For 350 million years, fish have breathed liquid through gills. Mammals evolved lungs to breathe air. Rarely, circumstances can occur when a mammal needs to `turn back the clock' to breathe through a special liquid medium.
This is particularly true if surface tension at the air±liquid interface of the lung is increased, as in acute lung injury. In this condition, surface tension increases because the pulmonary surfactant system is damaged, causing alveolar collapse, atelectasis, increased right-to-left shunt and hypoxaemia.