Among the things which are present in the body fluids should be mentioned the two gases oxygen and carbon dioxide. We should expect oxygen to be present in the body fluids, because it is necessary for the metabolism of cells and can get to them only by being carried in the blood stream. We have seen in the red corpuscles the special method by which an abundance of oxygen is transported. Carbon dioxide is the gaseous product of the oxidation of carbon and is found in large amounts wherever there is burning, since carbon is the chief constituent of all fuel and whenever carbon is burned, carbon dioxide is formed. Since the fuel materials that are burned in the living cells consist largely of carbon, carbon dioxide is produced in them as well. They have to get rid of it, and the only way they can do so is by passing it out into the fluids that surround them, which in turn pass it on to the blood. The way in which the body handles these two gases makes up a special subdivision of the subject of physiology and will be so treated in the chapter on respiration. CHAPTER XIV THE CONVEYER SYSTEM OF THE BODY In the last chapter we talked about the body fluids and saw that they can be subdivided into the tissue fluids, which surround the cells, and the blood, which is inclosed in a system of pipes and which carries the materials to and from the tissue fluids. We now have to take up this matter of transporting the material in more detail. The first step will be to see how materials that are in the blood get from it to the tissue fluids, and how materials that are in the tissue fluids get from them into the blood. Unless these interchanges can take place freely, there is no way in which the blood can serve as a conveyer system. In order to see how the interchanges are carried on, we shall have to look first at some features of the construction of the system of pipes through which the blood flows. As we all know, the large blood vessels are either arteries or veins, the arteries being blood vessels which are carrying blood away from the heart, and the veins vessels that are carrying blood toward the heart. If we start with an artery and trace it through the body, we find that it is continually giving off branches which in turn give off smaller branches, until finally the subdivisions are so small that we cannot trace them any further with the naked eye. In the days before the microscope was discovered, there was a great deal of question as to how these finest branches ended. At first no one suspected that there was connection between the fine subdivisions of the arteries and the fine subdivisions of the veins through which the blood could pass.