The microcirculation comprises the smallest arterioles, and the exchange vessels, including the capillaries and the postcapillary venules. The transfer of gases, water, nutrients, waste materials and other substances between the blood and body tissues carried out by the exchange vessels is the ultimate function of the cardiovascular system.

Organization of the Microcirculation

Blood enters the microcirculation via small arterioles, the walls of which contain smooth muscle cells. These vessels are densely innervated by the sympathetic system, particularly in the splanchnic and cutaneous vascular beds. Sympathetically mediated constriction of each small arteriole reduces the blood flow to many capillaries.

In the vast majority of tissues, the smallest or terminal arterioles divide to give rise to sets of capillaries (Figure 20a, left). The terminal arteriole itself acts as a functional precapillary sphincter for its entire cluster of capillaries. Terminal arterioles are not innervated, and their tone is controlled by local metabolic factors (see Chapter 23). Under basal conditions, terminal arterioles constrict and relax periodically. This vasomotion causes the flow of blood through the cluster of capillaries to fluctuate.

In a few tissues, however (e.g. mesentery), capillaries branch from thoroughfare vessels which run from small arterioles to venules (Figure 20a, right). The proximal (arteriolar) end of such a vessel is termed a metarteriole, and it is wrapped intermittently in smooth muscle cells. The capillaries have a ring of smooth muscle called a precapillary sphincter at their origin, but thereafter lack smooth muscle cells. Constriction of the precapillary sphincter controls the flow of blood through that capillary.

The capillaries join to form postcapillary venules, which also lack smooth muscle cells. These merge to form venules, which contain smooth muscle cells and are sympathetically innervated.

Movement of Solutes Across the Capillary Wall

Water, gases and solutes (e.g. electrolytes, glucose, proteins) cross the walls of exchange vessels mainly by diffusion, a passive process by which substances move down their concentration gradients. O₂ and CO₂ can diffuse through the lipid bilayers of the endothelial cells. These and other lipophilic substances (e.g. general anaesthetics) therefore cross the capillary wall very rapidly. However, the lipid bilayer is impermeable to electrolytes and small hydrophilic (lipid-insoluble) molecules such as glucose, which therefore cross the walls of continuous capillaries (Figure 20b, bottom) 1000–10 000 times more slowly than does O₂. Hydrophilic molecules cross the capillary wall mainly by diffusing between the endothe­lial cells. This process is slowed by tight junctions between the endothelial cells which impede diffusion through the intercellular clefts. Diffusion is also retarded by the glycocalyx

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