[Effect of water immersion as a model of weightlessness on lung closing volume]

The effect of 7-day water immersion combined at night with head-down (6 degrees) tilting on the volumes of lung expiratory closing was examined in 6 healthy male test subjects, aged 25-35. During immersion they showed, along with a stable reduction of the lung vital and functional residual capacity (by 4.2 and 14.8%, respectively), an increase in the fraction parameters of the expiratory closing volumes. The increase in the closing volume (up to 40%) (P less than 0.05) was particularly distinct on immersion day 2. Subsequent changes indicated gradual recovery of the closing volumes and a decline in regional nonuniformity of lung ventilation. The time-course variations in the closing volumes may be attributed to an increase in the intrapulmonary blood volume at the early stage of adaptation to immersion and to a decrease in the nonuniformity of the ventilation-perfusion ratios.