Cardiovascular responses to dynamic exercise with acute anemia in humans

We hypothesized that reducing arterial O₂ content (${\text{Ca}}_{O_2}$ ) by lowering the hemoglobin concentration ([Hb]) would result in a higher blood flow, as observed with a low $P{O}_2$ , and maintenance of O₂ delivery. Seven young healthy men were studied twice, at rest and during two-legged submaximal and peak dynamic knee extensor exercise in a control condition (mean control [Hb] 144 g/l) and after 1–1.5 liters of whole blood had been withdrawn and replaced with albumin {mean drop in [Hb] 29 g/l (range 19–38 g/l); low [Hb]}. Limb blood flow (LBF) was higher (P < 0.01) with low [Hb] during submaximal exercise (i.e., at 30 W, LBF was 2.5 ± 0.1 and 3.0 ± 0.1 l/min for control [Hb] and low [Hb], respectively;P < 0.01), resulting in a maintained O₂ delivery and O₂ uptake for a given workload. However, at peak exercise, LBF was unaltered (6.5 ± 0.4 and 6.6 ± 0.6 l/min for control [Hb] and low [Hb], respectively), which resulted in an 18% reduction in O₂ delivery (P < 0.01). This occurred despite peak cardiac output in neither condition reaching >75% of maximal cardiac output (∼26 l/min). It is concluded that a low Ca_{O2 induces an elevation in submaximal muscle blood flow and that O2 delivery to contracting muscles is tightly regulated.}