The cardiac cycle is the sequence of events that occur when the heart beats. The cycle has two main phases: diastole, when the heart ventricles are relaxed, and systole, when the ventricles contract. One cardiac cycle is defined as the contraction of the two atria followed by contraction of the two ventricles.
Description
The heart is a muscular organ that works as a pumping system. It takes in blood with reduced levels of oxygen from the veins (deoxygenated blood), and delivers it to the lungs for oxygenation. When it receives the oxygenated blood back from the lungs, it pumps the blood back into the arteries to be distributed throughout the body.
The heart is divided into four hollow chambers, two on the left and two on the right. The right chambers are the right atrium and the right ventricle. They receive blood from the veins. The left chambers are the left atrium and the left ventricle. They receive blood from the pulmonary circulation, and the left ventricle forces blood into the systemic circulation.
In a cardiac cycle, blood enters the right atrium of the heart from the superior and inferior vena cavae, and flows across the tricuspid valve into the right ventricle. From the right ventricle the blood flows into the pulmonary artery, which is separated from the ventricle by the pulmonary valve. After oxygenation in the lungs, blood returns to the heart via four pulmonary veins that enter the left atrium. From the left atrium, blood flows across the mitral valve and into the left ventricle. From the left ventricle blood is ejected across the aortic valve into the aorta. Together, the mitral and triscupsid valves are known as the atrioventricular valves and the aortic and pulmonary valves as the semilunar valves.
From a mechanical point of view, the cardiac cycle is due to blood movement occurring as a result of pressure differences within the chambers of the heart. In order for blood to flow through a blood vessel or across and heart valve, there must be a force acting on the blood. This force is provided by the difference in blood pressure (a pressure gradient) across these structures by the contractions of the heart. Each heart beat, or cardiac cycle, is divided into two phases of contraction and relaxation, stimulated by electrical impulses from the sinoatrial node (SA node), a patch of tissue in the heart that sets the rate of contractions. It contracts itself and then sends nerve impulse to the atria. The time during which ventricular contraction occurs is called systole. The time between ventricular contractions, during which ventricular filling occurs, is called diastole (also known as the relaxation phase).
In early diastole, the ventricles relax, the semilunar valves close, the atrioventricular valves open and the ventricles fill with blood. In mid diastole, the atria and ventricles are relaxed, the semilunar valves are closed, the atrioventricular valves are open, and the ventricles keep filling with blood. In late diastole, the SA node sends an electrical impulse to the atria, which causes the atria to contract and the ventricles to fill with more blood. The electrical signal that causes contraction moves from the atria toward the ventricles. Before it does, though, it reaches the atrioventricular node (AV node). The AV node delays the signal so that the ventricle can contract all at once rather than a little bit at a time.
Prior to systole, the electrical signal passes from the AV node down the AV bundle, also known as the bundle of His, to the Purkinje fibers. The fibers allow the fast spread of the electrical signal to all parts of the ventricles, and the electrical signal causes the ventricles to contract. Systole begins with the closure of the atrioventricular valves. During systole, the ventricles contract, the semi- lunar valves open, and blood is pumped from the ventricles to the aorta.
Blood pressure is highest during systole, and lowest during diastole. It has two components, the systolic and diastolic pressure. Normal systolic pressure for an adult is 120 mm Hg, and normal diastolic pressure is 80 mm Hg. These values are commonly recorded as 120/80.
The normal heart beats at a rate of about 72 beats per minute (with a range of 60-100 beats per minute), but can vary with normal daily activity.
The cardiac cycle produces well-known sounds that can be clearly heard with a stethoscope. The first heart sound is associated with closure of the atrioventricular valves and signals the start of ventricular systole. The second heart sound is associated with the closure of the semilunar valves and indicates the start of ventricular diastole. A third heart sound is due to the rapid phase of ventricular filling, and a fourth heart sound is due to atrial systole. The two last sounds are usually not loud enough to hear, so neither of them is heard under normal exam procedures.
Role in human health
The role of the cardiac cycle is essential to maintain life, as the heart distributes the oxygen-carrying blood required for the functioning of the body.
Common diseases and disorders
Although diseases of the valves are common, and other cardiac disorders can also disrupt normal blood flow, they do not cause an abnormal cardiac cycle per se. Since the normal sequence of events occurs, even though it's impaired, it is considered a normal cardiac cycle (but not a normal heart beat). Most of the common abnormalities in the cardiac cycle are caused by disturbances in electrical conduction of the heart.
Disturbances in the electrical cycle are known as arrhythmias. However, two types of disturbances, sinus tachycardia and sinus bradycardia, do not affect the sequence of events, because they speed up (tachycardia)
or slow down (bradycardia) the beating of the heart. The other arrthymias have one of two origins. They can be caused by a lack of normal electrical conduction. For example, a lack of electrical signaling from the sinoatrial node can lead to the placement of an exogenous pacemaker (something that causes the heart to beat other than the sinoatrial node). The other cause of arrhythmias is abnormally strong ectopic (in the wrong place) electrical activity. For example, premature atrial contractions can be caused by excessive electrical activity somewhere in the atrium other than the sinoatrial node. Some arrhythmias may be a combination of the two.
Cardiac cycle disturbances can be complex, but can be dissected by an electrocardiogram (EKG or ECG). They can be caused by a wide variety of problems, including, but not limited to, coronary heart disease, damage to the heart muscle secondary to a heart attack, genetic heart defects, valvular disease of the heart, and medications. Following is a list of common arrhythmias. Atrial arrhythmias:
Guyton, A. C., G. Guyton, and F. Hall. Textbook of Medical Physiology, Ninth ed. Philadelphia: W.B. Saunders Co.,1995.
KEY TERMS
Aorta—The largest artery of the body that originates from the left ventricle of the heart, arches over the heart to the left, and descends just in front of the spinal column. The aorta divides into three arteries: the brachiocephalic artery (that supplies blood to the brain and head), the left carotid artery, and the left subclavian artery.
Aortic valve—The heart valve that divides the left ventricle and the aorta. Blood from the left ventricle is ejected across the aortic valve into the aorta for further distribution. It opens during contraction of the left ventricle and closes afterwards to prevent the backward flow of blood from the aorta.
Arterial blood pressure—The ejection of blood into the aorta by the left ventricle results in the characteristic aortic pressure pulse. The maximum of the aortic pressure pulse is called the systolic pressure and the lowest pressure in the aorta is called the diastolic pressure.
Arterial pulse pressure—The arterial pulse pressure is the difference between the systolic and diastolic arterial pressures.
Artery—Blood vessel that carries blood away from the heart for distribution throughout the body.
Atrioventricular node (AV node)—Nodal tissue located on the right side of the partition that divides the atria, near the bottom of the right atrium. It delays impulses from the SA node, thus allowing the atria to empty themselves.
Atrioventricular valves—Heart valves located between the atria and the ventricles. They are the mitral valve and the tricuspid valve.
Diastole—The time in between ventricular contractions during which ventricular filling occurs. Also called the relaxation phase.
Diastolic pulse pressure—The pressure exerted on the walls of the arteries during diastole. A normal value ranges around 90 mmHg.
Heart—In humans, the heart is divided into four chambers: the right atrium and ventricle and the left atrium and ventricle. Blood flows from the veins into the right atrium, then to the right ventricle and into the lungs for oxygenation, from where it is returned to the left atrium, then to the left ventricle for distribution to the body via arteries.
Mitral valve—The heart valve that divides the left atrium and the left ventricles. It opens during contraction of the left atrium to allow blood flow into the left ventricle and closes to prevent the backward flow of blood to the left atrium.
Oxygenation—The process of adding oxygen to something. Oxygen is added to the blood in the lungs. Oxygen contained in the inhaled air is delivered to the blood, where the oxygen binds to a protein called hemoglobin (which functions as an oxygen-carrier).
Pulmonary artery—Short blood vessel that carries deoxygenated blood from the heart to the lungs.
Pulmonary vein—One of four blood vessels that carry oxygenated blood from the lungs to the heart.
Pulmonary valve—The heart valve that divides the right ventricle from the pulmonary artery.
Semilunar valves—Heart valves shaped like a half- moon that located between the aorta and the left ventricle and between the pulmonary artery and the right ventricle. They are the aortic valve and the pulmonary valve.
Sinoatrial node (SA node)—Also called the pacemaker of the heart. It consists of nodal tissue located in the upper wall of the right atrium. It controls the rate of contraction of the heart by generating nerve impulses that travel throughout the heart wall causing both atria to contract.
Systole—The time during which ventricular contraction occurs.
Systolic pulse pressure—The pressure exerted on the walls of the arteries during the contraction of the heart. A normal value ranges around 150 mmHg.
Tricuspid valve—The heart valve, named for its three cusps, that divides the right atrium from the right ventricle. Blood flows from the right atrium, across the tricuspid valve, and into the right ventricle. When closed, it prevents the blood from flowing back into the right atrium.
Vein—Blood vessel that returns blood to the heart from the body. All the veins from the body converge into two major veins that lead to the right atrium of the heart. These veins are the superior vena cava and the inferior vena cava. The pulmonary vein carries the blood from the right ventricle of the heart into the lungs.
Tortora, Gerard, and Sandra Grabowski. "The Cardiovascular System: The Heart." In Principles of Anatomy and Physiology, 8th ed. New York: Harper Collins, 1996.
ORGANIZATIONS
American Heart Association National Center. 7272 Greenville Avenue, Dallas, Texas 75231. (800) AHA-USA1. <http://www.americanheart.org>.
OTHER
Anderson, R. The Cardiac Cycle. Windows/Macintosh CD- rom, 1996.
