Introduction to the Cardiac Cycle

The Cardiac Cycle: Understanding Heart Function Introduction The cardiac cycle is the sequence of mechanical and electrical events that allows your heart to fill with blood, pump it out to the body, and then repeat. This cycle is fundamental to understanding how the cardiovascular system works. During each cycle, the heart must coordinate the movement of blood through four chambers and across four valves—all with precise timing to maintain steady blood flow. The Two Main Phases: Diastole and Systole The cardiac cycle consists of two primary phases: Diastole is the relaxation phase. During diastole, the heart chambers relax, allowing them to fill with blood from the veins. Systole is the contraction phase. During systole, the heart chambers contract, pumping blood out to the lungs and body. At a resting heart rate of 75 beats per minute, one complete cardiac cycle takes approximately 0.8 seconds. Interestingly, diastole occupies roughly two-thirds of this time, while systole takes up roughly one-third. This means your heart spends more time filling than it does pumping—an important principle for maintaining adequate blood supply. The Heart's Valve System Before diving into the detailed events of the cardiac cycle, it's essential to understand the four valves that control blood flow through the heart. Think of these valves as one-way doors that open and close based on pressure differences. Atrioventricular (AV) Valves lie between the atria and ventricles: The tricuspid valve sits on the right side (between right atrium and right ventricle) The mitral valve sits on the left side (between left atrium and left ventricle) These valves open when atrial pressure exceeds ventricular pressure, allowing blood to flow downward from atria to ventricles. They close when ventricular pressure rises during contraction, preventing backflow. Semilunar Valves separate the ventricles from the arteries they pump into: The pulmonary valve on the right (between right ventricle and pulmonary trunk) The aortic valve on the left (between left ventricle and aorta) These valves open when ventricular pressure exceeds arterial pressure, allowing blood to be ejected. They snap shut when pressure reverses, preventing blood from flowing back into the ventricles. Diastole: The Heart Fills Diastole occurs in three sequential phases, shown clearly in the diagram above. Let's walk through each: Atrial Diastole and Ventricular Diastole When the cycle begins, all four heart chambers are relaxed. The atria receive blood passively from the veins: The right atrium fills from the superior and inferior vena cava (bringing deoxygenated blood from the body) The left atrium fills from the pulmonary veins (bringing oxygenated blood from the lungs) As the atria relax, the AV valves open, allowing blood to flow passively from the atria into the relaxed ventricles. This passive filling is called ventricular diastole. During this phase, the semilunar valves remain firmly closed, and roughly 70-80% of ventricular filling occurs naturally due to the pressure difference. Atrial Systole (the "Atrial Kick") At the end of diastole, the atria contract. This atrial contraction, called atrial systole or the "atrial kick," pushes the remaining blood into the ventricles, completing ventricular filling. This active component accounts for about 20-30% of total ventricular filling and becomes increasingly important during exercise when heart rate is elevated. Systole: The Heart Pumps Systole also consists of distinct phases. This is where the heart actually ejects blood: Isovolumetric Contraction Systole begins when the ventricles start to contract in response to electrical signals. Initially, ventricular pressure rises rapidly, but all four valves are closed. This creates a critical moment: the AV valves close because ventricular pressure now exceeds atrial pressure (preventing backflow into the atria), but the semilunar valves haven't opened yet because ventricular pressure hasn't risen above arterial pressure. During this isovolumetric contraction phase, the volume of blood in the ventricles doesn't change—it's simply being squeezed harder and harder, like pressing on a closed syringe. This phase lasts only about 50 milliseconds but is crucial for building pressure. Ventricular Ejection Once ventricular pressure exceeds arterial pressure, the semilunar valves open: The pulmonary valve opens on the right The aortic valve opens on the left Blood is now actively ejected from the ventricles into the arteries. The right ventricle pumps blood to the lungs via the pulmonary artery, while the left ventricle pumps blood to the body via the aorta. This phase is the longest part of systole and is what we typically think of when we imagine the heart "beating." Isovolumetric Relaxation As the ventricles finish contracting, they begin to relax, and ventricular pressure drops rapidly. This sudden pressure drop causes the semilunar valves to snap shut—producing the second heart sound (S2), which you hear as the "dup" in "lub-dup." During isovolumetric relaxation, all valves are again closed (the semilunar valves just closed, and the AV valves haven't opened yet because ventricular pressure is still higher than atrial pressure). Like the contraction phase, the volume doesn't change—the ventricles are simply relaxing while sealed closed. This phase lasts until ventricular pressure drops below atrial pressure, at which point the AV valves open and the cycle returns to diastole. Timing and Heart Rate The duration of the cardiac cycle changes with heart rate. When heart rate increases, both diastole and systole shorten—but diastole shortens proportionally more than systole. This is clinically important: during exercise, when your heart needs to beat faster, the main reduction comes from less filling time, not less pumping time. However, the body compensates through other mechanisms to maintain adequate filling despite the shorter diastolic period. Electrical Events: The ECG Correlates The mechanical events of the cardiac cycle are triggered and coordinated by electrical signals. The electrocardiogram (ECG) records these electrical events: P Wave: Represents atrial depolarization (the electrical signal that initiates atrial contraction). It occurs just before atrial systole begins. QRS Complex: Represents ventricular depolarization. This large, distinctive complex signals the start of ventricular contraction. The "Q" is a small downward deflection, "R" is a large upward spike, and "S" is a small downward deflection after the R wave. T Wave: Represents ventricular repolarization, indicating that the ventricles are relaxing electrically and mechanically. A crucial concept: electrical depolarization precedes mechanical contraction by a brief moment. The electrical signal must travel through the heart tissue before the mechanical squeeze happens. Similarly, electrical repolarization precedes mechanical relaxation. This is why we see the P wave before atrial contraction, the QRS complex before ventricular contraction, and the T wave during ventricular relaxation. Heart Sounds: Listening to the Cycle The two main heart sounds you hear with a stethoscope are directly caused by valve closure: First Heart Sound (S1 or "lub"): Produced by closure of the AV valves (tricuspid and mitral) at the beginning of systole. When ventricular pressure rises above atrial pressure, these valves slam shut with a low-pitched sound. Second Heart Sound (S2 or "dup"): Produced by closure of the semilunar valves (aortic and pulmonary) at the beginning of diastole. When ventricular pressure drops below arterial pressure after systole ends, these valves snap shut with a higher-pitched sound. The sequence "lub-dup, lub-dup" that you hear is essentially the sound of these valves opening and closing in their coordinated sequence. Clinical Application: Cardiac Output Understanding the cardiac cycle provides the foundation for one of the most important calculations in cardiovascular physiology: $$\text{Cardiac Output (CO)} = \text{Stroke Volume (SV)} \times \text{Heart Rate (HR)}$$ Stroke volume is the amount of blood ejected by one ventricle with each contraction (typically about 70 mL at rest). Cardiac output is the total amount of blood pumped by the heart per minute. At rest with a heart rate of 75 beats per minute and a stroke volume of 70 mL, the cardiac output is about 5.25 liters per minute—roughly the total blood volume circulating through your body every minute. Any disease or drug that alters the normal sequence of the cardiac cycle can impair cardiac function. For example: Valve disease can prevent proper opening or closing, disrupting blood flow Abnormal electrical conduction can disrupt the coordination between atrial and ventricular contraction Weakened heart muscle can reduce stroke volume The cardiac cycle provides the framework for understanding these pathological conditions and how treatments might restore normal function.