Tissue Perfusion

Capillaries have the largest total cross-sectional area, so the slowest blood flow. They consist of a single layer of endothelial cells and are the site of transfer of fluids from the blood into the interstitial fluids. Their slow flow rate ensures adequate exchange of nutrients and wastes with the tissues (perfusion).

Capillary hydrostatic pressure (CHP, or blood pressure) drives fluid into the interstitial space to bathe the tissues. Perfusion pressure is the drop in hydrostatic pressure from arteriole to venule. Blood colloid osmotic (oncotic) pressure (BCOP) is due to proteins in the blood. It retains fluids in the circulation. The difference between blood pressure and oncotic pressure is net filtration pressure.

The direction of fluid movement is determined by a balance of hydrostatic and osmotic pressures (Starling forces). On the arteriole side of a capillary bed, blood pressure is greater than oncotic pressure (CHP > BCOP; net filtration pressure is positive), so fluid flows out of the capillaries into the tissues. On the venule side, blood pressure is greatly reduced so it is now less than oncotic pressure (CHP < BCOP; net filtration pressure is negative) and extracellular fluid (ECF) flow back into the capillary bed.

In the brain, endothelial cells of the capillaries are surrounded by glial cells (astrocytes) that reduce permeability and tight-junctions between cells prevent transfer of toxins from blood to brain. This is the blood-brain barrier.