Analysis of salt and water intake by continuous determination of blood volume and plasma sodium concentration

Seven rats were exposed to a hot environment (36 degrees C) for about 5 h to induce thermal dehydration. They were allowed to recover from their fluid loss; both tap water and 1.8% NaCl solution were provided simultaneously as drinking fluids. In the recovery stage, these animals initially consumed low NaCl solution (about 41 mEq/l) for the first 90 min and then shifted to drinking slightly hypotonic NaCl solution (about 122 mEq/l) on the average. To analyze the mechanism driving the above shift, changes in blood volume (BV), plasma Na concentration ([Na]), and the amount of water and 1.8% NaCl solution consumed were measured continuously. After 4 h of recovery, BV increased by +1.1 ml/100 g body wt. and reached 5.7 ml/100 g body wt. (set as 100%) while plasma [Na] reduced by -5.5 mEq/l or to 141.5 mEq/l (mean values of 7 rats). The shift of [Na] of drinking water was observed when plasma [Na] reduced to -3.4 mEq/l or 62.3% of the 4th-hour level, while BV recovery remained only +0.5 ml or 46.3%. A response to urinary output started later when plasma [Na] reached 142.6 mEq/l or 83.1%, while BV recovery remained +5.4 ml or 76%. These results indicate that the regulation of plasma [Na] takes precedence over the regulation of BV. This conclusion is in agreement with the view that the blood osmolality change directly modifies the cell volume whereas circulatory function under decreased blood volume can be regulated by change of the vascular compliance.