Which feature characterizes the lungs of mammals?

Question

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Mammalian lungs are characterized by tidal ventilation, which means that air enters and exits the lungs through the same pathway. This is different from birds, who have unidirectional air flow with the help of air sacs. Mammals also have complete emptying of air during exhalation, unlike reptiles who always have some residual air left in their lungs. Crosscurrent and countercurrent air flow are features found in fish gills.

Which Feature Characterizes the Lungs of Mammals