2. Oxygen–Haemoglobin Dissociation Curve Flashcards
Draw the oxygen–haemoglobin dissociation curve (OHDC).
The OHDC is a graph relating
the percentage of haemoglobin saturated
with oxygen to the partial pressure of oxygen (PO2).
Points on the X
kpa 3.5 5.3 13.3
% O2 Sat 50 75 97
Normal oxyhaemoglobin
dissociation curve
> Arterial PO2 is 13.3 kPa with a Hb saturation of 97%
(it is not 100% due to venous admixture constituting physiological shunt).
> Venous PO2 is 5.3 kPa with a Hb saturation of 75%.
> P50 is 3.5 kPa
(this is the PO2 at which Hb is 50% saturated and it is the conventional point used to compare the oxygen affinity of Hb).
Explain the shape of the OHDC.
The OHDC has a characteristic
sigmoid shape due to the binding
characteristics of haemoglobin to oxygen:
1 > Allosteric modulation When oxygen binds to haemoglobin, the two β chains move closer together and change the position of the haem moieties that assume a ‘relaxed’ or R state.
When oxygen dissociates from haemoglobin,
the reverse happens and
the haem moieties take up a ‘tense’ or T state.
> Cooperative binding
When oxygen binds to haemoglobin
the R state is favoured, which has
an increased affinity for oxygen
and so facilitates the uptake of additional oxygen.
The affinity of haemoglobin for
the fourth oxygen molecule is,
therefore, much greater than that for the first.
What are the major physiological factors that determine the position of the OHDC?
Right shift
x 8
> Factors that shift the OHDC to the right:
This facilitates the unloading
of oxygen into tissues and
the P50 value is higher than 3.5 kPa:
1• ↓ pH
2• ↑ Temperature
3• ↑ 2,3-Diphosphoglycerate
4• ↑ PaCO2
5• HbS
6• Anaemia
7• Pregnancy
8 • Post-acclimatisation to altitude.
> Factors that shift the OHDC to the left:
> Factors that shift the OHDC to the left:
This facilitates the uptake of oxygen
from the lungs and the
P50 value is lower than 3.5 kPa:
1 • ↑ pH
2 • ↓ Temperature
3 • ↓ 2,3-Diphosphoglycerate
4• ↓ PaCO2
5• HbF
6• Methaemoglobin
7• Carboxyhaemoglobin
8• Stored blood.
What is the Bohr effect?
This describes the right shift in the OHDC in association with increased
PaCO2 and hydrogen ion concentration.
What is the double Bohr effect?
This refers to the situation in the placenta
where the Bohr effect operates in
both the maternal and fetal circulations.
The increase in PCO2 in the maternal
intervillous sinuses assists oxygen unloading.
The decrease in PCO2 on the
fetal side of the circulation assists oxygen loading.
The Bohr effect facilitates the reciprocal exchange
of oxygen for carbon dioxide.
The double Bohr effect means that the oxygen dissociation curves for maternal HbA and fetal
HbF move apart −
i.e. right shift (maternal); left shift (fetal).
What is the Haldane effect?
This describes the increased ability of
deoxygenated haemoglobin to carry Carbon dioxide.
Conversely, oxygenated blood has a reduced capacity to carry carbon dioxide.
The Haldane effect occurs because
deoxygenated haemoglobin is a better
proton acceptor than oxyhaemoglobin.
How does the OHDC compare with the myoglobin dissociation curve?
What is myoglobin
What does it consist of
Where does the ODC lie
How is this beneficial
Myoglobin is an oxygen-carrying protein
found in skeletal muscles
(it gives muscle its dark red appearance).
It consists of a single polypeptide chain
associated with a haem moiety.
Unlike haemoglobin,
it can only bind one molecule of oxygen
and, therefore,
its dissociation curve is a rectangular hyperbola.
Myoglobin also has a higher affinity
for oxygen than haemoglobin, and so its
dissociation curve lies to the left of the OHDC.
Myoglobin takes up oxygen from the
circulating haemoglobin and releases
it into exercising muscle tissues at very low PO2,
thus providing a source of oxygen during periods of sustained muscle contractions when blood flow to these muscles may be constricted due to blood vessel compression.
