Early changes in the forearm circulation following transient increase of local external pressure*

1. The pressure at the surface of a segment of forearm enclosed in a plethysmograph was abruptly raised from atmospheric level to +40 mm Hg, held at the new level for 4 sec, and abruptly dropped to atmospheric level. 2. Forearm circumference (Vf) equivalent to the volume of a small segment of forearm, was monitored with a mercury-in-rubber strain gauge. Pressure was measured in the cylinder (Pp) in veins exposed to external compression (Pv, e), and in the radial artery exposed to compression (Pra). 3. Forearm blood flow was measured by venous occlusion plethysmography before, and after, release of external compression. There was, on average, over the 3rd and 4th second after release of pressure, a 2·4-fold increase of inflow, as compared with resting level. 4. By the 15th second after release of compression, forearm blood flow had returned to its previous resting level. 5. The increase in blood flow after compression appears to be due to active reduction in vascular resistance, for refilling of the arteries and arterioles would be completed before the increased flow was recorded; venous backflow can be excluded, and the pressure difference for flow (arterial minus venous) is virtually unchanged.