It may be said that at rest all the factors of respiration, gaseous exchange, carbon dioxide tension, and the mechanical factors, are normal in persons with a collapsed lung, that the reaction to carbon dioxide is normal up to the point at which the respiration is trebled, or sometimes quadrupled, but that beyond that point a limit may be reached.

The ventilation of the lungs can be accomplished in an entirely normal manner in spite of a greatly reduced vital capacity. The only difference between normal persons and persons with a collapsed lung is that the latter when called upon to increase their ventilation reach their limit a little sooner than the former.

From these findings we might deduce that there will be no dyspnea except after moderate exertion. And this deduction is borne out by the histories of the patients.

In other words, in the lungs, as in other organs, there is a large factor of safety, one lung being as efficient as two, except when the work done calls for more than a threefold increase in the normal ventilation.

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