explain the roles of the sinoatrial node, the atrioventricular node and the Purkyne tissue in the cardiac cycle (knowledge of nervous and hormonal control is not expected)

The Heart – Structure, Cardiac Cycle and Electrical Conduction System

1. External and Internal Structure of the Heart

The heart is a muscular pump that works as a closed double‑circulation system.

• Four chambers

  • Right atrium (RA) – receives de‑oxygenated blood from the body.
  • Right ventricle (RV) – pumps this blood to the lungs via the pulmonary artery.
  • Left atrium (LA) – receives oxygen‑rich blood from the lungs.
  • Left ventricle (LV) – pumps blood to the systemic circulation through the aorta.

• Major vessels

  • Superior and inferior vena cava – empty de‑oxygenated blood into the RA.
  • Pulmonary veins (usually four) – bring oxygenated blood from the lungs into the LA.
  • Pulmonary artery – carries de‑oxygenated blood from the RV to the lungs.
  • Aorta – distributes oxygen‑rich blood from the LV to the body.

• Valves

  • Atrioventricular (AV) valves: tricuspid (right) and mitral/bicuspid (left) separate atria from ventricles.
  • Semilunar valves: pulmonary valve (right) and aortic valve (left) guard the ventricular outflows.

• Wall thickness

  • Atrial walls are thin – they only need to push blood into the ventricles.
  • Ventricular walls are thick; the left‑ventricular wall is the thickest because it must generate high systemic pressure, whereas the right‑ventricular wall is thinner for the low‑pressure pulmonary circuit.

2. Double‑Circulation Concept

The heart forms two linked circuits:

• Pulmonary circuit – RA → RV → pulmonary artery → lungs → pulmonary veins → LA.
• Systemic circuit – LA → LV → aorta → body tissues → superior/inferior vena cava → RA.

This arrangement ensures that de‑oxygenated blood is re‑oxygenated in the lungs before being delivered to the rest of the body.

3. The Cardiac Cycle – Mechanical Phases, Pressure Changes and Valve Actions

4. Electrical Conduction System – Structures and Their Roles

Sinoatrial (SA) Node

• Location: Upper wall of the right atrium, near the opening of the superior vena cava.
• Primary role: Natural pacemaker; generates spontaneous depolarisations at 60–100 beats min⁻¹.
• Effect on the cycle: Initiates the impulse that spreads across both atria, producing atrial systole.

Atrioventricular (AV) Node

• Location: Lower part of the interatrial septum, just above the tricuspid valve.
• Primary role: Provides a brief delay (~0.1 s) to allow the atria to finish emptying blood into the ventricles.
• Effect on the cycle: Ensures the AV valves close before ventricular contraction begins, coordinating atrial and ventricular systole.

Bundle of His

• Location: Extends from the AV node down the interventricular septum.
• Primary role: Conducts the impulse from the AV node toward the right and left bundle branches.

Right and Left Bundle Branches

• Location: Run along the interventricular septum toward the apex of the heart.
• Primary role: Transmit the impulse to the respective ventricular walls.

Purkinje Tissue (Fibres)

• Location: Sub‑endocardial layer of the ventricular walls, branching from the bundle branches.
• Primary role: Rapidly distributes the impulse throughout the ventricular myocardium, producing a coordinated and simultaneous ventricular contraction.

5. Sequence of Electrical and Mechanical Events in One Cardiac Cycle

1. SA node depolarises → impulse spreads across atria → atrial systole (blood moves into ventricles).
2. Impulse reaches AV node → ~0.1 s delay.
3. Impulse travels down the Bundle of His → divides into right and left bundle branches.
4. Impulse spreads through Purkinje fibres → rapid ventricular depolarisation → isovolumetric contraction followed by ejection.
5. Ventricular repolarisation → isovolumetric relaxation → ventricular filling.
6. SA node readies the next impulse, and the cycle repeats.

6. Comparison of the Conduction Elements

7. Key Points to Remember

• The SA node is the heart’s natural pacemaker and starts every cardiac cycle.
• The AV node provides a crucial delay, synchronising atrial emptying with ventricular filling.
• The Bundle of His, bundle branches and Purkinje fibres form a fast‑conducting pathway that ensures ventricular contraction is rapid and simultaneous.
• Electrical events are directly linked to mechanical phases; the first “lub” follows AV‑valve closure (end of isovolumetric contraction) and the second “dub” follows semilunar‑valve closure (end of isovolumetric relaxation).