Where the heart lies

Where the heart lies The heart lies within the mediastinum, a cavity that contains the tissues and organs separating the two pleural sacs. In most people, two-thirds of the heart extends to the left of the body's midline. Where the heart lies

As a person ages, his heart usually becomes slightly smaller and loses its contractile strength and efficiency. In persons with hypertension, a moderate increase in left ventricular wall thickness may occur. As the myocardium of the aging heart becomes more irritable, extrasystoles may occur, along with sinus arrhythmias and sinus bradycardias. In addition, increased fibrous tissue infiltrates the sinoatrial (SA) node and internodal atrial tracts, which may cause atrial fibrillation and flutter. By age 70, cardiac output at rest has diminished by 30% to 35% in many people. Where the heart lies The heart wall, which encases the heart, is made up of three layers: epicardium, myocardium, and endocardium. The epicardium, the outermost layer, consists of squamous epithelial cells overlying connective tissue. The myocardium, the middle and thickest layer, makes up the largest portion of the heart's wall. This layer of muscle tissue contracts with each heartbeat. The endocardium, the heart wall's innermost layer, consists of a thin layer of endothelial tissue that lines the heart valves and chambers. (See Layers of the heart wall, page 4.) P.4Layers of the heart wall This cross section of the heart wall shows its various layers.