Every cell must have oxygen for its metabolism. This it must get from the tissue fluid upon which it fronts, and tissue fluid in turn must get it from the circulating blood. The blood in turn has to get it from somewhere and the place from which the blood gets it is called a lung or a gill, according as the animal breathes air directly or gets its oxygen out of the water. The purpose of the present chapter is to trace oxygen from the air through the blood to the tissue fluids and so to the living cells. We saw in the chapter on “Blood” that we have a special substance, the hemoglobin, which helps in the transportation of oxygen. We shall have occasion here to see how it does this. It is well to bear in mind that for practical purposes we include the methods by which carbon dioxide is gotten rid of along with the study of the supply of oxygen, so that although this chapter carries the heading “The Service of Supply of Oxygen,” we shall also study in it how the carbon dioxide is carried away. This is a convenient way of dealing with the subject, because the two gases are handled in very much the same manner; it is also made necessary by the fact that the control of the apparatus by which these gases are handled is so interwoven that the transportation of one could not well be studied without giving attention also to the transportation of the other. The problem of the oxygen supply and of the removal of the carbon dioxide is in theory very simple; the air contains a large percentage of oxygen and a very small percentage (three parts in ten thousand) of carbon dioxide. If the blood is exposed to air with nothing between but a very delicate membrane, oxygen will diffuse from the air into the blood until the blood has taken up all that it is capable of holding. As the blood circulates around the body and comes to the tissue spaces where there is little or no oxygen, because the living cells are constantly taking it up and using it, the oxygen which previously diffused into the blood will diffuse out into the tissue spaces. The only special arrangements that have to be provided are a sufficiently great surface of exposure to the air, so that no matter how rapidly the blood may be flowing it shall be able to take up all the oxygen it can hold, and an arrangement for making sure that the blood can hold and carry as much oxygen as the tissues need. The first of these requirements is met by the special construction of the lung or gill; the second by having present in the blood stream a chemical substance (hemoglobin) which automatically takes up large quantities of oxygen and so insures that sufficient shall be transported.