Human Heart — Structure and Blood Vessels

The heart is the muscular pump that drives the entire circulation, roughly the size of its owner's closed fist, located in the thoracic cavity between the two lungs and tilted slightly towards the left side of the body. It is enclosed within a tough, double-layered membranous sac, the pericardium, which contains a small amount of fluid between its two layers to reduce friction as the beating heart moves within it.

Internally, the human heart is divided into four separate chambers by a system of muscular partitions (septa). The two upper chambers, the right atrium and the left atrium, have comparatively thin walls, since their job is simply to receive incoming blood and pass it on to the ventricle just below; they are separated from each other by the inter-atrial septum. The two lower chambers, the right ventricle and the left ventricle, have much thicker, more muscular walls, since their job is to generate the force needed to pump blood out of the heart altogether; they are separated from each other by the inter-ventricular septum. Notably, the left ventricle's wall is markedly thicker than the right ventricle's, because the left ventricle must generate enough pressure to drive blood all the way around the systemic circuit to the entire body, whereas the right ventricle only has to pump blood the comparatively short distance to the adjoining lungs. Each atrium is separated from the ventricle directly beneath it by an atrio-ventricular septum, in which the corresponding atrio-ventricular valve is set.

Four valves within the heart ensure that blood always flows in one direction and is never allowed to flow backwards. The tricuspid valve, made of three flap-like cusps, guards the opening between the right atrium and the right ventricle. The bicuspid valve, also called the mitral valve because of its resemblance to a bishop's mitre, has two cusps and guards the opening between the left atrium and the left ventricle. A further pair of pocket-shaped semilunar valves guards the exit of each ventricle into its great artery: the pulmonary semilunar valve at the base of the pulmonary artery, where it leaves the right ventricle, and the aortic semilunar valve at the base of the aorta, where it leaves the left ventricle. All four valves open and close purely in response to the pressure differences generated on either side of them as the heart contracts and relaxes, snapping shut the instant pressure downstream threatens to push blood back the way it came. …