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The heart functions as a pump at the centre of the circulatory system. In humans it is located in the chest cavity, between the lungs, comparedrather thana bitless than a bit to the left. The heart consists of four chambers surrounded by a very strong muscular wall, the myocardium. The upper chambers, the right and left atria, lowerreceiverepeltransfer receive blood entering the heart, and the lower chambers, the right and left ventricles pump the blood out of the heart, via the pulmonary and the systemic circulatory systems. The two systems work as followingfollowedfollowsfollow follows . Blood from the body enters the right atrium, beingishashad is passed into the right ventricle and from there is propelled through the pulmonary artery to the lungs. In the lungs the blood releases carbon dioxide and absorbs oxygen and is then dissolvedtransporteddischargedmultiplied transported back to the heart into the left atrium. From here it passes into the left ventricle, which pumps the oxygenated blood around the body.