Nitrogen 76.95 Oxygen 20.61 Carbon dioxide .03 Argon 1.00 Water vapor (average) 1.40 Air expired from the lungs contains:-- Nitrogen 76.95 Oxygen 15.67 Carbon dioxide 4.38 Water vapor 2 Argon 1 In other words, there is a loss between 4 and 5 per cent oxygen, and nearly a corresponding gain in carbon dioxide, in expired air. There are also some other organic substances present. [Illustration: The respiration of cells.] Cell Respiration.--It has been shown, in the case of very simple animals, such as the _amoeba_, that when oxidation takes place in a cell, work results from this oxidation. The oxygen taken into the lungs is not used there, but is carried by the blood to such parts of the body as need oxygen to oxidize food materials in the cells. Since work is done in the cells of the body, food and oxygen are therefore required. The quantity of oxygen used by the body is nearly dependent on the amount of work performed. Oxygen is constantly taken from the blood by tissues in a state of rest and is used up when the body is at work. This is suggested by the fact that in a given time a man, when working, gives off more oxygen (in carbon dioxide) than he takes in during that time. While work is being done certain wastes are formed in the cell. Carbon dioxide is given off when carbon is burned. But when proteins are burned, another waste product containing nitrogen is formed. This must be passed off from the cells, as it is a poison. Here again the lymph and blood, the common carriers, take the waste material to points where it may be _excreted_ or passed out of the body. The Mechanics of Respiration. The Pleura.--The lungs are covered with a thin elastic membrane, the _pleura_. This forms a bag in which the lungs are hung. Between the walls of the bag and the lungs is a space filled with lymph. By this means the lungs are prevented from rubbing against the walls of the chest. [Illustration: The chest cavity (_a_) at the time of a full breath; (_b_), after an expiration. Explain how the cavity for lungs is made larger.]