The big idea: Your heart does not wait for a message from your brain to beat.
A small patch of special muscle called the sinoatrial (SA) node starts every heartbeat on its own. Because the beat begins inside the heart muscle itself, we say the heart is myogenic.
The SA node is the heart's natural pacemaker — it sets the rhythm that all the heart muscle follows.
The heart's four chambers. The SA node — the heart's natural pacemaker — sits in the wall of the RIGHT ATRIUM; each beat begins here and the wave of contraction spreads across the atria first, then down to the ventricles.
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- Myogenic
- The heartbeat starts within the heart muscle itself, not from a nerve impulse sent by the brain.
- Sinoatrial (SA) node
- A patch of special muscle in the wall of the right atrium that starts each heartbeat. It is the heart's natural pacemaker.
- Pacemaker
- The structure that sets the rhythm of the heartbeat. In a healthy heart this is the SA node.
- Atrioventricular (AV) node
- A patch of tissue at the border of the atria and ventricles that briefly delays the impulse, then passes it on to the ventricles.
- Cardiac muscle
- The special muscle the heart wall is made of. Its cells are joined so an electrical impulse passes quickly from cell to cell.
Pacemaker = rhythm-setter: The SA node does not pump blood itself — it just fires the signal that tells the muscle when to contract.
Think of it as the drummer keeping time for the whole band: it sets the beat, and every muscle cell follows.
A heartbeat must be coordinated: the atria need to contract first to fill the ventricles, and only then should the ventricles contract to push blood out.
The way the electrical impulse spreads makes this happen in the right order.
The route of the impulse: 1. The SA node fires an electrical impulse.
2. The impulse spreads across both atria, so they contract and push blood down into the ventricles.
3. The impulse reaches the AV node, which delays it for a fraction of a second.
4. That delay lets the atria finish emptying before the ventricles contract.
5. The impulse then travels down and up through the ventricle walls, so both ventricles contract together and push blood out into the arteries.
| Step | Where | What happens |
|---|---|---|
| 1. SA node fires | Wall of the right atrium | The sinoatrial (SA) node sets off an electrical impulse on its own (the heart is myogenic). |
| 2. Atria contract | Across both atria | The impulse spreads through the atrial walls, so both atria contract and push blood into the ventricles. |
| 3. AV node delays | Border of atria and ventricles | The atrioventricular (AV) node receives the impulse and holds it for a fraction of a second. |
| 4. Impulse travels down | Down the centre wall, then up the ventricle walls | Conducting fibres carry the impulse to the base of the ventricles. |
| 5. Ventricles contract | Across both ventricles | Both ventricles contract from the bottom up, pushing blood out into the arteries. |
Why the AV node delay matters: Without the AV node's brief delay, the ventricles would try to contract at the same time as the atria — before they had filled with blood.
The delay keeps the two halves of the cardiac cycle separate and in order: atria first, ventricles a moment later. This is why the AV node is essential to a coordinated beat.
Cardiac muscle helps the signal travel: Cardiac-muscle cells are joined end-to-end by intercalated discs that contain gap junctions — tiny channels between neighbouring cells.
These let the electrical impulse pass directly and quickly from one cell to the next, so a whole region of the heart contracts almost as one. This structural feature is what aids conduction of the impulse through the heart wall.
Speeding it up and slowing it down: The SA node sets a resting rhythm by itself, but the body can change the heart rate to match what you are doing.
Two nerves from the brain's medulla reach the SA node: one speeds it up (during exercise or stress) and one slows it down (during rest).
The hormone adrenaline, released into the blood, also reaches the SA node and speeds it up — this is why your heart races when you are frightened or excited.
| Way heart rate is changed | Signal | Effect on the SA node |
|---|---|---|
| Sympathetic nerve | Nerve impulses from the medulla (brain) | Speeds the SA node up → faster heart rate |
| Parasympathetic nerve | Nerve impulses from the medulla (brain) | Slows the SA node down → slower heart rate |
| Adrenaline (hormone) | Released into the blood by the adrenal glands | Speeds the SA node up → faster heart rate |
Raises heart rate
- Sympathetic nerve impulses from the medulla
- The hormone adrenaline in the blood
- Happens during exercise, stress or fright
- Delivers oxygen and glucose to muscles faster
Lowers heart rate
- Parasympathetic nerve impulses from the medulla
- Less adrenaline in the blood
- Happens during rest and recovery
- Saves energy when high output is not needed
A memory hook: SA node = Starts the beat (Above, in the atria). AV node = After, passing the signal from Atria to Ventricles (with a delay).
And: adrenaline always speeds the heart up — it never slows it down.
When the natural pacemaker fails: If a person's SA node is faulty and the heart beats too slowly or irregularly, doctors can fit an artificial pacemaker — a small device implanted near the heart.
It sends out regular electrical impulses that take over from the SA node and keep the heartbeat at a normal, steady rhythm.
| Feature | Natural pacemaker (SA node) | Artificial pacemaker (implant) |
|---|---|---|
| What it is | A patch of special muscle in the right-atrium wall | A small electronic device implanted near the heart |
| Job | Starts each heartbeat and sets the rhythm | Sends regular electrical impulses to keep a normal rhythm |
| When it's needed | Works in a healthy heart | Used when the SA node is faulty / the rhythm is too slow or irregular |
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How this is tested: On Paper 3 a 4-mark Explain question is a favourite: explain how the signal from the SA node spreads to coordinate the heartbeat — score the SA node firing, the atria contracting, the AV node delay, and then the ventricles contracting.
A 2-mark Explain question often asks for the role of the AV node (it delays the impulse so the atria empty before the ventricles contract).
On Paper 1A you may be asked to identify what always raises heart rate (adrenaline), or — using an ECG-style trace — to state what the heart is doing during a labelled wave.
IB-style question — how the SA node coordinates the beat
Explain how an electrical signal from the sinoatrial node spreads through the heart to coordinate a single heartbeat. [4]
How to score all four marks
- The SA node starts it. The SA node (in the right-atrium wall) fires an electrical impulse on its own — the heart is myogenic.
- The atria contract. The impulse spreads across both atria, making them contract and push blood into the ventricles.
- The AV node delays the impulse. The signal reaches the AV node, which delays it briefly so the atria can finish emptying before the ventricles contract.
- The ventricles contract. The impulse then travels through the ventricle walls, so both ventricles contract together and push blood out into the arteries. (Award 1 mark per distinct point, up to 4.)
Final answer
The SA node fires an impulse; it spreads across the atria so they contract; the AV node delays it; then the impulse reaches the ventricles so they contract a moment later — giving a coordinated beat.
✓ Why this scores full marks: Each step is a separate, ordered event — SA node fires, atria contract, AV node delays, ventricles contract.
A 4-mark Explain needs the sequence in the right order, not just naming the SA and AV nodes.
Where it all happens: the SA node fires in the right-atrium wall, the AV node relays the signal at the atria-to-ventricle border, and the impulse then sweeps down through both ventricle walls so they contract together.
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