OCR · A-Level · Biology
Course version: H420 · Version from Sept 2025
(k) the effects of hormones and nervous mechanisms on heart rate
Full specification wording
Plant and animal responses: (k) the effects of hormones and nervous mechanisms on heart rate
Heart rate is set by the sinoatrial node (SAN) in the right atrium, which acts as the pacemaker and fires without nervous input.
Checked against the OCR H420 specification (opens in a new tab) on .
- Point 10 of 12
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Heart rate is set by the sinoatrial node (SAN) in the right atrium, which acts as the pacemaker and fires without nervous input. The autonomic nervous system modulates this intrinsic rhythm: sympathetic nerves release noradrenaline to speed the SAN and increase heart rate, while parasympathetic nerves (vagus) release acetylcholine to slow it. Adrenaline from the adrenal medulla raises heart rate during fight-or-flight. Chemoreceptors detect changes in blood CO₂ and pH, baroreceptors detect blood pressure changes, and the medulla oblongata integrates these inputs and adjusts autonomic outflow. A worked example: during exercise, CO₂ rises, chemoreceptors signal the medulla, sympathetic outflow increases and heart rate rises; at rest, vagal tone dominates and heart rate falls.
What earns the marks
- The SAN in the right atrium is the pacemaker and generates the intrinsic rhythm without nervous stimulation.
- Sympathetic stimulation releases noradrenaline, increasing SAN firing rate and raising heart rate.
- Parasympathetic (vagus) stimulation releases acetylcholine, slowing SAN firing and lowering heart rate.
- Adrenaline from the adrenal medulla increases heart rate as part of the fight-or-flight response.
- Chemoreceptors detect changes in blood CO₂/pH and baroreceptors detect blood pressure; the medulla oblongata integrates these and adjusts autonomic outflow.
Common mistakes and how to put them right
Thinking the heart needs a nerve impulse to beat.
Put it right: The SAN generates its own rhythm, and nerves only modify the rate.
Confusing the transmitters: noradrenaline speeds the heart (sympathetic) while acetylcholine slows it (parasympathetic).
Assuming adrenaline acts directly on the SAN as a neurotransmitter.
Put it right: It is a hormone carried in the blood that also raises heart rate.
Exam tips
- Link each mechanism to a receptor, a pathway and an effect on SAN firing rate so answers are causal, not just descriptive.
- Use a stimulus such as exercise or a rise in blood CO₂ to show how the medulla adjusts autonomic outflow.
- Keep sympathetic and parasympathetic effects clearly separated to avoid contradictory statements.
Try the lesson
The start of the Plant and animal responses: (k) the effects of hormones and nervous mechanisms on heart rate lesson, made from checked teaching notes for this specification point.
Have a go first
Which division of the nervous system increases heart rate during exercise?
Listen
Heart rate is set by the sinoatrial node (SAN) in the right atrium, which acts as the pacemaker and fires without nervous input. The autonomic nervous system modulates this intrinsic rhythm: sympathetic nerves release noradrenaline to speed the SAN and increase heart rate, while parasympathetic nerves (vagus) release acetylcholine to slow it. Adrenaline from the adrenal medulla raises heart rate during fight-or-flight. Chemoreceptors detect changes in blood CO₂ and pH, baroreceptors detect blood pressure changes, and the medulla oblongata integrates these inputs and adjusts autonomic outflow. A worked example: during exercise, CO₂ rises, chemoreceptors signal the medulla, sympathetic outflow increases and heart rate rises; at rest, vagal tone dominates and heart rate falls. A strong answer earns marks for these points. The SAN in the right atrium is the pacemaker and generates the intrinsic rhythm without nervous stimulation. Sympathetic stimulation releases noradrenaline, increasing SAN firing rate and raising heart rate. Parasympathetic (vagus) stimulation releases acetylcholine, slowing SAN firing and lowering heart rate. Adrenaline from the adrenal medulla increases heart rate as part of the fight-or-flight response. Exam tip: Link each mechanism to a receptor, a pathway and an effect on SAN firing rate so answers are causal, not just descriptive.
Key words
Say what each word means before you turn the card.
SAN as intrinsic pacemaker
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Autonomic control of heart rate
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Adrenaline and fight-or-flight
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Receptors and medulla integration
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See every definition
- SAN as intrinsic pacemaker
- The sinoatrial node in the right atrium acts as the pacemaker and fires without nervous input, setting the intrinsic rhythm.
- Autonomic control of heart rate
- The autonomic nervous system modulates the intrinsic rhythm, with sympathetic nerves speeding the SAN and parasympathetic nerves slowing it.
- Adrenaline and fight-or-flight
- Adrenaline from the adrenal medulla raises heart rate during fight-or-flight.
- Receptors and medulla integration
- Chemoreceptors detect changes in blood CO₂ and pH and baroreceptors detect blood pressure changes, and the medulla oblongata integrates these inputs and adjusts autonomic outflow.
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