The roles and locations of chemoreceptors and pressure receptors and the…

The idea

Heart rate is adjusted by the cardiovascular centre in the medulla oblongata. Chemoreceptors in the aortic and carotid bodies detect a rise in blood carbon dioxide, which lowers blood pH by raising hydrogen ion concentration, and send impulses to the centre. The centre then sends more impulses along sympathetic neurones to the SAN, raising heart rate so blood flows faster and carbon dioxide is removed at the lungs more quickly. Pressure receptors, the baroreceptors in the aortic arch and carotid sinus, detect blood pressure instead: high pressure produces more impulses along the parasympathetic or vagus neurones to the SAN, slowing the heart, while low pressure produces more sympathetic activity and speeds it up. Both pathways are autonomic, so involuntary, and both converge on the same effector, the SAN, which is why the two branches act antagonistically.

What earns the marks

• An increase in carbon dioxide concentration, or a fall in pH, is detected by chemoreceptors in the aortic and carotid bodies.
• Impulses are sent to the cardiac or cardiovascular centre in the medulla oblongata.
• More impulses along the sympathetic neurones, or fewer impulses along the parasympathetic or vagus neurones, change the frequency of impulses reaching the SAN.
• The impulses reach the SAN as the effector, which sets the heart rate.
• For a slowing response, mirror the chain exactly: receptor, medulla, parasympathetic or vagus, SAN.

Common mistakes and how to put them right

• Saying receptors detect a fall in oxygen, when the credited stimulus is a rise in carbon dioxide or a fall in pH.

• Writing about 'an impulse' or 'one impulse' when describing sympathetic or parasympathetic changes.

  Put it right: The change is in the frequency of impulses, not the presence or absence of a single impulse.

• Using 'signals' or 'messages' in place of 'impulses' or 'action potentials'.

• Describing a hormonal route through adrenaline when the question asks for the nervous route through the medulla.

• Sending the sympathetic neurones to the AVN or to the ventricle walls rather than to the SAN.

Exam tips

• Write 'more impulses along the sympathetic' or 'fewer along the parasympathetic'; the mark is on frequency, not on the nerve firing.
• Always finish the chain at the SAN, because it is a separate mark and is the one most often left out.
• For a slowing response, mirror the chain exactly: receptor, medulla, parasympathetic or vagus, SAN.