Capillary Exchange

Capillary exchange occurs through diffusion, transcytosis, and bulk flow. Diffusion is the most common mechanism, where oxygen and nutrients move from blood into interstitial fluid down their concentration gradient, while carbon dioxide and waste move in the opposite direction. Transcytosis involves lipid-insoluble molecules like insulin being enclosed in vesicles and transported across endothelial cells. Bulk flow is a pressure-driven movement of multiple substances from high to low pressure areas.

Small lipid-soluble substances like steroid hormones diffuse directly through endothelial cell membranes. Water-soluble molecules such as glucose and amino acids pass through fenestrations or intercellular clefts. Larger molecules use facilitated diffusion via membrane-specific channels. Some large proteins enter and exit endothelial cells through endocytosis and exocytosis, while water moves by osmosis across the semipermeable capillary membrane.

Filtration occurs when fluid moves from the capillary into interstitial fluid under high hydrostatic pressure, typically at the arterial end of the capillary. Reabsorption is the opposite process, where fluid moves from interstitial fluid back into the capillary, driven primarily by osmotic pressure at the venous end. Both processes are pressure-driven mechanisms controlled by the balance between hydrostatic and osmotic pressures.

Blood colloidal osmotic pressure (BCOP) is the pressure created by colloidal proteins suspended in blood plasma. These proteins cannot cross the semipermeable capillary membrane, creating a higher solute concentration in blood than in tissue fluid. This concentration gradient draws water back into the capillary during reabsorption, carrying dissolved molecules with it and maintaining fluid balance between blood and tissues.

Net filtration pressure (NFP) varies at different points along the capillary. At the arterial end, NFP is approximately 10 mm Hg, promoting fluid movement out of the capillary. At the midpoint, NFP equals zero, meaning fluid enters and exits at equal rates. At the venous end, NFP becomes negative (−7 mm Hg), causing water reabsorption. This variation results from changing capillary hydrostatic pressure while blood colloidal osmotic pressure remains constant.

Bulk flow moves large numbers of ions, small particles, and molecules simultaneously from high to low pressure areas, making it more efficient than diffusion for transporting multiple substances. While diffusion relies on concentration gradients and moves substances individually, bulk flow is pressure-driven and can transport many substances together. This dual mechanism ensures adequate delivery of nutrients and removal of waste products throughout the body.

Capillary hydrostatic pressure (CHP) decreases progressively as fluid exits the capillary along its length. At the arterial end, CHP is approximately 35 mm Hg, supporting filtration. By the midpoint, CHP drops to 25 mm Hg, equaling blood colloidal osmotic pressure. At the venous end, CHP decreases further to about 18 mm Hg due to fluid loss, creating conditions that favor reabsorption and can lead to blood pressure imbalances and circulatory shock if severely disrupted.