The muscular pump in the centre of the chest that beats continuously and rhythmically to send blood to the lungs and the rest of the body. Much of the heart consists of myocardium, a special type of muscle. The heart muscle is supplied with oxygen and nutrients by 2 coronary arteries. The internal surface of the heart is lined with a smooth membrane, called endocardium, and the entire heart is enclosed in a tough, membranous bag, the pericardium. Inside the heart there are 4 chambers. A thick central muscular wall, the septum, divides the heart cavity into right and left halves. Each half consists of an upper chamber, called an atrium, and a larger lower chamber, called a ventricle. The right atrium receives deoxygenated blood from the entire body via 2 large veins called the venae cavae. This blood is transferred to the right ventricle and pumped to the lungs via the pulmonary artery to be oxygenated and to lose carbon dioxide. The left atrium of the heart receives oxygenated blood from the lungs (via the pulmonary veins); this blood is transferred to the left ventricle and then pumped to all tissues in the body. One-way valves at the exits from each chamber ensure that blood flows in only 1 direction (see heart valves). As resistance to blood flow through the general circulation is much greater than resistance through the lungs, the left side of the heart must contract more forcibly than the right, and has greater muscular bulk.


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