SEASONAL VARIATION OF WATER AND ELECTROLYTE IN SERUM WITH RESPECT TO HOMEOSTASIS

The seasonal variation of body fluid, i. e., hemodilution in summer and hemoconcentration in winter, was studied with respect to homeostasis.The water loading test (ingestion of 20ml/kg of water) and the sweating test (sweat rate of 10-20ml/kg-hr.) were performed with the same ten subjects both in summer and in winter. Serum specific gravity, hematocrit, serum osmolality, sodium, potassium, chloride, ADH, and circulating blood volume were measured both under basal conditions and after each experimental stress. Significant seasonal changes were observed under basal conditions in the levels of ADH, sodium, and the osmolality. These differences were maintained in the hemodilution after water loading and the hemoconcentration after sweating. The magnitude of the response to the stress, the magnitude of the seasonal variation, and the magnitude of individual variation in the eight variables show very high correlations with each other. From these correlations it was concluded that the deviation of the internal environment after stress, seasonal variation of the internal environment, and individual or daily variation of the internal environment are all controlled by the same homeostatic mechanism. It was also suggested that the seasonal change of the set point for homeostasis is a result of repeated changes in the internal environment induced by the climate.