Capillary

Capillaries are the smallest blood vessels in the microcirculation, connecting arterioles to venules and serving as the primary sites for the exchange of gases, nutrients, and waste products between blood and tissues.

Each nutrient artery entering an organ typically branches six to eight times before the vessels become small enough to be classified as arterioles, with a diameter ranging from 10 to 15 μm. These arterioles then branch an additional two to five times, gradually reducing their diameter to about 5–9 μm at the terminal ends, where they deliver blood into the capillary network. Venules, which collect blood from capillaries, are generally larger than arterioles and possess a thinner muscular coat. Despite their weaker musculature, the lower intraluminal pressure within venules allows them to contract considerably.

The walls of capillaries are composed of a single layer of endothelial cells surrounded externally by a delicate basement membrane. This simple structure results in a total wall thickness of approximately 0.5 μm and an internal diameter between 4 and 9 μm, allowing efficient exchange between the blood and surrounding tissues. Along the capillary walls, intercellular clefts are located at the junctions between endothelial cells. These small openings permit the passage of water, electrolytes, glucose, and other small solutes, while larger molecules are generally restricted. In addition to these clefts, capillaries contain plasmalemmal vesicles, or caveolae, which are believed to play a role in endocytosis and transcytosis—processes that transport substances across the endothelial cell membrane.

Capillaries exhibit specialized structures depending on the organ and its functional needs. In the brain, endothelial cells are joined by tight junctions that limit permeability, allowing only small molecules such as oxygen, carbon dioxide, and water to pass; this selective barrier forms blood-brain barrier.

In contrast, liver capillaries possess wide intercellular clefts that permit nearly all dissolved plasma substances to move freely into liver tissue. The capillaries of the gastrointestinal tract have clefts of intermediate size, which provide moderate permeability suited for nutrient absorption. Meanwhile, glomerular capillaries in the kidneys are fenestrated, with small pores that penetrate the endothelial cells. These openings facilitate the rapid filtration of small molecules and ions while generally excluding larger plasma proteins.

Blood flow through capillaries is not continuous but intermittent, a phenomenon known as vasomotion. This rhythmic alternation between flow and pause results from the periodic contraction of metarterioles and precapillary sphincters. Vasomotion is primarily regulated by the local concentration of oxygen within tissues and by other metabolic factors that influence microvascular blood flow, ensuring that perfusion meets the varying demands of cellular activity.