The heart is located between the lungs behind the sternum and above the diaphragm. It's only about the size of a fist, and it weighs 7 to 15 ounces. It's a hollow, muscular organ. It is surrounded by a sac called the pericardium. There is a fluid between the heart and the pericardium called pericardial fluid. It is lubricating fluid, which reduces friction between the pericardium and heart. The thick middle layer of the heart is called myocardium. The smooth inner surface of the heart chambers is endocardium.
2.5.1 The Heart Chambers
Internally, the heart is divided by a vertical portion into two halves, the right and the left. The vertical partition is called septum. The heart consists of four chambers, two on the top and two on the bottom. The two bottom chambers are called the right ventricle and the left ventricle. They pump blood out of the heart. The two top chambers are the right atrium and the left atrium, the plural of which is atria. They receive the blood entering the heart. The thickness of the walls of each chamber is different. The atria have comparatively thin walls as they have to force blood into the ventricles and this does not require much power. On the other hand, the ventricles have to force blood out of the heart hence they have relatively thick walls, especially the left ventricle which has to pump blood around the whole body. The right ventricle has thinner than the left ventricle in a ratio of 1:3; it pumps blood to the lungs, which are at a short distance from the heart. The human heart is myogenic (A heart that does not require neural input to beat is called myogenic heart.)
2.5.2 The Heart Valves
The heart also has four valves, which separate the top chambers from the bottom chambers and also move blood.
Atrioventricular valves separate the atria from the ventricles. Each valve is composed of cusps or flaps. The valve between the right atrium and right ventricle has three cusps and is called tricuspid valve.
The valve between left atrium and left ventricle has two cusps and is called bicuspid or mitral valve. Tricuspid valve separates the right side, and the bicuspid valve separates the left side. Two additional valves separate the ventricles from the large blood vessels that carry blood leaving the heart. The pulmonic semilunar valve is between the right ventricle and pulmonary artery. It carries blood to the lungs. The aortic semilunar valve is between the left ventricle and the aorta. It carries blood to the body.
The atria receive blood and the ventricles distribute it. Blood from the head, neck and arms is returned to the right atrium by superior vena cava. Blood from lower parts of the body is brought back by the inferior vena cava to the right atrium. Thus, the right atrium receives deoxygenated blood from the superior and inferior vena cava. When the right atrium contracts, the blood flows into the right ventricle through the tricuspid valve.