Dynamics of Capillary Exchange

Capillary Exchange

Substances cross capillary walls by:
Diffusion
Transcytosis
Bulk flow

Diffusion

Substances such as oxygen, carbon dioxide, glucose, amino acids, and some hormones cross capillary walls via simple diffusion

Transcytosis

Large, lipid-insoluble molecules (like insulin) cross capillary walls in vesicles via transcytosis

Bulk flow is a passive process in which large numbers of ions, molecules, or particles in a fluid move together in the same direction
Bulk flow occurs from an area of higher pressure to an area of lower pressure, and it continues as long as a pressure difference exists
Bulk flow is more important for regulation of the relative volumes of blood and interstitial fluid

Filtration is pressure-driven movement of fluid and solutes from blood capillaries into interstitial fluid
Blood hydrostatic pressure (BHP) and interstitial fluid osmotic pressure (IFOP) promote filtration
Reabsorption is pressure-driven movement of fluid and solutes from interstitial fluid into blood capillaries
Interstitial fluid hydrostatic pressure (IFHP) and blood colloid osmotic pressure (BCOP) promote reabsorption