The heart (latin: cor) is a thick, muscular organ with four cavitated parts located in the middle portion of the inferior mediastinum. The heart acts as a central pump of the cardiovascular system that maintains unidirectional flow of blood.
The heart has a cone shape with the base of the heart directed upward, to the right and posteriorly, while the apex of the heart is directed downward, to the left and anteriorly. The heart is composed of four chambers - two atria (right and left) and two ventricles (right and left). The left atrium and the left ventricle together form the arterial part of the heart, while the right atrium and the right ventricle compose the venous part.
The wall of the heart is formed by three layers - the endocardium (inner layer), myocardium (middle layer) and epicardium (outer layer). The endocardium is the epithelial layer lining all the cavities of the heart, the myocardium is the thick muscular layer formed by cardiac muscles, and the epicardium is a serous membrane of the heart.
The heart is surrounded by a closed fibrous sac called the pericardium. The pericardium has an outer (fibrous) layer and an inner (serous) layer. The serous layer has two lamina - the parietal and the visceral laminae. The visceral lamina of the serous pericardium is the same epicardium that forms the outer layer of the heart. The space between the parietal and visceral laminae of the serous pericardium is called the pericardial cavity, and it contains small amount of serous fluid that decreases the surface tension and lubricates, allowing free movement of the heart during contraction.
The constant and rythmic contractions of the heart are due the conducting system of the heart, with the rythm generating normally in the sinuatrial node. The conducting system coordinates the contractions of the myocardium of the atria and ventricles of the heart. However, the heart rate can be altered by nerve impulses from the sympathetic and parasympathetic parts of the nervous system, which provide innervation to the heart. The parasympathetic nerve fibers supplying the heart originate from the vagus nerve (CN X) and terminate at the sinuatrial and atrioventricular nodes, slowing the heart rate and reducing the force of the heartbeat. The sympathetic innervation comes from the cervical ganglia of the sympathetic trunk via the cardiac nerves, causing the rate and force of cardiac muscle contraction to increase.