Transcapillary Fluid Absorption and Other Vasular Reactions in the Human Forearm during Reduction of the Circulating Blood Volume

The responses of the human forearm vascular bed to a reduction of circulation blood volume, produced by pooling about 600 to 800 ml of blood in the legs, were followed by a plethysmographic technique. The pattern of vascular response was similar to that previously found in anesthetized animals after hemorrhage. It involved a reflex constriction of resistance and capacitance vessels and a net transcapillary absorption of extravascular fluid. The capillary filtration coefficient averaged 0.0061 ml/min × 100 g tissue × mm Hg , and did not change significantly during the pooling of blood, indicating no major change of precapillary sphincter tone. The absorption of extravascular fluid can be related to a fall in capillary hydrostatic pressure, which is, in turn, due to a reflex increase of the pre‐ to postcapillary resistance ratio. This process seems to aid greatly in the restoration of plasma volume. A reflex resetting of the pre‐to postcapillary resistance ratio might therefore consitute an important component in the overalll cardiovascular compensatory adjustments to reduced circulating blood volume in man.