Heart

Structure

The heart is a hollow, muscular, cone-shaped organ consisting of four chambers. These chambers are the two upper atria (entrance chambers) and the two lower ventricles (exit chambers).

Borders:

The borders of the heart refer to the 2D relations of the anterior surface of the heart and great vessels as they appear in situ. The four borders are the superior, inferior, right, and left borders. As the heart is rotated, the majority of the anterior surface consists of the right atrium and ventricle. The superior border consists of the atria, the base of the superior vena cava, ascending aorta, and pulmonary trunk. The inferior border is formed primarily by the right ventricle and the apex of the left ventricle. The right border consists of the right atrium, while the left border consists of the left ventricle.

External features:

The atria and ventricles are separated externally by the coronary sulcus while the ventricles are separated by the anterior and posterior interventricular sulci. Each of these sulci contains the vasculature of the heart (coronary arteries and cardiac veins).

The heart wall:

The wall of the heart is made of three layers, an outer epicardium, a middle myocardium, and an inner endocardium. Additionally, it has a fibrous skeleton that separates the electrical impulses between the atria and ventricles, while also providing structural support to the valves.

The epicardium consists of the visceral layer of the serous pericardium and contains areolar connective tissue with some adipocytes. The coronary vessels are found within the adipose tissue of the epicardium.

The myocardium is the thickest layer of the heart and consists of cardiac muscle tissue (cardiac myocytes). The cardiac myocytes are the contractile cells of the heart, which provide the pumping force for blood circulation. Myocytes have an oblique arrangement, encircling the cone-shaped ventricles to provide a maximum contractile force, and “wrings” the blood from the chambers (increased ejection fraction). Capillaries and small lymphatic vessels can also be found within the myocardium, supplying and draining the myocytes.

The endocardium is an epithelial lining on the inner surface of the heart chambers. It’s a simple squamous epithelium and is continuous with the endothelium of the blood vessels. Functionally, the endocardium and vascular endothelium are identical, i.e., they provide a smooth, frictionless lining for blood cells traveling through the cardiovascular system and help to eliminate clotting due to stagnation or turbulent blood flow.

The valves:

The heart valves ensure that blood flows only in one direction. The atrioventricular junctions are guarded by the right and left atrioventricular valves. The pulmonary and aortic valves sit between the outflow tract of the ventricles, i.e., the pulmonary trunk and aorta. They’re aided structurally by the fibrous skeleton of the heart.

Conducting system:

The conducting system of the heart is formed by specialized myocytes that transduce contractile impulses. The sinuatrial node (the pacemaker), located in the right atrium, initiates contraction. It receives both parasympathetic and sympathetic fibers as described below. Its impulse travel to the atrioventricular node, then along the atrioventricular bundle which divides into left and right branches. These branches give of the Purkinje fibers which radiate throughout the ventricular walls.

Anatomical Relations

The heart is placed obliquely within the middle mediastinum of the thorax and may be conceptualized as roughly pyramidal in shape. Therefore, it has four surfaces, a base, and an apex. Unlike the heart’s borders, the surfaces refer to the hearts 3D anatomical relations.

The anterior or sternocostal surface is primarily the right atrium and ventricle, while the inferior or diaphragmatic surface consists of the left ventricle. The medial aspects of the left and right lung and their associated pleural linings form the pulmonary or lateral surfaces of the heart.

The base is the posterior aspect of the heart consisting of the left atrium at the level T5–T8. It’s separated from the vertebral column by the structures of the posterior mediastinum including the esophagus, thoracic duct, and descending aorta. The apex of the heart is the tip of the left ventricle and sits at the left midclavicular line, near the fifth intercostal space.

Function

The heart is constantly pumping blood through the pulmonary and systemic circulatory systems and continuously alters its output according to physiological demand; where cardiac output is the function of heart rate and stroke volume. Both atria contract simultaneously and are stimulated by an impulse arising from the sinuatrial node. Additionally, both ventricles contract concurrently and are stimulated by an impulse arising from the atrioventricular node.

The right atrial contraction squeezes the blood through the right atrioventricular valve and into the right ventricle. During right ventricular contraction, blood is forced through the pulmonary valve and into the pulmonary trunk, which carries the blood to the lungs. As the blood reaches the alveoli in the lungs, gaseous exchange occurs, and the blood is oxygenated. This oxygenated blood is then returned to the left side of the heart, the left atrium, via the pulmonary veins.

The left atrium contracts and blood flows through the left atrioventricular valve and into the left ventricle. During ventricular contraction, blood is ejected from the ventricles, through the aortic valve and into the aorta. Just superior to the aortic valve, a small amount of blood is siphoned into the coronary arteries to supply the tissue of the heart itself (coronary circulation).

The rest of the blood continues along the aorta, branches of which carry the oxygenated blood to the tissues throughout the body (systemic circulation). When the blood has emptied its oxygen load, it enters the veins and is returned to the superior and inferior venae cavae. Through these vessels and, in the case of the cardiac tissue, through the coronary sinus, the blood is returned to the right side of the heart (pulmonary circulation) and the cycle begins again.

Arterial Supply

The heart is supplied by the coronary arteries. The left and right coronary arteries branch directly from the ascending aorta, less than 1 cm superior to the aortic semilunar valve.

The vessels and their respective branches are usually embedded in the epicardial adipose tissue. The branches that lie within the atrioventricular and interventricular sulci are useful external anatomical landmarks for locating the divisions of the chambers of the heart.

The right coronary artery supplies the right atrium, right ventricle, the inferior (diaphragmatic) surface of the left ventricle, posterior one third of the interventricular septum, and the atrioventricular node. The left coronary artery supplies a small portion of the right ventricle, the majority of the left ventricle and the anterior two-thirds of the interventricular septum. The coronary vessel that gives off the inferior interventricular branch (or posterior descending artery), thus supplying the sinuatrial node, is described as the side of coronary dominance. In 67% of people, the right coronary artery is the dominant coronary artery (Moore et al., 2013).

Venous Drainage

The heart tissue is drained by a series of cardiac veins, primarily the great, middle, and small cardiac veins. These drain the muscular tissue of the heart into a large coronary sinus. The coronary sinus lies within the posterior coronary sulcus and drains the cardiac veins into the right atrium.

Innervation

As involuntary muscle, the heart is innervated by the sympathetic and parasympathetic divisions of the autonomic nervous system, which form the cardiac plexus. The cardiac plexus is divided into superficial and deep components, which are located around the arch of the aorta and trachea. The right and left coronary plexuses accompany the right and left coronary arteries and innervate the right and left atria and ventricles, respectively. The atrial plexuses provide additional innervation to the atria on both sides. Both the coronary and atrial plexuses are derived from the cardiac plexus.

The heart is constantly adjusting to its physiological demand via parasympathetic and sympathetic innervation. Parasympathetic fibers originate from the vagus nerve (CN X). Preganglionic sympathetic fibers originate from the lateral horns of spinal gray matter in segments T1-T5 of the spinal cord, while postganglionic fibers extend from the cervical, upper thoracic, and cardiac ganglia. The signals sent to the heart via the sympathetic and parasympathetic divisions are regulated by cardiac control centers in the medulla oblongata.

Lymphatic Drainage

A series of small lymphatic vessels are found within the myocardium, and on the surface of the heart and drain into the subendocardial, myocardial, and subepicardial plexuses. These plexuses coalesce into larger cardiac collecting trunks that course superiorly, following the course of the coronary arteries. The collecting trunks find their way to the inferior tracheobronchial and brachiocephalic lymph nodes, where lymph is eventually drained into the bronchomediastinal trunks, and to the right lymphatic duct and thoracic duct.

List of Clinical Correlates

- Abnormalities of cardiac position

- Septal defects (atrial and ventricular)

- Valvular heart disease

- Fibrillation

- Myocardial infarction

- Tetralogy of Fallot

- Coronary artery disease

- Angina

- Coronary bypass

- Electrocardiogram

- Angiogram

- Artificial pacemaker

- Defibrillation

References

Moore, K. L., Dalley, A. F. and Agur, A. M. R. (2013) Clinically Oriented Anatomy. Clinically Oriented Anatomy 7th edn.: Wolters Kluwer Health/Lippincott Williams & Wilkins.