Cardiac Cycle Definition

The cardiac cycle is the series of electrical impulses and muscle contractions that pressurizes different chambers of the heart, causing blood to flood in one direction. The cardiac cycle varies in different organisms, due to changes in the structure of the heart. Some organisms have a three-chambered heart, which consists of the sinus venosus, atrium, and ventricle. Most tetrapods have developed a more efficient heart, which can supply a greater pressure of blood to the organisms. The “four-chambered heart”, is actually just a modification of the three chambered heart. The sinus venosus is reduced to the sinoatrial node, located on the right atrium. The atrium and ventricles are divided in the four-chambered heart, allowing a separate pathway to be established to the lungs, allowing for greater oxygenation of the blood. The separate circulations pathways are known as pulmonary (lungs) and systemic (body) circulation.

The following cardiac cycle phases are representative of the mammalian, four-chambered heart. The cardiac cycle of animals with three-chambered hearts is similar, except the atria and ventricles are not divided completely, if at all. In most animals, the heartbeat is regulated by nerves in the sinoatrial node, and carried out by nerves throughout the heart. Heart muscles cells are also connected laterally, allowing them to pass the nerve impulse received to all their neighbors, creating rhythmic contractions. Hagfish, and other organisms that have a more ancestral heart, simply use the heart to move liquid through their body at a slow rate. In organisms such as this, the cardiac cycle is much less distinguished because the heart does not set up a specific rhythm.

Cardiac Cycle Phases

The two main phases of the cardiac cycle are systole and diastole, and follow each other in sequence. Typically, the cardiac cycle starts with diastole, or the relaxation of all the heart muscles. During diastole, blood returns to the heart, and begins to fill the atria and ventricles. The lack of pressure in the ventricle allows the mitral and tricuspid valves to open, which allow blood from the atria into the left and right ventricles, respectively. This phase of the cardiac cycle can be seen in the image below.

Heart diastole

A signal sent to the sinoatrial node induces the muscles of both atria to contract. In unison, this forces blood out of the atria and into the ventricles. Most of the blood leaves the atria at this point in the cardiac cycle. As the atria squeeze, the action potential is passed through the muscles and nerves of the heart to the ventricles. Another wave of contraction starts, and the ventricles enter ventricular systole, and begin contracting themselves. The increased pressure in the ventricles closes the mitral and tricuspid valves, and opens the aortic and pulmonary valves. This can be seen in the image below.

Heart systole

The ventricles contract hard, pushing most of blood volume they contain into the pulmonary and systemic circulation. The aorta is the main artery that feeds oxygenated blood to the body, and is attached to the left ventricle. The pulmonary artery exits the right ventricle and carries unoxygenated blood to the lungs. This blood then returns and enters the left atrium, to be pumped out to the body. The body used the oxygen and return the blood to the right atrium, for the cycle to start over. While it may take a while for one pump of blood to circle the body, the heart continues the cardiac cycle indefinitely, to ensure the movement of nutrients and oxygen in the body, as well as remove toxic metabolic wastes. The entire cardiac cycle can be seen in the following animation, which tracts the cardiac cycle along with the pressure and volume of different chambers.

Cardiac Cycle Animated

An import tool to measure the cardiac cycle is the electrocardiogram, which can be seen as the green line on the above animated graph. The electrocardiogram is a signal which can be measured by sensitive medical electronics, and provides a glimpse of the cardiac cycle, and the actions taking place in the heart. The “QRS” on the green line indicates significant points in the signal, and correspond to the contraction of the ventricles. The study of electrocardiograms can lead to important insights into the functioning of the heart, and significance is put only on the presence of a signal, but the strength of the signal and the spacing of the events.

Related Biology Terms

  • Systole – The phase of heart contraction during the cardiac cycle.
  • Diastole – The period of relaxation in the heart muscles, which allows the heart to fill with blood.
  • Systolic Pressure – The pressure in the blood vessels when the heart contracts, and pressure is maximized.
  • Diastolic Pressure – The minimum pressure in the blood vessels, caused by relaxation of the heart.


1. Obesity is a health concern for a number of reasons. A large component that makes it more dangerous for a person’s heart is the increased viscosity of the blood. In other words, the heavier a person gets, the thicker their blood becomes. Why is this a heart issue?
A. The heart simply cannot pump thick blood
B. The heart will become too muscular
C. The blood will become so thick it will permanently seal the heart’s valves

Answer to Question #1

2. A miracle of modern science is the heart transplant. Doctors, using the still viable heart from a diseased person, can replace the heart of someone who has a bad or damaged heart. The doctors simply put the patient on life support, remove their heart, put the new one in and shock it back into rhythm. Why does electricity reactivate the heart?
A. Nerves work by passing electrical signals
B. Muscles are stimulated by electrical signals
C. Once reactivated, the heart tissue can excite itself

Answer to Question #2

3. A pacemaker is a small electrical device inserted into the chest, which sends a repetitive signal to the sinoatrial node. Why would this be necessary?
A. The cardiac cycle is off, which is controlled by the node
B. The extra electricity helps stimulate stronger heart contractions, for weak blood pressure
C. The electricity counters the heartbeat, slowing the heart for people with high blood pressure

Answer to Question #3

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