Ten days of head down tilt: effects of isotonic and hypertonic saline loads in normal man

The initial response to bed rest involves an increase in central blood volume leading to a an enhanced renal excretion of fluid and electrolytes. Within 24 hours of head-down bed rest a new steady state condition occurs with a sustained reduction of plasma volume, extracellular fluid volume, total body water, and body weight. It was the purpose of the present study to elucidate the volume homeostatic mechanisms during head-down bed rest by investigating the endocrine and renal responses to a load of sodium chloride given as either an isotonic or a hypertonic solution.