Effects of sustained intrathoracic vascular distension on body fluid distribution and renal excretion in man

Intrathoracic blood volume was increased by prolonged immersion in thermo-indifferent (34δ C) water. Urinary excretion patterns of free water and electrolytes during immersion were compared with those for an identical period of the previous day when the subjects were performing routine activity. Plasma volume changes during immersion were compared with the concomitant urine volume which under these conditions can be equated with total fluid loss. The nature of the immersion diuresis depended on the state of hydration. Normally hydrated subjects showed a rise in free water clearance whereas a hydropenic group increased urine volume by an augmentation of osmolar clearance. Sodium excretion during immersion rose from 118±48 (SD) to 180±51.7 (SD) μeq./h×kg in normally hydrated subjects (p>0.05) and from 66.8±22.5 (SD) to 152±43.3 (SD) μeq./h×kg (p<0.01) in the hydropenic group. Immersion led to plasma volume reduction in all cases. Plasma volume reduction constituted a much greater percentage of the urine volume in hydropenic subjects (98.8±35.4 (SD)%) than in the normally hydrated ones (19.3±8.56 (SD)%). It is concluded that engorgement of the intrathoracic volume-sensitive vascular areas may not only lead to increased fluid elimination by the kidney but at the same time to a shift of fluid from plasma into the interstitial space. Both effects serve the homeostatic control of blood volume.