As before shown, the blood as it flows through the tissues loses most of its oxygen, and carbon dioxid takes its place. Now if the blood is to maintain its efficiency in this respect, it must always be receiving new supplies of oxygen, and also have some mode of throwing off its excess of carbon dioxid. This, then, is the double function of the process of respiration. Again, the blood sent out from the left side of the heart is of a bright scarlet color. After its work is done, and the blood returns to the right side of the heart, it is of a dark purple color. This change in color takes place in the capillaries, and is due to the fact that there the blood gives up most of its oxygen to the tissues and receives from them a great deal of carbon dioxid. In brief, while passing through the capillaries of the lungs the blood has been changed from the venous to the arterial blood. That is to say, the blood in its progress through the lungs has rid itself of its excess of carbon dioxid and obtained a fresh supply of oxygen.[36] 214. Effects of Respiration upon the Air in the Lungs. It is well known that if two different liquids be placed in a vessel in contact with each other and left undisturbed, they do not remain separate, but gradually mix, and in time will be perfectly combined. This is called diffusion of liquids. The same thing occurs with gases, though the process is not visible. This is known as the diffusion of gases. It is also true that two liquids will mingle when separated from each other by a membrane (sec. 129). In a similar manner two gases, especially if of different densities, may mingle even when separated from each other by a membrane. In a general way this explains the respiratory changes that occur in the blood in the lungs. Blood containing oxygen and carbon dioxid is flowing in countless tiny streams through the walls of the air cells of the lungs. The air cells themselves contain a mixture of the same two gases. A thin, moist membrane, well adapted to allow gaseous diffusion, separates the blood from the air. This membrane is the delicate wall of the capillaries and the epithelium of the air cells. By experiment it has been found that the pressure of oxygen in the blood is less than that in the air cells, and that the pressure of carbon dioxid gas in the blood is greater than that in the air cells. As a result, a diffusion of gases ensues. The blood gains oxygen and loses carbon dioxid, while the air cells lose oxygen and gain the latter gas. Illustration: Fig. 92.—Capillary Network of the Air Cells and Origin of the Pulmonary Veins. A, small branch of pulmonary artery; B, twigs of the pulmonary artery anastomosing to form peripheral network of the primitive air cells; C, capillary network around the walls of the air sacs; D, branches of network converging for form the veinlets of the pulmonary veins.