The heart is a pair of muscular pumps united in a single hollow organ. It is enclosed by the pericardium and resides in the thoracic cavity, in the middle mediastinum, between the lungs and their pleural coverings. The heart lies obliquely behind the sternal body and the adjoining cartilages and ribs. It has a general shape that resembles that of a pyramid, and a series of borders and surfaces. Its base faces posteriorly and its apex projects forwards, downwards, and to the left.
The heart's self-adjusting suction and pressure pumps work together to distribute blood to all parts of the body. The right side of the heart receives poorly oxygenated venous blood from the body via the superior and inferior vena cava, and then pumps it to the pulmonary trunk and arteries to reach the lungs for oxygenation. The left side of the heart, on the other hand, receives well oxygenated arterial blood from the lungs via the pulmonary veins, and pumps it into the aorta to be distributed.
The heart consists of four chambers: the right and left atria, and the right and left ventricles. Each pump consists of one atrium and one ventricle separated by a valve. Commonly, the right atrium and ventricle are referred together as the right heart and their left counterparts as the left heart. The atria are thin walled and receive blood coming into the heart, whereas the thick walled ventricles expel the blood out.
There are four valves present in the heart. The mitral valve (bicuspid valve) and the tricuspid valve are located between the atria and the ventricles, while the aortic valve and the pulmonary valve are located within the arteries leaving the heart. The normal lub dub sounds of the heart are produced by the closure of the one way valves, which prevents blood from flowing backward during heart contractions.
The wall of each heart chamber consists of three layers. The endocardium is a thin, internal layer that lines the heart and its valves. The myocardium is a thick, middle layer comprised of cardiac muscles. The epicardium is a thin, external layer formed by the visceral layer of the serous pericardium. The myocardium makes up most of the walls, especially in the ventricles. This is because more force is needed to pump blood through the body than through the lungs. The muscle fibers of the heart are anchored to the cardiac skeleton.
The cardiac skeletal framework is a complex collection of dense fibrous connective tissue that forms four fibrous rings. These rings surround the two atrioventricular orifices, the aortic orifice, and the pulmonary trunk opening. The cardiac skeleton helps maintain the integrity of the openings it surrounds, and provides a site of attachment for the myocardium and valves. It also serves as a dense connective tissue partition that insulates the atria from the ventricles so that they contract independently. The atrioventricular (AV) bundle is the only connection between these two groups of myocardium.
The heart's vascular supply is provided by the right and left coronary arteries that arise from the ascending aorta. It is drained by the coronary sinus and its tributaries, the anterior cardiac and small cardiac veins. The heart is innervated by both parasympathetic and sympathetic fibers from the autonomic nervous system.