Hydrostatic Pressure

Hydrostatic pressure is the pressure a fluid exerts on the walls of its container. In the circulation, it is the pressure of the blood pushing against the walls of the vessels, and it is generated by the contraction of the heart's ventricles.In a capillary, hydrostatic pressure pushes fluid out through the gaps between the endothelial cells. It therefore acts in the opposite direction to oncotic pressure, which draws water back in.Hydrostatic pressure is not constant along a capillary. Blood enters from an arteriole, so pressure is highest at the arterial end. As blood moves along the narrow capillary, pressure falls, because of friction with the capillary wall and because fluid is lost from the blood.Worked example. Typical values are about 4.6 kPa at the arterial end and about 2.0 kPa at the venous end. The fall in hydrostatic pressure along the capillary is:4.6 kPa − 2.0 kPa = 2.6 kPaThe tissue fluid also has a small hydrostatic pressure of its own, which pushes slightly back against the capillary wall, but at this level it is usually treated as negligible.Anything that raises the blood pressure in the capillaries increases hydrostatic pressure and so increases the amount of fluid pushed out. For example, people with raised blood pressure (hypertension) or with heart failure, where blood backs up in the veins, form more tissue fluid than can be drained, and may develop oedema.