Control of Heart Rate

Which part of the brain controls heart rate?

• The cardiovascular centre in the medulla oblongata.

Which branch of the autonomic nervous system increases heart rate?

• The sympathetic nervous system.

What initiates the heartbeat?

• The sinoatrial node (SAN), located in the wall of the right atrium.

• It acts as the heart's natural pacemaker, generating electrical impulses.

• This is intrinsic control – the heart muscle itself (myogenic muscle) initiates the contraction

Which part of the brain controls heart rate via the nervous system?

• The medulla oblongata.

• It contains the cardiovascular control centre.

How does the sympathetic nervous system affect heart rate?

• Increases heart rate.

• Accelerator nerves release noradrenaline at the SAN.

• Prepares the body for activity ('fight or flight').

What are baroreceptors and how do they influence heart rate?

• Pressure receptors located in the walls of the aorta and carotid arteries.

• Detect changes in blood pressure.

• If blood pressure is high, they send signals to the medulla oblongata to decrease heart rate (via parasympathetic nerves).

• If blood pressure is low, they send signals to increase heart rate (via sympathetic nerves).

What are chemoreceptors and how do they influence heart rate?

• Chemical receptors located in the aorta, carotid arteries, and the medulla oblongata itself.

• Detect changes in blood pH, carbon dioxide levels, and oxygen levels.

• High CO₂ / low pH (acidic blood, e.g., during exercise) signals the medulla oblongata to increase heart rate (via sympathetic nerves) to speed up CO₂ removal and O₂ delivery.

• Low O₂ also signals an increase in heart rate.

Which hormone primarily controls heart rate?

• Adrenaline

Where is adrenaline released from?

• The adrenal medulla (part of the adrenal glands located above the kidneys).

How does adrenaline affect heart rate?

• Increases heart rate and stroke volume.

• Binds to receptors on the SAN, increasing the frequency of electrical impulses.

• Part of the 'fight or flight' response, preparing the body for intense physical activity.

Summarise the main factors increasing and decreasing heart rate.

• Increase Heart Rate:

  • Sympathetic nervous system (noradrenaline)

  • Adrenaline (hormone)

  • Detected low blood pressure (baroreceptors)

  • Detected high CO₂ / low pH / low O₂ (chemoreceptors)

• Decrease Heart Rate:

  • Parasympathetic nervous system (acetylcholine)

  • Detected high blood pressure (baroreceptors)

Explain the roles of the sympathetic and parasympathetic nervous systems in regulating heart rate during and after exercise. (4 marks)

• During exercise, chemoreceptors detect increased CO₂/low pH, signalling the medulla oblongata (1 mark).

• The medulla increases signals via the sympathetic nervous system.

• Sympathetic nerves release noradrenaline at the SAN, increasing heart rate to meet increased O₂ demand (1 mark).

• After exercise, CO₂/pH levels normalise. The medulla increases signals via the parasympathetic nervous system (vagus nerve).

• Parasympathetic nerves release acetylcholine at the SAN, decreasing heart rate back to resting levels (1 mark).