Go ahead, vent your spleen!

摘要

For diving and exercise physiologists, 'venting your spleen' has a whole different meaning than it does to the layman. In diving mammals and heavy exercisers, the spleen serves as an oxygen reservoir, storing highly viscous 'thick blood', rich in red blood cells, during periods of rest, and injecting these stored red blood cells into the general circulation when oxygen levels are stressed and increased transport is required. Such injections of oxygenated red blood cells occur during periods of heavy exercise in some mammals, such as horses, and are particularly well developed in diving mammals. The spleen of the Arctic Weddell seal contains so much extra oxygenated blood that it often is referred to as a SCUBA tank: in fact, the release of red blood cells into the circulation is so tightly regulated that arterial oxygen content doesn't decrease for me first 15-18 min underwater. Splenic emptying under these circumstances is due to an active contraction of the spleen triggered by adreneric innervation; spleen volume in some diving seals may reduce by as much as 85%! However, in general, humans have a less well-developed dive response and a relatively small splenic reservoir (about 8% of total body red blood cells versus more than 20 litres of sequestered RBCs in the Weddell seal), and the human spleen is poorly innervated by adrenergic fibers. It has thus been suggested that the observed decrease in human spleen volume following sympathetic activity is due to passive collapse as blood supply to the spleen decreases rather than to an adaptive, active contraction. Recent work by D. Bakovic and coworkers set out to investigate whether splenic contraction is part of the human dive response too.