The heart is a four-chambered muscular pump that is located within the mediastinum in the thoracic cavity. It lies between the lungs and just superior to the diaphragm. The apex of the heart is the inferior pointed end, which extends toward the left side of the thoracic cavity at the level of the fifth rib. The base of the heart is the superior portion, which is attached to several large blood vessels at the level of the second rib. The heart is about the size of a closed fist.

Protective Coverings

The heart and the bases of the attached blood vessels are enveloped by membranes that are collectively called the pericardium (per-i-kar’-de-um). An external, loosely fitting pericardial sac separates the heart from surrounding tissues and allows space for the heart to expand and contract as it pumps blood. The pericardial sac consists of two membranes: an external fibrous pericardium and an internal parietal layer of serous pericardium.

The fibrous pericardium is a tough, unyielding membrane composed of dense irregular connective tissue. It is attached to the diaphragm, internal surfaces of the sternum and thoracic vertebrae, and to adjacent connective tissues. The delicate parietal pericardium lines the internal surface of the fibrous pericardium. At the bases of the large vessels (base of the heart), the parietal layer of serous pericardium folds back to form the epicardium (visceral layer of serous pericardium), which forms the thin membrane that tightly adheres to the surface of the heart. The potential space between the parietal pericardium and the epicardium is the pericardial cavity. This cavity is filled with pericardial fluid, which reduces the friction between the two layers of the pericardium when the heart contracts and expands.

The Heart Wall

The wall of the heart consists of a thick layer of cardiac muscle tissue, the myocardium (mi-o-kar i-de-um), sandwiched between two thin membranes. Contractions of the myocardium provide the force that pumps the blood through the blood vessels. The epicardium is the thin membrane that is firmly attached to the external surface of the myocardium. Blood vessels that nourish the heart itself are located within the epicardium. The internal surface of the myocardium is covered with a simple squamous epithelium called the endocardium. The endocardium not only lines the chambers and valves of the heart, but also is continuous with the internal lining of the blood vessels attached to the heart.

Heart Chambers

The two superior chambers are the atria (a^tre-ah) (singular, atrium), which receive blood being returned to the heart by the veins. The two inferior chambers are the ventricles (veni-tri-kuls), which pump blood into the arteries. There is no opening between the two atria or between the two ventricles. The atria are separated from each other by a partition called the interatrial septum. The ventricles are separated by the interventricular septum, a thick partition of cardiac muscle tissue. The heart is a double pump. The right atrium and right ventricle compose the right pump. The left atrium and left ventricle compose the left pump.

The walls of the atria are much thinner than the walls of the ventricles. Differences in thickness are due to differences in the amount of cardiac muscle tissue that is present, which in turn reflects the work required of each chamber. Atrial walls possess less cardiac muscle tissue because blood movement from atria to ventricles is mostly passive, so that force from contraction is not as essential. The ventricles have more cardiac muscle tissue in order to create enough force to push blood superiorly out of the heart. The left ventricle has a thicker, more muscular wall than the right ventricle because it must pump blood throughout the entire body, except the lungs, whereas the right ventricle pumps blood only to the lungs.

Functions of The Heart Chambers

Chamber Function
Right atrium Receives deoxygenated blood from the superior and inferior venae cavae and the coronary sinus, and passes this blood through the tricuspid valve to the right ventricle
Right ventricle Receives deoxygenated blood from the right atrium and pumps this blood through the pulmonary valve into the pulmonary trunk
Left atrium Receives oxygenated blood from the pulmonary veins and passes this blood through the mitral valve to the left ventricle
Left ventricle Receives oxygenated blood from the left atrium and pumps this blood through the aortic valve into the aorta

Heart Valves

Like all pumps, the heart contains valves that allow the blood to flow in only one direction through the heart. The two types of heart valves are atrioventricular valves (AV valves) and semilunar valves.

Atrioventricular Valves

The opening between each atrium and its corresponding ventricle is guarded by an atrioventricular (ea-tree-eo-ven- triki-u-lar) valve that is formed of dense irregular connective tissue. Each valve allows blood to flow from the atrium into the ventricle but prevents a backflow of blood from the ventricle into the atrium. The AV valve between the right atrium and the right ventricle is the tricuspid (tri-kus i-pid), or right atrioventricular, valve. Its name indicates that it is composed of three cusps, or flaps, of tissue. The mitral (mistral), or left atrioventricular, valve consists of two cusps and is located between the left atrium and the left ventricle.

The AV valves originate from rings of thick, dense irregular connective tissue that support the junction of the ventricles with the atria and the large arteries attached to the ventricles. This supporting dense irregular tissue is called the fibrous skeleton of the heart. The fibrous skeleton not only provides structural support but also serves as insulation separating the electrical activity of the atria and ventricles. This insulation enables the atria and ventricles to contract independently.

Thin strands of dense irregular connective tissue, the chordae tendineae (kor-de- ten-di-ne-ee), extend from the valve cusps to the papillary muscles, small mounds of cardiac muscle tissue that project from the internal walls of the ventricles. The chordae tendineae prevent the valve cusps from being forced into the atria during ventricular contraction. In fact, they are normally just the right length to allow the cusps to press against each other and tightly close the opening during ventricular contraction.

The Functions of The Heart Valves

Valve Location Function
Atrioventricular Valves
Tricuspid valve Opening between the right atrium and right ventricle Prevents backflow of blood from the right ventricle into the right atrium
Mitral valve Opening between the left atrium and left ventricle Prevents the backflow of blood from the left ventricle into the left atrium
Semilunar Valves
Pulmonary valve Entrance to the pulmonary trunk Prevents backflow of blood from the pulmonary trunk into the right ventricle
Aortic valve Entrance to the aorta Prevents backflow of blood from the aorta into the left ventricle

Flow of Blood Through The Heart

Blood is oxygenated as it flows through the lungs and becomes deoxygenated as it releases oxygen to body tissues.

The right atrium receives deoxygenated blood from all parts of the body except the lungs via three veins: the superior and inferior venae cavae and the coronary sinus. The superior vena cava (ve’-nah ka’-vah) returns blood from the head, neck, shoulders, upper limbs, and thoracic and abdominal walls. The inferior vena cava returns blood from the inferior trunk and lower limbs. The coronary sinus drains deoxygenated blood from cardiac muscle tissue. Simultaneously, the left atrium receives oxygenated blood returning to the heart from the lungs via the pulmonary veins.

Blood flows from the left and right atria into the corresponding ventricles. About 70% of the blood flow into the ventricles is passive, and about 30% results from atrial contraction.

After blood has flowed from the atria into their respective ventricles, the ventricles contract. The right ventricle pumps deoxygenated blood into the pulmonary trunk. The pulmonary trunk branches to form the left and right pulmonary arteries, which carry blood to the lungs. The left ventricle pumps oxygenated blood into the aorta (a-oL-tah). The aorta branches to form smaller arteries that carry blood to all parts of the body except the lungs.

Because the heart is a double pump, there are two basic pathways, or circuits, of blood flow. The pulmonary circuit carries deoxy- genated blood from the right ventricle to the lungs and returns oxygenated blood from the lungs to the left atrium. The systemic circuit carries oxygenated blood from the left ventricle to all parts of the body except the lungs and returns deoxygenated blood to the right atrium.

Blood Supply To The Heart

The heart requires a constant supply of blood to nourish its own tissues. Blood is supplied by left and right coronary arteries which branch from the aorta just distal to the aortic valve. Blockage of a coronary artery may result in a heart attack. After passing through capillaries in cardiac muscle tissue, blood is returned via cardiac veins, which lie next to the coronary arteries. These veins empty into the coronary sinus, which drains into the right atrium. Also, study the relationships of the atria, ventricles, and large blood vessels associated with the heart.