Head down tilt combined with breathing assistance by the "IRON LUNG." A new simulation model for cardiovascular deconditioning, skin, and kidney function in weightlessness?

In 1951 Gauer, Henry and Sieker proposed that "central hyper-volemia" might produce a diuresis that serves to reduce blood volume, a postulated negative-feedback system for the control of blood volume. Recent surprising results from human spaceflight indicate that although a central hyper-volemia takes place in weightlessness, an increase in salt and urine excretion cannot be observed. We hypothesised that on earth the mediastinum is shaped by gravity because of the gravity dependent hydrostatic gradient in the blood filled cavities and that its unloading by weightlessness reduces the wall stretch in the atria, thus resulting in reduced sodium and urine excretion. Therefore we have applied the principle of the "Iron Lung" (lung pressure manipulator LPM) in combination with a simulation method of weightlessness, head down tilt bed rest (HDT), to test this hypothesis. We found that similar to weightlessness, not only urinary excretion, but also evaporative water loss was reduced and that diastolic blood pressure increased.