Effect of dehydration on the volumes of body fluid compartments in horses

Maintenance of plasma volume during dehydration is essential for survival in mammals. Supplemental fluid may be drawn from the interstitial, intracellular and transcellular (primarily gastrointestinal) compartments. We have studied the changes in the volumes of these compartments in arid-adapted (Namib) and in subtropical farm horses from the Transvaal (Boerperd) in response to dehydration, in order to identify mechanisms which protect plasma volume in these horses. As a fraction of total fluid loss, about 6% was lost from the plasma, about 27% from the interstitial space and 67% from the combined intracellular/transcellular space. The Namib horses showed a tendency to lose, as a fraction of total fluid loss, more fluid from the interstitial space (29%) and less from the transcellular space (65%) than the Boerperd (21% and 73% respectively). The Namib horses absorbed intestinal fluid more rapidly than the Boerperds, as indexed by a larger difference in tritium content of the body water pool, between the dehydrated and rehydrated states, over the same period of rehydration. This difference was not significant (p < 0•115). We conclude that horses maintain their plasma volume effectively, as do arid-adapted species such as donkeys, camels, desert sheep and goats during acute dehydration, and that Namib horses tended to be more competent in this respect than Boerperd.