Human Circulatory System

The human heart, first accurately described by Raymond de Vieussens in 1706, is built from a unique muscle type - cardiac muscle - found nowhere else in the body. It sits in the thoracic cavity with its apex tilted slightly to the left, weighs about 300 g in an adult, and is roughly the size of a closed fist. The heart has four chambers: two smaller upper chambers, the atria (or auricles), and two larger, thicker-walled lower chambers, the ventricles - the ventricle walls are thicker partly because of internal muscular ridges called papillary muscles. The heart wall itself is three-layered: an outer epicardium, a middle, muscular myocardium, and an inner endocardium, with a fluid-filled pericardial space (containing pericardial fluid) surrounding the whole organ (see Figure 7.7a-b). An inter-atrial septum separates the two atria and an inter-ventricular septum separates the two ventricles, keeping oxygenated and deoxygenated blood from mixing. Each atrium communicates with the ventricle below it through an atrioventricular (AV) aperture guarded by an AV valve: the right AV opening is guarded by the three-flapped tricuspid valve, and the left AV opening is guarded by the two-flapped bicuspid (mitral) valve; both valve types allow blood to flow only one way, atria to ventricles. Where the ventricles open into the great arteries, semilunar valves - the pulmonary valve and the aortic valve, each made of three half-moon-shaped cusps - allow one-way flow from ventricles outward and prevent any backflow; these valves are opened and closed by fibrous chordae tendineae, which run from …