Diving mammals may enhance dive duration by injecting extra erythrocytes into the circulation by spleen contraction. This mechanism may also be important for apneic duration in humans. We studied the speed and magnitude of spleen volume changes evoked by serial apneas, and the associated changes in hematocrit (Hct) and hemoglobin (Hb) concentration, diving response and apneic duration. Three maximal apneas separated by 2 min rest elicited spleen contraction in all ten subjects, by a mean of 49 (27) ml (18%; P<0.001). During the same period, Hct and Hb rose by 2.2 and 2.4% respectively (P<0.01 and P<0.001), and apneic duration rose by 20 s (22% P<0.05). The mean heart rate reduction of the diving response was 15%, which remained the same throughout the apnea series. While the diving response was completely reversed between the apneas, spleen size was not recovered until 8-9 min after the final apnea corresponding with recovery of Hct and Hb. Thus, although the spleen contraction may be associated with the cardiovascular diving response, it is likely to be triggered by different mechanisms, and it may remain activated between dives spaced by short pauses. The two adjustments may provide a fast, quickly reversed, and a slow, but long-lasting, way of shifting to a diving mode in humans.

Human splenic contraction occurs both during apnea and maximal exercise, increasing the circulating erythrocyte volume. We investigated the hematological responses to 3 maximal apneas performed by elite apneic divers, elite cross-country skiers, and untrained subjects. Post-apnea hemoglobin concentration had increased in all groups, but especially in divers. The increases disappeared within 10 min of recovery. Apneic duration across apneas also increased the most in divers. Responses in divers could be more pronounced as a result of apnea training.