The effects of position and skin temperature on the capillary pressures in the fingers and toes.

1. Direct measurements of the capillary pressure (P(c)) were made in capillaries at the base of the nails of the fingers and toes of two subjects (the authors of this paper).2. With the hand or foot at heart level, P(c) varied over the range of 7-70 cm H(2)O with mean values of 43 cm H(2)O in both the fingers and the toes. P(c) was higher in the arterial limb (mean 49 cm H(2)O) than in the venous limb (mean 34 cm H(2)O) of the capillary loops. The plasma colloid osmotic pressures for the two subjects were 33 and 34 cm H(2)O.3. For capillaries at heart level there was a strong positive correlation between P(c) and skin temperature when the latter was varied over the range 23-36 degrees C.4. When the hand or foot was lowered, P(c) increased less than the local arterial (P(a)) and venous pressures (P(v)). Furthermore the variation in P(c) was reduced. In fourteen measurements of P(c) made on capillaries in the toes of standing subjects, P(c) was no more than 10 cm H(2)O greater than P(v). It is argued that the increase in the ratio (P(a) - P(c))/(P(c) - P(v)) with hydrostatic load represents an increase in the ratio of pre- to post-capillary resistance.5. When P(v) was increased by inflating a sphygmomanometer cuff around the upper arm, (P(a) - P(c))/(P(c) - P(v)) increased in the hand held at heart level. These changes were similar to those seen with changes in position.6. The implications of the results are discussed with respect to fluid balance between the blood and tissues. It is argued that since P(c) in the warm hand was never less than the plasma colloid osmotic pressure, fluid is not reabsorbed from the tissues into the capillaries of the warm skin of the hand even at heart level. Compensatory changes in the circulation appear to minimise the filtration of fluid into the feet of the standing subject but the mechanism of these changes remains obscure.