Equine plasma and blood volumes decrease with dehydration but subsequently increase with exercise.

The effects of dehydration and 40 min of exercise at approximately 40% of maximal O2 consumption on plasma volume (PV) and blood volume (BV) were studied in six horses. Horses were exercised while euhydrated (C); 4 h after administration of furosemide (1.0 mg/kg i.v.; FDH), which induced isotonic dehydration; and after 30 h without water (DDH), which caused hypertonic dehydration. Dehydration resulted in decreases of 6.3 and 9.9% for PV and BV, respectively, with FDH and 10.7 and 8.5%, respectively, with DDH. During exercise in C, PV and BV increased by 12.7 and 20.0%, respectively; during exercise with FDH, they increased by 11.7 and 26.1%, respectively; and during exercise with DDH, PV decreased by 1.3% from predehydration values, while BV increased by 18.7%. Hematocrit and total plasma protein concentration rose to higher values throughout exercise in FDH and DDH than in C; plasma [Na+] was higher in DDH than in FDH and C, [Cl-] was higher in DDH and lower in FDH than in C, and [K+] was lower in FDH and DDH than in C through exercise and recovery. From these results, we conclude that increases in PV and BV are normal features of low-intensity exercise in the horse. The increases in BV not only augment O2 carriage but also help maintain circulating volume. These increases can be modified by preexercise dehydration, the nature of which affects the extent of modification.