The cardiac cycle is the repeating sequence of heart chamber contraction and relaxation that occurs during each heartbeat. Its two main ventricular phases are systole, when the ventricles contract and eject blood, and diastole, when they relax and fill with blood for the next contraction.
Each phase involves coordinated changes in pressure, heart valve position, and blood flow. Understanding this sequence connects cardiovascular physiology with human anatomy and explains how the heart maintains circulation through the lungs and the rest of the body.
What Is the Cardiac Cycle?
The cardiac cycle describes all the mechanical events that occur from the beginning of one heartbeat to the beginning of the next. It includes atrial contraction, ventricular contraction, relaxation, valve movements, and changes in blood pressure within the heart.
These mechanical events are coordinated by electrical impulses. The cardiac conduction system initiates and distributes the signals that activate the heart muscle.
The amount of blood pumped depends partly on ventricular filling and contraction strength. Cardiac output and stroke volume explains how the volume ejected with each beat contributes to total blood flow per minute.
Blood returning from the systemic circulation also influences filling. Venous return and valve function describes how blood moves back toward the heart and how venous valves help limit backward flow.
Systole and Diastole: The Two Main Phases
Although both the atria and ventricles undergo contraction and relaxation, the terms systole and diastole commonly refer to ventricular activity when describing the overall cardiac cycle.
| Feature | Ventricular systole | Ventricular diastole |
|---|---|---|
| Main activity | Ventricular contraction | Ventricular relaxation and filling |
| Ventricular pressure | Rises, then falls | Falls and remains relatively low during filling |
| AV valves | Normally closed | Open during ventricular filling |
| Semilunar valves | Open during ejection | Normally closed |
| Blood movement | From ventricles into arteries | From atria into ventricles |
Neither phase is a single event. Each contains shorter stages determined by pressure differences and the timing of valve opening and closure.
Diastole: How the Ventricles Fill With Blood
Ventricular diastole begins when the ventricles relax after ejecting blood. As ventricular pressure falls below the pressure in the aorta and pulmonary artery, the semilunar valves close.
Initially, all four heart valves are closed while ventricular pressure continues to decrease. This brief stage is called isovolumetric relaxation because ventricular volume remains nearly constant.
When ventricular pressure falls below atrial pressure, the atrioventricular valves open. Blood then flows from the atria into the ventricles.
Most ventricular filling occurs passively during early diastole. Filling slows as pressures become more similar, and atrial contraction near the end of diastole adds more blood to the ventricles.
This final contribution is often called the atrial kick. Its importance increases when ventricular relaxation or filling is impaired.
Systole: How the Ventricles Eject Blood
Ventricular systole begins when electrical activation triggers ventricular muscle contraction. Rising ventricular pressure causes the mitral and tricuspid valves to close, preventing blood from moving backward into the atria.
At first, the semilunar valves remain closed. The ventricles develop pressure without changing their blood volume substantially, a stage called isovolumetric contraction.
Once ventricular pressure exceeds pressure in the pulmonary artery or aorta, the corresponding semilunar valve opens and blood is ejected.
- Right ventricle: Pumps blood through the pulmonary valve into the pulmonary artery and toward the lungs.
- Left ventricle: Pumps blood through the aortic valve into the aorta and systemic circulation.
As contraction ends, ventricular pressure falls. When arterial pressure becomes greater than ventricular pressure, the semilunar valves close and the next diastolic phase begins.
The Complete Cardiac Cycle in Sequence
The cycle can be divided into six major stages, beginning with ventricular filling:
- Rapid ventricular filling: The AV valves are open, and blood flows from the atria into the relaxed ventricles.
- Reduced filling and atrial systole: Passive filling slows, followed by atrial contraction that completes ventricular filling.
- Isovolumetric contraction: Ventricular pressure rises while all valves remain closed.
- Ventricular ejection: The aortic and pulmonary valves open, allowing blood to enter the arteries.
- Isovolumetric relaxation: The semilunar valves close, and ventricular pressure falls while all valves remain closed.
- Filling resumes: The AV valves reopen when atrial pressure exceeds ventricular pressure, beginning the next filling phase.
This sequence repeats continuously, although the duration of individual phases changes with heart rate.
How Heart Valves Control Blood Flow
The heart contains four valves that help maintain one-way blood flow. Their leaflets open and close primarily because of pressure differences rather than direct muscular contraction.
- Tricuspid valve: Separates the right atrium from the right ventricle.
- Mitral valve: Separates the left atrium from the left ventricle.
- Pulmonary valve: Controls flow from the right ventricle into the pulmonary artery.
- Aortic valve: Controls flow from the left ventricle into the aorta.
The AV valves close as ventricular pressure rises, while the semilunar valves open when ventricular pressure exceeds arterial pressure. Understanding how the heart pumps blood helps connect these valve movements with circulation through the lungs and body.
Electrical Control of the Cardiac Cycle
The heart’s electrical conduction system coordinates the timing of atrial and ventricular contractions.
The sinoatrial node initiates an impulse that spreads through the atria. The atrioventricular node briefly delays conduction before the impulse travels through the bundle of His, bundle branches, and Purkinje fibers to activate the ventricles.
Electrical events can be recorded using an electrocardiogram. Electrocardiogram basics explains how the P wave, QRS complex, and T wave correspond to different stages of cardiac electrical activity.
These electrical signals precede the mechanical events of contraction and relaxation, allowing coordinated chamber activity.
Pressure Changes During Systole and Diastole
Blood moves through the heart because of pressure differences between chambers and connected blood vessels.
During ventricular filling, atrial pressure exceeds ventricular pressure, allowing the AV valves to remain open. During ventricular ejection, ventricular pressure rises above pressure in the pulmonary artery or aorta, opening the semilunar valves.
These pressure changes also explain the familiar systolic and diastolic blood pressure measurements. Systolic arterial pressure is the peak pressure during ventricular ejection, while diastolic arterial pressure is the lowest arterial pressure between beats.
However, blood pressure measurements taken in an artery are not the same as the pressure inside the ventricles throughout the cardiac cycle.
Heart Sounds: What Causes the Lub-Dub?
The two main heart sounds occur around the times when heart valves close, producing vibrations in the heart and surrounding blood.
- First heart sound (S1): Associated mainly with closure of the mitral and tricuspid valves at the beginning of ventricular systole.
- Second heart sound (S2): Associated mainly with closure of the aortic and pulmonary valves at the end of ventricular ejection.
Clinicians listen to heart sounds to assess cardiac timing and identify possible abnormalities, including certain valve disorders.
A Practical Example: One Heartbeat at Rest
Consider a resting heart rate of 75 beats per minute. At this rate, one complete cardiac cycle lasts approximately 0.8 seconds.
In a commonly used textbook approximation, ventricular systole occupies about 0.3 seconds and ventricular diastole about 0.5 seconds. These durations vary with heart rate and physiological conditions.
If the left ventricle ejects approximately 70 milliliters of blood per beat, multiplying that stroke volume by 75 beats per minute gives a cardiac output of about 5.25 liters per minute.
During exercise, heart rate rises and the cycle becomes shorter. Diastole generally shortens proportionally more than systole, leaving less time for ventricular filling between contractions.
Conditions That Can Disrupt the Cardiac Cycle
Normal cardiac function depends on coordinated electrical activity, healthy valves, adequate filling, and effective ventricular contraction.
- Arrhythmias: Abnormal electrical rhythms can disrupt the timing of atrial and ventricular contractions.
- Valve stenosis: Narrowed valves can restrict forward blood flow and increase pressure demands.
- Valve regurgitation: Incomplete valve closure allows blood to flow backward.
- Heart failure: The ventricles may have difficulty ejecting blood, filling adequately, or both.
- Hypertension: Elevated arterial pressure can increase the workload on the left ventricle.
These conditions can alter pressure relationships, ventricular filling, or stroke volume. Their effects depend on the specific disorder and its severity.
Frequently Asked Questions
What is the difference between systole and diastole?
Systole is the contraction phase in which the ventricles develop pressure and eject blood. Diastole is the relaxation and filling phase in which ventricular pressure falls and blood enters the ventricles.
Do all four heart valves open at the same time?
No. AV valves open during ventricular filling, while semilunar valves open during ventricular ejection. During isovolumetric contraction and relaxation, all four valves are normally closed.
Does the heart rest completely during diastole?
No. During ventricular diastole, the ventricles relax and refill, but blood continues moving through the heart and circulation. The atria also contract near the end of ventricular diastole to contribute to filling.
The cardiac cycle coordinates ventricular filling during diastole with blood ejection during systole through precisely timed changes in pressure and valve position. Electrical activation initiates these mechanical events, while valve closure maintains the direction of blood flow. Together, these processes determine how effectively each heartbeat supplies blood to the lungs and body.
This article is for general education and is not medical advice. Talk to a qualified health professional about any medical concern.
Sources: Guyton and Hall Textbook of Medical Physiology; Braunwald’s Heart Disease: A Textbook of Cardiovascular Medicine; OpenStax Anatomy and Physiology 2e.
