Angiology: The Vascular System

Angiology is the description of the organs of circulation of the blood and lymph—the heart and vessels. The heart is the central hollow muscular organ which functions as a suction and force pump; the differences in pressure caused by its contraction and relaxation determine the circulation of the blood and lymph. It is situated in the middle mediastinal space of the thorax, between the two lungs, and is inclosed in a fibro-serous sac—the pericardium. The vessels are tubular and run through almost all parts of the body. They are designated according to their contents as blood- and lymph vessels.

The blood-vascular system consists of: (1) the arteries, which convey blood from the heart to the tissues; (2) the capillaries, microscopic tubes in the tissues which permit of the necessary interchange between the blood and the tissues; (3) the veins, which convey the blood back to the heart.

The blood-vessels are divided into the pulmonary and the systemic. The pulmonary artery conveys the blood from the right ventricle of the heart to the lungs, where it is arterialized, and is returned by the pulmonary veins to the left atrium of the heart, and passes into the left ventricle. The systemic arteries convey the blood from the left ventricle all over the body, whence it is returned by the venæ cavæ to the right atrium, and passes into the right ventricle.

It should be noted, however, that the lungs receive arterial blood through the systemic bronchial arteries. This blood is returned in part by the bronchial veins to the right atrium (indirectly), in part by the pulmonary veins to the left atrium.

The term portal system is often applied to the portal vein and its tributaries which come from the stomach, intestine, pancreas, and spleen. The vein enters the liver, where it branches like an artery, so that the blood in this subsidiary system passes through a second set of capillaries before being conveyed to the heart by the hepatic veins and the posterior vena cava.

The arteries (Arteriæ), as a rule, divide at an acute angle, giving off finer and finer branches. In some cases branches come off at a right angle, and others are recurrent, i. e., run in a direction opposite to that of the parent stem. The intercommunication of branches of adjacent arteries is termed anastomosis. Most commonly the connections are made by a network of numerous fine branches (Plexus vasculosus). Relatively large communicating branches (Rami communicantes) occur in certain places; they may be transverse or in the form of arches. Wide-meshed networks of vessels are termed Retia vasculosa. Terminal or end arteries are such as form isolated networks, i. e., do not anastomose with adjacent arteries. A rete mirabile is a network intercalated in the course of an artery. A collateral vessel (Vas collaterale) is one which pursues a course near and similar to that of a larger vessel.

The veins (Venæ) are in general arranged like the arteries, but are usually of greater caliber. When a vein accompanies an artery, it is termed a vena comitans or satellite vein; in many places two veins accompany a single artery. The primitive venous trunks do not run with the arteries, and most of the superficial veins (Venæ cutaneæ) pursue independent courses. They anastomose even more freely than the arteries, and large communicating branches are very common. The veins form very rich plexuses (Plexus venosi) in many places. Some veins which are without independent walls and are inclosed by dense membranes and run usually in bony grooves are termed (venous) sinuses; examples of this are the sinuses of the dura mater of the brain.

Structure of Arteries.—The wall consists of three coats. The external coat (Tunica externa) consists chiefly of fibrous connective tissue. In the deeper part are some elastic fibers, and in some arteries there are also longitudinal unstriped muscle-fibers. The middle coat (Tunica media) is composed of unstriped muscle and elastic tissue in medium-sized arteries. In small vessels there is only the muscular tissue, and in the largest trunks only elastic tissue. The internal coat or intima (Tunica intima) consists of a layer of endothelial cells, resting on an elastic membrane. The sheath (Vagina vasis) is a condensation of the surrounding connective tissue, and is attached more or less closely to the external coat.

Structure of Veins.—The walls of veins are similar in structure to those of the arteries, but are very much thinner, so that veins collapse more or less completely when empty, while arteries do not. The middle coat is very thin and consists to a large extent of ordinary connective tissue. The intima is also less elastic than in the arteries. In many veins this coat forms semilunar valves, the free edges of which are directed toward the heart. They are most numerous in the veins of the skin and the deep veins of the extremities (except the foot), while in most veins of the body cavities and viscera they are absent or occur only where the veins open into larger ones or where two veins join.

The walls of the vessels are supplied with blood by numerous small arteries, called vasa vasorum. These arise from branches of the artery which they supply or from adjacent arteries, ramify in the external coat, and enter the middle coat also.

The nerves of the vessels consist of both medullated and non-medullated fibers. They form plexuses around the vessels, from which fibers pass mainly to the muscular tissue of the middle coat.