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Proliferation of pneumocyte II cells in prolonged exposure to 1% CO2.

Guinea pigs were exposed to 1% CO2 in a mixture of 21% O2, balance N2 and were killed at weekly intervals, together with control animals housed in the same type of environmental chamber and exposed to normal ambient CO2. The exposed animals showed a persistent elevation of PaCO2, averaging about 4 mmHg, and a small decrease in pH (-0.04 units). During the whole exposure period standard bicarbonate remained 1-1.5 mEq below control levels, indicating a failure of the kidney to increase bicarbonate reabsorption. Electron microscopic studies after 4 and 6 weeks of exposure to 1% CO2 showed ultrastructural changes of the lungs, consisting of marked increases in the size and number of pneumocyte II cells that were still present two weeks and to a lesser extent four weeks after recovery. Changes in the pneumocyte II cell were postulated to be compensatory reactions to impairing CO2 effects on the alveolar lining cell (Type I cell).