<P> The heart pumps blood with a rhythm determined by a group of pacemaking cells in the sinoatrial node . These generate a current that causes contraction of the heart, traveling through the atrioventricular node and along the conduction system of the heart . The heart receives blood low in oxygen from the systemic circulation, which enters the right atrium from the superior and inferior venae 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 . The heart beats 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 . </P> <P> 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 are a result of coronary artery disease and stroke . Risk factors include: smoking, being overweight, little exercise, high cholesterol, high blood pressure, and poorly controlled diabetes, among others . Cardiovascular diseases frequently do not have symptoms or may cause chest pain or shortness of breath . Diagnosis of heart disease is often done by the taking of a medical history, listening to the heart - sounds with a stethoscope, ECG, and ultrasound . Specialists who focus on diseases of the heart are called cardiologists, although many specialties of medicine may be involved in treatment . </P> <P> The human heart is situated in the middle mediastinum, at the level of thoracic vertebrae T5 - T8 . A double - membraned sac called the pericardium surrounds the heart and attaches to the mediastinum . The back surface of the heart lies near the vertebral column, and the front surface sits behind the sternum and rib cartilages . The upper part of the heart is the attachment point for several large blood vessels--the venae cavae, aorta and pulmonary trunk . The upper part of the heart is located at the level of the third costal cartilage . The lower tip of the heart, the apex, lies 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 . </P> <P> 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 and larger, 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 cone - shaped, with its base positioned upwards and tapering down to the apex . An adult heart has a mass of 250--350 grams (9--12 oz). The heart is often described as 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, although this description is disputed, as the heart is likely to be slightly larger . 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 . </P>

Where is heart located in the human body