Blood vessels

The walls of blood vessels may contain varying amounts of fibrous tissue, elastic tissue, and smooth muscle. All blood vessels are lined with a single layer of endothelial cells forming the endothelium. The fibrous connective tissue provides structural support and stiffens the vessel. The elastic connective tissue allows vessels to expand and hold more blood. It also allows the vessels to recoil and exert pressure on blood within the vessels, which pushes this blood forward. Most blood vessels contain smooth muscle arranged in circular or spiral layers. Therefore, contraction of vascular smooth muscle, or vasoconstriction, narrows the diameter of the vessel and decreases the flow of blood through it. Relaxation of vascular smooth muscle, or vasodilation, widens the diameter of the vessel and increases the flow of blood through it. The smooth muscle of the vessel is innervated by the autonomic nervous system and is, therefore, physiologically regulated. Furthermore, this is where endogenous vasoactive substances and pharmacological agents exert their effects. The endothelium has several important physiological functions, including contributing to the regulation of blood pressure, blood vessel growth, and the exchange of materials between blood and the interstitial fluid of the tissues.

The circulatory system is composed of several anatomically and functionally distinct blood vessels including: (1) arteries, (2) arterioles, (3) capillaries, and (4) veins.

Arteries carry blood away from the heart (see Figure 15.1). These vessels contain fibrous connective tissue that strengthens them and enables them to withstand the high blood pressures generated by the heart. In general, the arteries function as a system of conduits, or pipes, transporting the blood under high pressure toward the tissues. There is little smooth muscle and therefore little physiological regulation of vessel diameter in these vessels.

Another noteworthy anatomical feature of the arteries is the presence of elastic connective tissue. When the heart contracts and ejects the blood, a portion of the stroke volume flows toward the capillaries. However, much of the stroke volume ejected during systole is retained in the distensible arteries. When the heart relaxes, the arteries recoil and exert pressure on the blood within them, forcing this "stored" blood to flow forward. In this way, a steady flow of blood toward the capillaries is maintained throughout the entire cardiac cycle.

As the arteries travel toward the peripheral organs and tissues, they branch and become smaller. Furthermore, the walls of the vessels become less elastic and more muscular. The smallest arterial vessels, arterioles, are composed almost entirely of smooth muscle with a lining of endothelium.

Figure 15.1 The circulatory system. Arteries carry blood away from the heart. The smallest arterial vessels, the arterioles, are composed mainly of smooth muscle and are the major resistance vessels in the circuit. The capillaries are the site of exchange between blood and tissues. Veins carry blood back toward the heart. The small veins are the major compliance vessels in the circuit and, under resting conditions, contain 64% of the blood volume.

Figure 15.1 The circulatory system. Arteries carry blood away from the heart. The smallest arterial vessels, the arterioles, are composed mainly of smooth muscle and are the major resistance vessels in the circuit. The capillaries are the site of exchange between blood and tissues. Veins carry blood back toward the heart. The small veins are the major compliance vessels in the circuit and, under resting conditions, contain 64% of the blood volume.

Therefore, depending upon the degree of constriction of the vascular smooth muscle, these vessels may alter their diameter, and consequently their blood flow, across a very wide range. For this reason, the arterioles are the major resistance vessels in the circulatory system. In fact, the primary function of arterioles is to regulate the distribution of the cardiac output and to determine which tissues receive more blood and which receive less, depending upon the tissue's and the body's needs.

From the arterioles, blood flows through capillaries, the smallest vessels in the circulatory system. The capillaries are the site of exchange between blood and the interstitial fluid surrounding the cells of the tissues. The primary mechanism of exchange is simple diffusion as substances move across the capillary walls "down" their concentration gradients, or from an area of high concentration to an area of low concentration. Two important factors influencing the process of diffusion include surface area and thickness of the barrier. As the surface area increases, so does diffusion. Approximately 10 billion capillaries are in the adult human body with a total exchange surface area of more than 6300 square meters — the equivalent of almost two football fields. Furthermore, most tissue cells are not more than 20 mm away from the nearest capillary. The capillaries also have the thinnest walls of all the blood vessels; they are composed of only a flat layer of endothelium, one cell thick, supported by a thin acellular matrix referred to as the basement membrane, with a total thickness of only 0.5 mm. As such, the anatomical characteristics of the capillaries, which maximize the exchange surface area and minimize the thickness of the barrier, render these vessels ideally suited for the exchange of materials by simple diffusion.

Following the exchange of substances with the tissues, blood begins its route back to the heart through the venous system. Blood flows from the capillaries into the venules, small vessels consisting mainly of a layer of endothelium and fibrous connective tissue. From the venules, the blood flows into veins that become larger as they travel toward the heart. As with the arteries, the walls of these vessels consist of a layer of endothelium, elastic connective tissue, smooth muscle, and fibrous connective tissue. However, veins have much thinner walls and wider diameters than the arteries they accompany. These vessels are very distensible and are capable of holding large volumes of blood at a very low pressure. For this reason, the veins are the major compliance vessels of the circulatory system (see Figure 15.1). In fact, approximately 64% of the blood volume is contained within the veins under resting conditions. During exercise, the pumping action of the contracting skeletal muscles and the smooth muscle in the walls of the veins forces this blood toward the heart and increases venous return. Therefore, the veins are referred to as blood reservoirs and play an important role in regulation of venous return and, consequently, cardiac output.

Another important anatomical characteristic of veins is the presence of valves, which ensure the one-way flow of blood back toward the heart. They are most abundant in the lower limbs, where the effects of gravity on the circulatory system are most prevalent and would tend to cause pooling of blood in the feet and ankles. Finally, the large veins and the venae cavae return blood to the right atrium of the heart. As with the large arteries and aorta, these vessels function primarily as conduits. There is little smooth muscle and therefore little physiological regulation of their diameter.

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