Respiratory Adaptations in Marine Mammals

Abstract

This paper is a discussion of some of the possible structural and functional modifications of the lung which represent adaptations in mammals living in the sea. Lung capacities of marine mammals seem to be larger than terrestrial mammals especially if they are compared on a lean weight basis. It is proposed that at least in some this represents an important increase in buoyancy which enables these mammals to rest at sea. The importance of the lung as an O2 store during dives is considered, and it seems that it would be important only to those species that have a low breath-hold tolerance. In their case the O2 present in the fully inflated lung is from four times to equal that in the blood. In those species with a large breath-hold tolerance the lung O2 store is a small fraction of blood stores. Several experiments are discussed which indicate that during dives to depth gas exchange between the blood and lungs is low. One of the reasons suggested is compression collapse of the alveoli. This occurs because of the apparent rigidity of the airways which even in the terminal segments possess an unusual amount of muscular or cartilaginous support. The reinforcement insures that during compression the airways will not occlude and trap gas in the alveoli. In fact, in some species, especially otariids and cetaceans, the airways seem overly strong and an additional function is suggested. Studies of mechanical properties of sea lion and whale lungs show that they may be capable of high expiratory flow rates at low volumes. This feature of the lung would make possible an exchange of a large gas volume in very short periods. Such an ability is consistent with the ventilatory behavior of many marine mammals.