Cardiac Cycle Definition
A cardiac cycle is defined as the events that take place for pumping deoxygenated blood in the lungs and oxygenated blood to the body with through the heart. Diastole and systole are the major events of the cardiac cycle.
Diastole is relaxation during which the heart’s right and left chamber (atrial and ventricular) is filled with blood and systole is compression during which the heart chamber is pumped out blood into). Atrial systole followed by ventricular systole, atrial diastole, and ventricular diastole, are the phases of cardiac cycle.
Heart Anatomy Labeled Diagram
The heart has 4 chambers divided into two atria (left and right) and two ventricles (left and right). The left side receives blood from the lungs through the pulmonary vein and sends it to the body through the aorta (systemic) and the Right side receives blood through the vena cava and sends it to the lung via the pulmonary artery. The cardiac muscle contraction results from the electrochemical stimulus in the myocardium.
Valves present in the heart ensure the blood flow inaccurate direction heart has four valves mitral valve, tricuspid valve, pulmonary valve, and aortic valve).
In order to pump the blood to diastole and systole take place. Diastole causes ventricular filling and systole cause the ejection of blood, it occurs in both left and right side of the heart via the same mechanism. changes in pressure and volume during the cardiac cycle are illustrated in the form of a Wiggers diagram.
Overview of Cardiac Cycle
Changes in pressure and volume cause excitation and contraction of cardiac muscle occur due to Ca+2 ions influx the myocytes cause depolarization, perpetuating conduction. The conduction arises from the sinoatrial node (SA) located in the right atrium which generates electrical stimulus, both atria contract together.
Signal flows to the atrioventricular node (AV) located between atria and the ventricles where AV slows it down (gives time to atria systole before ventricle systole. Electric stimulus continues down to through the bundle of His, and pathway and into the ventricles causes contraction.
Cardiac Cycle Steps
• Atrial Diastole: The closure of the aortic valve and pulmonary valve (between the ventricles and the major blood vessels) and the cardiac myocytes don’t contract so relaxation and filling of ventricles takes place. The blood enters the atria from the vein due to pressure gradient force between veins and relaxed atria (moves from higher pressure to lower pressure) the pressure in the atria increases.
• Atrial Systole: Electrical depolarization of atria myocardium and leads to contraction During Atrial systole the pressure in atria increases and the blood gets pumped into ventricles. at the start of systole blood passively (inflow) flows from atria to ventricle so the ventricle is already filled 70% during diastole. The atrial contraction fills the remaining 30% space in the ventricle with blood. It lasts for 100ms then muscle relaxes (diastole) before ventricular systole.
• Ventricular Systole: The ventricular systole starts with the mitral valve and tricuspid valve (between the atria and the ventricles) closure.
i. Events in Ventricular Systole
a. Isovolumic Contraction: The atrioventricular valves (mitral valve and tricuspid valve) and semi-luminar valve (The pulmonary valve and aortic valve) are closed. This period is the isovolumic contraction period where pressure builds up but the blood does not leave the ventricle.
b. Rapid Ventricular Ejection: Ventricular muscle contraction makes ventricular pressure greater than the pulmonary trunk and the aorta. As for ventricular systole due to contraction of heart muscle pressure in ventricles increases which lead to opens of pulmonary and aortic semi-luminar valve and ejection of blood into the pulmonary trunk (from the right ventricle and into the aorta from the left ventricle takes place. Pressure in the left ventricle is greater by both ventricle pump same volume of blood.
a. Isovolumic Relaxation: After the ventricle contraction phase, the atrioventricular valves have not yet opened
b. Late Ventricular Diastole: relaxation causes very low pressure in the ventricle than atria which open atrioventricular valves which completed the cardiac cycle.
Healthy individuals produce two audible heart sounds, S1 and S2. S1 also called as first heart sound is generated due to the closing of atrioventricular valves during ventricular contraction. S2 is the second heart sound caused by the closure of semilunar valves during ventricular diastole. There also S3 sound which is rarely heard which is believed to be the sound of blood flowing in the heart. If it S3 is heard in later stage of life it can be the cause of underlying condition.
Clinical Significance of Cardiac Cycle
The doctor checks to heartbeat rhythm by auscultation (listening to the sound of heartbeat with a stethoscope). Changes in heart sounds and observation of its timing can help in the determination of underlying cardiac conditions.
Changes in heart sound like murmur (unusual sound in the heart). Murmur takes place in systole (between S1 and S2) or diastole (after S2 and before the next S1) is also determined. Example systolic murmurs involve blood leaving the ventricle can be a symptom of mitral regurgitation, aortic stenosis, and diastolic murmurs involve blood entering the ventricles can be a symptom of mitral stenosis, aortic regurgitation.
ii. Cardiac Imaging
An echocardiogram is a type of cardiac imaging method based on ultrasound technology that uses sound waves to create pictures of the heart. It is important for understanding the cardiac cycle.
It Provides visualization of cardiac wall motion, valve function, and blood pumping and beating which helps in analyzing and compare the cardiac cycle of a patient. It can be useful for the diagnosis of underlying conditions related to the heart for example heart failure, congenital heart defects before birth, infective endocarditis, pericardial effusion, etc.
Cardiac Cycle Citations
- How myofilament strain and strain rate lead the dance of the cardiac cycle. Arch Biochem Biophys . 2019 Mar 30;664:62-67.
- Heart regeneration and the cardiomyocyte cell cycle. Pflugers Arch . 2018 Feb;470(2):241-248.
- Assessment of Cardiac Function–Basic Principles and Approaches. Compr Physiol . 2015 Sep 20;5(4):1911-46.