Cardiovascular system: Carbon dioxide.

The idea

Carbon dioxide is a product of aerobic respiration in working muscles and acts as a powerful chemical stimulus for cardiovascular adjustment. As exercise intensity rises, more carbon dioxide enters the blood, and chemoreceptors detect the resulting changes in blood chemistry. Increased carbon dioxide, together with the associated fall in pH, raises sympathetic stimulation and reduces parasympathetic stimulation of the sinoatrial node, so heart rate increases. Carbon dioxide also contributes to local vasodilation in active muscle, increasing blood flow where it is needed, while the sympathetic nervous system redirects flow away from less active areas. A useful method is to follow one molecule: produced in muscle, carried in blood, detected by chemoreceptors, sympathetic activity rises, sinoatrial node fires faster, heart rate and cardiac output increase.

What earns the marks

• Carbon dioxide is produced by aerobic respiration in exercising muscle.
• Rising carbon dioxide in the blood is detected by chemoreceptors.
• Increased carbon dioxide and the associated fall in pH increase sympathetic stimulation of the sinoatrial node.
• The result is an increased heart rate and cardiac output during exercise.
• Carbon dioxide contributes to local vasodilation in active muscle, increasing blood flow to working tissue.
• Vascular shunting directs blood towards active muscle and away from less active areas.

Common mistakes and how to put them right

• Treating carbon dioxide as a fuel for exercise.

  Put it right: It is a waste product of aerobic respiration that acts as a stimulus for cardiovascular adjustment.

• Confusing chemoreceptors with baroreceptors.

  Put it right: Chemoreceptors detect chemical changes such as carbon dioxide and pH, while baroreceptors detect changes in blood pressure.

• Assuming carbon dioxide only affects breathing.

  Put it right: It also drives cardiovascular changes, including increased heart rate and local vasodilation in active muscle.

• Forgetting the direction of the pH change.

  Put it right: Increased carbon dioxide is associated with a fall in pH, not a rise.

Exam tips

• Name the receptor type and the structure it acts on, for example chemoreceptors leading to increased sympathetic stimulation of the sinoatrial node.
• Link the carbon dioxide stimulus to a measurable outcome such as increased heart rate, stroke volume or cardiac output.
• Use the term vasodilation accurately and state where it occurs, for example in active muscle.
• Keep the sequence of cause and effect explicit so the marker can follow the pathway from production to response.