Microcirculation

The microcirculation comprises arterioles, metarterioles, capillaries, and venules — the exchange territory of the cardiovascular system. Capillaries are the primary exchange vessels, performing gas, nutrient, and waste transfer. Their function depends on several structural and physiological features.

Arterioles control total flow into the capillary bed by adjusting resistance via smooth muscle contraction. Metarterioles are branches of the smallest arterioles that connect either to the capillary bed or directly to venules; direct arteriovenous connections in skin allow rapid heat removal. Precapillary sphincters — thin rings of smooth muscle or pericytes at the origin of capillaries — control which capillaries are perfused (capillary recruitment). Capillaries and venules lack smooth muscle but have pericytes capable of regulating flow.

Substances cross capillaries by several routes. O2 and CO2 (lipid-soluble) diffuse directly through the endothelium. Ions and polar molecules cross via water-filled intercellular clefts (tight junctions limit this in continuous capillaries). Large molecules use vesicular transcytosis. Fenestrated and sinusoidal capillaries allow passage through structural pores and gaps respectively. The capillary brush border carries a negative charge, repelling negatively charged molecules such as albumin.

Exchange is driven by concentration gradients generated by cellular metabolism. Fick's Law applies: rate of diffusion depends on surface area, concentration gradient, membrane thickness, and diffusion constant. During exercise, metabolic demand increases → higher concentration gradients → more diffusion. Simultaneously, capillary recruitment increases total surface area and shortens diffusion distance. Blood transit time through capillaries falls from a resting 0.5-2 s to ~0.25 s during heavy exercise, yet exchange is still adequate because gradients are steeper.

The Laplace relationship explains why thin-walled capillaries resist bursting: T = Pr/2w; the very small radius (r) keeps wall tension low despite moderate intraluminal pressure. On the venous side of capillaries, hydrostatic pressure is low relative to oncotic pressure → net reabsorption of interstitial fluid back into the capillary (Starling equilibrium).