Cardiac Cycle and Phases of the Heartbeat: MDCAT Biology notes

What is the cardiac cycle?

The cardiac cycle is the sequence of events in one complete heartbeat: one contraction (systole) and one relaxation (diastole) of the heart chambers. At a resting rate of about 75 beats per minute, one cycle lasts about 0.8 second:

$$\text{duration}=\frac{60\ \text{s}}{75}=0.8\ \text{s}$$

Three phases of the heartbeat

Phase	Atria	Ventricles	AV valves	Semilunar valves
Atrial systole	Contract, topping up ventricles	Relaxed, filling	Open	Closed
Ventricular systole	Relaxed, filling from veins	Contract	Closed	Open (blood ejected)
Diastole (complete)	Relaxed	Relaxed	Open	Closed

Pressure changes explain the valves

• At the start of ventricular systole, ventricular pressure rises above atrial pressure, so the tricuspid and bicuspid valves close, preventing backflow into the atria.
• When ventricular pressure rises above the pressure in the aorta and pulmonary trunk, the semilunar valves open and blood is ejected.
• When the ventricles relax, their pressure falls below arterial pressure and the semilunar valves close. When it falls below atrial pressure, the AV valves open and the chambers fill passively. During diastole the heart relaxes and fills with blood.

Heart sounds

• "Lub" (first sound): closure of the atrioventricular valves at the start of ventricular systole.
• "Dub" (second sound): closure of the semilunar valves at the start of ventricular diastole.

Conducting system: control of the heartbeat

Cardiac muscle is myogenic: the beat starts within the heart itself.

1. The sinoatrial (SA) node in the wall of the right atrium generates the impulse. It is the pacemaker and sets the rhythm.
2. The impulse spreads over both atria, which contract together.
3. It reaches the atrioventricular (AV) node, which delays the impulse briefly so the atria finish emptying before the ventricles contract.
4. From the AV node it travels down the bundle of His (AV bundle) in the septum and through the Purkinje fibres in the ventricle walls, so the ventricles contract from the apex upward.

The rate set by the SA node can be speeded up by sympathetic nerves and slowed by the vagus (parasympathetic) nerve.

Common MDCAT traps

• Semilunar valves open in ventricular systole; AV valves close in ventricular systole.
• "Lub" is AV valve closure, not semilunar valve closure.
• The AV node delays the impulse; the SA node starts it. AV valves are not part of the conducting system.
• In atrial systole the ventricles are still relaxed.
• One cycle is about 0.8 s, not 0.08 or 8 s.

Quick revision

• Three stages: atrial systole, ventricular systole, diastole.
• Pacemaker: SA node.
• AV node delay lets atria empty first.
• Diastole: chambers relax and fill.
• Cardiac cycle about 0.8 s at 75 beats per minute.