7th February 2024

Summary

Normal curve

Image Source: Oregon State Education (2017). 22-5-transport-of-gases, Licensed under a creative commons licence. Available for free at: https://open.oregonstate.education/aandp/chapter/22-5-transport-of-gases/(Accessed: 9 January 2024).

Things to note:

• Arterial blood:  SpO2 is 100%, PaO2 is 13.3kPa
• Mixed venous blood: SpO2 is 75%, PaO2 is 5.3kPa
• P50: SpO2 at 50%, PaO2 is 3.5kPa

P50 is the partial pressure of oxygen when SpO2 is at 50%. This is used to compare the oxygen-haemoglobin dissociation curve in different situations. We will discuss this next.

Shifting the curve to the right

When the oxygen-haemoglobin dissociation curve is shifted to the right, the affinity of haemoglobin to oxygen is reduced. Oxyhaemoglobin has a higher tendency to unload oxygen.

Factors that shift the oxygen-haemoglobin dissociation curve to the right include:

• Increased 2-3 diphosphoglycerate (produced during metabolism)
• Acidosis
• Increased pCO2
• Increased temperature

Shifting curve to the left

When the oxygen-haemoglobin dissociation curve is shifted to the left, the affinity of haemoglobin to oxygen is increased.

Factors that shift the oxygen-haemoglobin dissociation curve to the left include:

• Hypothermia
• Alkalosis
• Decrease pCO2.
• Foetal Hb
• MetHb
• COHb

Image Source: Wikimedia Commons (2006). Oxyhaemoglobin dissociation curve, Licensed under a creative commons licence. https://upload.wikimedia.org/wikipedia/commons/8/8a/Oxyhaemoglobin_dissociation_curve.png  (Accessed: 4 January 2024).

Bohr effect

The Bohr effect describes the shift in the position of the oxygen-haemoglobin dissociation curve at different partial pressures of CO2. The increased partial pressure of CO2 shifts the oxygen-haemoglobin dissociation curve to the right and increases the unloading of O2 by oxyhaemoglobin. This is attributed to the effect of changes in pCO2 to H+ concentration.

Haldane effect

The Haldane effect describes the increased ability of deoxyhaemoglobin to carry CO2 compared to oxyhaemoglobin.