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The heart is a muscular organ in humans and other animals, which pumps blood through the blood vessels of the circulatory system. Blood provides the body with oxygen and nutrients, and also assists in the removal of metabolic wastes. The heart is located in the middle compartment of the mediastinum in the chest. In humans, other mammals, and birds, the heart is divided into four chambers: upper left and right atria; and lower left and right ventricles. Commonly the right atrium and ventricle are referred together as the ''right heart'' and their left counterparts as the ''left heart''. Fish in contrast have two chambers, an atrium and a ventricle, while reptiles have three chambers.〔 In a healthy heart blood flows one way through the heart due to heart valves, which prevent backflow.〔 The heart is enclosed in a protective sac, the pericardium, which also contains a small amount of fluid. The wall of the heart is made up of three layers: epicardium, myocardium, and endocardium. The heart pumps blood through both circulatory systems. Blood low in oxygen from the systemic circulation enters the right atrium from the superior and inferior vena cavae and passes to the right ventricle. From here it is pumped into the pulmonary circulation, through the lungs where it receives oxygen and gives off carbon dioxide. Oxygenated blood then returns to the left atrium, passes through the left ventricle and is pumped out through the aorta to the systemic circulation−where the oxygen is used and metabolized to carbon dioxide. In addition the blood carries nutrients from the liver and gastrointestinal tract to various organs of the body, while transporting waste to the liver and kidneys. Normally with each heartbeat the right ventricle pumps the same amount of blood into the lungs as the left ventricle pumps to the body. Veins transport blood to the heart and carry deoxygenated blood - except for the pulmonary and portal veins. Arteries transport blood away from the heart, and apart from the pulmonary artery hold oxygenated blood. Their increased distance from the heart cause veins to have lower pressures than arteries.〔 The heart contracts at a resting rate close to 72 beats per minute. Exercise temporarily increases the rate, but lowers resting heart rate in the long term, and is good for heart health. Cardiovascular diseases (CVD) are the most common cause of death globally as of 2008, accounting for 30% of deaths.〔 Of these more than three quarters follow coronary artery disease and stroke. Risk factors include: smoking, being overweight, little exercise, high cholesterol, high blood pressure, and poorly controlled diabetes, among others. Diagnosis of CVD is often done by listening to the heart-sounds with a stethoscope, ECG or by ultrasound.〔 Specialists who focus on diseases of the heart are called cardiologists, although many specialties of medicine may be involved in treatment.〔 ==Structure== The heart is situated in the middle mediastinum behind the breastbone in the chest, at the level of thoracic vertebrae T5-T8. The largest part of the heart is usually slightly offset to the left side of the chest (though occasionally it may be offset to the right) and is felt to be on the left because the left heart is stronger, since it pumps to all body parts. Because the heart is between the lungs, the left lung is smaller than the right lung and has a cardiac notch in its border to accommodate the heart. The heart is supplied by the coronary circulation and is enclosed in a double-membraned sac–the pericardium. This attaches to the mediastinum, providing anchorage for the heart. The back surface of the heart lies near to the vertebral column, and the front surface sits deep to the sternum and costal cartilages.〔 Two of the great veins – the venae cavae, and the great arteries, the aorta and pulmonary artery, are attached to the upper part of the heart, called the base, which is located at the level of the third costal cartilage.〔 The lower tip of the heart, the apex, lies just to the left of the sternum (8 to 9 cm from the midsternal line) between the junction of the fourth and fifth ribs near their articulation with the costal cartilages.〔 The right side of the heart is deflected forwards, and the left deflected to the back.〔 The heart is cone-shaped, with its base positioned upwards and tapering down to the apex.〔 A stethoscope can be placed directly over the apex so that the heartbeats can be counted. An adult heart has a mass of 250–350 grams (9–12 oz).〔http://health.howstuffworks.com/human-body/systems/circulatory/heart1.htm〕 The heart is typically the size of a fist: 12 cm (5 in) in length, 8 cm (3.5 in) wide, and 6 cm (2.5 in) in thickness.〔 Well-trained athletes can have much larger hearts due to the effects of exercise on the heart muscle, similar to the response of skeletal muscle.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「heart」の詳細全文を読む スポンサード リンク
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