Regional circulation and capillary filtration in relation to capillary exchange in the foot and calf of the newborn infant.

The ultimate purpose of the circulatory system is the maintenance of an exchange of oxygen, water, electrolytes, nutrients and metabolites across capillary walls, which is adequate to the rate of cellular metabolism. The movement of molecules between the circulating blood and the extravascular tissue spaceb is known to occur generally at rates many times that of blood flow itself. The capillary endothelium is considered to take no active part in this transfer, but to behave as a porous membrane through which molecules may pass with extreme rapidity. It is also a well-established fact that diffusion predominates in this transcapillary exchange, taking place at a rate in accordance with the chemical conccntration gradient and physicochemical properties of the particular substance [21]. The molecules of water diffuse freely between the plasma and the tissue fluid. Nevertheless. net movements of fluid across the capillary membrane take place by filtration in response to disequilibrium of hydrostatic and osmotic forces, acting over the capillary wall. This nowadays generally accepted concept of the exchange of fluid across capillary membranes goes