Recovery of blood volume and osmolality after thermal dehydration in rats

Changes in blood volume and fluid intake were assessed continuously during recovery from thermal dehydration (approximately 6 ml/100 g body wt) in rats. Catheters chronically implanted into the jugular vein and descending aorta were connected with the system for continuous monitoring of blood volume. Changes in blood volume and fluid consumption were recorded for 2-6 h while tap water (TW group) or 0.45% NaCl solution (Na group) was provided ad libitum. As soon as the fluids became available, the rats in both groups drank a relatively large volume (2-3 ml/100 g) within 10 min. Thereafter, Na group consumed approximately 1 ml X 100 g body wt-1 X 30-60 min-1 and regained almost all water loss in 2 h, whereas TW group drank only approximately 50% of their water deficit. The percent of ingested fluid that remained in vascular space was 17% on average in both groups. Thus TW group replenished 80% of the depleted blood volume, whereas Na group regained almost all of it. Rehydration for 2 h decreased plasma Na concentration to the control level in TW group, whereas in Na group the level remained high. These results suggest that the lower fluid intake in TW group is due to the dilutional inhibition of drinking and that the osmoregulation has priority over blood volume regulation in restitution from thermal dehydration.