Circulation

in physiology, the passage of blood from the heart into the arteries and from them into the veins and back through the veins into the heart. The human circulatory system consists of the heart, the arteries, the capillaries, the veins, and the lymphatic system.

Complete circulation is of two parts: The blood passes from the left ventricle to the arteries of the body, then through the capillaries to the veins, and returns to the right auricle. This passage is called systemic or body circulation. Without stopping, the blood passes into the right ventricle and is forced through the pulmonary artery into the capillaries of the lungs, whence it returns through the pulmonary veins into the left auricle, and through it, into the left ventricle. The trip to the lungs and return is called the pulmonary circulation.

The vessels that carry the blood away from the heart are called arteries. Those that carry the blood to the auricles of the heart are called veins. The arteries and veins correspond very generally in number, size, and location. In a general way an artery may be said to have a corresponding vein; but the walls of the arteries are thicker, stronger, and more elastic than those of the veins. The heart forces the blood out through the ventricles with considerable pressure. Near the heart, the pressure in the arteries is about a fifth greater than atmosphere pressure, but diminishes as the blood travels from the heart.

The great artery of the systemic circulation is the aorta. It rises from the left ventricle in an arch and passes down back of the heart, through the body cavity in front of the backbone. More than thirty branches go out from the aorta to the trunk and the internal organs. At the lower end of the abdominal cavity the aorta divides into two main branches, one going to each leg. Three branches rising from the arch of the aorta carry blood to the arms, the neck, and the head. The arteries that rise, one on each side of the windpipe, are called carotid arteries. The large vessel carrying blood from the right ventricle to the lungs is called the pulmonary artery.

The vein which carries returning blood from the legs, the internal organs, and the trunk, is called the ascending, or inferior, vena cava. The blood from the head, neck, and arms returns through the descending, or superior, vena cava. The veins corresponding to the two carotid arteries are called jugular veins. The arteries branch again and again, until they are too small to be seen without the aid of a microscope. The veins branch likewise.

The blood passes from the ends of the arteries into the veins by small blood vessels known as capillaries, which are just large enough to allow the corpuscles of the blood to pass through. The name is derived from a Latin word meaning hair, but the capillaries are very much smaller than the finest hairs. They have the thinnest of walls, and are not to exceed a microscopic fraction of an inch in length. The veins and arteries are mere carriers. They are water-tight--we may say blood-tight--but the capillaries permit the escape of oxygen, white cells, and plasma. They also allow waste matter to pass from the tissues into the blood.

The arteries of the systemic circulation carry arterial blood--blood red with oxygen and rich with nourishment. The veins of the systemic circulation carry back dark venous blood--blood laden with carbon dioxide and the wastes of the tissues. The artery of the pulmonary system carries dark venous blood to the lungs. The four pulmonary veins bring bright arterial blood to the heart. The outgoing current of blood is driven by the muscular action of the heart. The walls of the arteries are smooth. The walls of the veins and lymphatic vessels, however, are furnished with valves which open toward the heart. The pulse of the blood in the arteries, breathing--every movement of the body that disturbs the blood or the lymph--causes these liquids to pass on toward the heart. The valves prevent a return.

There are numerous ordinarily minute crevices and cavities within the tissues, known as lymph spaces. In many parts of the body lymph capillaries form a network. The lymph vessels combine to form large ducts which empty into the veins. The largest of these is the left thoracic duct. It is about as large as a penholder. It carries lymph from the legs, the left side of the body, the neck, the head, and the arms. These lymph ducts empty into the large veins of the neck. The lymph vessels thus form a second drainage system.

The circulation of blood was not understood by the ancients. While it was known that the blood of arteries was connected with the heart, it was thought that the dark blood of the veins came from the liver, and that the two systems had no connection. Servetus, in 1553, discovered that blood passes from the heart to the body and back again; but the facts of circulation were not finally known until published by Harvey in 1628. The manner in which the blood manages to pass from the arteries to the veins was made known in 1661. The main features of the lymphatic system were made out at about