child respires very much faster than the adult An adult respires fifteen or sixteen times in a minute, but the activity of respiration, and so the activity of all those changes that are produced by respiration, and the amount of the purification of the blood that can be produced by respiration, may be increased in various ways.

Exercise is one great agent in increasing the activity of respiration. It is also increased by cold. We always respire more quickly in cold weather, and you can see why. It is because in cold weather we lose heat faster, and so require to make more heat to keep ourselves sufficiently warm.

The first use then of the respiratory apparatus is to supply oxygen to burn substances in the blood, for this combination is a kind of burning, and so to produce the animal heat upon which our lives depend, which is converted into all the forces which we exert, whether muscular or mental.

All the forces of our life are derived from the animal heat produced in the way just described - viz. by burning the different substances in the blood with oxygen gas. I use the word burning advisedly, because it is precisely the same kind of process that we use for all our methods of lighting and warming, as they all consist of taking substances capable of combining with the oxygen of the air, and making them combine with it, as in burning coal, gas, or candles.

And another important use of this apparatus is to get rid of certain substances from the blood which are superfluous in it, and which are injurious to the system, thereby helping to purify the blood, so that it may be of a proper quality to nourish the different tissues of the body.