Respiratory function and mechanics in pinnipeds and cetaceans

Abstract

In this Review, we focus on the functional properties of the respiratory system of pinnipeds and cetaceans, and briefly summarize the underlying anatomy; in doing so, we provide an overview of what is currently known about their respiratory physiology and mechanics. While exposure to high pressure is a common challenge among breath-hold divers, there is a large variation in respiratory anatomy, function and capacity between species -how are these traits adapted to allow the animals to withstand the physiological challenges faced during dives? The ultra-deep diving feats of some marine mammals defy our current understanding of respiratory physiology and lung mechanics. These animals cope daily with lung compression, alveolar collapse, transient hyperoxia and extreme hypoxia. By improving our understanding of respiratory physiology under these conditions, we will be better able to define the physiological constraints imposed on these animals, and how these limitations may affect the survival of marine mammals in a changing environment. Many of the respiratory traits to survive exposure to an extreme environment may inspire novel treatments for a variety of respiratory problems in humans.

Ventilation and gas exchange before and after voluntary static surface breath-holds in clinically healthy bottlenose dolphins, Tursiops truncatus

“…During voluntary breaths, exhalation is passive and mainly driven by the elastic recoil of the chest, whereas the inspiratory flow is active for both voluntary and forced breaths (see fig. 5A in Fahlman et al, 2017a). As the respiratory effort increases, a greater proportion of the expiratory phase becomes active where the respiratory muscles assist in generating high V̇(see fig.…”

“…Past measurements have shown that cetaceans are capable of exchanging as much as 90% of their TLC est (Fahlman et al, , 2017aKooyman and Cornell, 1981;Olsen et al, 1969;Ridgway et al, 1969). From these estimates, numerous studies have assumed that the V T is close to TLC est , and during exercise or during recovery from a dive it may be logical to assume that V T is close to TLC est (Dolphin, 1987a;Fahlman et al, 2016;Folkow and Blix, 1992).…”

“…As the respiratory effort increases, a greater proportion of the expiratory phase becomes active where the respiratory muscles assist in generating high V̇(see fig. 5B,C in Fahlman et al, 2017a). The relative importance of the diaphragm, cranio-cervical, thoracic and lumbo-pelvic muscles to drive inspiration during rest in dolphins is not known (Cotten et al, 2008;Dearolf, 2003).…”

Adaptations to deep and prolonged diving in phocid seals

“…Assuming 15% oxygen in the lung air and that 50% of lung volume is expired before the dive, the lung oxygen store in hooded seals amounts to 6 ml O 2 kg −1 , compared with some 9 ml O 2 kg −1 in the fully expanded lungs of man. However, at depth, as the alveoli of the lungs collapse because of the hydrostatic pressure (Kooyman et al, 1970;Falke et al, 1985;Moore et al, 2011), the blood is shunted through the lung with minimal opportunity for gas exchange (Sinett et al, 1978;Kooyman and Sinnett, 1982;Falke et al, 1985, Fahlman et al, 2017. Accordingly, Miller et al (2006) have shown that in seals the surfactants in the lungs are not primarily there to reduce surface tension to very low values, as in terrestrial mammals, but also have an anti-adhesive function, enabling the lungs to reopen following collapse during deep diving.…”

“…The collapse of the lungs at depth has, in fact, a real advantage: Scholander (1940) suggested, and it was later shown by Kooyman and associates (for references, see Kooyman, 1963;Fahlman et al, 2017), that because the airways are reinforced with an unusual amount of cartilage that extends to the openings of the alveolar sacs, it is the alveoli that collapse first under pressure. The alveolar air is thereby shifted into the airways, in which gas exchange is no longer possible; hence accumulation of N 2 , which causes decompression sickness in humans, is mitigated, but marine mammals may still have to manage high N 2 loads (Hooker et al, 2012).…”