Download Gas Exchange – additional higher level material

Gas Exchange – additional higher level material
What is partial pressure?
The partial pressure of a gas is a measure of its concentration. It is expressed in kPa
(kilopascals). The partial pressure of oxygen, written as pO2 is a measure of how much of
the whole atmospheric pressure is due to the oxygen present in it.
Oxygen dissociation curves
The ability of the blood to transport enough oxygen to meet the needs of the body is due to
the affinity of haemoglobin for oxygen. Graphs can be plotted showing the percentage
saturation of oxygen against different partial pressures.
The curve is sigmoid – S-shaped. Where it rises steeply, a small increase in the partial
pressure of oxygen achives a relatively high percentage saturation of the blood. The flat
part of the curve at the top corresponds to the situation in the lungs. Over this range, a
high saturation is maintained even if the partial pressure of oxygen in the alveoli falls, thus
favouring the loading of haemoglobin with oxygen in the lungs.
The oxygen dissociation curve also facilitates the unloading of oxygen in the tissues. The
steep part of the curve corresponds to the range of oxygen partial pressures found in the
tissues. Over this part of the curve, a small drop in oxygen partial pressure will bring about
a comparatively large fall in the percentage saturation of the blood. Thus, if the partial
pressure of oxygen falls as a result of the tissues using oxygen at a faster rate, the
haemoglobin gives up more of its oxygen.
The shape of the oxygen dissociation curve ensures that the red blood cells take up
oxygen in the lungs and release it in the tissues.
Percentage saturation of haemoglobin with
oxygen
The effect of carbon dioxide on the oxygen dissociation curve
A
pCO2 2.7kPa
B
pCO2 5.3kPa
C
pCO2 10.7kPa
100
80
60
40
20
0
0
2
4
6
8
10
12
14
Partial Pressure of oxygen/kPa
The above graph shows the effect of different concentrations of carbon dioxide on the
oxygen dissociation curve for human haemoglobin. High CO2 levels shift the curve to the
right, ie it lowers the affinity of haemoglobin for oxygen. This is called the BOHR EFFECT.
When the Bohr effect is operating, the haemoglobin must be exposed to a higher partial
pressure of oxygen in order to become fully saturated. At the same time, however, it will
release its oxygen at higher partial pressures of oxygen. In other words, carbon dioxide
makes haemoglobin less efficient at taking up oxygen, but more efficient at
releasing it.
The release of oxygen is therefore favoured in the tissues, where the partial pressure of
CO2 tends naturally to be high as a result of its continual release from the respiring cells.
In the lungs the partial pressure of CO2 is lower due to its continual escape into the
atmosphere, thus favouring oxygen uptake.
Oxygen dissociation curves of myoglobin and other blood pigments
So far we have learned that the further an oxygen dissociation curve is to the left, the more
firmly the pigment absorbs and holds on to the oxygen. Certain types of blood pigment eg,
myoglobin, readily take up oxygen even when the partial pressure of oxygen is low. The
oxygen dissociation curve for myoglobin is well to the left of haemoglobin (see graph).
Myoglobin has a similar chemical structure to haemoglobin and is found in muscles, where
it remains fully saturated with oxygen at partial pressures well below that required for
haemoglobin to give up its oxygen. Myoglobin stores oxygen, releasing it when the partial
pressure of oxygen falls very low, as in the case of severe muscular exertion. Commonly
found in seals and other diving mammals that are liable to suffer from oxygen shortage.
Foetal haemoglobin has an oxygen dissociation curve situated to the left of the adult
haemoglobin. The reason for this is that the foetal blood has to pick up oxygen from the
mother’s blood across the placenta, which can only happen if the foetal haemoglobin has a
higher affinity for oxygen than the mother’s haemoglobin.
Carriage of carbon dioxide
Plasma
CO2
CO2 + H2O
H+
HCO3-
HHb
Hb
Cl-
Tissue Cells
H2CO3
HbO2
O2
O2
Carbon dioxide diffuses from the tissues into red blood cells where it combines with water
to form carbonic acid, H2CO3. This is normally a very slow reaction, but in the red blood it
is greatly accelerated by the presence of the enzyme carbonic anhydrase. Because of
this enzyme, most of the CO2 enters the RBCs rather than remaining in the plasma. The
carbonic acid then dissociates into hydrogen-carbonate and hydrogen ions.
If the hydrogen ions were allowed to accumulate they would increase the acidity of the cell
and kill it. However, they are buffered by the haemoglobin itself. Their presence
encourages the oxyhaemoglobin to dissociate into haemoglobin and oxygen. The
oxygen then diffuses out of the RBC into the tissues. The haemoglobin combines with the
hydrogen ions forming a weak acid, heamoglobinic acid, HHb. The Bohr effect,
therefore, is not due to carbon dioxide as such, but to the hydrogen ions resulting from its
presence.
The carriage of carbon dioxide in this way leads to an accumulation of hydrogencarbonate
ions in the RBC. The plasma membrane of the RBC is highly permeable to these negative
ions and they readily diffuse into the plasma. However, the membrane is relatively
impermeable to positive ions, resulting in a net positive charge inside the cell.
Electroneutrality is maintained by an inward movement of chloride ions from the plasma –
the chloride shift.
Most CO2 is carried in this way, however, not all. Some combines with amino groups in
the haemoglobin molecule to form carbaminohaemoglobin, HbCO2. About 5% of CO2
never enters the RBCs but dissolves in the plasma and is carried in solution.
When the RBCs reach the lungs, the partial pressure of oxygen is high and the partial
pressure of carbon dioxide is low. With this sudden change in the equilibrium conditions,
all the reactions are reversed. This results in oxygen being taken up by RBCs and carbon
dioxide released.