Cambridge Notes, Past Papers, Revision Questions

Cambridge A-Level Biology 9700 – The Heart: Cardiac Cycle

The Heart

Objective

Describe the cardiac cycle, with reference to the relationship between blood pressure changes during systole and diastole and the opening and closing of the heart valves.

Overview of the Cardiac Cycle

The cardiac cycle consists of a repeating sequence of mechanical and electrical events that result in the pumping of blood. It can be divided into two main phases:

Systole – contraction of the ventricles, causing ejection of blood into the arteries.

Diastole – relaxation of the ventricles, allowing filling of the chambers from the atria.

Each phase is associated with characteristic changes in pressure within the chambers and major vessels, which in turn control the opening and closing of the atrioventricular (AV) and semilunar valves.

Pressure Changes and \cdot alve Dynamics

During the cycle, pressure gradients drive blood flow. A valve opens when the pressure behind it exceeds the pressure ahead of it, and closes when the reverse gradient develops.

Key pressure relationships:

When \$P{\text{ventricle}} > P{\text{atrium}}\$, the A \cdot valves (tricuspid and mitral) close.

When \$P{\text{ventricle}} > P{\text{aorta}}\$ (or \$P_{\text{pulmonary artery}}\$), the semilunar valves (aortic and pulmonary) open.

When \$P{\text{ventricle}} < P{\text{aorta}}\$ (or \$P_{\text{pulmonary artery}}\$), the semilunar valves close.

When \$P{\text{ventricle}} < P{\text{atrium}}\$, the A \cdot valves open.

The pressure gradient that drives flow can be expressed as:

\$\Delta P = P{\text{upstream}} - P{\text{downstream}}\$

Detailed Sequence of Events

Phase	Ventricular Pressure (mmHg)	Aortic / Pulmonary Pressure (mmHg)	A \cdot Valves	Semilunar \cdot alves
Isovolumetric Contraction	Rises rapidly from \overline{5} to 80	\overline{80} (aorta) / \overline{15} (pulmonary)	Closed (pressure > atrium)	Closed (pressure < aorta)
Ejection (Rapid)	80–120 (left) / 25–30 (right)	80–120 (aorta) / 15–25 (pulmonary)	Closed	Open (ventricular pressure > arterial pressure)
Ejection (Reduced)	120 → 80 (left) / 30 → 25 (right)	120 → 80 (aorta) / 25 → 15 (pulmonary)	Closed	Open, then begins to close as pressures equalise
Isovolumetric Relaxation	Falls rapidly from \overline{80} to 5	\overline{80} (aorta) / \overline{15} (pulmonary)	Closed (ventricular pressure still > atrial)	Closed (pressure < arterial)
Ventricular Filling (Rapid)	\overline{5}–12	\overline{80} (aorta) / \overline{15} (pulmonary)	Open (atrial pressure > ventricular)	Closed
Ventricular Filling (Diastasis)	\overline{5}–12	\overline{80} (aorta) / \overline{15} (pulmonary)	Open	Closed

Key Points to Remember

The heart sounds “lub” and “dub” correspond to the closing of the A \cdot valves and semilunar valves respectively.

Systolic blood pressure is measured when the semilunar valves are open and the ventricles are contracting.

Diastolic blood pressure is measured when the ventricles are relaxed and the semilunar valves are closed.

Any disruption to the normal pressure gradients (e.g., valve stenosis) alters the timing of valve movements and can impair cardiac output.

Suggested diagram: Schematic of the heart showing the sequence of valve opening/closing during the cardiac cycle, with pressure curves for the left ventricle and aorta.