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Structure File:Real-time MRI - Thorax.ogvPlay media Real-time MRI of the human heart Location of human heart in the thorax; apex at a point 9cm to left of midsternum.[12]

The heart is situated in the middle of the 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 (though occasionally it may be offset to the right). The heart is usually felt to be on the left side because the left heart is stronger, since it pumps to all body parts. The left lung in turn is smaller than the right lung because it has to accommodate the heart. The heart is supplied by the coronary circulation and is enclosed in the pericardial sac.

The pericardium encloses the heart and also attaches to the mediastinum via the pericardiac pleura, providing anchorage for the heart.[13] The back surface of the heart lies near to the vertebral column, and the front surface sits deep to the sternum and costal cartilages.[7] The two great veins, the venae cavae, and the great arteries, the aorta and pulmonary trunk, are attached to the upper surface of the heart, called the base, which is located at the level of the third costal cartilage.[7] The lower tip of the heart, the apex, lies just to the left of the sternum between the junction of the fourth and fifth ribs near their articulation with the costal cartilages.[7] The right side of the heart is deflected forwards, and the left side is deflected to the back.[7]

The shape of the heart is similar to a pinecone, rather broad at the base and tapering to the apex.[7] A stethoscope can be placed directly over the apex so that the beats can be counted. An adult heart has a mass of 250–350 grams (9–12 oz).[14] 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.[7] 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.[7]