Anatomy & Physiology

Anatomy & Physiology Study Guide

©2018 Achieve Page 258 20.3 Capillaries The true work of the cardiovascular system is accomplished in the microscopic capillaries that permeate most tissues. These delicate vessels weave throughout active tissues, forming intricate networks that surround muscle fibers, radiate through connective tissues, and branch beneath the basal laminae of epithelia. Capillaries are the only blood vessels whose capillary walls permit exchange between the blood and the surrounding interstitial fluids, a vessel characteristic unique to capillaries. Capillary walls are thin; thus, diffusion distances are short. Hence, exchange can occur quickly. Also, there is sufficient time for the diffusion or active transport of materials across the capillary walls because blood flows through relatively slowly. Thus, the histological structure of capillaries permits a two-way exchange of substances between blood and interstitial fluid. A typical capillary possesses neither a tunica media nor a tunica externa and consists solely of an endothelial tube inside a delicate basal lamina. The average diameter of a capillary is a mere 8 µm, very close to that of a single red blood cell. There are two major types of capillaries: continuous capillaries and fenestrated capillaries. Continuous Capillaries Many regions of the body are supplied by continuous capillaries. In these vessels, the endothelial layer is a complete lining. Continuous capillaries are located in all tissues except epithelia and cartilage. Continuous capillaries are selectively permeable; they prevent the loss of blood cells and plasma proteins while permitting the diffusion of water, small solutes, and lipid-soluble materials into the surrounding interstitial fluid. Also, some exchange may occur between blood and interstitial fluid by bulk transport—the movement of vesicles that form through endocytosis at the inner endothelial surface. Fenestrated Capillaries Fenestrated capillaries are capillaries that contain “windows,” or pores, that infiltrate the endothelial lining. The pores allow the rapid exchange of water and solutes as big as small peptides between plasma and interstitial fluid. Examples of fenestrated capillaries include The choroid plexus of the brain and the blood vessels in a variety of endocrine organs, such as the hypothalamus and the pituitary, pineal, and thyroid glands, are examples of these porous vessels. Fenestrated capillaries are also located along absorptive areas of the intestinal tract and at filtration areas in the kidneys. Both the quantity of pores and their permeability characteristics may differ from one region of the capillary to another. Sinusoids resemble fenestrated capillaries that are flattened and irregularly shaped. In contrast to fenestrated capillaries, sinusoids commonly have gaps between adjacent endothelial cells, and the basal lamina is either thinner or absent. As a result, sinusoids permit the free exchange of water and solutes as large as plasma proteins between blood and interstitial fluid. Capillary Beds Capillaries function as a portion of an interconnected network called a capillary bed , or capillary plexus . A single arteriole gives rise to dozens of capillaries that empty into several venules, the smallest vessels of the venous system. Precapillary sphincters guard the entrance to each capillary. of 368