9.14: Cardiac Cycle

Atrial Filling

In this segment of the physiological process, both the atria and the ventricles are in a state of relaxation, and all four valves within the heart are closed. The ventricles hold the lowest blood volume, roughly 50-60 mL, termed the end-systolic volume (ESV). During this phase, the right atrium receives deoxygenated blood from the superior and inferior venae cavae as well as blood from the coronary sinus. Similarly, the left atrium welcomes oxygenated blood from the four pulmonary veins. The movement of blood from these veins into the atria hinges on the pressure discrepancies between them.

Ordinarily, the pressure within the atria remains lower than the pressure within the veins that transport blood into these chambers. As blood steadily fills the atria, the pressure within these chambers rises due to the accumulating blood volume. Consequently, this pressure increment prompts the AV valves to open, facilitating the passive filling of blood into the ventricles. Notably, gravity also plays a role in aiding this process. This mechanism of blood flow allows approximately 70 to 80 percent of the blood to fill the ventricles during this phase.

Atrial Systole and Diastole

The progression of depolarization through the atria, followed by the subsequent contraction of atrial muscle fibers, generates an increase in pressure within the atria. This surge in pressure prompts the atria to propel the remaining blood into the ventricles, constituting a phase known as atrial systole. As the atrial systole begins, the ventricles are already filled to about 70 to 80 percent of their capacity, courtesy of the passive inflow of blood. An additional contribution of 20 to 30 percent filling is attributed to atrial contraction, often called the "atrial kick". This phase of atrial systole has a duration of approximately 100 milliseconds. Following atrial systole, the atrial muscle fibers transition back to atrial diastole.

Ventricular Systole

Ventricular systole, as depicted in Figures 18 – 19, ensues after ventricular depolarization and encompasses a duration of 270 milliseconds. Initially, internal pressures within the ventricles rise during the contraction of the ventricular muscles. However, these pressures remain insufficient to prompt the opening of the semilunar valves. The opening of these valves necessitates pressures higher than those in the arteries they guard. Concurrently, the ventricular pressures exceed those in the now-relaxed and diastolic atria. The augmented ventricular pressure causes blood to retrograde towards the atria, instigating the sudden closure of the tricuspid and mitral valves. This swift valve closure results in an audible sound termed the first heart sound, recognized as "lubb."

As the ejection of blood from the ventricles is yet to commence during this initial stage, the blood volume within each chamber remains unaltered with all four valves sealed. This specific blood volume is termed the end-diastolic volume (EDV) or preload, constituting the largest amount of blood within each ventricle during a cardiac cycle, approximately 130 mL. When all valves are closed, and the ventricles hold their maximum blood quantity, the primary ventricular systole phase is designated as isovolumic or isovolumetric contraction (Figure 19).

Figure 18 | ECG Tracing Correlated to the Cardiac Cycle | ³²

This diagram depicts the relationship between an ECG tracing and the electrical and mechanical events during one cardiac cycle. 1) Depolarization begins in SA node. 2) Atrial depolarization spreads from the SA node towards the AV node, 3) PR interval reflects the time the electrical impulse takes to travel from the SA node through the AV node, 4) The QRS complex represents the rapid depolarization of the right and left ventricles 5) The ST segment connects the QRS complex and the T wave; it represents the period when the ventricles are depolarized, 6) The T wave represents the repolarization of the ventricles.

Figure 19 | Phases of the Cardiac Cycle | ³³ The cycle diagram depicts one heartbeat of the continuously repeating cardiac cycle. 1) The cardiac cycle begins when atrial systole begins - Atrial contract forces a small amount of additional blood into relaxed ventricles. 2) Atrial systole ends and atrial diastole begins. 3) Ventricular systole - first phase: Ventricular contraction pushes AV valves closed but does not create enough pressure to open semilunar valves. 4) Ventricular systole second phase: As ventricular pressure rises and exceeds pressure in the arteries, the semilunar valves open and blood is ejected. 5) Ventricular diastole early: As ventricles relax, pressure in ventricles drops; blood flows back against cusps of semilunar valves and forces them closed. Blood flows in the relaxed atria. 6) Ventricular diastole late: All chambers are relaxed. Ventricles fill passively. Correlations to the ECG are highlighted. RA, Right Atrium; LA, Left Atrium; RV, Right Ventricle; LV; Left Ventricle. 

In the subsequent phase of ventricular systole, known as the ventricular ejection phase, the ongoing contraction of ventricular muscle fibers propels the internal pressures in these chambers to surpass those in the pulmonary trunk and aorta. This pressure increase facilitates the opening of the pulmonary and aortic semilunar valves. Notably, the force generated by the left ventricle surpasses that of the right ventricle due to the inherently higher pressure in the aorta. Specifically, the left ventricle generates an approximate pressure of 120 mm Hg, while the right ventricle exerts around 35 mm Hg. Despite this pressure discrepancy, both ventricles release the same amount of blood. This ejected blood volume from each ventricle is termed the stroke volume (SV), commonly falling within the 70 to 80 mL range.

After ejection, each ventricle retains a residual blood volume of 50 to 60 mL, referred to as the ESV. The ejection fraction is the portion of blood expelled from the heart with each contraction. This fraction is determined by dividing the stroke volume by the EDV. Typically ranging from 55 to 70 percent, with an average of 58 percent, ejection fraction quantifies cardiac pumping efficiency.

Ventricular Diastole

Ventricular diastole constitutes the most prolonged cardiac cycle phase, lasting approximately 430 milliseconds. It follows the repolarization of the ventricles as depicted in Figures 19 & 21 and occurs immediately after the blood ejection phase. Diastole is categorized into two distinct stages: early diastole and late diastole. During the initial phase of ventricular diastole, as the ventricular muscle fibers unwind, the internal pressure within the ventricles begins to decline. As this pressure descends below that within the pulmonary trunk and the aorta, a tendency arises for blood to flow backward into the ventricles, leading to a slight dip, termed the dicrotic notch, in the blood pressure waveforms (Figure 22). To prevent this retrograde blood flow, the semilunar valves swiftly close, causing the second heart sound, often described as "dub." Since the AV valves remain shut during this period, no additional blood volume enters the ventricles. Hence, the initial phase of ventricular diastole is termed the isovolumic or isovolumetric ventricular relaxation phase (Figure 19).

During the subsequent stage of ventricular diastole, known as late ventricular diastole, the relaxation of the ventricular myocardium persists, leading to a further decrease in pressure within the ventricles. This pressure eventually falls below the pressure found within the atria. Consequently, blood initiates its flow from the atria into the ventricles, causing the tricuspid and mitral valves to open. Meanwhile, the semilunar valves remain shut (Figure 19). 

A left ventricular pressure-volume loop, often referred to as LV loop, is a graphical representation to illustrate the dynamic changes in pressure and volume within the left ventricle throughout the cardiac cycle (Figure 20). This loop provides valuable insights into the mechanical function of the left ventricle and is particularly useful in assessing cardiac performance and diagnosing various heart conditions.³⁴

Figure 20 | Left Ventricular Pressure-Volume Loop | ³⁵ Left Panel summarizes volume changes during a cardiac cycle in the bottom graph and pressure changes in the left ventricle in the upper illustration.

Right Panel: The horizontal axis of the loop represents ventricular volume in mL. The vertical axis represents left ventricular pressure, usually measured in mmHg. Pressure values start at their lowest during ventricular diastole (relaxation) and increase as the ventricle contracts during systole (contraction). A. Ventricular Filling: The left ventricle continues to relax, and as pressure decreases, it falls below the left atrial pressure. The mitral valve opens, allowing blood to flow from the left atrium into the left ventricle. B. Isovolumetric Contraction: This phase begins at the lower right corner of the loop. During this phase, the ventricle contracts, causing an increase in pressure while the volume remains relatively constant. Both the mitral valve (bicuspid valve) and aortic valve are closed. C. Ejection Phase: The aortic valve opens as left ventricular pressure rises and exceeds aortic pressure. Blood is ejected from the left ventricle into the aorta, leading to a decrease in ventricular volume but a further increase in pressure. This phase forms the ascending portion of the loop. D. Isovolumetric Relaxation: After ejection, the aortic valve closes, marking the onset of ventricular relaxation. During this phase, the ventricular volume remains relatively constant while the pressure decreases (descending portion of the loop). Both the mitral and aortic valves are closed. 

Figure 21 | The relationship between the cardiac cycle and the ECG | ³⁶ This figure illustrates the relationship between the electrical activity of the heart, as recorded by an electrocardiogram (ECG), and the mechanical events of the cardiac cycle, specifically atrial and ventricular systole (contraction) and diastole (relaxation).