83
Chapter four: Pressure regulation
mounted on it (Kooyman 1965), and later on carbon-coated paper with a quartz motor
allowing recording durations of 25 days (Naito et al. 1990). These analog systems were
superseded by digital solid-state recorders. Early limitations were in terms of memory
capacity (and thus, temporal resolution of data sampling), number of parallel sensors
which could be supported, 8-bit resolution (allowing only 255 measurements, e.g., 1000 m
depth could only be recorded to the nearest 4 m), and instrument size (Ropert-Coudert
and Wilson 2005).
Most of these impediments have become vastly improved, and tags can now combine
hydrophone recordings, video recordings, accelerometers, speed sensors, and oceanographic measurements alongside records of animal depth. Thus, these instruments not
only log a detailed diary of the behavior and physiology of the animal but can also provide
a detailed record of their external environment.
4.3.2 Observation of lung structure and dynamics
For semi-aquatic marine mammals such as pinnipeds, several aspects of the diving response
have been effectively studied in lab-based settings. In terms of investigations examining
the effect of pressure on diving animals, these have included sub-cellular based studies
(Castellini and Castellini 2004) but also whole animal studies (Kooyman et al. 1972; Kooyman
and Sinnett 1982) using hyperbaric chambers. Such studies have examined a natomical
changes, for example, the flexibility of the trachea under pressure (Kooyman et al. 1970), but
also changes in pulmonary shunt and its effects on blood gas content under breath-hold and
pressure (Kooyman et al. 1972; Kooyman and Sinnett 1982).
The field of medical imaging (ultrasound, CT, and MRI) is becoming ever more refined,
and there is great scope for the application of this type of study to investigations of diving animals. Recent work has used a hyperbaric chamber inside a CT scanner to obtain 3D
images of the changes in lung compression of dead marine mammals as pressure was
increased (Moore et al. 2011). There has been some progress on development of a MRIcompatible hyperbaric chamber and, combined with trained animals that dive on cue, there
is potential for work on the respiratory alterations caused by pressure.
While imaging studies may become feasible in the future, recent work has investigated
the structural properties of the respiratory system in anesthetized pinnipeds (Fahlman
et al. 2014a). The static compliance (pressure–volume relationship) of the lung and chest
were estimated in anesthetized individuals during manual ventilation. The results agreed
with Scholander’s suggestion that the chest provides little resistance to compression and is
able to compress to very low volumes. In cetaceans, anesthesia is uncommon and studies
on respiratory physiology and lung mechanics have been performed in conscious animals
(Fahlman et al. 2015; Kooyman and Cornell 1981; Olsen et al. 1969). Both anatomical studies (Cotten et al. 2008) and work on live animals using a custom-made pneumotachometer
(Fahlman et al. 2015) suggest that exhalation during voluntary breaths is passive while
inhalation requires substantial work from the respiratory muscles. Thus, the elastic recoil
of the chest may help empty the lung to low volumes which would be beneficial to avoid
negative pressures (lung squeeze) to develop inside the lung during diving. In addition,
works on excised lungs (Kooyman and Sinnett 1979) and live dolphins (Fahlman et al.
2015; Kooyman and Cornell 1981) suggest that the respiratory architecture may play a role
in their unusual ability to respire. For example, respiratory flow rates are as much as three
times higher than those in the terrestrial champion, the horse. The vital capacity can be as
much as 80%–90% of the total lung capacity and exchanged in as little as 200–300 ms. The
unusual anatomy of the respiratory system of odontocetes may enable rapid and efficient
Chapter four: Pressure regulation
mounted on it (Kooyman 1965), and later on carbon-coated paper with a quartz motor
allowing recording durations of 25 days (Naito et al. 1990). These analog systems were
superseded by digital solid-state recorders. Early limitations were in terms of memory
capacity (and thus, temporal resolution of data sampling), number of parallel sensors
which could be supported, 8-bit resolution (allowing only 255 measurements, e.g., 1000 m
depth could only be recorded to the nearest 4 m), and instrument size (Ropert-Coudert
and Wilson 2005).
Most of these impediments have become vastly improved, and tags can now combine
hydrophone recordings, video recordings, accelerometers, speed sensors, and oceanographic measurements alongside records of animal depth. Thus, these instruments not
only log a detailed diary of the behavior and physiology of the animal but can also provide
a detailed record of their external environment.
4.3.2 Observation of lung structure and dynamics
For semi-aquatic marine mammals such as pinnipeds, several aspects of the diving response
have been effectively studied in lab-based settings. In terms of investigations examining
the effect of pressure on diving animals, these have included sub-cellular based studies
(Castellini and Castellini 2004) but also whole animal studies (Kooyman et al. 1972; Kooyman
and Sinnett 1982) using hyperbaric chambers. Such studies have examined a natomical
changes, for example, the flexibility of the trachea under pressure (Kooyman et al. 1970), but
also changes in pulmonary shunt and its effects on blood gas content under breath-hold and
pressure (Kooyman et al. 1972; Kooyman and Sinnett 1982).
The field of medical imaging (ultrasound, CT, and MRI) is becoming ever more refined,
and there is great scope for the application of this type of study to investigations of diving animals. Recent work has used a hyperbaric chamber inside a CT scanner to obtain 3D
images of the changes in lung compression of dead marine mammals as pressure was
increased (Moore et al. 2011). There has been some progress on development of a MRIcompatible hyperbaric chamber and, combined with trained animals that dive on cue, there
is potential for work on the respiratory alterations caused by pressure.
While imaging studies may become feasible in the future, recent work has investigated
the structural properties of the respiratory system in anesthetized pinnipeds (Fahlman
et al. 2014a). The static compliance (pressure–volume relationship) of the lung and chest
were estimated in anesthetized individuals during manual ventilation. The results agreed
with Scholander’s suggestion that the chest provides little resistance to compression and is
able to compress to very low volumes. In cetaceans, anesthesia is uncommon and studies
on respiratory physiology and lung mechanics have been performed in conscious animals
(Fahlman et al. 2015; Kooyman and Cornell 1981; Olsen et al. 1969). Both anatomical studies (Cotten et al. 2008) and work on live animals using a custom-made pneumotachometer
(Fahlman et al. 2015) suggest that exhalation during voluntary breaths is passive while
inhalation requires substantial work from the respiratory muscles. Thus, the elastic recoil
of the chest may help empty the lung to low volumes which would be beneficial to avoid
negative pressures (lung squeeze) to develop inside the lung during diving. In addition,
works on excised lungs (Kooyman and Sinnett 1979) and live dolphins (Fahlman et al.
2015; Kooyman and Cornell 1981) suggest that the respiratory architecture may play a role
in their unusual ability to respire. For example, respiratory flow rates are as much as three
times higher than those in the terrestrial champion, the horse. The vital capacity can be as
much as 80%–90% of the total lung capacity and exchanged in as little as 200–300 ms. The
unusual anatomy of the respiratory system of odontocetes may enable rapid and efficient