Fig. 10.1 ). These layers are present in all vessels but are most apparent in larger vessels and particularly arteries. The intima consists of an EC monolayer on a basement membrane with minimal underlying ECM; it is separated from the media by a dense elastic membrane called the internal elastic lamina. The media is composed predominantly of SMCs and ECM, surrounded by loose connective tissue, nerve fibers, and smaller vessels of the adventitia. An external elastic lamina is present in some arteries and defines the transition between media and adventitia. Diffusion of oxygen and nutrients from the lumen is adequate to sustain thin-walled vessels and the innermost SMCs of all vessels. In large and medium-sized vessels, however, small arterioles within the adventitia (called vasa vasorum — literally, “vessels of the vessels”) perfuse the outer half to two-thirds of the media.
http://ebooksmedicine.net
Fig. 10.1 Regionalvascularspecializations.Althoughallvesselssharethesamegeneralconstituents,thethicknessandcompositionofthevariouslayersdifferasafunctionofhemodynamicforcesandtissuerequirements.
Arteries are divided into three types based on their size and structure:
Large elastic arteries (e.g., aorta, arch vessels, iliac and pulmonary arteries). In these vessels, elastic fibers alternate with SMCs throughout the media, which expands during systole (storing some of the energy of each cardiac contraction), and recoils during diastole to propel blood distally. With age and/or diseases, such as diabetes and hypertension, the elasticity is lost; the vessels become “stiff pipes” that transmit high arterial pressures to distal organs, or become dilated and tortuous (ectatic) and prone to rupture.
Medium-sized muscular arteries (e.g., coronary and renal arteries). Here, the media is composed primarily of SMCs, with elastin limited to the internal and external elastic lamina. The medial SMCs are circularly or spirally arranged around the lumen, and regional blood flow is regulated by SMC contraction (vasoconstriction) and relaxation (vasodilation) controlled by the autonomic nervous system and local metabolic factors (e.g., acidosis). ECs also modulate arterial SMC tone, for example, by releasing nitric oxide (NO, causing vasodilation) or endothelin (causing vasoconstriction).
• Small arteries (2 mm or less in diameter) and arterioles (20 to 100 µm in diameter) lie within the connective tissue of organs. The media in these vessels is mostly composed of SMCs. Arterioles are where blood flow resistance is regulated. As pressures drop during passage through arterioles, the velocity of blood flow is sharply reduced, and flow becomes steady rather than pulsatile. Because the resistance to fluid flow is inversely proportional to the fourth power of the diameter (i.e., halving the diameter increases resistance 16-fold), small changes in arteriolar lumen size have profound effects on blood pressure.
Capillaries have lumen diameters slightly smaller than those of red blood cells (7 to 8 µm). These vessels are lined by ECs and partially surrounded by SMC-like cells called pericytes. Collectively, capillary beds have a very large total cross-sectional area and a low rate of blood flow. With their thin walls and slow flow, capillaries are ideally suited to the rapid exchange of diffusible substances between blood and tissue. The capillary network of most tissues is necessarily very rich, because diffusion of oxygen and nutrients is not efficient beyond 100 µm; metabolically active tissues (e.g., heart) have the highest capillary density.
Veins receive blood from the capillary beds as postcapillary venules, which anastomose to form collecting venules and progressively larger veins. The vascular leakage (edema) and leukocyte emigration characteristic of inflammation occurs preferentially in postcapillary venules (Chapter 3).