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The heart and lungs (from an older edition of Gray's Anatomy)

The heart (Latin cor) is a hollow, muscular organ in vertebrates that pumps blood through the blood vessels by repeated, rhythmic contractions, or a similar structure in annelids, mollusks, and arthropods. The term cardiac means "related to the heart", from the Greek kardia (καρδια) for "heart". The human heart is the most studied organ in biology.

Early development

At 21 days after conception, the human heart rate begins beating a 75-80 beats per minute and accelerates linearly for the first month of beating.

The human embryonic heart (EHR) begins beating at approximately 21 days after conception, or five weeks after the last normal menstrual period (LMP), which is the date normally used to date pregnancy. The human heart begins beating at a rate near the mother’s, about 75-80 beats per minute (bpm). It then accelerates linearly for the first month of beating, peaking at 165-185 bpm during the early 7th week, (early 9th week after the LMP). This acceleration is approximately 3.3 bpm per day, or about 10 bpm every three days, an increase of 100 bpm in the first month. [1]

After peaking at about 9.2 weeks after the LMP, it decelerates to about 150 bpm (+/-25 bpm) during the 15 week after the LMP. After the 15th week the deceleration slows reaching an average rate of about 145 (+/-25 bpm) bpm at term. The regression formula which describes this acceleration before the embryo reaches 25 mm in crown-rump length or 9.2 LMP weeks is:

Age in days from LMP = EHR(0.3)+6


There is no difference in male and female heart rates before birth. [2]

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In the human body, the heart is normally situated slightly to the left of the middle of the thorax, underneath the sternum (breastbone). It is enclosed by a sac known as the pericardium and is surrounded by the lungs. The apex is the blunt point at the base of the heart. A stethoscope can be placed directly over the apex and count the beats. In normal adults, its mass is 250-350 g, but extremely diseased hearts can be up to 1000 g in mass. It consists of four chambers, the two upper atria (singular: atrium) and the two lower ventricles.

A septum divides the right atrium and ventricle from the left atrium and ventricle, preventing blood from passing between them. Valves between the atria and ventricles (atrioventricular valves) maintain coordinated unidirectional flow of blood from the atria to the ventricles.The ventricular systole consists of the contraction of the ventricles and flow of blood into the circulatory system. Again, once all the blood empties from the ventricles, the pulmonary and aortic semilunar valves close. Finally complete cardiac diastole involves relaxation of the atria and ventricles in preparation for refilling with circulating blood.

The function of the right side of the heart (see right heart) is to collect deoxygenated blood from the body and pump it into the lungs so that carbon dioxide can be dropped off and oxygen picked up. this happens through a process called diffusion. The left side (see left heart) collects oxygenated blood from the lungs and pumps it out to the body. On both sides, the lower ventricles are thicker than the upper atria.

Anterior (frontal) view of the opened heart. White arrows indicate normal blood flow. (SVG version)

Oxygen-depleted or deoxygenated blood from the body enters the right atrium through two great veins, the superior vena cava which drains the upper part of the body and the inferior vena cava that drains the lower part. The blood then passes through the tricuspid valve to the right ventricle. The right ventricle pumps the deoxygenated blood to the lungs, through the pulmonary artery. In the lungs gaseous exchange takes places and the blood releases carbon dioxide into the lung cavity and picks up oxygen. The oxygenated blood then flows through pulmonary veins to the left atrium. From the left atrium this newly oxygenated blood passes through the mitral valve to enter the left ventricle. The left ventricle then pumps the blood through the aorta to the entire body. Even the lungs take some of the blood supply from the aorta via bronchial arteries.

The left ventricle is much more muscular (1.3 - 1.5 cm thick) than the right (0.3 - 0.5 cm thick) as it has to pump blood around the entire body, which involves exerting a considerable force to overcome the vascular pressure. As the right ventricle needs to pump blood only to the lungs, it requires less muscle.

Even though the ventricles lie below the atria, the two vessels through which the blood exits the heart (the pulmonary artery and the aorta) leave the heart at its top side.

The contractile nature of the heart is due to the presence of cardiac muscle in its wall which can work continuously without fatigue. The heart wall is made of three distinct layers. The first is the outer epicardium which is composed of a layer of flattened epithelial cells and connective tissue. Beneath this is a much thicker myocardium made up of cardiac muscle. The endocardium is a further layer of flattened epithelial cells and connective tissue which lines the chambers of the heart.

The blood supply to the heart itself is supplied by the left and right coronary arteries, which branch off from the aorta.

The cardiac cycle

See main page cardiac cycle

Atrial systole

Ventricular systole

The function of the heart is to pump blood around the body. Every single beat of the heart involves a sequence of events known as the cardiac cycle, which consists of three major stages: atrial systole, ventricular systole and complete cardiac diastole. The atrial systole consists of the contraction of the atria and the corresponding influx of blood into the ventricles. Once the blood has fully left the atria, the atrioventricular valves, which are situated between the atria and ventricular chambers, close. This prevents any backflow into the atria. It is the closing of the valves that produces the familiar beating sounds of the heart, commonly referred to as the "lub-dub" sound due to the closing of the semilunar and atrioventricular valves.

The ventricular systole consists of the contraction of the ventricles and flow of blood into the circulatory system. Again, once all the blood empties from the ventricles, the pulmonary and aortic semilunar valves close. Finally complete cardiac diastole involves relaxation of the atria and ventricles in preparation for refilling with circulating blood.

Regulation of the cardiac cycle

Cardiac muscle is self-exciting. This is in contrast with skeletal muscle, which requires either conscious or reflex nervous stimuli. The heart's rhythmic contractions occur spontaneously, although the frequency or heart rate can be changed by nervous or hormonal influences such as exercise or the perception of danger.

The rhythmic sequence of contractions is coordinated by the sinoatrial and atrioventricular nodes. The sinoatrial node, often known as the cardiac pacemaker, is located in the upper wall of the right atrium and is responsible for the wave of electrical stimulation (See action potential) that initiates atria contraction. Once the wave reaches the atrioventricular node, situated in the lower right atrium, it is conducted through the bundles of His and causes contraction of the ventricles. The time taken for the wave to reach this node from the sinoatrial nerve creates a delay between contraction of the two chambers and ensures that each contraction is coordinated simultaneously throughout all of the heart. In the event of severe pathology, the Purkinje fibers can also act as a pacemaker; this is usually not the case because their rate of spontaneous firing is considerably lower than that of the other pacemakers and hence is overridden.

Other physiological functions

The heart also secretes ANF (atrial natriuretic factor), a powerful peptide hormone, that affects the blood vessels, the adrenal glands, the kidneys and the regulatory regions of the brain to regulate blood pressure and volume.

Diseases and treatments

The study of diseases of the heart is known as cardiology. Important diseases of the heart include:

If a coronary artery is blocked or narrowed, the problem spot can be bypassed with coronary artery bypass surgery or it can be widened with angioplasty.

Beta blockers are drugs that lower the heart rate and blood pressure and reduce the heart's oxygen requirements. Nitroglycerin and other compounds that give off nitric oxide are used to treat heart disease as they cause the dilation of coronary vessels.

At Groote Schuur Hospital in Cape Town, South Africa, 53-year-old Louis Washkansky on December 3, 1967 became the first human to receive a heart transplant (however he died 18 days later from double pneumonia). The transplant team was headed by Christiaan Barnard.

See also: Cardiology diagnostic tests and procedures

First aid

See cardiac arrest for emergencies involving the heart

If a person is encountered in cardiac arrest (no heartbeat), cardiopulmonary resuscitation (CPR) should be started, and help called. If an automated external defibrillator is available, this device may automatically administer defibrillation if this is indicated.

The hearts of other animals

Main article: Circulatory system#Types of circulatory systems


The heart of fish have only two chambers: one atrium and one ventricle. In fish, the system has only one circuit. The blood pumps through the gills and on to the body tissues without returning to the heart.

Amphibians and reptiles have a three-chambered heart: two atria and one ventricle. Blood from both the body and the lungs is pumped into the single ventricle, where unoxygenated and oxygenated blood may mix. The blood from the single ventricle is then pumped to both the lungs and the body tissues, a type of double circulation.

Birds and mammals show complete separation of the heart into two pumps, for a total of four heart chambers; it is thought that the four-chambered heart of birds evolved independently of that of mammals.


The earthworm has a series of multiple primitive hearts. This section is a stub. You can help by adding to it.


Smaller animals have faster heartbeats. This is evident within a species as well, as the young beat their hearts faster than the adults. See "Early development" above for information about the early human heart rates. The Gray Whale's heart beats 9 times per minute, Harbour Seal 10 when diving, 140 when on land, elephant 25, human 70, sparrow 500, shrew 600, and hummingbird 1,200 when hovering. An ectothermic animal will usually have a slower, and more variable heartbeat than an endothermic animal of similar size.

Food use

The hearts of cattle, sheep, pigs and certain fowl are consumed as food in many countries. They are counted among offal, but being a muscle, the taste of heart is much more like regular meat than that of other offal. It resembles venison in structure and taste.

Different species have different heart chambers. It can vary from one to four chambers (2 atria and 2 ventricle)

As an icon

For more details on this topic, see Heart (symbol).

The heart may also be illustrated as an icon (), symbolizing love.

See also

External links

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