The number of the capillaries is inconceivable. Those in the lungs alone, placed in a continuous line, would reach thousands of miles. The thin walls of the capillaries are admirably adapted for the important interchanges that take place between the blood and the tissues. 190. The Circulation of the Blood. It is now well to study the circulation as a whole, tracing the course of the blood from a certain point until it returns to the same point. We may conveniently begin with the portion of blood contained at any moment in the right auricle. The superior and inferior venæ cavæ are busily filling the auricle with dark, impure blood. When it is full, it contracts. The passage leading to the right ventricle lies open, and through it the blood pours till the ventricle is full. Instantly this begins, in its turn, to contract. The tricuspid valve at once closes, and blocks the way backward. The blood is now forced through the open semilunar valves into the pulmonary artery. The pulmonary artery, bringing venous blood, by its alternate expansion and recoil, draws the blood along until it reaches the pulmonary capillaries. These tiny tubes surround the air cells of the lungs, and here an exchange takes place. The impure, venous blood here gives up its _débris_ in the shape of carbon dioxid and water, and in return takes up a large amount of oxygen. Thus the blood brought to the lungs by the pulmonary arteries leaves the lungs entirely different in character and appearance. This part of the circulation is often called the lesser or pulmonic circulation. The four pulmonary veins bring back bright, scarlet blood, and pour it into the left auricle of the heart, whence it passes through the mitral valve into the left ventricle. As soon as the left ventricle is full, it contracts. The mitral valve instantly closes and blocks the passage backward into the auricle; the blood, having no other way open, is forced through the semilunar valves into the aorta. Now red in color from its fresh oxygen, and laden with nutritive materials, it is distributed by the arteries to the various tissues of the body. Here it gives up its oxygen, and certain nutritive materials to build up the tissues, and receives certain products of waste, and, changed to a purple color, passes from the capillaries into the veins. Illustration: Fig. 75.—Diagram illustrating the Circulation. 1, right auricle; 2, left auricle; 3, right ventricle; 4, left ventricle; 5, vena cava superior; 6, vena cava inferior; 7, pulmonary arteries; 8, lungs; 9, pulmonary veins; 10, aorta; 11, alimentary canal; 12, liver; 13, hepatic artery; 14, portal vein; 15, hepatic vein.