Because of the need for the early development of a transport system within the embryo, the organs of the vascular system are among the first to appear and to assume their functional role. In fact, this system is established in its basic form by the fourth week of embryonic life. At approximately the 18th day of gestation, cells begin to group together between the outer skin (ectoderm) and the inner skin (endoderm) of the embryo. These cells soon become rearranged so that the more peripheral ones join to form a continuous flattened sheet enclosing more centrally placed cells; these cells remain suspended in a fluid medium as primitive blood cells. The tubes then expand and unite to form a network; the primitive blood vessels thus appear.
The blood vessels consist of a closed system of tubes that transport blood to all parts of the body and back to the heart. As in any biologic system, structure and function of the vessels are so closely related that one cannot be discussed without the other’s being taken into account.
Arteries transport blood to body tissues under high pressure, which is exerted by the pumping action of the heart. The heart forces blood into these elastic tubes, which recoil, sending blood on in pulsating waves. It is, therefore, imperative that the vessels possess strong, elastic walls to ensure fast, efficient blood flow to the tissues.
The wall of an artery consists of three layers. The tunica intima, the innermost layer, consists of an inner surface of smooth endothelium covered by a surface of elastic tissues. The tunica media, or middle coat, is thicker in arteries, particularly in the large arteries, and consists of smooth muscle cells intermingled with elastic fibres. The muscle cells and elastic fibres circle the vessel. In larger vessels the tunica media is composed primarily of elastic fibres. As arteries become smaller, the number of elastic fibres decreases while the number of smooth muscle fibres increases. The tunica adventitia, the outermost layer, is the strongest of the three layers. It is composed of collagenous and elastic fibres. (Collagen is a connective-tissue protein.) The tunica adventitia provides a limiting barrier, protecting the vessel from overexpansion. Also characteristic of this layer is the presence of small blood vessels called the vasa vasorum that supply the walls of larger arteries and veins. In contrast, the inner and middle layers are nourished by diffusion from the blood as it is transported. The thicker, more elastic wall of arteries enables them to expand with the pulse and to regain their original size.
The transition from artery to arteriole is a gradual one, marked by a progressive thinning of the vessel wall and a decrease in the size of the lumen, or passageway. In arterioles, the tunica intima is still present as a lining covered by a layer of thin longitudinal fibres; however, the tunica media no longer contains elastic fibres and is composed of only a single layer of circular or spiral smooth muscle fibres. The tunica adventitia consists of connective tissue elements.
The small arteries and arterioles act as control valves through which blood is released into the capillaries. The strong muscular wall is capable of closing the passageway or permitting it to expand to several times its normal size, thereby vastly altering blood flow to the capillaries. Blood flow is by this device directed to tissues that require it most.
As the arterioles become smaller in size, the three coats become less and less definite, with the smallest arterioles consisting of little more than endothelium, or lining, surrounded by a layer of smooth muscle. The microscopic capillary tubules consist of a single layer of endothelium that is a continuation of the innermost lining cells of arteries and veins.
As the capillaries converge, small venules are formed whose function it is to collect blood from the capillary beds (i.e., the networks of capillaries). The venules consist of an endothelial tube supported by a small amount of collagenous tissue and, in the larger venules, by a few smooth muscle fibres as well. As venules continue to increase in size, they begin to exhibit wall structure that is characteristic of arteries, though they are much thinner.
In veins, which function to conduct blood from the peripheral tissues to the heart, an endothelial lining is surrounded by the tunica media, which contains less muscle and elastic tissue than is found in the arterial wall. The outermost layer, tunica adventitia, is composed chiefly of connective tissue. Blood pressure in these vessels is extremely low as compared with that in the arterial system, and blood must exit at an even lower pressure. This creates a need for a special mechanism to keep blood moving on its return to the heart.
To achieve this, many veins possess a unique system of valves. These valves, formed by semilunar folds in the tunica intima, are present in pairs and serve to direct the flow of blood to the heart, particularly in an upward direction. As blood flows toward the heart, the flaps of the valves flatten against the wall of the vein; they then billow out to block the opening as the pressure of the blood and surrounding tissues fills the valve pocket. These valves are more abundant in the veins of the extremities than in any other parts of the body.
The veins are more distensible than arteries, and their walls are so constructed as to enable them to expand or contract. A major function of their contractility appears to be to decrease the capacity of the cardiovascular system by constriction of the peripheral vessels in response to the heart’s inability to pump sufficient blood.
Veins tend to follow a course parallel to that of arteries but are present in greater number. Their channels are larger than those of arteries, and their walls are thinner. About 60 percent of the blood volume is in the systemic circulation, and 40 percent is normally present in the veins.
The pulmonary circuit consists of the right ventricle; the exiting pulmonary artery and its branches; the arterioles, capillaries, and venules of the lungs; and the pulmonary veins that empty into the left atrium.
The pulmonary trunk, the common stem of the pulmonary arteries, arises from the upper surface of the right ventricle and extends four to five centimetres beyond this origin before dividing into the right and left pulmonary arteries, which supply the lungs. The pulmonary valve, which has two leaflets, or cusps, guards the opening between the right ventricle and the pulmonary trunk. The trunk is relatively thin-walled for an artery, having walls approximately twice the thickness of the vena cava and one-third that of the aorta. The right and left pulmonary arteries are short but possess a relatively large diameter. The walls are distensible, allowing the vessels to accommodate the stroke volume of the right ventricle, which is a necessary function equal to that of the left ventricle.
The pulmonary trunk passes diagonally upward to the left across the route of the aorta. Between the fifth and sixth thoracic vertebrae (at about the level of the bottom of the breastbone), the trunk divides into two branches—the right and left pulmonary arteries—which enter the lungs. After entering the lungs, the branches go through a process of subdivision, the final branches being capillaries. Capillaries surrounding the air sacs (alveoli) of the lungs pick up oxygen and release carbon dioxide. The capillaries carrying oxygenated blood join larger and larger vessels until they reach the pulmonary veins, which carry oxygenated blood from the lungs to the left atrium of the heart.