Some effects on the blood vessels of the human forearm of local exposure to pressures below atmospheric.

1. Measurements were made of the initial rate of change of volume of the forearm when the limb was suddenly exposed to various pressures below atmospheric; this was taken to indicate the arterial inflow under these conditions.2. The relationship between arterial inflow and the change in external pressure was curvilinear, with the convexity towards the pressure axis. At -90 mm Hg inflow was increased more than fivefold.3. With short repeated exposures to negative pressure the inflow to the limb showed a progressive decrease for successive exposures until a steady value was obtained.4. The relationship between arterial inflow and the reduction of external pressure during short repeated periods of exposure (6th-10th exposures) was linear from ambient to -80 mm Hg.5. Measurements of forearm blood flow after one, four and ten exposures to reduced external pressure showed that vasoconstriction developed within 20 sec of the end of the exposure and took about 60 sec to disappear. There were only slight differences in the degree of vasoconstriction induced by the different numbers of exposures.