Volume fraction of the lung occupied by collapsed alveoli (gray) or normal pulmonary areas (white). Animals were randomly assigned to hypovolemia (HYPO), normovolemia (NORMO) or hypervolemia (HYPER) with recruitment maneuver (RM-CPAP) or not (NR). All values were computed in 10 random, noncoincident fields per rat. Values are mean ± standard error of the mean of six animals in each group. †Significantly different from NR (P < 0.05). **Significantly different from HYPO-NR (P < 0.05). ‡Significantly different from HYPO-RM-CPAP (P < 0.05). #Significantly different from NORMO-NR (P < 0.05). §Significantly different from NORMO-RM-CPAP (P < 0.05).