Briefly describe the factors that affect the partial pressure of carbon dioxide in mixed venous blood.
Candidates were expected to provide a definition of important terms such as mixed
venous. Many candidates provided much information about the partial pressure of
carbon dioxide in arterial blood without discussing the factors which alter the mixed
Partial pressure of CO2 in mixed venous blood depends on the CO2 content of the
mixed venous blood, which in turn represents a balance between CO2 production in
the tissues and the CO2 content in arterial blood. Good answers demonstrated an
understanding of this and provided relevant details about these aspects. The partial
pressure of CO2 is related to the CO2 content by the CO2 dissociation curve, the
position of which is determined by the state of oxygenation of haemoglobin, the
Haldane effect. CO2 production is related to aerobic metabolism in cells and total
production is defined by the metabolic rate. Examples of increased and decreased
CO2 production gained additional marks. The partial pressure of CO2 in mixed
venous blood is related to the partial pressure or content of CO2 in arterial blood.
This is determined mainly by alveolar ventilation under the control of chemoreceptors
and the brainstem respiratory centre.
Syllabus: B1h, 2c
Recommended sources: Applied Respiratory Physiology, Nunn 5th edition, Chp 10
pages 222 to 239
VCO2 = CO × k × (PvCO2 - PaCO2)
- VCO2 is the rate of CO2 production,
- CO is the cardiac output,
- PvCO2 - PaCO2 is the arteriovenous CO2 difference, and
- k is a coefficient used to describe the near-linear relationship between CO2 content and partial pressure in the blood.
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