Relationship between aerobic power, blood volume, and thermoregulatory responses to exercise-heat stress

To clarify the relationship between aerobic power (V̇O2(max)), blood volume (BV), and thermoregulatory responses to exercise-heat stress, we analyzed the cross-sectional relationship between the resting BV, plasma volume (PV), erythrocyte volume (EV), V̇O(2max), forearm blood flow (FBF), and sweating responses during exercise in a hot environment (31 °C, 50% relative humidity). Twelve college-aged male subjects with a mean maximal oxygen uptake of 48 (range 42-59) mL · kg^-1 · min^-1, a mean PV of 54 (range 42-72) mL · kg^-1, a mean EV of 31 (range 23-43) mL · kg^-1, and a mean BV of 85 (range 67-115) mL · kg^-1 (measured by the Evans Blue dye dilution method) performed three sessions of 20-min cycle exercise at two levels of intensity (40% and 60% V̇O(2max). The BV, PV, and EV correlated positively with peak FBF (r = 0.596-0.711, P < 0.05), the increase of FBF in response to a unit rise in esophageal temperature (Tes; peak ΔFBF/peak ΔTes) (r = 0.592-0.656, P < 0.05) and with total sweat loss (TSL) (r = 0.599-0.634, P < 0.05) during the exercise. The V̇O(2max) correlated with TSL during exercise at 40% V̇O(2max) (r = 0.578, P < 0.05), but not with peak FBF and peak ΔFBF/peak ΔTes. The V̇O(2max) per lean body mass also showed a significant positive correlation with BV (r = 0.769, P < 0.01), PV (r = 0.706, P < 0.05), and with EV (r = 0.841, P < 0.001). The peak ΔFBF/peak ΔTes was correlated positively with peak FBF (r = 0.5970.830, P < 0.05-0.01) and negatively with peak Tes (r = 0.641-0.769, P < 0.05-0.01) during the exercise at the two levels. However, the chest sweat rate (CSR), TSL, and the increase of CSR in response to a unit rise in Tes (peak ΔCSR/peak ΔTes) showed no correlation with peak Tes during the exercise at the two levels. These findings suggest that 1) heat dissipation responses during exercise were related more to blood volume than aerobic power and 2) skin blood flow was related more to body temperature than sweating responses during exercise under mild heat stress.