[Change in human kidney activity in passive orthostatism and head-down tilting]

Changes in the natriuretic and water excretory renal functions of healthy people during tilt table tests--with the head-end lifted upwards at an angle of 45 degrees and lowered at an angle of -15 degrees--were investigated. The renal excretion of sodium, water and osmotically active substances decreased drastically during the first 20 min of the headdown tilting and continued to fall down during the next 20 min. These changes were induced by an increase in the rate of tubular reabsorption of filtrate, sodium and osmotically free water rather than by a significant decrease of glomerular filtration rate. After tilting to the horizontal position circulation and glomerular filtration parameters rapidly returned to the normal. However, sodium clearance, diuresis and osmolar clearance remained decreased till the end of the experiment due to the further increase in the tubular reabsorption rate of sodium and water. During head-down tilting an increase in natriuresis, diuresis and osmolar clearance was seen which was related to an increase in the glomerular filtration rate and a decrease in the tubular reabsorption rate of filtrate, sodium and osmotically free water.