Purpose: This study investigated whether maximal oxygen uptake (V˙O_2max) and exercise capacity are affected by small acute blood loss (150 mL) and elucidated compensatory mechanisms. Methods: Thirteen male subjects (V˙O_2max, 63 ± 9 mL·kg⁻¹·min⁻¹; mean ± SD) performed incremental exercise to exhaustion on a cycle ergometer in three experimental conditions: in euvolemia (control; blood volume [BV], 6.0 ± 0.7 L) and immediately after acute BV reductions of 150 mL (BVR_150mL) and 450 mL (BVR_450mL). Changes in plasma volume (PV) and BV during exercise were calculated from hematocrit, hemoglobin concentration, and hemoglobin mass (carbon monoxide rebreathing). Results: The reduction in V˙O_2max per milliliter of BVR was 2.5-fold larger after BVR_450mL compared with BVR_150mL (−0.7 ± 0.3 vs −0.3 ± 0.6 mL·min⁻¹·mL⁻¹, P = 0.029). V˙O_2max was not significantly changed after BVR_150mL (−1% ± 2%, P = 0.124) but reduced by 7% ± 3% after BVR_450mL (P < 0.001) compared with control. Peak power output only decreased after BVR_450mL (P < 0.001). At maximal exercise, BV was restored after BVR_150mL compared with control (−50 ± 185 mL, P = 0.375) attributed to PV restoration, which was, however, insufficient in restoring BV after BVR_450mL (−281 ± 184 mL, P < 0.001). The peak heart rate tended to increase (3 ± 5 bpm, P = 0.062), whereas the O₂ pulse (−2 ± 1 mL per beat, P < 0.001) and vastus lateralis tissue oxygenation index (−4% ± 8% points, P = 0.080) were reduced after BVR_450mL, suggesting decreased stroke volume and increased leg O₂ extraction. Conclusion: The deteriorations of V˙O_2max and of maximal exercise capacity accelerate with the magnitude of acute blood loss, likely because of a rapid PV restoration sufficient to establish euvolemia after a small but not after a moderate blood loss.

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