*Interchange of Gases in the Lungs.*—During each inspiration the air from the outside fills the entire system of bronchial tubes, but the alveoli are largely filled, at the same time, by the air which the last expiratory effort has left in the passages. By the action of currents and eddies and by the rapid diffusion of gas particles, the air from the outside mixes with that in the alveoli and comes in contact with the membranous walls. Here the oxygen, after being dissolved by the moisture in the membrane, diffuses into the blood. The carbon dioxide, on the other hand, being in excess in the blood, diffuses toward the air in the alveoli. The interchange of gases at the lungs, however, is not fully understood, and it is possible that other forces than osmosis play a part. [Fig. 43] Fig. 43—*Diagram* illustrating lung capacity. *Capacity of the Lungs.*—The air which passes into and from the lungs in ordinary breathing, called the _tidal_ air, is but a small part of the whole amount of air which the lungs contain. Even after a forced expiration the lungs are almost half full; the air which remains is called the _residual_ air. The air which is expelled from the lungs by a forced expiration, less the tidal air, is called the _reserve_, or supplemental, air. These several quantities are easily estimated. (See Practical Work.) In the average individual the total capacity of the lungs (with the chest in repose) is about one gallon. In forced inspirations this capacity may be increased about one third, the excess being known as the _complemental_ air (Fig. 43). [Fig. 44] Fig. 44—*Diagram* illustrating internal respiration and its dependence on external respiration. (Modified from Hall.) (See text.) *Internal, or Cell, Respiration.*—The oxygen which enters the blood in the lungs leaves it in the tissues, passing through the lymph into the cells (Fig. 44). At the same time the carbon dioxide which is being formed at the cells passes into the blood. An exchange of gases is thus taking place between the cells and the blood, similar to that taking place between the blood and the air. This exchange is known as _internal_, or cell, respiration. By internal respiration the oxygen reaches the place where it is to serve its purpose, and the carbon dioxide begins its movement toward the exterior of the body. This "breathing by the cells" is, therefore, _the final and essential act of respiration_. Breathing by the lungs is simply the means by which the taking up of oxygen and *the* giving off of carbon dioxide by the cells is made possible. HYGIENE OF RESPIRATORY ORGANS