Gas Transport In The Blood Page 2
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1
2
3DPG, Hb is 50% saturated at 1 mmHg P
instead of at 26 mmHg. The concentration of 2,3
O
2
DPG varies slightly in the erythrocyte, rising with glycolysis in anaerobic conditions and
thus promoting the release of oxygen in the presence of hypoxia.
G-6-P
↓
2,3-DPG mutase
3-phosphoglyceraldehyde
↓
2,3-bisphosphoglycerate (2,3-DPG)
1,3-bisphosphoglycerate
2,3-DPG phosphatase
↓
3-phosphoglycerate
↓
pyruvate
A rise in P
or in H
+
ion concentration also promotes the release of oxygen
CO
2
(moving the dissociation curve to the right). This occurs as both CO
and H
+
compete to bind
2
to Hb, which plays a major role in pH buffering. CO
reacts with the
NH
groups of Hb,
α
2
3
reversibly forming a carbamate which forms salt bridges and helps stabilize the T form. H
+
similarly binds more readily to aspartate and histidine residues which display a rise in pKa
with the conformational change from R to T state. This linkage of the affinity for oxygen
and H
+
and CO
binding sites on Hb through conformational change is known as the Bohr
2
Effect.
Temperature rise reduces the affinity of Hb for oxygen, producing a right shift in
tissues which are substantially above normal temperature, such as exercising muscles.
Carbon monoxide binds to Hb about 240 times as avidly as oxygen, having a P
of
50
about 0.1 mmHg. It coordinates similarly with the Fe
2+
ion and moves the oxygen
dissociation curve to the left.
Other factors which move the curve to the left include high altitude (due to
alkalosis), neonatal haemoglobin and thalassaemia. Factors which move the curve to the
right include: Hb S, anaemia, hyperthyroidism and normal physiology in the infant (not
neonate). More detail in
Monitoring
(3.B.2)
c. Describe the carbon dioxide carriage in blood including the Haldane effect and
chloride shift.
CO
is carried in three ways in blood, as dissolved CO
, as HCO
-
, and combined
2
2
3
with proteins as carbamino compounds. It is far more soluble in blood than O
, with about
2
0.06ml/100ml/mmHg dissolving. In solution it is in equilibrium with carbonic acid and
bicarbonate ion:
CO
+ H
O ↔ H
CO
↔ HCO
-
+ H
+
2
2
2
3
3
which reacts only slowly in plasma but rapidly within red cells where carbonic anhydrase
catalyzes the first reaction.
-
-
HCO
formed within red cells diffuses easily back into plasma in exchange for Cl
3
ion, according to the Gibbs-Donnan equilibrium in which diffusible ions distribute
themselves such that their concentration ratios are equal between compartments. This
movement of Cl
-
is known as the chloride shift.
The H
+
ion formed inside red cells does not diffuse readily into plasma as the cell
membrane is relatively impermeable to cations. It partly buffered by binding to
deoxygenated Hb, helping to stabilize the T form. This buffering allows a greater amount of
-
CO
to be carried as HCO
than would otherwise be possible. The net increase in CO
2
3
2
carrying capacity of blood when it is deoxygenated is known as the Haldane effect.
Some CO
is also carried in combination with globin by reacting with terminal
2
-
NH
groups to form carbamates: Hb·NH·COO
. This reaction is also facilitated by the
2
deoxygenation of haemoglobin.
-
Of the total CO
content of arterial blood, 90% is as HCO
, and 5% each dissolved
2
3
CO
and carbamino compounds. However, of the amount of CO
exchanged between tissues
2
2
-
and lungs, only 60% is carried as HCO
, 30% as carbamino compounds and 10% dissolved.
3
Respiratory gas transport
1.B.7.2
James Mitchell (December 24, 2003)
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