Effective osmotic pressure of the plasma proteins and other quantities associated with the capillary circulation in the hindlimbs of cats and dogs.

The principal factors regulating the exchange of fluid between capillaries and tissue spaces were outlined by Starling (1) following his experiments on the absorption of fluid from the connective tissue spaces. Starling’s experiments, performed on perfused hindlimbs of dogs, gave rise to one of the most widely recognized hypotheses in physiology and medicine. According to the ‘Starling Hypothesis’, the direction and rate of fluid transfer between plasma and tissue fluids are determined by three factors; a> the hydrostatic pressures on each side of the capillary membranes, b) the protein osmotic pressures of plasma and tissue fluids acting across the capillary membranes, and c) the physical properties of the capillary membranes considered as mechanical filters. Convincing quantitative evidence in support of the Starling theory was obtained by Landis (2) from direct measurements of the hydrostatic pressure and rates of fluid movement in individual capillaries of the frog’s mesentery. In this preparation the rate of fluid movement across the capillary membrane is, on the average, proportional to the difference between t?ne mean hydrostatic pressure in the capillary and the protein osmotic pressure of the plasma as measured in vitro. This behavior represents a special case of the Starling theory in which tissue pressure opposing filtration is negligible and the capillary membranes are almost completely impermeable to the plasma proteins; it implies that the protein concentration in tissue fluid immediately outside the capillary wall is too low to play a significant role in determining the osmotic balance. No such direct measurements have been made in mammalian capillaries. Circumstantial evidence, reviewed by Landis (3), has in general lent support to the