Cardiac Cycle Systole and Diastole

Understanding the precise sequence of pressure and volume changes during the cardiac cycle is fundamental to grasping how the heart functions as a pump. This knowledge is not merely academic; it is the direct basis for interpreting heart sounds, diagnosing valvular disorders, and understanding the pathophysiology of heart failure. For the MCAT and your medical career, mastering this cycle provides the framework for cardiovascular physiology, linking electrical events on an EKG to the mechanical work of contraction and the flow of blood.

The Phases of the Cardiac Cycle

The cardiac cycle is the complete sequence of events from the beginning of one heartbeat to the beginning of the next. It is traditionally divided into phases based on ventricular activity, but it's crucial to remember that the atria operate on a slightly offset timetable. We will follow one complete cycle for the left ventricle, noting that events occur nearly simultaneously in the right heart, albeit at much lower pressures.

1. Atrial Systole: The "Atrial Kick"

This phase begins with the depolarization of the atria (the P wave on an EKG). Atrial systole is the active contraction of the atrial myocardium. Prior to this, the ventricles are already about 70-80% filled passively from venous return. Atrial contraction provides the final 20-30% of ventricular filling, known as the "atrial kick." This phase ends with the closure of the atrioventricular (AV) valves (mitral and tricuspid) once ventricular pressure exceeds atrial pressure. In conditions like atrial fibrillation, the loss of this coordinated kick can lead to a noticeable decrease in cardiac output.

2. Isovolumetric Ventricular Contraction

This phase marks the beginning of ventricular systole. Triggered by the QRS complex on the EKG, the ventricles begin to contract isometrically—meaning muscle tension increases without a change in muscle length or chamber volume. Why no volume change? Because both the AV valves and the semilunar valves (aortic and pulmonary) are closed. The AV valves closed at the end of atrial systole, and the semilunar valves remain closed because ventricular pressure is still below aortic/pulmonary artery pressure. This is a period of rapidly rising ventricular pressure with no ejection of blood, hence "isovolumetric."

3. Ventricular Ejection

Ventricular ejection begins the moment ventricular pressure surpasses the pressure in the great arteries, forcing the aortic and pulmonary valves open. This phase is subdivided:

• Rapid Ejection: Blood is expelled quickly into the arteries as the ventricle continues to contract forcefully. Ventricular volume decreases rapidly.
• Reduced Ejection: Ventricular contraction wanes, and the rate of ejection slows. Throughout ejection, ventricular pressure first rises slightly above arterial pressure and then begins to fall. The phase ends when ventricular pressure drops below arterial pressure, causing the semilunar valves to snap shut. The volume of blood ejected is the stroke volume, and the blood remaining in the ventricle is the end-systolic volume.

4. Isovolumetric Ventricular Relaxation

This is the diastolic counterpart to isovolumetric contraction. Following the T wave on the EKG (ventricular repolarization), the ventricles relax. Pressure falls precipitously. However, for a brief period, all four heart valves are again closed—the semilunar valves have just closed, and the AV valves are not yet open because ventricular pressure is still higher than atrial pressure. Therefore, ventricular volume does not change; it is "isovolumetric" relaxation. This phase ends when ventricular pressure drops below atrial pressure.

5. Ventricular Filling

This passive phase of diastole is where the majority of ventricular filling occurs. As the AV valves open, blood stored in the atria during systole flows rapidly into the relaxing ventricles (rapid filling). This flow then slows (diastasis), until the next atrial systole completes the cycle. The volume in the ventricle at the end of filling, just before atrial contraction, is the end-diastolic volume.

Heart Sounds: The Audible Landmarks

The closure of heart valves generates vibrations audible as heart sounds. For the MCAT, the two primary sounds are non-negotiable:

• S1 ("lub"): Caused primarily by the closure of the AV valves (mitral and tricuspid) at the onset of ventricular systole. It signals the end of atrial systole and the beginning of isovolumetric contraction. It is best heard at the apex of the heart.
• S2 ("dub"): Caused by the closure of the semilunar valves (aortic and pulmonary) at the onset of ventricular diastole. It marks the end of ventricular ejection and the beginning of isovolumetric relaxation. It is best heard at the base of the heart (second intercostal space).

Understanding the timing of S1 and S2 allows you to mentally map the entire cycle. The time between S1 and S2 is systole; the time between S2 and the next S1 is diastole.

The Pressure-Volume Loop: A Synthesis Tool

The pressure-volume (PV) loop is a powerful graphical model that integrates all the events of the cardiac cycle for one ventricle. It plots ventricular pressure on the y-axis against ventricular volume on the x-axis through one complete cycle. Tracing the loop clockwise:

1. Bottom Segment (Filling): Represents ventricular filling (phase 5 and 1). Volume increases at low pressure (diastolic filling), with a small upward blip from atrial systole.
2. Right Vertical Segment (Isovolumetric Contraction): Phase 2. Volume is constant (vertical line) as pressure rockets upward.
3. Top Segment (Ejection): Phase 3. Pressure first rises then falls as volume decreases, forming a curved, descending line.
4. Left Vertical Segment (Isovolumetric Relaxation): Phase 4. Volume is constant as pressure plummets back to the starting point.

The area inside this loop represents the stroke work performed by the ventricle—the mechanical energy expended to eject the stroke volume. Key volumes are read directly from the x-axis: the rightmost point is the end-diastolic volume (EDV), and the leftmost point is the end-systolic volume (ESV). The width of the loop (EDV - ESV) equals the stroke volume.

Common Pitfalls

1. Confusing Valve Timing with Muscle Contraction: A common error is thinking valves open because muscles command them to. Valves are passive; they open and close solely based on pressure gradients. The AV valves open when atrial pressure > ventricular pressure, and close when ventricular pressure > atrial pressure.
2. Misattributing Heart Sounds: S1 is not the "sound of contraction." It is the sound of AV valve closure, which is a consequence of the pressure rise from contraction. Similarly, S2 is the sound of semilunar valve closure due to falling ventricular pressure, not "the sound of relaxation."
3. Overlooking the "Isovolumetric" Condition: The isovolumetric phases are critical for building pressure gradients. Students often forget that during these short periods, the ventricular volume is truly constant—no blood enters or leaves.
4. Misreading the PV Loop Direction: The loop is always traced clockwise. Starting the cycle at the wrong point (e.g., at ejection) or going counter-clockwise indicates a fundamental misunderstanding of the pressure-volume sequence.

Summary

• The cardiac cycle is a tightly coordinated sequence of pressure changes, valve openings/closures, and volume shifts divided into systole (contraction) and diastole (relaxation).
• Key phases include atrial systole, isovolumetric ventricular contraction, ventricular ejection, isovolumetric relaxation, and passive ventricular filling.
• Heart sounds S1 (AV valves) and S2 (semilunar valves) are essential auditory markers for the transition between systole and diastole.
• The pressure-volume loop provides an integrated visual model of the cycle, with the loop's area representing ventricular stroke work and its width representing stroke volume.
• For the MCAT, always remember: valve function is driven by pressure gradients, not active neural signals. Mastering this pressure-driven narrative is key to excelling in cardiovascular questions.