37
Chapter two: Oxygen stores and diving
air volume and (2) the net extraction of O 2 from the diving air volume. The diving air
volume is not necessarily the same as the total lung capacity. The total lung capacity is
the maximum amount of air in the lungs after maximum inspiration. Total lung capacity
has been measured in a number of marine mammals, using a variety of methods including inflation of excised lungs, helium dilution techniques, nitrogen washout, tidal volume
measurements, and inspiratory capacity (for further review, see Ponganis 2015). Most total
lung capacity measurements have been made by inflation of excised lungs, including bottlenose whales (H. ampullatus), pygmy and dwarf sperm whales (K. breviceps, K. sima), and
sea otters (Enhydra lutris) (Piscitelli et al. 2010, 2013).
The diving air volume is the volume of air in the lungs at the start of a dive. Some marine
mammals, such as many pinnipeds, dive after exhalation. As a result, diving lung volume is
far less than the total lung capacity. In simulated dives in pressure chambers, the diving lung
volumes of phocid seals and sea lions have been estimated to be about 50% total lung capacity (Kooyman et al. 1973; Kooyman and Sinnett 1982). Such values, obtained from restrained
animals, may not be representative of the initial lung volumes of free-diving animals.
However, these values have been used extensively in calculations of O 2 stores. Cetaceans, in
contrast, dive on inspiration. Thus, it is often assumed that their diving lung volume is near
total lung capacity. Although measurements on cetaceans are not common, diving lung volumes determined by buoyancy-swim velocity calculations are similar to total lung capacity
measured in excised lungs of some deep-diving whales (Scholander 1940, Miller et al. 2004).
The true, usable, respiratory O 2 store is based on the amount of O 2 extracted from the
diving lung volume. It is unlikely that the O 2 in the lungs can be completely depleted.
Thus, the net extraction of O 2 is key to estimating the usable respiratory O 2 store. The net
O 2 extraction is the difference between the O 2 fraction at the start of the dive (initial O 2
fraction) and the lowest possible O 2 at the end of dives. This difference is assumed to be
15% in diving mammals (Kooyman 1989). At the start of dives, O 2 fractions are likely near
20%, or slightly less for pinnipeds which exhale before diving. The expiratory O 2 fractions
measured at the end of forced submersion and forced dives in seals and other marine
mammals were approximately 2%–4%, consistent with the assumption of 15% net O 2 fraction (Scholander 1940; Ridgway et al. 1969; Kooyman et al. 1973; Ponganis et al. 1993).
2.3.2 Measuring the blood O 2 store
To calculate the total blood O 2 store, a number of parameters must be measured: blood
volume (BV), mass, Hb concentration, Hb carrying capacity (1.34 ml O 2 g Hb −1 ), arterial
and venous Hb saturation (SaO 2 and SvO 2 ), and the assumption that one-third of the blood
volume is arterial and two-thirds venous (Lenfant et al. 1970; Kooyman 1989). The amount
of O 2 in the total blood volume can be calculated by the following equation:
Blood O
33BV dl kg
Mass kg Hb g dl
1 34 ml O g Hb
2
1
1
2
1
=
´
´
´
-
-
-
( .
(
)
( )
(
) . (
)
0
´ ´
+
´
´
´
-
-
SaO
67BV (dl kg ) Mass kg Hb (g dl ) 1 34 ml O g Hb
2
1
1
2
(%))
( .
( )
. (
0
- -
´
1
2
SvO
)
( %))
(2.1)
Note that BV is in dl kg –1 , not in ml kg –1 .
The usable blood O 2 store, however, is dependent both on the initial Hb saturation of
blood (initial SvO 2 and SaO 2 ) and on how much O 2 can be extracted from the blood on both
the arterial side and venous side during a dive. Typically, it is assumed that the SaO 2 can
Chapter two: Oxygen stores and diving
air volume and (2) the net extraction of O 2 from the diving air volume. The diving air
volume is not necessarily the same as the total lung capacity. The total lung capacity is
the maximum amount of air in the lungs after maximum inspiration. Total lung capacity
has been measured in a number of marine mammals, using a variety of methods including inflation of excised lungs, helium dilution techniques, nitrogen washout, tidal volume
measurements, and inspiratory capacity (for further review, see Ponganis 2015). Most total
lung capacity measurements have been made by inflation of excised lungs, including bottlenose whales (H. ampullatus), pygmy and dwarf sperm whales (K. breviceps, K. sima), and
sea otters (Enhydra lutris) (Piscitelli et al. 2010, 2013).
The diving air volume is the volume of air in the lungs at the start of a dive. Some marine
mammals, such as many pinnipeds, dive after exhalation. As a result, diving lung volume is
far less than the total lung capacity. In simulated dives in pressure chambers, the diving lung
volumes of phocid seals and sea lions have been estimated to be about 50% total lung capacity (Kooyman et al. 1973; Kooyman and Sinnett 1982). Such values, obtained from restrained
animals, may not be representative of the initial lung volumes of free-diving animals.
However, these values have been used extensively in calculations of O 2 stores. Cetaceans, in
contrast, dive on inspiration. Thus, it is often assumed that their diving lung volume is near
total lung capacity. Although measurements on cetaceans are not common, diving lung volumes determined by buoyancy-swim velocity calculations are similar to total lung capacity
measured in excised lungs of some deep-diving whales (Scholander 1940, Miller et al. 2004).
The true, usable, respiratory O 2 store is based on the amount of O 2 extracted from the
diving lung volume. It is unlikely that the O 2 in the lungs can be completely depleted.
Thus, the net extraction of O 2 is key to estimating the usable respiratory O 2 store. The net
O 2 extraction is the difference between the O 2 fraction at the start of the dive (initial O 2
fraction) and the lowest possible O 2 at the end of dives. This difference is assumed to be
15% in diving mammals (Kooyman 1989). At the start of dives, O 2 fractions are likely near
20%, or slightly less for pinnipeds which exhale before diving. The expiratory O 2 fractions
measured at the end of forced submersion and forced dives in seals and other marine
mammals were approximately 2%–4%, consistent with the assumption of 15% net O 2 fraction (Scholander 1940; Ridgway et al. 1969; Kooyman et al. 1973; Ponganis et al. 1993).
2.3.2 Measuring the blood O 2 store
To calculate the total blood O 2 store, a number of parameters must be measured: blood
volume (BV), mass, Hb concentration, Hb carrying capacity (1.34 ml O 2 g Hb −1 ), arterial
and venous Hb saturation (SaO 2 and SvO 2 ), and the assumption that one-third of the blood
volume is arterial and two-thirds venous (Lenfant et al. 1970; Kooyman 1989). The amount
of O 2 in the total blood volume can be calculated by the following equation:
Blood O
33BV dl kg
Mass kg Hb g dl
1 34 ml O g Hb
2
1
1
2
1
=
´
´
´
-
-
-
( .
(
)
( )
(
) . (
)
0
´ ´
+
´
´
´
-
-
SaO
67BV (dl kg ) Mass kg Hb (g dl ) 1 34 ml O g Hb
2
1
1
2
(%))
( .
( )
. (
0
- -
´
1
2
SvO
)
( %))
(2.1)
Note that BV is in dl kg –1 , not in ml kg –1 .
The usable blood O 2 store, however, is dependent both on the initial Hb saturation of
blood (initial SvO 2 and SaO 2 ) and on how much O 2 can be extracted from the blood on both
the arterial side and venous side during a dive. Typically, it is assumed that the SaO 2 can