Oxygen-Haemoglobin Dissociation Curve
Exercise changes to gaseous exchange
What changes take place during respiration?
Both external and internal respiration increase during exercise in order to
increase the supply of O2 to the working muscles, but to understand how, we
need to have a basic understanding of an oxygen-haemoglobin dissociation
curve.
Oxygen-haemoglobin dissociation curve
An oxygen-haemoglobin dissociation curve informs us of the amount of
haemoglobin saturated with oxygen. Haemoglobin that is fully bound or
loaded with oxygen is termed saturated or associated, whereas oxygen
unloading from haemoglobin is called dissociation.
Where do we need the association and dissociation of oxygen and
haemoglobin to take place in order to maintain an efficient supply of oxygen to
the working muscles during exercise?
 we need association to happen in the lungs so that blood can carry the
oxygen to the muscle capillaries where…..
 …..it can dissociate and unload the oxygen to the muscle tissue to provide
energy for work
When seen in the form of a graph, the further to the right that the graph line is
the greater the dissociation of oxygen. Hence, more oxygen is unloaded from
the haemoglobin in the blood to the muscle tissue. (p98/99)
External Respiration
During exercise, working muscles need more O2 and produce more CO2.
Hence venous blood returning to the lungs has a higher partial pressure (PP)
of CO2 and lower partial pressure (PP) of O2. In contrast alveolar air has a
higher PP of O2 and a lower PP of CO2.
Oxygen-Haemoglobin Dissociation Curve
This has the effect of increasing the diffusion gradient for both O2 and CO2
between the alveoli-capillary membrane resulting in both a quicker and
greater amount of gaseous exchange.
The high PP of O2 in the alveoli and the low PP of O2 in the capillaries
ensures haemoglobin is almost fully saturated with O2.
The O2 and CO2 will diffuse across until the partial pressures are equal,
hence, the greater the diffusion gradient the greater the amount of O2 and
CO2 exchanged.
Internal respiration
A greater O2 dissociation in the muscle tissue during exercise is required in
order to increase the supply of O2 to the working muscles.
Four factors all have the effect of shifting the dissociation curve to the right (ie
increasing the dissociation of O2 from haemoglobin in the blood capillaries to
the muscle tissue).
1. increase in blood and muscle temperature
2. decrease in partial pressure of oxygen within muscle increasing the O2
diffusion gradient
3. increase in partial pressure of CO2 increasing the CO2 diffusion
gradient
4. Bohr effect – increase in acidity (lower pH)
All of these factors increase during exercise. The effect is that the working
muscles:
 Generate more heat when working
 Use more O2 to provide energy, lowering the PP of O2
 Produce greater amounts of CO2 as a waste product
 Increase lactic acid levels – increases acidity – lowers the pH
These four factors increase the dissociation of O2 from haemoglobin, thus
increasing the supply of O2 to the working muscles … delaying fatigue …
increasing the possible duration of performance.