The revivified (arterial) blood now passes to all parts of the body and is brought into the most intimate relation with the minutest portion of every tissue. Several things happen in the course of time, all of which, whether the combination or cleavage of food compounds, or the oxidations that result in complete combustion, are brought about by the protoplasmic activity of the cells of which the body tissues are an aggregation.

138. Builds Tissue

In the first place, the new supply of nutritive substances is used by the living cells in a way we do not wholly understand, to rebuild worn-out tissue and to form new growth. With the young animal, much material is appropriated in the latter way. In the case of mammals, there is furnished to the mammary gland the nutrients out of which the milk is formed through the special activities of that gland.

139. Function Of Oxygen

Moreover, itis in the tissues that the oxygen which was taken up in the lungs is used to slowly oxidize a portion of the food. This combustion is believed not to take place by contact of the oxygen and nutrients in the blood-vessels, but it occurs through cell activity by progressive steps throughout the minute divisions of the muscles and other tissues of the body. The tissue cells undoubtedly obtain their energy from oxidation of the nutriment furnished to them. Notwithstanding this oxidation may be very gradual and occupy much time, its ultimate products are, for the most part, similar to those which result from the rapid combustion of fuel. In the fireplace, starch, sugar, cellulose, fats, and similar bodies would be burned to carbonic acid and water, and this is what takes place in the animal to the extent that these nutrients are not used constructively.

140. Protein Not Wholly Oxidized

When the protein is not stored as such, but is broken up, the result in the animal is somewhat different from that in the furnace, because in the former the oxidation is not complete. In the animal, the proteins are partially oxidized to carbonic acid and water, but a portion of their substance passes from the body principally in the form of urea and uric acid, which are the prominent constituents of urine. These compounds carry with them a certain proportion of carbon and hydrogen which, in ordinary fuel combustion, would more fully unite with oxygen. The heat production from protein is therefore less in the animal than in the furnace.

Oxidations in the human body are continuous, but not uniform. They vary with the mass, age, and habits of the individual, with the exercise the individual is taking, and with the amount of food that must be disposed of. The quantity of oxygen needed is therefore variable, and when the demand for it with a given individual is largely increased, the heart pumps faster, more blood passes through the lungs, the breathing is more rapid, and the supply of oxygen is in this way augmented.