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Splenic contraction, catecholamine release, and blood volume redistribution during exercise in man Stewart, Ian Braidwood

Abstract

Numerous mammals have the ability to autotransfuse oxygenated red cells from the spleen into circulation during times of stress. This enhancement of the oxygen transport system has benefited mammals capable of heavy work (such as the thoroughbred horse, fox and greyhound) in an improved endurance performance. During times of inactivity the spleen in these mammals can sequester up to 50% of the total red cell volume. This reduces the viscosity of the blood and work of the heart. In humans the spleen is capable of storing considerably less red cells and is primarily a lymphoid organ. Although significant volume reductions have been observed with physical stress, the mechanism responsible remains unidentified and represents a primary objective of this dissertation. To determine the mechanism responsible for splenic volume reduction nine male subjects participated in a two part study. Initially they undertook three cycling bouts of 5, 10, and 15 minute duration at 60% VO^max. Spleen size and volume was assessed by scintography before and immediately following each cycling bout. Although there was a significant decrease in spleen size and volume with exercise, no significant differences were observed between the three durations. In the second part of the study the same subjects undertook an incremental ride to exhaustion. The spleen size and volume were measured prior to exercise and during one hour of passive recovery. Blood samples were collected in conjunction with spleen imaging and analyzed for plasma catecholamine concentrations. The spleen was restored to its pre-exercise value 20 minutes following the completion of exercise. The volume of the spleen during this time was inversely related to plasma catecholamine levels. It is apparent that the spleen will contract in response to an intensity dependent signal and that sympathetic stimulation, as reflected in changes in plasma catecholamine concentrations, is involved in this effect. In a simultaneous study, the effect of splenic contraction during exercise on the circulating red cell volume and indirect calculations of plasma volume change (Hb/Hct) was determined. Radio nuclide measurements of red cell and plasma volume were performed, and a blood count taken, prior to and immediately following exercise. The release of 50 mls of red cells from the spleen during exercise accounted for a 2.2% increase in the total circulating red cell volume. The indirect calculations of plasma volume were unaffected by the minor increase in peripheral hematocrit and were equivalent to radio labelled measurements. In conjunction the results of this dissertation imply that the release of red cells during exercise is not a primary function of the human spleen.

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