6.1.12 Investigating Variables that Affect Heart Rate

Effect of pH and Carbon Dioxide

• High levels of carbon dioxide lower blood pH because CO2 reacts with water to form carbonic acid.
• Chemoreceptors in the carotid arteries, aorta, and medulla oblongata detect this change.
• The chemoreceptors send impulses to the medulla oblongata, specifically to the cardioacceleratory centre.
• The medulla sends more frequent impulses via the sympathetic pathway to the sinoatrial node (SAN).
• This increases the heart rate, pumping blood faster to the lungs to expel CO2 and restore pH levels.

Effect of Blood Pressure

• Baroreceptors in the carotid sinus and aorta monitor blood pressure.
• If blood pressure is too high:
  • Baroreceptors send more frequent impulses to the medulla, activating the cardioinhibitory centre.
  • Impulses are sent via the parasympathetic pathway to the SAN, slowing heart rate and lowering blood pressure.
• If blood pressure is too low:
  • Baroreceptors send fewer impulses, prompting the medulla to stimulate the sympathetic pathway, which increases heart rate.

Key Feedback Mechanisms

1. High CO2 or Low pH → Increased heart rate to remove CO2.
2. High Blood Pressure → Decreased heart rate to lower pressure.
3. Low Blood Pressure → Increased heart rate to restore pressure.

Restoration of Normal Conditions

Once CO2 levels, pH, or blood pressure are back to normal, negative feedback mechanisms reduce the frequency of impulses, and the heart rate returns to its resting level.

Practical Investigation Tip

To measure the effect of variables like CO2 on heart rate:

4. Use a respirometer to adjust CO2 levels in the breathing air.
5. Record heart rate changes with a pulse monitor.
6. Ensure variables like temperature and subject activity remain constant to avoid confounding results.