1. After a 4 min period of maximal exercise in 10 normal subjects (14 studies), there was a consistent decrease in total blood volume and a consistent increase in erythrocyte indices, which were maximal immediately after exercise. Peripheral platelet and leucocyte counts increased, but did not reach maximal values until 5–10 min after the end of exercise.

2. The distributions of 99mTc-Iabelled erythrocytes (five studies), 111In-labelled platelets (five studies) and 111In-labelled granulocytes (four studies) were monitored with a γ-camera immediately after injection and before and after maximal exercise performed 60 min after injection.

3. Labelled erythrocytes equilibrated rapidly between the spleen and circulating blood after injection, whereas labelled platelets and granulocytes equilibrated more slowly. After exercise, each cell type was released from the spleen with a time course that was the reciprocal of the time course of the corresponding cell count in peripheral blood. Thus, whereas the radioactivity of 99mTc-labelled erythrocytes in the spleen, which fell to 0.46 (SD 0.09) of the pre-exercise value, increased towards its baseline value as soon as exercise was completed, the radioactivities of 111In-labelled platelets and 111In-labelled granulocytes decreased, to respective minimum values of 0.61 (0.09) and 0.63 (0.09) of the pre-exercise levels, 5–10 min after the end of exercise. The exercise-induced changes in lung radioactivity for each cell type, and their time courses, broadly reflected those in the corresponding cell counts in peripheral blood. Liver radioactivity tended to decrease for each cell type.

4. Because of the dissociation of the time courses of splenic expulsion of erythrocytes (intrasplenic transit time about 1 min) from those of platelets and granulocytes (intrasplenic transit times both of 8–10 min), we conclude that blood cell expulsion from the spleen in man is not the result of active splenic contraction but probably the passive result of a decrease in splenic blood flow.

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