The CO2 then diffuses out of the red blood cell, through the plasma, moves into the lung gas and
is excreted. The bicarbonate ion that reacts with the Bohr H+ entered the red blood cell via
chloride-bicarbonate exchange. In this manner, the oxygenation of haemoglobin is linked (via the
Bohr H+) to the removal of CO2.
The red blood cell then travels in the circulation, from the lungs, to the systemic tissues.
In the tissues, CO2 (a metabolic waste product) diffuses from the tissues into the plasma and then
into the red blood cell. In the red blood cell CO2 reacts with water (carbonic anhydrase catalysis)
to form an H+ ion and a bicarbonate ion. The H+ ion then binds to haemoglobin causing the
release of oxygen. The oxygen then diffuses out of the red blood cell, into the plasma and then
into the cells (systemic tissues). The bicarbonate that was formed is quickly moved out of the red
blood cell, into the plasma, by a chloride bicarbonate exchanger. The bicarbonate remains in the
plasma until the blood comes to the lungs when once again it moves back into the red cell to
participate in the reactions that lead to CO2 excretion.
The Oxygen Equilibrium (Dissociation) Curve
The oxygen equilibrium curve (also called the oxygen dissociation curve) is used to measure the
extent of blood oxygenation (haemoglobin saturation with oxygen) as well as the maximum
amount of oxygen in the blood and the affinity of haemoglobin for oxygen.
The curve plots the saturation of haemoglobin with oxygen (O2-Hb Saturation) as a function of
the partial pressure of oxygen (PO2) in the arterial blood (mmHg = Torr).
The curve is sigmoidal. It starts our relatively flat at low PO2 then becomes steeper and finally
plateaus at high levels of PO2.
Haemoglobin consists of 4 subunits. The binding of oxygen to one subunit increases the affinity
of the other subunits to bind oxygen. The binding to a second subunit increases the affinity of
binding to a third and binding to the third increases the affinity of binding to the fourth.
The curve can also plot total oxygen content levels (units could be vol %; molO2 per mol Hb,
etc..) as a function of the partial pressure of oxygen in the arterial blood (mmHg = Torr) rather
than plotting O2-Hb saturation on the y-axis.
One of the power point slides illustrates an example of how oxygen-Hb saturation can be 100% in
three different cases but each case has a different level of oxygen content.
The three cases are: 1) A normal person. 2) A case of anaemia in which the number of red blood
cells (and therefore haemoglobin molecules) is reduced compared to a normal person and 3) A
case of polycythaemia in which the number of red blood cells (and Hb molecules) is elevated
compared to a normal person.