The body consists of a pump section and a stimulating portion, which anatomically closely related but functionally should be clearly distinguished. The heart of an adult weighs about 300 grams and is as big as a fist. The heart wall consists of four layers: the endocardium, the myocardium (muscle layer of the heart), and epicardium as the outermost layer of the pericardium (pericardium). The heart has two ventricles or atria furthermore (plural) and two ventricles, or chambers: a right atrium and right ventricle and an atrium and a left a left ventricle. Between the atrium and ventricle occurs in both halves of a heart valve, as well as between ventricles and arteries.