Cardiac Cycle Phases

Why This Matters

The cardiac cycle is the rhythmic sequence of contractions and relaxations that keeps blood flowing through your body every second of your life. Exams focus on more than just "the heart pumps blood." You'll be tested on pressure-volume relationships, valve mechanics, and the precise timing that makes the heart an efficient pump. Understanding why valves open and close when they do, and how pressure gradients drive blood flow, connects directly to concepts like Starling's Law, cardiac output, and clinical conditions like heart murmurs.

Think of the cardiac cycle as a pressure story: blood always moves from high pressure to low pressure, and valves are passive gates that respond to these pressure differences. When you study these phases, focus on what's happening to pressure, which valves are open or closed, and whether volume is changing. Don't just memorize phase names. Know what mechanical principle each phase demonstrates and how it contributes to efficient circulation.

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Systolic Phases: When the Heart Contracts

Systole refers to contraction, the active, energy-requiring phase where the myocardium generates force. The key principle: contraction increases pressure, and pressure gradients determine valve behavior and blood movement.

Atrial Systole

• Completes ventricular filling. The atria contract to "top off" the ventricles, contributing roughly 20–30% of total ventricular volume.
• AV valves are open because atrial pressure exceeds ventricular pressure, allowing blood to flow downward into the ventricles.
• Often called the "atrial kick." This contribution becomes critical during exercise or in patients with heart failure, where passive filling alone isn't enough to adequately load the ventricles.

Isovolumetric Contraction

• All four valves are closed. Ventricular pressure is rising but hasn't yet exceeded arterial pressure, so semilunar valves remain shut. AV valves already closed because ventricular pressure just surpassed atrial pressure.
• Volume stays constant while pressure increases dramatically. This is pure pressure-building with no blood movement.
• The first heart sound (S1, "lub") occurs here as the AV valves snap shut, marking the beginning of ventricular systole.

Rapid Ejection

• Semilunar valves open once ventricular pressure exceeds pressure in the aorta (~80 mmHg on the left side) and pulmonary artery (~10 mmHg on the right side).
• Maximum blood flow rate. Approximately two-thirds of stroke volume is ejected during this brief, powerful phase.
• Ventricular volume drops steeply as blood rushes into the great arteries. This is the heart doing its primary job.

Reduced Ejection

• Ejection continues but slows as the pressure gradient between ventricles and arteries narrows.
• Myocardial contraction is waning. The ventricles are running out of contractile force, and ventricular pressure begins to decline.
• Prepares for valve closure. As ventricular pressure falls below arterial pressure, blood briefly tries to flow backward, catching the semilunar valve cusps and pushing them shut.

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Diastolic Phases: When the Heart Relaxes

Diastole is relaxation, the phase where chambers refill. Relaxation decreases pressure, creating gradients that draw blood into the chambers without requiring active contraction.

Atrial Diastole

• Atria passively fill with blood returning from the pulmonary veins (left atrium) and venae cavae (right atrium).
• AV valves are closed during most of ventricular systole, so blood accumulates in the atria.
• Atrial pressure gradually rises as blood pools, building the pressure gradient that will eventually push the AV valves open once ventricular relaxation drops ventricular pressure low enough.

Isovolumetric Relaxation

• All four valves are closed. Ventricular pressure is falling but hasn't dropped below atrial pressure yet, so AV valves remain shut. Semilunar valves just closed.
• Volume remains constant while pressure plummets. This is the mirror image of isovolumetric contraction.
• The second heart sound (S2, "dub") occurs here as semilunar valves snap shut, preventing arterial backflow. S2 marks the very beginning of this phase.

Rapid Ventricular Filling

• AV valves open when ventricular pressure drops below atrial pressure, and blood rushes in passively.
• Accounts for roughly 70–80% of ventricular filling. Most filling requires no atrial contraction at all; the pressure gradient does the work.
• A third heart sound (S3) may occur in this phase. It's considered normal in children and young adults but can be pathological in older patients (suggesting volume overload or decreased ventricular compliance).

Reduced Ventricular Filling (Diastasis)

• Filling rate slows as the pressure gradient between atria and ventricles equalizes.
• Diastasis is the term for this brief pause where very little blood movement occurs before atrial systole begins the next cycle.
• This phase shortens the most during tachycardia. That's why high heart rates can compromise filling and reduce stroke volume.

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Pressure-Volume Relationships: The Mechanical Story

Understanding how pressure and volume change together reveals why the cardiac cycle works. These relationships are tested through pressure-volume loops and Wiggers diagrams.

Ventricular Systole (Overall)

• Two sub-phases: isovolumetric contraction (pressure builds, no flow) followed by ejection (blood moves out).
• AV valves close at the start (producing S1) when ventricular pressure exceeds atrial pressure.
• End-systolic volume (ESV) is the blood remaining in the ventricles after ejection. A typical left ventricular ESV is about 50–60 mL. ESV is a key variable in calculating ejection fraction: $EF = \frac{EDV - ESV}{EDV} \times 100$

Ventricular Diastole (Overall)

• Two sub-phases: isovolumetric relaxation (pressure drops, no flow) followed by filling (blood enters).
• Semilunar valves close at the start (producing S2) when arterial pressure exceeds ventricular pressure.
• End-diastolic volume (EDV) is the blood in the ventricles after filling is complete. A typical left ventricular EDV is about 120 mL. Increased EDV stretches the myocardium and increases stroke volume via the Frank-Starling mechanism (greater preload → greater contractile force).

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Quick Reference Table

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Self-Check Questions

1. Which two phases share the characteristic of having all four heart valves closed simultaneously, and what distinguishes them from each other?

2. If a patient has a heart murmur heard during ventricular systole, which valves might be malfunctioning, and during which specific phases would you expect to hear it?

3. Compare rapid ventricular filling with atrial systole. How do their contributions to ventricular filling differ, and why does the "atrial kick" become more important during tachycardia?

4. On a pressure-volume loop, which phases correspond to the vertical segments (where volume doesn't change), and what is happening to pressure during each?

5. Why does the second heart sound (S2) occur at the beginning of isovolumetric relaxation rather than at the end of reduced ejection? What pressure relationship triggers semilunar valve closure?