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

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Abstract
Direct measurements of the capillary pressure (Pc) were made in capillaries at the base of the nails of the fingers and toes of 2 subjects. With the hand or foot at heart level, Pc varied over the range of 7-70 cm H2O with mean values of 43 cm H2O in both the fingers and the toes. Pc was higher in the arterial limb (mean 49 cm H2O) than in the venous limb (mean 34 cm H2O) of the capillary loops. The plasma colloid osmotic pressures for the 2 subjects were 33 and 34 cm H2O. For capillaries at heart level there was a strong positive correlation between Pc and skin temperature when the latter was varied over the range 23-36.degree. C. When the hand or foot was lowered, Pc increased less than the local arterial (Pa) and venous pressures (Pv). Furthermore the variation in Pc was reduced. In 14 measurements of Pc made on capillaries in the toes of standing subjects, Pc was no more than 10 cm H2O greater than Pv. The increase in the ratio (Pa-Pc)/(Pc-Pv) with hydrostatic load possibly represents an increase in the ratio of pre- to post-capillary resistance. When Pv was increased by inflating a sphygmomanometer cuff around the upper arm, (Pa-Pc)/(Pc-Pv) increased in the hand held at heart level. These changes were similar to those seen with changes in position. The implications of the results are discussed with respect to fluid balance between the blood and tissues. Since Pc 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 minimize the filtration of fluid into the feet of the standing subject but the mechanism of these changes remains obscure.