Fundamental Mechanics of the Cardiac Cycle

Introduction to the Cardiac Cycle What Is the Cardiac Cycle? The cardiac cycle is the complete sequence of events from one heartbeat to the next—essentially, all the mechanical activity your heart performs during a single contraction and relaxation period. Understanding this cycle is fundamental to cardiac physiology because it explains how the heart functions as a pump to circulate blood throughout your body. How Heart Rate Affects Cycle Duration The duration of the cardiac cycle is directly tied to your heart rate. The relationship is inverse: as heart rate increases, each cycle becomes shorter. This can be expressed mathematically as: $$\text{Cycle duration} \propto \frac{1}{\text{Heart rate}}$$ For a healthy person at rest with a typical heart rate of 70–75 beats per minute, each cardiac cycle lasts approximately 0.8 seconds. If someone's heart rate increases to 100 beats per minute during exercise, the cycle duration shortens accordingly. The Two Main Periods: Diastole and Systole The cardiac cycle consists of two alternating periods: Diastole is the relaxation phase when the heart muscle relaxes and the chambers fill with blood returning from the body and lungs. Systole is the contraction phase when the heart muscle contracts and actively pumps blood out of the heart into the arteries. These two phases don't occur simultaneously in all parts of the heart—atria and ventricles contract at different times, which is crucial for efficient pumping. This coordinated sequence is what makes the heart an effective pump. The Heart's Structure: Chambers and Valves The Four Chambers The heart has four chambers arranged in two pairs: Right atrium – receives deoxygenated blood from the body Right ventricle – pumps deoxygenated blood to the lungs Left atrium – receives oxygenated blood from the lungs Left ventricle – pumps oxygenated blood to the body The right and left sides operate in parallel, each receiving blood on the atrial side and pumping it out on the ventricular side. The Atrioventricular Valves The atrioventricular (AV) valves sit between the atria and ventricles and open during ventricular diastole (relaxation) to allow blood to flow from atria into ventricles. Mitral valve – between left atrium and left ventricle (also called the bicuspid valve) Tricuspid valve – between right atrium and right ventricle These valves prevent backflow when the ventricles contract. They close when ventricular pressure rises above atrial pressure, preventing blood from flowing backward into the atria. The Semilunar Valves The semilunar valves sit between the ventricles and the major arteries. They open during ventricular systole (contraction) to allow blood ejection: Aortic valve – between left ventricle and aorta (the main artery to the body) Pulmonary valve – between right ventricle and pulmonary artery (which carries blood to the lungs) These valves are pushed open by ventricular pressure and close when arterial pressure exceeds ventricular pressure, again preventing backflow. The Complete Cardiac Cycle: Step by Step To understand how the heart works as a coordinated pump, it's essential to follow the events in sequence. The cycle is often divided into distinct phases based on valve activity and pressure changes. Phase 1: Ventricular Diastole – Early Filling The cycle begins as the ventricles relax and expand. The atrioventricular valves are open, and blood passively flows from the atria into the ventricles. This phase accounts for most of the ventricular filling—about 70% of the blood that will be ejected enters during this passive filling period. Importantly, both atria and ventricles are relaxed during this phase, so blood fills the ventricles simply due to pressure gradients from blood returning from the lungs and body. Phase 2: Atrial Systole and Late Ventricular Diastole Near the end of ventricular diastole, the atria contract (this is atrial systole). This forceful contraction pushes the remaining blood—about 30%—into the ventricles under pressure. This is sometimes called the "topping-off" phase because it completes ventricular filling. The atrioventricular valves remain open during this phase. Understanding that atrial systole actually occurs while the ventricles are still relaxing can be tricky—they don't contract simultaneously. Phase 3: Isovolumic Contraction After atrial systole ends, the ventricles begin to contract vigorously. As ventricular pressure rises rapidly, it exceeds the pressure in the atria, forcing the atrioventricular valves to slam shut. Now here's the key concept: the semilunar valves aren't open yet because ventricular pressure hasn't exceeded arterial pressure. So all four valves are momentarily closed. During this isovolumic contraction phase ("iso-" means "same" and "-volumic" means "volume"), the ventricular volume doesn't change—the muscle contracts but no blood enters or leaves because all valves are closed. This is an important but brief phase that typically lasts only a fraction of a second. Phase 4: Ventricular Ejection Once ventricular pressure rises above the pressure in the aorta (or pulmonary artery on the right side), the semilunar valves are forced open. Blood is now ejected forcefully from the ventricles into the arteries. The left ventricle ejects oxygenated blood into the aorta, which distributes it throughout the body The right ventricle ejects deoxygenated blood into the pulmonary artery, which carries it to the lungs The ventricular volume decreases noticeably during this phase as blood is pumped out. This phase is divided into two parts: rapid ejection (when pressure is highest and lots of blood flows out) and slow ejection (as ventricular pressure falls toward the end of systole). Phase 5: Isovolumic Relaxation After systole, the ventricles begin to relax. Ventricular pressure drops below the pressure in the aorta and pulmonary artery, causing the semilunar valves to snap shut. This valve closure is responsible for the audible "dup" sound of your heartbeat. Again, all four valves are closed momentarily. During isovolumic relaxation, the ventricular volume remains constant because no blood can enter or leave—a second isovolumic phase during which the muscle relaxes but no volume change occurs. This is another "in-between" phase that sets up for the next filling period. Phase 6: Return to Ventricular Diastole As the ventricles continue to relax, ventricular pressure falls below atrial pressure. The atrioventricular valves open again, and the cycle returns to Phase 1 as blood once more flows passively from atria into ventricles. The cycle repeats. Understanding Diastole and Systole as Functional Periods While we've discussed the cardiac cycle in detailed phases, it's also useful to think about diastole and systole as the larger functional periods: Ventricular Systole encompasses isovolumic contraction, ventricular ejection, and the first part of isovolumic relaxation—essentially, all the time when the ventricles are actively contracting or just beginning to relax. Ventricular Diastole encompasses the rest of isovolumic relaxation, passive ventricular filling, and atrial systole—all the time when the ventricles are relaxing and filling with blood. The key insight is that atrial systole occurs during ventricular diastole—the atria and ventricles don't contract at the same time. This overlapping activity ensures efficient blood movement through the heart. The graph shows how pressures in different chambers and the aorta change throughout the cycle, along with volume changes in the ventricles and the electrical activity recorded as an electrocardiogram (ECG). Notice how valve opening and closing corresponds precisely with pressure crossovers—when pressure in one chamber exceeds another, the valve between them opens; when it reverses, the valve closes.