Cardiovascular System
Atrioventricular node delay
The atrioventricular (AV) node, located at the junction between the atria and the ventricles, introduces a brief delay to the electrical signal, so the atria can finish contracting first.
Remember
The AV node deliberately slows the signal. That gives the atria time to finish emptying their blood into the ventricles, and then the ventricles contract.
Each heartbeat starts at the sinoatrial (SA) node, the heart's natural pacemaker in the wall of the right atrium. Its pacemaker cells depolarise spontaneously (their electrical charge changes by itself), and the electrical impulse that results spreads across the atria and makes them contract. An impulse is an electrical signal that controls contraction of the muscular tissue of the heart.
Between the atria and the ventricles lies a fibrous band of tissue, and the impulse is stopped from crossing it directly. The AV node is the only normal electrical link between the two. There the impulse is deliberately held back for a short time before it moves on to the bundle of His (specialised conducting fibres) and the Purkinje fibres (fine fibres that carry the impulse through the ventricular walls).
The atria first top up the ventricles with blood, and then the ventricles eject it. During the delay the atria finish contracting and emptying, so the ventricles are well filled when ventricular systole (their contraction) begins. Well-filled ventricles eject more blood, which gives a bigger stroke volume (the volume of blood pumped out by a ventricle with each beat). Without the pause, the atria and ventricles would contract at nearly the same moment and the ventricles would be only partly filled.
After the AV node, activation of the ventricles starts at the apex (the pointed lower tip) and spreads upwards, a bit like squeezing toothpaste up from the bottom of its tube. That upward spread depends on the fibres that follow the node (the bundle of His and the Purkinje fibres), while the delay serves the timing between atria and ventricles.
Memory aid
the AV node is a lift operator who waits for the last passengers (the atrial blood) to get in before closing the doors (ventricular contraction)
Think of the AV node as a lift operator who keeps the doors open until the blood from the atria has got in.
Key facts
- Route
- The SA node starts the impulse, which reaches the atria, then the AV node, then the bundle of His and the Purkinje fibres, and then the ventricles.
- Barrier
- A fibrous band separates the atria from the ventricles, so the AV node is the only normal electrical link between them.
- Delay
- The AV node deliberately slows the impulse rather than speeding it up.
- Purpose
- The atria finish contracting and emptying into the ventricles before ventricular contraction begins.
- Benefit
- Because the ventricles are then well filled, each contraction ejects more blood, giving a bigger stroke volume.
- Ventricular spread
- Activation that starts at the apex and spreads upwards depends on the fibres after the AV node, the bundle of His and the Purkinje fibres.
Exam traps
- The AV node does not stop the impulse spreading through the atrial muscle. That spread is what makes the atria contract, and the node acts afterwards, at the junction with the ventricles.
- Contraction from the apex (the pointed lower tip) upwards is a genuine feature, but it depends on the fibres that follow the AV node and not on the delay itself.
- The AV node is the normal route from the atria to the ventricles, and its delay is about timing and not about blocking backward travel.
- The delay comes after the atria have been activated, so it cannot be what gives the ventricles time to fill before the atria contract.
- Spontaneous depolarisation belongs to pacemaker cells such as those of the sinoatrial node. In a healthy heart the AV node passes the impulse on and does not start the ventricles.
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