Every day in the adult, non-renal capillaries filter approximately 20 litres of water with the intercellular space. The vast majority of this is reabsorbed by the same capillaries. This capillary exchange is a dynamic process that has the role of supplying extravascular cells with the substances essential for survival e.g. water and glucose, while removing waste products.
Starling hypothesised that the extent of water and small solute transit in any one direction across a capillary is dependent upon the balance of two opposing forces.
Hydrostatic pressure is the intravascular force per unit surface area of capillary wall pushing water out of the vessel. This 'filtration force' is dependent on the blood pressure, itself dependent on the force with which the heart is pumping and the local vascular resistance.
Forces causing water to move into the intravascular compartment include the oncotic pressure and the interstitial tissue hydrostatic pressure. The oncotic pressure derives from the fact that generally, capillaries are impermeable to large proteins and blood cells; the latter act as an osmotic load attracting water back inwards.
The forces are normally balanced so that filtration occurs across the arterial end of a capillary and reabsoption across the venous end. Disruption of any element can cause the extremes of tissue dehydration, as in skin turgor, or oedema.
The transfer of small metabolites such as oxygen to cells is not dependent on the fluid shifts across the capillary membrane. For simple molecules, diffusion down a concentration gradient suffices.
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