Capillary Exchange.

Capillaries are composed of a single layer of epithelium surrounding a lumen of a few micrometers. The average capillary is only about 1 mm long. The capillary beds are the region of exchange of materials with the tissues. O₂ and nutrients are supplied to the cells, and CO₂ and other waste products are taken away. Some materials are transported across the endothelial membrane by diffusion, but material (especially water) also leaves through pores in the capillary walls.

One might suppose that the presence of pores in the capillary wall would imply huge amounts of bulk flow, but the degree to which material moves through these pores is a function of opposing hydrostatic and osmotic pressures. Although premeant solute osmotic pressure of blood and extracellular fluid is roughly constant (25 mm Hg). Osmotic pressure prevents too much water from leaving capillaries through the pores. Hydrostatic pressure of the blood system drops from 100-200 mm Hg coming out of the heart to 35 mm Hg on the arterial side of the capillary beds, to 10 mm Hg on the venous side. This pressure disposition means that there will be net movement of water from the blood into the extracellular compartment on the arterial side of the capillary bed, and back into the blood on the venous side, where osmotic pressure exceeds hydrostatic pressure.

The point in the capillary system at which osmotic exceeds hydostatic pressure favouring absorption over irrigation varies depending on blood flow through the capillary; vasodilation increases blood flow and hydrostatic pressure and may favour irrigation over the entire capillary length, whereas vasoconstriction produces an almost entirely absorptive capillary. Vasodilation and vasocontriction are under local (and more general control) control. For example, highly active tissues, tend to produce characteristic waste products in the surrounding extracellular tissues. Some of these products (e.g. CO₂) act as vasodilators thereby bringing increased blood flow and irrigation into the area. Conversely, O₂ may accumulate around inactive tissues and acts as a vasoconstrictor, bringing absorptive processes into play. The irrigation-absorption balance in any tissue will be a mosaic, which ensures that appropriate materials are brought to and transported from individual cells as needed.