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Fig. 5. Dependent variables during simulated shallow resting dive cycles in an
adult Weddell seal model. A dive (onset indicated by downward broken arrows)
and the preceding and succeeding surface intervals (onset at upward broken
arrows) are shown. Parameter values were chosen for the purposes of
illustration to yield a 16.4 min dive with 5.9 min surface intervals, typical
diving behaviour for an adult Weddell seal:
 n=75%
 max,
 s=81.4 1 min-1,
d=6.7 1 min-1
(zero flow in muscle), corresponding to a mean surface heart rate of 146 beats
min-1 and mean diving heart rate 10.5 beats min-1. (A)
Alveolar ventilation (A,
dark blue) and heart rate (fH, red). (B) Total chemical respiratory
drive (chem, dark blue) and
central neural respiratory drive
(n, red). Horizontal light
green line is the chemical drive threshold (Tchem).
Ventilation occurs only when
chem and/or
n are above
Tchem, so that Tchem functions as an
`apnoeic threshold' of respiratory drive.
n is assumed to be
inhibited (e.g. by reflexes in the upper respiratory tract) during dives. (C)
Central chemoreflex drive
(c, dark blue) and
peripheral chemoreflex drive
(p, red), which represent
the components of chem.
c is determined by partial
pressure of CO2 in the brain
PBCO2(t) and
p is determined by partial
pressures of both O2 and CO2 in the arterial blood
flowing through the peripheral chemoreceptor
[PaO2(t-tc) and
PaCO2(t-tc), respectively].
(D) Partial pressures of CO2 in various parts of the model system:
orange, PBCO2[t] at the central chemoreceptor;
light blue, PACO2(t) in the
lung and pulmonary capillary blood; dark blue,
PaCO2(t-tc) at the
peripheral chemoreceptor; dark green,
P CO2(t-tv)
in mixed venous blood at the lung. (E) Partial pressures of O2 in
various parts of the model system: light blue,
PAO2(t) in the lung and
pulmonary capillary blood; dark green,
PO2(t-tv)
in mixed venous blood at the lung. (F) Oxygen saturation of `arterial' blood
in the pulmonary capillaries [SaO2(t),
light blue], mixed venous blood at the lung
[SO2(t-tv),
dark green], and skeletal muscle myoglobin
[SmO2(t), red].
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Tc, mmHg) at each of four
different levels of peripheral chemoreceptor threshold
(Tp, mmHg). Tc during dives was always
37.4 mmHg. Tp was constant across the dive cycle: circles,
Tp=39.1 mmHg; triangles, Tp=37.4 mmHg;
squares, Tp=35.7 mmHg; diamonds, Tp=34
mmHg. (B) The effect of variation in Tp on
ts and td at constant
, see text
for details); white diamonds, effect of hypoxia
(FIO 2=0.15) on the responses to
varying