Cardiac Cycle Phases and Events

Understanding the cardiac cycle is not just academic; it is the cornerstone of cardiovascular physiology that you must master for the MCAT and your future clinical practice. This intricate sequence of pressure changes, valve movements, and blood flow dictates how the heart functions as a pump, and disruptions in this cycle underlie common conditions like heart failure, valve disorders, and hypertension. A firm grasp of its phases will enable you to interpret heart sounds, EKGs, and hemodynamic data with confidence.

Defining Systole and Diastole: The Heart's Fundamental Rhythm

The cardiac cycle is divided into two overarching phases: systole and diastole. Systole refers to the period of ventricular contraction and ejection of blood. Diastole is the period of ventricular relaxation and filling. It is critical to remember that these terms specifically describe ventricular activity. Atrial systole and diastole occur separately, timed to optimize ventricular filling. The entire cycle is a continuous, repeating process driven by pressure gradients across the heart's four chambers and the great vessels. For the MCAT, you will often be presented with graphs plotting pressure versus time in the left ventricle, aorta, and left atrium; recognizing which phase corresponds to each segment of these curves is a key testable skill.

A Step-by-Step Walkthrough of Ventricular Systole

Ventricular systole begins with the closure of the atrioventricular (AV) valves—the mitral and tricuspid valves—producing the first heart sound (S1). This moment initiates the phase of isovolumetric contraction. Here, the ventricles contract forcefully, but because all valves (AV and semilunar) are closed, no blood is ejected. Ventricular pressure rises rapidly while volume remains constant. This phase continues until ventricular pressure exceeds the pressure in the aorta (on the left) and pulmonary artery (on the right).

Once that pressure threshold is crossed, the aortic and pulmonary valves open, marking the start of ventricular ejection. Ejection is often subdivided into a rapid ejection phase, where about 70% of the stroke volume is swiftly expelled, followed by a reduced ejection phase. During this time, ventricular pressure peaks and then begins to fall as blood exits. A common MCAT trap is to associate the entire period of ventricular contraction with ejection; remember, isovolumetric contraction is part of systole but involves no ejection.

The Transition to Ventricular Diastole: Relaxation and Filling

Systole ends when ventricular pressure falls below the pressure in the great vessels, causing the aortic and pulmonary valves to snap shut. This closure produces the second heart sound (S2) and begins the phase of isovolumetric relaxation. The ventricles now relax without changing volume, as all valves are again closed. Ventricular pressure drops rapidly until it falls below the pressure in the atria.

This pressure crossover opens the AV valves, initiating ventricular filling. Filling occurs in three stages:

1. Rapid ventricular filling: Blood stored in the atria rushes into the relaxing ventricles due to the pressure gradient. This rapid inflow can sometimes cause a third heart sound (S3), which may be normal in youth but pathological in older adults.
2. Diastasis: This is a period of slow, passive filling where blood moves from the veins through the atria and into the ventricles at a lower rate. Its duration is highly dependent on heart rate; at faster rates, diastasis is shortened or absent.
3. Atrial systole: The atria contract, delivering a final "atrial kick" that contributes about 20-30% of the total ventricular filling volume. This is particularly important for optimizing stroke volume during exercise or in conditions with stiff ventricles.

Pressure-Volume Relationships and the Complete Timeline

The valve events are entirely governed by pressure differences. A useful mnemonic is that valves open and close to prevent backflow, not on a fixed timer. For instance, the mitral valve closes when ventricular pressure exceeds atrial pressure, and the aortic valve opens when ventricular pressure surpasses aortic pressure. In a standard pressure-volume loop for the left ventricle, the four corners correspond to the end of filling (max volume), end of isovolumetric contraction (max pressure at constant volume), end of ejection (min volume), and end of isovolumetric relaxation (min pressure at constant volume).

At a resting heart rate of 75 beats per minute, the entire cardiac cycle lasts approximately 0.8 seconds. Of this, systole occupies about 0.3 seconds, and diastole about 0.5 seconds. As heart rate increases, diastole shortens disproportionately more than systole, which can compromise filling time and, consequently, cardiac output. This is a crucial concept for understanding exercise physiology and diseases like tachycardia.

Clinical Correlations and MCAT Application

For the MCAT, you must be able to apply this knowledge to novel graphs or scenarios. A frequent question type involves identifying a phase based on a snapshot of chamber pressures and valve states. For example, if you see that left ventricular pressure is rising sharply while aortic pressure is steady and higher, you should recognize this as isovolumetric contraction—the valves are closed, so no blood is entering the aorta yet. Another common application is linking cycle events to heart sounds, EKG waves, or jugular venous pulses. Remember that the QRS complex on an EKG signifies the start of ventricular depolarization and thus immediately precedes mechanical systole.

From a clinical perspective, knowing the cycle allows you to pinpoint where pathologies manifest. Aortic stenosis, for instance, prolongs the isovolumetric contraction phase as the ventricle must generate higher pressure to open the stiff valve. Mitral regurgitation causes a systolic murmur because blood flows back into the atrium during ventricular contraction when the AV valve should be sealed.

Common Pitfalls

1. Confusing atrial and ventricular events: Systole and diastole refer specifically to the ventricles. Atrial systole occurs during late ventricular diastole. On an exam, always check which chamber the question is asking about.

Correction: Mentally separate the atrial and ventricular timelines. Ventricular diastole includes the moments when the atria are contracting (atrial systole).

1. Misidentifying isovolumetric phases: Students often think blood is moving during these phases because the heart is active. However, "isovolumetric" means volume is constant; all valves are closed.

Correction: Associate isovolumetric contraction with rising pressure after S1 and before ejection. Associate isovolumetric relaxation with falling pressure after S2 and before filling.

1. Overlooking the effect of heart rate: Assuming the 0.8-second cycle or the proportion of systole to diastole is fixed. In reality, increasing heart rate primarily shortens diastole, which can be a critical factor in disease.

Correction: Remember the adage, "diastole is sacrificed for rate." This has direct implications for coronary perfusion, which occurs primarily during diastole.

1. Forgetting the pressure gradient rule: Valves are passive structures; they do not actively open or close. They respond solely to pressure differences across them.

Correction: Before deciding a valve's state, compare the pressures upstream and downstream. If pressure behind the valve is greater, it will open; if pressure in front is greater, it will close.

Summary

• The cardiac cycle is a repeating sequence of systole (ventricular contraction and ejection) and diastole (ventricular relaxation and filling), lasting about 0.8 seconds at rest.
• Systole includes isovolumetric contraction (all valves closed, pressure rises) and ventricular ejection (semilunar valves open, blood is expelled).
• Diastole includes isovolumetric relaxation (all valves closed, pressure falls) and ventricular filling, which occurs in three subphases: rapid filling, diastasis, and atrial systole.
• Valve openings and closings are passive events driven entirely by pressure gradients between heart chambers and great vessels, producing the characteristic heart sounds.
• The timing of the cycle is dynamic; increased heart rate disproportionately reduces diastolic filling time, which is a key consideration in physiology and pathology.
• Mastery of pressure-volume relationships and the sequence of events is essential for interpreting clinical data and succeeding on MCAT biology and biochemistry sections.