The Cardiac Cycle: Pressure, Valves and Ventricular Volume

To understand the cardiac cycle, compare pressures before naming phases. A valve's position and the direction of flow follow the pressure difference across it. This guide uses a simplified left-heart model with normally functioning valves, suitable for undergraduate revision and postgraduate review.

Updated 1 November 2026.

Learning objectives

• Infer mitral and aortic valve states from pressure comparisons.
• Distinguish pressure changes from ventricular volume changes.
• Explain the two isovolumetric phases and place the first two heart sounds.
• Calculate stroke volume from end-diastolic and end-systolic volumes.

1. Start with two pressure comparisons

The mitral valve lies between the left atrium and left ventricle. It permits filling when atrial pressure exceeds ventricular pressure and closes as ventricular pressure overtakes atrial pressure. The aortic valve separates the left ventricle from the aorta; it opens for ejection when ventricular pressure exceeds aortic pressure.

Use the same reasoning on the right side, substituting the tricuspid valve, pulmonary valve and their neighbouring chambers and vessel. The pressure values differ between the two sides.

2. Four phases in a normal left-heart model

Atrial contraction contributes to the final part of ventricular filling. Ventricular systole includes both isovolumetric contraction and ejection; ventricular diastole includes isovolumetric relaxation and filling.

3. Why pressure can change while volume stays constant

After the mitral valve closes, the contracting ventricle must develop enough pressure to open the aortic valve. During that interval, both valves are closed: pressure rises while chamber volume remains constant. Isovolumetric describes the chamber's blood volume; it does not mean that every muscle fibre keeps exactly the same length.

After ejection ends and the aortic valve closes, ventricular relaxation lowers pressure. Filling cannot start until ventricular pressure falls sufficiently below atrial pressure to permit mitral opening. Both valves are again closed during this intervening interval, now at end-systolic volume.

4. Worked pressure exercise

Consider three snapshots in an illustrative normal left-heart cycle. All pressures are in mmHg. These are teaching values, not patient measurements or universal reference ranges.

1. Left atrium 10, left ventricle 40, aorta 80; ventricular pressure rising. Ventricular pressure exceeds atrial pressure but remains below aortic pressure. Both valves are closed: isovolumetric contraction.
2. Left atrium 10, left ventricle 100, aorta 90. The ventricular-to-aortic gradient permits aortic opening and ejection; the mitral valve remains closed.
3. Left atrium 10, left ventricle 20, aorta 80; ventricular pressure falling after ejection. Both valves are closed: isovolumetric relaxation. The trend and preceding event distinguish it from the first snapshot.

Study method: cover the phase names, compare the two pressure differences, then state what happens to ventricular volume. A pressure value alone may not identify the phase without its direction of change and position in the cycle.

5. Link the sequence to sounds and a pressure–volume loop

S1 is associated with atrioventricular valve closure near the start of ventricular systole; S2 is associated with semilunar valve closure at its end. An ECG records electrical activity, so its waves should be related to, rather than treated as identical to, mechanical events.

On a left ventricular pressure–volume loop, the horizontal axis is volume and the vertical axis is pressure. Isovolumetric segments are vertical because volume is constant. Loop width represents stroke volume: end-diastolic volume minus end-systolic volume.

Calculation: if end-diastolic volume is 125 mL and end-systolic volume is 55 mL, stroke volume is 125 − 55 = 70 mL per beat. The ventricle has not emptied completely.

6. Common misconceptions

• “Contraction immediately ejects blood.” First, pressure rises with both valves closed.
• “Relaxation immediately fills the ventricle.” An isovolumetric interval comes before mitral opening.
• “Isovolumetric means constant pressure.” It means constant chamber volume.
• “Papillary muscles pull the mitral valve open.” Their tension helps prevent leaflet prolapse during systole; opening follows the pressure relationship.

7. Revision questions with answers

1. Which valves are open during isovolumetric contraction? Neither the mitral nor aortic valve in this normal model.
2. What ends isovolumetric relaxation? Mitral opening as ventricular pressure falls below atrial pressure.
3. Why can a closed-valve ventricle change pressure? Its muscle is developing tension or relaxing while the enclosed blood volume stays constant.
4. Does a larger stroke volume necessarily mean a higher heart rate? No. Stroke volume is volume per beat; heart rate is beats per minute.

8. Choose your next resource

For focused reading, view The Cardiac Cycle PDF chapter. For broader cardiovascular study, compare the contents of Cardiovascular Physiology: A Clinically Integrated Approach. These are separate paid digital resources; check each listing for its scope and current price.

To try SelfHug's teaching style on another foundational topic, explore the free Homeostasis sample. It covers homeostasis, rather than the cardiac cycle. Find other free guides in the physiology resource hub.

References and further reading

• OpenStax, Anatomy and Physiology 2e, section 19.3: Cardiac Cycle — phase sequence and electrical–mechanical relationships.
• Richard E. Klabunde, Cardiovascular Physiology Concepts: Isovolumetric Contraction — pressure development, valve closure and papillary muscles.
• Klabunde: Isovolumetric Relaxation — pressure decline and the closed-valve interval.
• Klabunde: Ventricular Pressure–Volume Relationship — loop phases and stroke volume.

These original revision notes use a normal-valve model. Match the depth and terminology to your course textbook and teaching.