In the preceding chapters we have seen that the blood undergoes important changes as it courses through the different parts of its circuit. Where it comes in contact with the tissues it yields to them nutrient material for assimilation, and oxygen for their metabolism, and carries away from them some waste products. In the lungs it receives oxygen and gives off carbonic acid. While it flows through the minute vessels of the alimentary tract, some of the materials elaborated by the digestion of food are absorbed, and directly added to the blood; at the confluence of the great veins in the neck the stream, composed of lymph and chyle, is poured into the blood before it enters the heart, so as to be thoroughly mingled with it on its return from the general circulation. Moreover, in various glands, different substances are used in the manufacture of their secretions.

Thus there is a kind of material circulation, a constant income and output going on in the blood itself as it passes through the different parts of the body. The investigation of the exact changes which take place in the blood in each organ or part is surrounded with difficulty, and in many cases it is quite impossible to ascertain what changes occur. In some parts it may be made out by noting the results produced, or the substances given off or taken up by the blood, as seen in the changes found in the air after its exposure to the blood in the lungs, where we can definitely state that the blood has lost or gained certain materials, and is so far altered. In other parts, such as the muscles or the ductless glands, where, no doubt, profound changes in the blood occur, we have no separate outcome which we can analyze, and we must therefore trust altogether for the elucidation of the change going on in them to the differences which may be found to exist in the blood flowing to, and that flowing from, such an organ. For this purpose one can either examine samples of the blood from the artery and vein of the organ, while the ordinary circulation is going on, or, immediately after the removal of the organ, by causing the artificial stream of blood to flow through it; then the changes brought about in the blood in its passage through the organ will give the required information. It can be seen, from the foregoing enumeration of processes, that some organs have a double function as regards the blood. Thus, in the lung there is both renovation by taking in oxygen, and purification by getting rid of carbon dioxide. The textures in their internal respiration take the nutriment and oxygen, and give the blood C02 and various other waste products of tissue change.