Abstract
1. The pressure at the surface of a segment of forearm enclosed in a plethysmograph was abruptly reduced from atmospheric to -20 to -120 mm Hg.
2. Forearm circumference (equivalent to the volume of a small segment of forearm (Vf)) was measured with a strain gauge. Pressure was measured in the plethysmograph (Pp), in veins exposed (Pve) and not exposed (Pvne) to suction, in the brachial artery not exposed to suction (Pbane) and in forearm tissue (Pt).
3. Reduction of Pp caused increase of Vf. This was not due to gas evolution, since bubbles would not be liberated at the pressures employed. Nor was increase of Vf due to venous backflow since Pve fell, but Pvne did not, even with upper arm circulation occluded or when Pvne was raised by venous occlusion prior to reduction of Pp.
4. Reduction of Pp temporarily arrested venous outflow since Pve < Pvne < Pbane for 30 sec. With reduction of Pp 30 sec after occlusion of the upper-arm circulation, Pve < Pvne for > 1 min, indicating that arterial inflow was then minimal.
5. Increase of Vf, following reduction of Pp, was therefore due to inflow of arterial blood, of soft tissue or interstitial fluid. Interstitial fluid could flow from regions external to the plethysmograph, or enter as the result of filtration across capillaries. Occlusion of the upper arm circulation was not expected to interfere with motion of forearm soft tissue or the intratissue flow of interstitial fluid. It appears that capillary filtration is small compared with observed blood flow. Therefore subtraction of Vfoccl measured at intervals after reduction of Pp (upper arm circulation occluded) from Vf similarly obtained (but upper arm circulation free) appeared to give change of forearm volume due to inflow of arterial blood (ΔVb). V̇b, the volume inflow rate of arterial blood during suction, was then obtained.
6. Resting forearm flow was 1·8 ml./min/100 ml. in seven normal subjects (average mean arterial blood pressure 86 mm Hg). With Pp = -90 mm Hg, V̇b was 10·2 ml./min/100 ml. Suction therefore reduced vascular resistance, measured as (Pbane—Pve) /V̇b.
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Selected References
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