Active transport and passive liquid movement in isolated perfused rat lungs

The isolated perfused liquid-filled rat lung in a 'pleural bath' was the model used to study liquid exchange across the lung epithelium. Active transport and passive solute movement between the air space, the vascular perfusate, and the bath result in concentration changes of the three markers (Evans blue-tagged albumin, ²²Na⁺, and [³H]mannitol) instilled in the air space. A mathematical model was developed to estimate the active and passive solute transports and to interpret the results. Rat lungs were perfused at left atrial and pulmonary arterial pressures of 0 and 8 mmHg, respectively. Six rat lung experiments were conducted at 37°C and six at 4°C. The normothermic experiments demonstrate that active transport accounts for 26% of the Na⁺ movement out of the air space (17.3 ± 0.7 nm/s) and that passive mechanisms account for the remaining 74% (48.0 ± 5.7 nm/s). Hypothermia inhibits lung liquid clearance but does not affect passive solute movement, suggesting that lung liquid clearance is effected by active Na⁺ transport mechanisms.