Capillary filtration coefficient in the extremities of man in high environmental temperatures.

The capillary filtration coefficient was measured in human extremities by a simplified technique using the capacitance plethysmograph. The method was validated in terms of consistency of values obtained on the same individual at various times, the correspondence between values obtained on a group of normal persons, and the agreement between values reported here and those of earlier workers made under corresponding environmental conditions and duration of measurement after venous occlusion. The technique was used to evaluate the change in capillary filtration coefficient in the calves of subjects when ambient temperature was increased from 20° to 35°C. Corresponding measurements of total blood flow in this segment and determinations of the clearance of iodide ion from pretibial intradermal depots were made in both environments. Cutaneous blood flow increased by almost 170% and the capillary filtration coefficient by about 40%, whereas the clearance did not change. It is concluded that neither the capillary filtration coefficient nor the clearance of isotopes from tissue depots has been established as a valid measure of capillary surface area. The possibility that during venous occlusion, fluid accumulated primarily in the subcutaneous tissues of the extremity is suggested and discussed.