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Marine Mammal Physiology: Requisites for Ocean Living
response with pressure exposure, and showed reduced response in belugas compared to
humans following pressure exposure (Thompson and Romano 2015). Studies in rats and
rabbits have shown that animals that have been immune compromised have a reduced
risk of DCS (Kayar et al. 1997; Ward et al. 1990). This could potentially be an adaptation
protecting against dive-related pathologies (Thompson and Romano 2015). In addition,
platelets from elephant seals responded less to agonists which was suggested to be an
adaptation toward repeated pressure changes (Field et al. 2001). Cetaceans appear to lack a
number of clotting factors, for example, Hageman factor, common to terrestrial mammals,
leading to their more hypocoagulable blood (Robinson et al. 1969). Their absence may help
to improve microcirculation at depth and/or to reduce venous thrombosis, which has been
suggested important in human DCS (Montcalm-Smith et al. 2008; Pontier et al. 2011).
Pressure in the body can have a detrimental effect on blood pressure and blood flow to
the brain. Marine mammals possess extensive venous plexuses (Costidis and Rommel 2012).
Some, such as the retia mirabilia found in cetaceans and sirenians, might be related to diving ability (Vogl and Fisher 1982). In cetaceans, these are a series of vascular networks of
densely looped arteries primarily located along the base of the brain case, along and within
the vertebral column, and retro-pleurally lining the ventral aspect of the rib arches. It is the
only path of arterial blood to the brain in adult cetaceans and may be an adaptation allowing effective drainage of blood from the central nervous system during periods of elevated
pressure (D. Garcia-Parraga, 2015, pers. comm.). Alternatively, the venous portion of these
rete may allow intra-thoracic and vascular engorgement to prevent lung squeeze, similar
to that seen in humans (Brown and Butler 2000; Craig 1968) while the arterial rete may act
as a filter for arterial gas emboli preventing DCS (neuroprotective effect) (Nagel et al. 1968;
Ponganis et al. 2003; Scholander 1940).
4.2.4 Pressure and airspaces
It is the airspaces that cause the most problem to diving animals, and for this reason many
of the airspaces present in terrestrial mammals have been lost in the evolution of marine
mammals. There are three major airspaces that are a liability for divers—the lung, the
facial sinuses, and the middle ear.
Marine mammals have lost their facial sinuses and so avoid problems with these. The
middle ear is an air-filled rigid cavity with little or no compressibility. In pinnipeds, a
pressure differential is prevented by a complex vascular sinus lining the wall of the middle ear cavity (Odend’hal and Poulter 1966; Stenfors et al. 2001). The negative pressure
that develops in the middle ear during diving pulls blood into the venous sinus and helps
fill the internal volume. In cetaceans, a similar mechanism is found in the pterygoid and
peribullar sinuses possessing elaborate plexiform veins. More elaborate and voluminous
sinus vasculature is found in deep divers (e.g., physeterids, kogiids, ziphiids) compared to
shallow-diving delphinids (Fraser and Purves 1960).
4.2.5 Lungs and diving lung collapse
The major airspace that is affected by pressure during diving is the respiratory system. One
problem is lung squeeze, as pressure reduces the air volume within an incompressible rib
cage. Human divers face this problem with their largely incompressible ribs and chest wall
(Ferretti 2001; Lundgren and Miller 1999). Scholander (1940) suggested that marine mammals are able to circumvent this problem by having a highly compliant chest wall. In the
pinniped, volume–pressure curves (compliance) showed that the chest wall provides little
Marine Mammal Physiology: Requisites for Ocean Living
response with pressure exposure, and showed reduced response in belugas compared to
humans following pressure exposure (Thompson and Romano 2015). Studies in rats and
rabbits have shown that animals that have been immune compromised have a reduced
risk of DCS (Kayar et al. 1997; Ward et al. 1990). This could potentially be an adaptation
protecting against dive-related pathologies (Thompson and Romano 2015). In addition,
platelets from elephant seals responded less to agonists which was suggested to be an
adaptation toward repeated pressure changes (Field et al. 2001). Cetaceans appear to lack a
number of clotting factors, for example, Hageman factor, common to terrestrial mammals,
leading to their more hypocoagulable blood (Robinson et al. 1969). Their absence may help
to improve microcirculation at depth and/or to reduce venous thrombosis, which has been
suggested important in human DCS (Montcalm-Smith et al. 2008; Pontier et al. 2011).
Pressure in the body can have a detrimental effect on blood pressure and blood flow to
the brain. Marine mammals possess extensive venous plexuses (Costidis and Rommel 2012).
Some, such as the retia mirabilia found in cetaceans and sirenians, might be related to diving ability (Vogl and Fisher 1982). In cetaceans, these are a series of vascular networks of
densely looped arteries primarily located along the base of the brain case, along and within
the vertebral column, and retro-pleurally lining the ventral aspect of the rib arches. It is the
only path of arterial blood to the brain in adult cetaceans and may be an adaptation allowing effective drainage of blood from the central nervous system during periods of elevated
pressure (D. Garcia-Parraga, 2015, pers. comm.). Alternatively, the venous portion of these
rete may allow intra-thoracic and vascular engorgement to prevent lung squeeze, similar
to that seen in humans (Brown and Butler 2000; Craig 1968) while the arterial rete may act
as a filter for arterial gas emboli preventing DCS (neuroprotective effect) (Nagel et al. 1968;
Ponganis et al. 2003; Scholander 1940).
4.2.4 Pressure and airspaces
It is the airspaces that cause the most problem to diving animals, and for this reason many
of the airspaces present in terrestrial mammals have been lost in the evolution of marine
mammals. There are three major airspaces that are a liability for divers—the lung, the
facial sinuses, and the middle ear.
Marine mammals have lost their facial sinuses and so avoid problems with these. The
middle ear is an air-filled rigid cavity with little or no compressibility. In pinnipeds, a
pressure differential is prevented by a complex vascular sinus lining the wall of the middle ear cavity (Odend’hal and Poulter 1966; Stenfors et al. 2001). The negative pressure
that develops in the middle ear during diving pulls blood into the venous sinus and helps
fill the internal volume. In cetaceans, a similar mechanism is found in the pterygoid and
peribullar sinuses possessing elaborate plexiform veins. More elaborate and voluminous
sinus vasculature is found in deep divers (e.g., physeterids, kogiids, ziphiids) compared to
shallow-diving delphinids (Fraser and Purves 1960).
4.2.5 Lungs and diving lung collapse
The major airspace that is affected by pressure during diving is the respiratory system. One
problem is lung squeeze, as pressure reduces the air volume within an incompressible rib
cage. Human divers face this problem with their largely incompressible ribs and chest wall
(Ferretti 2001; Lundgren and Miller 1999). Scholander (1940) suggested that marine mammals are able to circumvent this problem by having a highly compliant chest wall. In the
pinniped, volume–pressure curves (compliance) showed that the chest wall provides little