Abstract
A nitrogen closed-circuit method has been used for the determination of the functional residual capacity (FRC) in the rat. Measurements were obtained in specific-pathogen-free (SPF) rats, which do not suffer from the specific endemic chronic bronchial disease that affects all ordinary rat colonies, and in ordinary laboratory rats (non-SPF). The range of FRC in tracheostomized rats was 3[long dash]6 ml. There was a high degree of correlation between FRC and the body weight in both the SPF and the non-SPF colonies. Age also correlated with F RC because it is related to weight. Comparison showed no significant difference in the relationship between FRC and body weight in the 2 colonies, so that animals of the same weight from either colony would have about the same FRC. Comparing the animals at the same age showed that FRC of the non-SPF animals are smaller because they weigh less at the same age.

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