Capillaries and Endothelium

Capillaries contain 6% of the circulating blood volume and measure 5 |im in diameter at their arteriolar end widening to 9 |im at the venous end. These dimensions are comparable with the 7 |im diameter of red blood cells, which decrease in diameter when traversing capillaries. A capillary consists of a tube formed by a single sheet of endothelial cells resting on a basement membrane. Associated with capillaries and venules are interstitial cells called pericytes, akin to renal mesangial cells. These cells release chemicals that mediate capillary permeability and also secrete the basement membrane (Figure CR.21).

Capillary Exchange

Capillaries have differing structures and permeabilities according to the tissues they serve. Most capillaries comprise a continuous layer of endothelial cells. Intercellular junctions and small pores representing about 0.02% of the capillary surface area permit the diffusion of small molecules < 8 nm in size. Larger molecules may cross through cell cytoplasm in vesicles and fat soluble molecules; water, oxygen and CO2 pass directly across the cell membrane. Fenestrations, gaps of 20-100 nm diameter, appear in the endothelial cells of endocrine glands, renal glomeruli and intestinal villi to facilitate secretion, filtration and absorption. In the sinusoids of the liver and in the spleen, the endothelium is discontinuous with gaps of > 1000 nm. As a result albumin escapes from hepatic sinusoids much more readily than in other tissues.

In summary, transfer of water and solutes across the endothelium occurs by several mechanisms:

• Diffusion—the main mechanism mediating the exchange of gases, and for the movement of small molecules such as glucose, urea, electrolytes and between circulation and tissues. It occurs at endothelial defects such as pores and fenestrations. Diffusion of gases and lipid soluble molecules also occurs directly across cell membranes

Figure CR.21 Capillaries and pericytes

• Filtration—the movement of water and small size solutes across the endothelium under the influence of hydrostatic and osmotic gradients. Transport of substances this way is only a fraction of that occurring by diffusion; however, a significant flow of water circulates between the circulation and interstitial fluid by filtration. About 2% of capillary plasma flow is filtered (20-40 ml/min)

• Pinocytosis—transports larger (> 30 nm) lipid insoluble molecules across the endothelium. Pinocytotic vesicles form at the cell membrane (endocytosis), migrate across the cell, and expel their contents at the opposite cell membrane (exocytosis). The numbers of pinocytotic vesicles seen varies between tissues and is greater at the arteriolar end than the venous end of capillaries

• Direct passage of lipid soluble compounds and gases occurs by diffusion across cell membranes, through the cells and into the interstitial fluid

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