Effects of forced diving on the spleen and hepatic sinus in northern elephant seal pups

In phocid seals, an increase in hematocrit (Hct) accompanies diving and per iods of apnea. The variability of phocid Hct suggests that the total red ce ll mass is not always in circulation, leading researchers to speculate on t he means of blood volume partitioning. The histology and disproportionate s ize of the phocid spleen implicates it as the likely site for RBC storage. We used magnetic resonance imaging on Northern elephant seals to demonstrat e a rapid contraction of the spleen and a simultaneous filling of the hepat ic sinus during forced dives (P < 0.0001, R-2 = 0.97). The resulting images are clear evidence demonstrating a functional relationship between the spl een and hepatic sinus. The transfer of blood from the spleen to the sinus p rovides an explanation for the disparity between the timing of diving-induc ed splenic contraction (approximate to1-3 min) and the occurrence of peak H ct (15-25 min). Facial immersion was accompanied by an immediate and profou nd splenic contraction, with no further significant decrease in splenic vol ume after min 2 (Tukey-Kramer HSD, P = 0.05). At the conclusion of the dive , the spleen had contracted to 16% of its predive volume (mean resting sple nic volume = 3,141 ml +/- 68.01 ml; 3.54% of body mass). In the postdive pe riod, the spleen required 18-22 min to achieve resting volume, indicating t hat this species may not have sufficient time to refill the spleen when rou tinely diving at sea, which is virtually continuous with interdive surface intervals between 1 and 3 min.