Gas Transport and Oxygen - Haemoglobin binding summary
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Gas Transport and Oxygen – Haemoglobin Binding
Haemoglobin consists of 4 subunits – 2 alpha subunits and 2 beta subunits. Each of these
subunits consists of a globin molecule (alpha globin or beta globin) plus a heme group. The
“heart” of the heme group is an iron (Fe) molecule that is the site of oxygen binding. The vast
majority of the oxygen in blood is bound to haemoglobin. Only about 1.5% of the oxygen in
blood is in the dissolved form within plasma. 98.5% is bound to haemoglobin. Haemoglobin only
exists in red blood cells. It is not found in the plasma or any other tissues.
Carbonic anhydrase is an enzyme that catalyses the reversible CO2 hydration/dehydration
reaction. It is found within the red blood cells.
H+ + HCO3- H20 + CO2
This reaction goes both ways depending upon the relative concentrations of the constituents on
Sometimes you see the intermediary carbonic acid (H2CO3) appear in the equation. However,
carbonic acid is so short-lived that it is impossible to ever quantify it. It is usually left out of the
H+ + HCO3- H2CO3 H20 + CO2
Oxygen and Carbon Dioxide Transport in the Blood
Deoxygenated haemoglobin has a H+ ion (proton) bound to it. This is called a Bohr proton. When
oxygen bind to haemoglobin (Hb) the H+ ion is released into solution.
O2 + H+Hb HbO2 + H+
The H+ ion in the above equation links that equation to the CO2 hydration/dehydration reaction. It
also means that oxygen transport in the blood is intimately linked to CO2 transport. O2 transport
affects CO2 transport and CO2 transport affects O2 transport.
H+ + HCO3- H20 + CO2
In the lungs, oxygen diffuses from the lung gas into the plasma and then into the red blood cells.
In the red blood cell, oxygen binds to deoxygenated haemoglobin. This causes haemoglobin to
become oxygenated but it also causes the release of a H+ ion (Bohr proton) from haemoglobin.
This H+ ion reacts with a bicarbonate ion (carbonic anhydrase catalysis) to form water and CO2.
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.