Chemoreceptors and Baroreceptors in Controlling Heart Rate
Laura Armstrong & Joe Wolfensohn
Teachers
Contents
Recall Questions
This topic requires prior knowledge of control of resting heart rate. You can test your knowledge on this below.
Why do the ventricles contract from the apex up?
So blood is forced up and into the arteries (and does not remain in the ventricles)
What is the role of the sympathetic nervous system in heart rate control?
It increases the heart rate.
What is the SAN and what does it do?
The sinoatrial node is the heart’s natural pacemaker; it initiates a wave of electrical activity that causes the atria to contract.
Topic Explainer Video
Check out this @JoeDoesBiology video that explains chemoreceptors and baroreceptors in controlling heart rate or read the full notes below. Once you've gone through the whole note, try out the practice questions!
Introduction to Chemoreceptors and Baroreceptors
- The medulla oblongata in the brain contains a region known as the cardiac centre.
- This part of the brain controls the SAN using two nerves in the autonomic nervous system:
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- Parasympathetic nerve (vagus nerve): slows heart rate.
- Sympathetic nerve (accelerans nerve): increases heart rate.
- At rest, the parasympathetic system is more active, keeping heart rate low.
Response to High CO₂ Levels (via Chemoreceptors)
- Chemoreceptors are receptors found in the walls of the aorta and carotid arteries (in the neck).
- They detect low pH caused by high CO₂ (CO₂ dissolves in the blood plasma which forms carbonic acid).
- Levels of CO₂ are increased by an increased rate of aerobic respiration in muscles cells during exercise.
- When CO₂ levels rise:
- Chemoreceptors send impulses to the cardiac centre in the medulla.
- The medulla then sends a higher frequency of impulses via the sympathetic nerve to the SAN in the right ventricle of the heart.
- This increases heart rate, helping to remove CO₂ faster via the lungs.
- It also helps to deliver more oxygen to respiring muscle cells.
Once CO₂ levels fall, the parasympathetic system slows the heart rate back down.
Response to Changes in Blood Pressure (via Baroreceptors)
- Baroreceptors are receptors also found in the walls of the aorta and carotid arteries.
- They detect pressure in artery walls.
- If blood pressure is too high:
- Baroreceptors send impulses to the cardiac centre in the medulla.
- The medulla then sends a higher frequency of impulses via the parasympathetic nerves to the SAN.
- This slows heart rate and decreases blood pressure.
- If blood pressure is too low:
- The medulla then sends a higher frequency of impulses via the sympathetic nerves to the SAN to increase heart rate and increase blood pressure.
Summary Table of Responses
| Stimulus | Receptor | Nervous Pathways | Effect on Heart |
| ↑ CO₂ (low pH) | Chemoreceptors | Sympathetic | ↑ HR |
| ↓ CO₂ (high pH) | Chemoreceptors | Parasympathetic | ↓ HR |
| ↑ BP | Baroreceptors | Parasympathetic | ↓ HR |
| ↓ BP | Baroreceptors | Sympathetic | ↑ HR |
Key Terms
- SAN (Sinoatrial Node): The pacemaker of the heart that initiates a wave of electrical activity.
- Medulla oblongata: Brain region that regulates heart rate via autonomic nerves.
- Chemoreceptors: Detect changes in blood CO₂ by sensing blood pH.
- Baroreceptors: Detect changes in blood pressure.
- Sympathetic nerve: Increases heart rate (fight or flight).
- Parasympathetic nerve: Decreases heart rate (rest and digest).
- Autonomic nervous system: A part of the peripheral nervous system that controls involuntary functions like heart rate, breathing, and digestion.
No answer provided.
Exam Tip
Be specific with pathways. Don’t just say “heart rate increases.”
Say: “A higher frequency of impulses are sent from the medulla along the sympathetic nerve to the SAN, increasing heart rate.”
No answer provided.
An increase in muscle activity causes an increase in heart rate. Describe and explain how. (4 marks)
1. Increase in CO₂ detected by chemoreceptors. Accept increase in acidity or decrease in pH
2. Send impulses to cardiac centre / medulla. Accept ‘action potentials’ for impulses.
3. More impulses from cardiac centre/medulla along sympathetic pathway/neurones/nerves
4. To the SAN which increases heart rate
Practice Question 1
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!
Practice Question 2
If you want to try out another one, check this video out and see how you do!