The capillaries serve as a medium to transmit the blood from the arteries to the veins; and it is through them that the blood brings nourishment to the surrounding tissues.  In brief, we may regard the whole body as consisting of countless groups of little islands surrounded by ever-flowing streams of blood.  The walls of the capillaries are of the most delicate structure, consisting of a single layer of cells loosely connected.  Thus there is allowed the most free interchange between the blood and the tissues, through the medium of the lymph.

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 venae cavae 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 debris 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.