<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 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 . </P> <P> The heart has four chambers, two upper atria, the receiving chambers, and two lower ventricles, the discharging chambers . The atria open into the ventricles via the atrioventricular valves, present in the atrioventricular septum . This distinction is visible also on the surface of the heart as the coronary sulcus . There is an ear - shaped structure in the upper right atrium called the right atrial appendage, or auricle, and another in the upper left atrium, the left atrial appendage . The right atrium and the right ventricle together are sometimes referred to as the right heart . Similarly, the left atrium and the left ventricle together are sometimes referred to as the left heart . The ventricles are separated from each other by the interventricular septum, visible on the surface of the heart as the anterior longitudinal sulcus and the posterior interventricular sulcus . </P> <P> The cardiac skeleton is made of dense connective tissue and this gives structure to the heart . It forms the atrioventricular septum which separates the atria from the ventricles, and the fibrous rings which serve as bases for the four heart valves . The cardiac skeleton also provides an important boundary in the heart's electrical conduction system since collagen cannot conduct electricity . The interatrial septum separates the atria and the interventricular septum separates the ventricles . The interventricular septum is much thicker than the interatrial septum, since the ventricles need to generate greater pressure when they contract . </P>

Where are the four chambers of the heart located
find me the text answering this question