ANATOMY OF THE VASCULAR SYSTEM
Arteries are thick-walled structures that carry blood from the heart to the tissues. The aorta, which has a diameter of approxi-mately 25 mm (1 inch), gives rise to numerous branches, which divide into smaller arteries that are about 4 mm (0.16 inch) in diameter by the time they reach the tissues. Within the tissues, the vessels divide further, diminishing to approximately 30 μ m in diameter; these vessels are called arterioles.
The walls of the arteries and arterioles are composed of three layers: the intima, an inner endothelial cell layer; the media, a middle layer of smooth elastic tissue; and the adventitia, an outer layer of connective tissue. The intima, a very thin layer, provides a smooth surface for contact with the flowing blood. The media makes up most of the vessel wall in the aorta and other large ar-teries of the body. This layer is composed chiefly of elastic and connective tissue fibers that give the vessels considerable strength and allow them to constrict and dilate to accommodate the blood ejected from the heart (stroke volume) and maintain an even, steady flow of blood. The adventitia is a layer of connective tis-sue that anchors the vessel to its surroundings. There is much less elastic tissue in the smaller arteries and arterioles, and the media in these vessels is composed primarily of smooth muscle.
Smooth muscle controls the diameter of the vessels by con-tracting and relaxing. Chemical, hormonal, and nervous system factors influence the activity of smooth muscle. Because arterioles can alter their diameter, thereby offering resistance to blood flow, they are often referred to as resistance vessels. Arterioles regulate the volume and pressure in the arterial system and the rate of blood flow to the capillaries. Because of the large amount of mus-cle, the walls of the arteries are relatively thick, accounting for ap-proximately 25% of the total diameter of the artery. The walls of the arterioles account for approximately 67% of the total diame-ter of arterioles.
The intima and the inner third of the smooth muscle layer are in such close contact with the blood that the blood vessel receives its nourishment by direct diffusion. The adventitia and the outer media layers have a limited vascular system for nourishment and require their own blood supply to meet metabolic needs.
Capillary walls, which lack smooth muscle and adventitia, are composed of a single layer of endothelial cells. This thin-walled structure permits rapid and efficient transport of nutrients to the cells and removal of metabolic wastes. The diameter of capillar-ies ranges from 5 to 10 μ m; this narrow channel requires red blood cells to alter their shape to pass through these vessels. Changes in a capillary’s diameter are passive and are influenced by contractile changes in the blood vessels that carry blood to and from a capillary. The capillary’s diameter also changes in response to chemical stimuli. In some tissues, a cuff of smooth muscle, called the precapillary sphincter, is located at the arteriolar end of the capillary and is responsible, along with the arteriole, for con-trolling capillary blood flow.
Some capillary beds, such as in the fingertips, contain arte-riovenous anastomoses, through which blood passes directly from the arterial to the venous system. These vessels are believed to regulate heat exchange between the body and the external environment.
The distribution of capillaries varies with the type of tissue. For example, skeletal tissue, which is metabolically active, has a denser capillary network than does cartilage, which is less active.
Capillaries join to form larger vessels called venules, which join to form veins. The venous system is therefore structurally analo-gous to the arterial system; venules correspond to arterioles, veins to arteries, and the vena cava to the aorta. Analogous types of vessels in the arterial and venous systems have approximately the same diameters (see Fig. 31-1).
The walls of the veins, in contrast to those of the arteries, are thinner and considerably less muscular. The wall of the average vein amounts to only 10% of the vein diameter, in contrast to 25% in the artery. The walls of a vein, like those of arteries, are com-posed of three layers, although these layers are not as well defined.
The thin, less muscular structure of the vein wall allows these vessels to distend more than arteries. Greater distensibility and compliance permit large volumes of blood to be stored in the veins under low pressure. For this reason, veins are referred to as capacitance vessels. Approximately 75% of total blood volume iscontained in the veins. The sympathetic nervous system, which innervates the vein musculature, can stimulate the veins to con-strict (venoconstriction), thereby reducing venous volume and increasing the volume of blood in the general circulation. Con-traction of skeletal muscles in the extremities creates the primary pumping action to facilitate venous blood flow back to the heart.
Some veins, unlike arteries, are equipped with valves. In gen-eral, veins that transport blood against the force of gravity, as in the lower extremities, have one-way bicuspid valves that interrupt the column of blood to prevent blood from seeping backward as it is propelled toward the heart. Valves are composed of endo-thelial leaflets, the competency of which depends on the integrity of the vein wall.
The lymphatic vessels are a complex network of thin-walled vessels similar to the blood capillaries. This network collects lymphatic fluid from tissues and organs and transports the fluid to the venous circulation. The lymphatic vessels converge into two main struc-tures: the thoracic duct and the right lymphatic duct. These ducts empty into the junction of the subclavian and the internal jugular veins. The right lymphatic duct conveys lymph primarily from the right side of the head, neck, thorax, and upper arms. The thoracic duct conveys lymph from the remainder of the body. Peripheral lymphatic vessels join larger lymph vessels and pass through re-gional lymph nodes before entering the venous circulation. The lymph nodes play an important role in filtering foreign particles.
The lymphatic vessels are permeable to large molecules and provide the only means by which interstitial proteins can return to the venous system. With muscular contraction, lymph vessels become distorted to create spaces between the endothelial cells, allowing protein and particles to enter. Muscular contraction of the lymphatic walls and surrounding tissues aids in propelling the lymph toward the venous drainage points.
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