Hand in hand with the increased consumption of oxygen, there is of course an increased production of carbon dioxide and of water, since the oxidation of fuel substances produces these waste products. The cells are always pouring both out into the tissue fluids, but to a very much greater extent when they are actively functioning. We need make no effort to keep track of the water, since it merely adds itself to the water already present, and we shall consider later how the water supplies of the body are handled. The carbon dioxide, however, must be gotten rid of, and the mechanism for getting rid of it must work efficiently, otherwise metabolism itself will be hampered, since it is a familiar law of chemical action that if the products of an action are allowed to accumulate they interfere with its further progress. The method of getting rid of carbon dioxide is by simple diffusion from the cells into the tissue fluids and from the tissue fluids into the blood. Carbon dioxide is many times as soluble as oxygen, so that a great deal more of it can be handled by merely dissolving. This is not sufficient, however, to take care of all the carbon dioxide; the remainder must go into chemical combination with some substances that are in the blood. There is no single conspicuous material for carrying carbon dioxide like the hemoglobin which transports the oxygen. There are, however, a number of compounds in the blood which are able to combine with carbon dioxide, among them the blood proteins of which much was made in Chapter XIII. The carbon dioxide distributes itself among these various substances and so is transported. It should be noted that the blood does not become saturated with carbon dioxide as it does with oxygen. Arterial blood ordinarily carries practically all the oxygen it is able to take up; venous blood on the other hand probably never comes anywhere near being as fully charged with carbon dioxide as it is able to be.