1. An investigation was carried out in five healthy lean adults to assess whether forearm and calf plethysmography largely reflect muscle blood flow as measured by 133Xe and whether there is substantial variability in the blood flow to muscles located at different sites in the body.

2. Blood flow to forearm and calf flexors and extensors, biceps, triceps and quadriceps was assessed using the 133Xe clearance technique. Blood flow to forearm skin and subcutaneous adipose tissue was also measured using the 133Xe clearance technique, whereas blood flow to the forearm and calf was measured using strain gauge plethysmography.

3. The mean blood flow to different muscles ranged from 1.4 ± 0.6 (gastrocnemius) to 1.8 ± 0.7 (forearm extensor) ml min−1 100 g−1 muscle (1.4 ± 0.6 and 1.9 ± 0.8 ml min−1 100 ml−1 muscle, respectively) but there were no significant differences between them. Forearm and calf blood flows (2.7 ± 0.3 and 3.0 ± 0.7 ml min−1 100 ml−1 limb tissue, respectively) were about 50% to more than 100% greater (P <0.025) than blood flow to the muscles within them (1.7 ± 0.5 and 1.4 ± 0.5ml min−1 100g−1 muscle, respectively, or 1.8 ± 0.6 and 1.5 ± 0.5 ml min−1 100 ml−1 muscle, respectively). In contrast, the blood flows to 100 g of forearm skin (9.1 ± 2.6 ml min−1 100 g−1) and adipose tissue (3.8 ± 1.1 ml min−1 100 g−1) were higher than the blood flow to 100 g of forearm (P <0.01 and not significant, respectively).

4. Although several possibilities can explain the discrepancy between muscle blood flow measured by 133Xe and blood flow to the distal limbs measured by plethysmography, the results suggest that non-muscular blood flow, especially that to skin, is substantially greater than muscular blood flow. Indeed, the overall blood flow to the forearm could be accounted for by summation of blood flows to individual constituent tissues, which were assumed to be present in proportions typical of lean subjects. The results have important implications in the use of arteriovenous catheterization studies for assessing flux of oxygen, carbon dioxide and metabolites across muscle.

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