Gas Transport In The Blood Page 3
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1
2
3d. Explain the carbon dioxide dissociation curve and its clinical implications.
The carbon dioxide
dissociation curve of blood is more
evenly sloped and steeper than that of
oxygen. The major component of the
60
total CO
concentration is HCO
-
ion
Total
2
3
52
which over the physiological range
(at 75% SaO
)
2
v ¯
varies almost linearly with P
.The
48
CO
2
a
(at 97% SaO
)
contribution of carbamino compounds
2
] ↑T
↑[HbO
varies very little with P
, but
40
CO
2
2
strongly with the proportion of
oxyhaemoglobin, favouring the uptake
of CO
in the tissues where P
is low.
O
2
2
A rise in temperature
20
reduces the solubility of CO
in blood.
2
The shape of the
dissociation curve makes CO
2
transport less dependent on V/Q
Dissolved
matching, as the contribution of high
0
46
20
40
60
80
V/Q alveoli can compensate for that of
P
(mmHg)
CO
2
low V/Q ones, as neither lies on a
“plateau” on the curve.
The substantial contribution
from CO
not bound to Hb makes CO
transport much less dependent upon Hb
2
2
concentration than O
transport is.
2
e. Describe the oxygen and carbon dioxide stores in the body.
The total body stores of oxygen are small compared with the basal requirements
for metabolism:
on air
on 100% O
2
lungs (at FRC)
270 ml
1800 ml
blood
820 ml
910 ml
interstitial fluid
50 ml
55 ml
myoglobin-bound 200 ml
200 ml
thus any change in gas exchange results in a rapid change in arterial and tissue P
(t
1
/
O
2
2
about 30 s). Breathing 100% O
results in an increase in the lung store, but increases the
2
blood content by only 50 ml bound to Hb and 50 ml dissolved. O
consumption can be
2
approximated using the Brody formula where BW is weight in kg and ˙ V O
is in ml/min:
2
˙ V O
0.73
= 10.15⋅ BW
2
The total body stores of carbon dioxide are very large and conform best to a multi-
compartment model. The blood and interstitial fluid of well-perfused organs represents a
rapid compartment which equilibrates with alveolar CO
in minutes. Less well perfused
2
organs such as skeletal muscle produce a medium compartment and poorly perfused tissue
(fat) and carbonates bound in bone compose the largest and very slow compartment.
The blood content of CO
is about 2.5 l and total body stores about 120 l. Because
2
of the multiple compartments, arterial P
does not equilibrate as quickly following a fall
CO
2
in ventilation as following a rise. Hyperventilation can deplete blood CO
rapidly (t
1
/
about
2
2
3 min). Apnoea causes a slower rise in P
, because the normal rate of production at rest is
CO
2
small compared with the capacity for excretion and equilibrates into the medium
compartment as well as blood and alveolar gas. P
rises 3-6 mmHg/min with a t
1
/
to
CO
2
2
equilibrium of about 15 min. In practice this allows for Ben-Jet ventilation with oxygen to
provide adequate oxygenation and build up a CO
surplus over 15 or 20 minutes without
2
harmful effects.
Respiratory gas transport
1.B.7.3
James Mitchell (December 24, 2003)
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