• Post last modified:December 30, 2022
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Cardiac cycle

The cardiac cycle is the performance of all of those events which occur, from the beginning of one heartbeat to the beginning of the next heartbeat.

The cardiac cycle consists of one heartbeat or one cycle of contraction and relaxation of the cardiac muscle. The cardiac cycle can be divided into two parts:

(i) One during which the heart muscle relaxes, called diastole.

(ii) Second during which the heart muscle contracts, called systole.

There are two atrial and two ventricle chambers of the heart. They are also called the left heart and the right heart, i.e. they paired as the left atrium with the left ventricle and the right atrium with the right ventricle. They work in concert to repeat the cardiac cycle continuously.

At the start of the cycle, i.e. during ventricular diastole, the heart relaxes and expands while receiving blood into both ventricles through both atria. Then, at the end of the ventricular diastole, both of the atria begin to contract (atrial systole), and each atrium pumps blood into the ventricles.

During ventricular systole, both of the ventricles contract, and two separated blood supplies begin from the heart: one to the lungs and one to all other body organs and organ systems. Atrial diastole is the part of the cardiac cycle during which both of the atria refills with blood.

The cardiac cycle consists of one cycle of contraction and relaxation of the cardiac muscle. The duration of a cardiac cycle is 0.8 sec.

The successive stages of the cardiac cycle are:

Atrial systole:

The atria contract due to the wave of contraction, stimulated by the SA node. The blood is forced into the ventricles as the bicuspid and tricuspid valves are open.

Ventricular systole:

The ventricles begin to contract due to a wave of contraction, stimulated by the AV node. The bicuspid and tricuspid valves close immediately producing part of the first heart sound, “lub”. When the ventricles complete their contraction, the blood flows into the pulmonary trunk and aorta as the semilunar valves open.

Ventricular diastole:

The ventricles relax and semilunar valves are closed. This causes the second heart sound “dup”. The tricuspid and bicuspid valves open again when the pressure in the ventricles falls and blood flows from the atria into the ventricles.

Cardiac Cycle Illustration
Cardiac Cycle Illustration

Cardiac Cycle Graphical Presentation
Cardiac Cycle Graphical Representation

Cardiac output

The volume of blood pumped by each ventricle per minute is called the cardiac output. It is determined by multiplying the heart rate with the volume of blood ejected by each ventricle during each beat, which is called the stroke volume.

  • Cardiac output = Heart rate * Stroke volume
  • Cardiac output = 72 beats/min. * 0.08 litre/beat ≈ 5.5litres/min.

Blood pressure

Blood pressure is the pressure against the walls of the blood vessels produced by the discharge of blood into them by contractions of the left ventricle. The blood pressure is high in the arteries, gradually drops in the arterioles and capillaries, and becomes very low in the veins. It is measured from the brachial artery by an instrument called a sphygmomanometer. The blood pressure is expressed as BP = 120/80 mm Hg for the normal human.

Electrocardiogram (ECG)

Electrocardiogram (ECG) is a graphic record of the electric current produced by the excitation of the cardiac muscles over a period of time, using electrodes placed over the skin. The instrument used to record the changes is an electrocardiograph.

A normal electrocardiogram (ECG) is composed of a P wave, a QRS wave (complex), and a T wave.

ECG: P wave, QRS wave ,T wave
  • The P wave is a small upward wave that indicates the depolarization of the atria (atrial contraction). It is caused by the activation of the SA node.
  • The QRS wave (complex) begins after a fraction of a second of the P wave. It represents ventricular depolarization (ventricular contraction).
  • The T wave is dome-shaped which indicates ventricular repolarization (ventricular relaxation).

ECG can also be represented as:

  • P is the atrial systole contraction pulse.
  • Q is a downward deflection immediately preceding the ventricular contraction.
  • R is the peak of the ventricular contraction.
  • S is the downward deflection immediately after the ventricular contraction.
  • T is the recovery of the ventricles.

Disorders of the Circulatory System

Angina pectoris:

It is a sudden recurring thoracic pain radiating to arms, especially the left arm when the demand for blood by the heart exceeds the supply of the coronary arteries. It can occur in excessive exercise.

Myocardial infarction:

It is the complication due to the reduced blood supply to the heart wall. It results in pain, perspiration, nausea, ECG changes.


It is wall thickening and narrowing of the lumen of medium and large-sized arteries. In atherosclerosis, yellowish plaques (atheromas) of cholesterol and other lipids are deposited within the internal wall and inner media of arteries.


It is sclerosis and the thickening of walls of smaller arteries and arterioles. Arteriosclerosis is common in elderly persons.