capillary exchange

Meaning: Capillary exchange is the transfer of gases, nutrients, wastes and fluid between blood and interstitial fluid across the thin endothelial wall. Solutes such as O₂ and CO₂ diffuse down concentration gradients. Fluid moves by bulk flow: hydrostatic pressure pushes it out, while oncotic pressure, the osmotic pull of plasma proteins like albumin that largely stay in the vessel, draws it in. Hydrostatic pressure falls along the capillary, so net filtration typically occurs at the arteriolar end and net reabsorption at the venular end. Lymph returns the excess.

Equation: Net filtration pressure = (Pc − Pi) − (πc − πi), where P is hydrostatic pressure and π is oncotic pressure (both in Pa) in the capillary (c) and interstitium (i). A positive value means fluid leaves the capillary.

On the MCAT: Passages give pressures at each end of a capillary and ask which way fluid moves, or ask what happens when one value changes. Low plasma albumin lowers πc, high venous pressure raises Pc, and blocked lymph drainage; each can cause edema (fluid buildup in tissue).

Common confusion: Oncotic pressure does not push; it pulls water toward the higher protein concentration, opposing hydrostatic pressure. Also, the fluid filtered out is typically not fully reabsorbed; the remainder drains into lymph.