''This page is about the muscular organ, the Heart. For other meanings of the word, see Heart (disambiguation).
heart (Latin, cor) is a hollow, muscular organ that pumpsblood through the blood vessels by repeated, rhythmic contractions. The term cardiac means "related to the heart", from the Greekcardia for "heart".
In the human body the heart is situated slightly to the left of the middle of the thorax, behind the sternum (breastbone). It is enclosed by a sac known as the pericardium and is surrounded by the lungs. In adults, it weighs about 300~350 g. It consists of four chambers, the two upper atria (singular: atrium) and the two lower ventricles.
A thick, muscular wall, the septum, divides the right atria and ventricle from the left atria and ventricle, keeping blood from passing between them. Valves between the atria and ventricles maintain coordinated unidirectional flow of blood from the upper atria to the lower ventricles.
The ventricles are the parts of the heart that pump blood around the body or to the lungs. They are thicker walled than the atria, and the contraction of the ventricle wall is much more important to move blood around.
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Oxygen-depleted blood from the body enters the right atrium through two veins, the superior vena cava and the inferior vena cava. The blood then passes to the right ventricle. The right ventricle pumps the deoxygenated blood to the lungs, through the pulmonary artery. After the blood loses carbon dioxide and picks up oxygen in the lungs, it flows through pulmonary veins to the left atrium. From the left atrium the newly oxygenated blood enters the left ventricle. The left ventricle is the main pumping chamber, sending blood through the aorta to all of the body except the lungs.
The left ventricle is much thicker than the right because it must pump blood around the entire body, which involves exerting a considerable force to overcome the pressure caused by the body. As the right ventricle must just pump blood 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 wall of the heart is very muscular and does not tire. It consists 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.
A large blood supply is necessary to power the heart itself. It is supplied by the left and right coronary arteries, which branch off from the aorta.
The ventricular systole consists of the contraction of the ventricles and flow of blood into the circulatory system. Again, once all the blood has left, the pulmonary and aortic semilunar valves close. Finally complete cardiac diastole involves the relaxation of the atria and ventricles in preparation for new blood to enter the heart.
The rhythmic sequence of contractions is coordinated by the sinoatrial node 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 the atria to contract. Once the wave reaches the atrioventricular node, situated in the wall between ventricular chambers, 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.
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.
The structure of the heart of other mammals is quite similar to that of humans, with four chambers. Birds also have a four-chambered heart, however it is thought that this evolved independently of that of mammals. Amphibians have a three-chambered heart. Fish have a single circulation system and a heart with two chambers. The hearts of arthropods and mollusks have a single chamber.
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