Circulatory system arterioles

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

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. 

As mentioned previously, the arterioles are the major resistance vessels in the circulatory system. Because the walls of these vessels contain primarily smooth muscle, they are capable of significant changes in their radius. Therefore, regulation of blood flow to the tissues is carried out by the arterioles. 

Myogenic mechanism. As discussed in Chapter 16 on the circulatory system, the myogenic mechanism involves contraction of vascular smooth muscle in response to stretch. For example, an increase in MAP would tend to increase RBF, leading to an increase in pressure within the afferent arteriole and distension, or stretch, of the vessel wall. Consequently, the vascular smooth muscle of the afferent arteriole contracts, increases the resistance of the vessel, and decreases RBF toward normal. 

The microcirculation is comprised of blood vessels (arterioles, capillaries, and venules) with diameters of less than approximately 150 /xm. The importance of the microcirculation is underscored by the fact that most of the hydrodynamic resistance of the circulatory system Hes in the microvessels (especially in arterioles) and most of the exchange of nutrients and waste products occurs at the level of the smallest microvessels. The subjects of microcirculatory research are blood flow and molecular transport in microvessels, mechanical interactions and molecular exchange between these vessels and the surrounding tissue, and regulation of blood flow and pressure and molecular transport. Quantitative knowledge of microcirculatory mechanics and mass transport has been accumulated primarily in the past 30 years owing to significant innovations in methods and techniques to measure microcirculatory parameters and methods to analyze microcirculatory data. The development of these methods has required joint efforts... 

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