A hollow muscular organ, divided into chambers (right and left atria, right and left ventricles) and enclosed within a fibrous sac (the pericardium) found within the thorax. It lies directly under the sternum, being protected by it and the adjacent ribs. It is the first organ to develop in the embryo (in humans by three weeks). In mammals it is separated into right and left halves concerned with pulmonary and systemic circulation respectively. It consists mainly of cardiac muscle (the myocardium) enclosed between two sheets of fibrous and elastic tissue, the epicardium and the endocardium. The myocardium forms a network of sheets and strands which have a characteristic arrangement in different parts of the heart. The heart also possesses a fibrous skeleton which surrounds and strengthens many of its openings. In humans, the ventricles have a capacity of between 90 and 120 ml. The right atrium receives deoxygenated blood from the body (via the superior and inferior venae cavae) and from the heart itself (via the coronary sinus) during diastole, and conveys it to the right ventricle via the right atrioventricular opening (guarded by the tricuspid valve). The right ventricle expels blood into the pulmonary trunk (guarded by the pulmonary valve) and thence to the lungs. The left atrium receives oxygenated blood from the lungs and conveys it to the left ventricle via the left atrioventricular opening (guarded by the mitral valve). The left ventricle forcibly expels blood into the aorta (guarded by the aortic valve) and thence to the rest of the body. In the fetus, blood bypasses the non-functioning lungs in two ways. Freshly oxygenated blood returning from the placenta enters the right atrium and passes directly into the left atrium through the foramen ovale, an opening in the inter-atrial septum, to be distributed to body tissues via the left ventricle. Deoxygenated blood returning to the right atrium passes into the right ventricle and pulmonary trunk and then into the descending aorta by a communicating channel (the ductus arteriosus). After birth, the foramen ovale closes (failure to do so leads to a hole in the heart, with the mixing of pulmonary and systemic blood), and the ductus arteriosus becomes obliterated. The conducting system of the mammalian heart consists of a pacemaker (the sinuatrial node), a relay station (the atrioventricular node), and the atrioventricular bundle within the interventricular septum. Both nodes are situated in the right atrium. The contractile impulse begins at the sinuatrial node, reaches the atrioventricular node via atrial muscle fibres, and reaches the ventricles via the atrioventricular bundle. Damage to the atrioventricular bundle (by disease or experimentally) causes the ventricles to contract independently and at a slower rate than the atria (known as heart block). The sinuatrial node has the highest degree of inherent rhythmicity, and determines the rate of the heart beat. It is influenced by the autonomic nervous system. The walls of the heart are supplied by the coronary arteries, and drained by the cardiac veins and by venae cordae minimae (small veins draining the deeper layers of myocardium). A heart attack (myocardial infarction) occurs when part of the coronary arterial supply becomes blocked by a blood clot (thrombus), leading to the cessation of blood flow and death of the myocardium.
The heart is a hollow, muscular organ in vertebrates, responsible for pumping blood through the blood vessels by repeated, rhythmic contractions, or a similar structure in annelids, mollusks, and arthropods. The term cardiac (as in cardiology) means "related to the heart" and comes from the Greek καρδιά, kardia, for "heart." The heart is composed of cardiac muscle, an involuntary muscle tissue which is found only within this organ.
Early development
The human embryonic heart begins beating 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). The embryonic heart rate (EHR) 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).
After peaking at about 9.2 weeks after the LMP, it decelerates to about 150 bpm (+/-25 bpm) during the 15th week after the LMP. 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 = EHR(0.3)+6
See: Embryonic Heart Rates Compared in Assisted and Non-Assisted Pregnancies
There is no difference in male and female heart rates before birth.
Structure
In the human body, the heart is normally situated slightly to the left of the middle of the thorax, underneath the breastbone (see diagrams). The heart is usually felt to be on the left side because the left heart (left ventricle) is stronger (it pumps to all body parts). The left lung is smaller than the right lung because the heart occupies more of the left hemithorax. In normal adults, the mass of the heart is 250-350 g (9-16 oz), but extremely diseased hearts can be up to 1000 g (2 lb) in mass due to hypertrophy. On the left is a picture of a fresh human heart which was removed from a 64-year-old British male.
The function of the right side of the heart (see right heart) is to collect deoxygenated blood, in the right atrium, from the body and pump it, via the right ventricle, into the lungs (pulmonary circulation) so that carbon dioxide can be dropped off and oxygen picked up (gas exchange). The left side (see left heart) collects oxygenated blood from the lungs into the left atrium. The heart's rhythmic contractions occur spontaneously, although the waves or nerves can be changed by nervous frequency influences such as exercise or the perception of danger. 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. 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 atrial natriuretic factor (ANF), a powerful peptide hormone that affects the blood vessels, the adrenal glands, the kidneys, and the regulatory regions of the brain in order to regulate blood pressure and volume.
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.
The hearts of other animals
Vertebrates
The hearts of fish have only two chambers: one atrium and one ventricle. The blood pumps through the gills and on to the bodily tissues before returning to the heart.
Amphibians and most reptiles have a three-chambered heart, in which oxygenated blood from the lungs and de-oxygenated blood from the respiring tissues enter by separate atria, and are directed via a spiral valve to the appropriate vessel—aorta for oxygenated blood and pulmonary artery for deoxygenated blood.
Mammals (a class of vertebrate), birds and crocodiles show complete separation of the heart into two pumps, for a total of four heart chambers;
Invertebrates
Many invertebrates, such as bivalves and arthropods, exhibit an open circulatory system where blood flows both in vessels and freely in the body cavity.
The earthworm doesn't have a heart; 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 72, sparrow 500, shrew 600, and hummingbird 1,200 when hovering. These heart rates usually vary on the animal's ratio of surface area to body mass;
Food use
The hearts of cattle, sheep, pigs, chickens and certain fowl are consumed as food in many countries.
As a symbol
The heart was historically seen by some as the seat of the soul and the organ responsible for human thought. Even though we now know that the heart has nothing to do with thought or love, people still carry on using the term "heart" metaphorically when talking about love.
The term "heart" can also refer to the core or center of anything i.e.
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