33
Chapter two: Oxygen stores and diving
O 2 store, and especially a high muscle O 2 content concentrated in the primary underwater locomotory muscles, will have implications for the nature of cardiovascular responses
required for effective utilization of O 2 stores during dives.
The muscle O 2 store is based on the concentration of Mb and the muscle mass.
In marine mammals, Mb concentrations vary almost 100-fold, from 0.1 g 100 g −1 muscle in
manatees to 9.5 g 100 g −1 muscle in hooded seals. The Mb concentration in marine mammals is often indicative of their diving capacity and dive patterns. The Mb concentration
is highest in the deep, long duration divers such as the hooded, northern elephant, ribbon,
harp, and Weddell seals (with Mb concentrations from 5.4 to 9.5 g 100 g −1 muscle) (Lenfant
et al. 1970; Ponganis et al. 1993; Burns et al. 2007; Hassrick et al. 2010). Among otariids, the
Galapagos (Zalophus wollebaeki) and California (Zalophus californianus) sea lions have some
of the highest Mb concentrations (5.3–5.4 g 100 g −1 muscle) (Weise and Costa 2007; VillegasAmtmann and Costa 2010). Mb content in the sperm whale, bottlenose whale, and the narwhal (Monodon monoceros) is also high, ranging from 5.4 to 7.9 g 100 g −1 muscle (Scholander
1940; Williams, Noren, and Glenn 2011).
2.2.2 The dive response
The dive response forms the basis for the management of O 2 stores, and, indeed, for all of
diving physiology. As exemplified in the forced submersion experiments of Scholander and
Irving, the heart rate decreases quickly and dramatically in these extreme situations, often
to heart rates of less than 10 beats per min in seals (Scholander 1940; Scholander et al. 1942).
Under these conditions, cardiac output is severely reduced, and there is widespread peripheral
vasoconstriction of the arterial vessels to maintain blood pressure. The reduced blood flow is
directed away from most organs, and is essentially distributed to the brain and heart (Blix
et al. 1983; Zapol 1987). This reduction in blood flow decreases blood O 2 consumption by most
organs and isolates the muscle from the circulation, so that muscle metabolism is dependent on the enhanced myoglobin-bound O 2 store and eventually on anaerobic glycolysis. This
severe reduction in heart rate and peripheral blood flow, the so-called dive reflex (see Figure
2.1), results in a very slow rate of blood O 2 depletion with conservation of the blood O 2 store
for metabolism of the brain and heart (Elsner et al. 1966; Kerem and Elsner 1973).
In free dives and spontaneous breath-holds, however, the reduction in heart rate and
peripheral blood flow (the dive response) is often variable and not as great as during forced
submersion (Elsner 1965; Thompson and Fedak 1993; Andrews et al. 1997). This was well
illustrated in Elsner’s studies of trained breath-holds of seals in the 1960s (Figure 2.1). In
free dives, although diving heart rates are variable, heart rates can be in the range of 20–40
beats min −1 (about 2–5 times those during forced submersions); gastrointestinal blood flow,
hepatic blood flow, renal blood flow, and even some muscle blood flow all appear to be
maintained in short-duration dives of Weddell seals (Davis et al. 1983; Guppy et al. 1986;
Hill et al. 1987; Guyton et al. 1995). At times, however, the heart rate can be quite low even
during free dives; during a 14 min dive of a gray seal, average dive heart rate was near
4 beats min −1 (Thompson and Fedak 1993). And, during deep dives of California sea lions
(Figure 2.2), the heart rate can be less than 10 beats per min at maximum depth (McDonald
and Ponganis 2014). Thus, the degrees of bradycardia and tissue blood flow reduction during free dives are variable and probably dependent on the nature and circumstances of a
given dive. Due to this variable dive response, the depletion rate of the blood O 2 store and
the duration of aerobic metabolism are variable in different dives. Similarly, the depletion
rate of the muscle O 2 store will also be variable, depending on locomotory requirements
during a given dive (Chapters 1 and 3) as well as any blood O 2 supplementation of muscle
Chapter two: Oxygen stores and diving
O 2 store, and especially a high muscle O 2 content concentrated in the primary underwater locomotory muscles, will have implications for the nature of cardiovascular responses
required for effective utilization of O 2 stores during dives.
The muscle O 2 store is based on the concentration of Mb and the muscle mass.
In marine mammals, Mb concentrations vary almost 100-fold, from 0.1 g 100 g −1 muscle in
manatees to 9.5 g 100 g −1 muscle in hooded seals. The Mb concentration in marine mammals is often indicative of their diving capacity and dive patterns. The Mb concentration
is highest in the deep, long duration divers such as the hooded, northern elephant, ribbon,
harp, and Weddell seals (with Mb concentrations from 5.4 to 9.5 g 100 g −1 muscle) (Lenfant
et al. 1970; Ponganis et al. 1993; Burns et al. 2007; Hassrick et al. 2010). Among otariids, the
Galapagos (Zalophus wollebaeki) and California (Zalophus californianus) sea lions have some
of the highest Mb concentrations (5.3–5.4 g 100 g −1 muscle) (Weise and Costa 2007; VillegasAmtmann and Costa 2010). Mb content in the sperm whale, bottlenose whale, and the narwhal (Monodon monoceros) is also high, ranging from 5.4 to 7.9 g 100 g −1 muscle (Scholander
1940; Williams, Noren, and Glenn 2011).
2.2.2 The dive response
The dive response forms the basis for the management of O 2 stores, and, indeed, for all of
diving physiology. As exemplified in the forced submersion experiments of Scholander and
Irving, the heart rate decreases quickly and dramatically in these extreme situations, often
to heart rates of less than 10 beats per min in seals (Scholander 1940; Scholander et al. 1942).
Under these conditions, cardiac output is severely reduced, and there is widespread peripheral
vasoconstriction of the arterial vessels to maintain blood pressure. The reduced blood flow is
directed away from most organs, and is essentially distributed to the brain and heart (Blix
et al. 1983; Zapol 1987). This reduction in blood flow decreases blood O 2 consumption by most
organs and isolates the muscle from the circulation, so that muscle metabolism is dependent on the enhanced myoglobin-bound O 2 store and eventually on anaerobic glycolysis. This
severe reduction in heart rate and peripheral blood flow, the so-called dive reflex (see Figure
2.1), results in a very slow rate of blood O 2 depletion with conservation of the blood O 2 store
for metabolism of the brain and heart (Elsner et al. 1966; Kerem and Elsner 1973).
In free dives and spontaneous breath-holds, however, the reduction in heart rate and
peripheral blood flow (the dive response) is often variable and not as great as during forced
submersion (Elsner 1965; Thompson and Fedak 1993; Andrews et al. 1997). This was well
illustrated in Elsner’s studies of trained breath-holds of seals in the 1960s (Figure 2.1). In
free dives, although diving heart rates are variable, heart rates can be in the range of 20–40
beats min −1 (about 2–5 times those during forced submersions); gastrointestinal blood flow,
hepatic blood flow, renal blood flow, and even some muscle blood flow all appear to be
maintained in short-duration dives of Weddell seals (Davis et al. 1983; Guppy et al. 1986;
Hill et al. 1987; Guyton et al. 1995). At times, however, the heart rate can be quite low even
during free dives; during a 14 min dive of a gray seal, average dive heart rate was near
4 beats min −1 (Thompson and Fedak 1993). And, during deep dives of California sea lions
(Figure 2.2), the heart rate can be less than 10 beats per min at maximum depth (McDonald
and Ponganis 2014). Thus, the degrees of bradycardia and tissue blood flow reduction during free dives are variable and probably dependent on the nature and circumstances of a
given dive. Due to this variable dive response, the depletion rate of the blood O 2 store and
the duration of aerobic metabolism are variable in different dives. Similarly, the depletion
rate of the muscle O 2 store will also be variable, depending on locomotory requirements
during a given dive (Chapters 1 and 3) as well as any blood O 2 supplementation of muscle