The heart is the only hollow muscle in the body. It is about the size of a fist. What is its function? To circulate the blood around our body by pumping it. This work is carried out in two chambers: the ventricles. Inside the heart, the ventricles are equipped with very muscular walls which can contract strongly. The left ventricle is the more powerful; it is this, in fact, which sends the blood through the whole body.  The blood first passes through the biggest artery, the aorta; this big vessel then sub-divides into small units, so that numerous little vessels (called capillaries) take over the blood. Also, the oxygen and food are distributed and the carbon monoxide and the waste is stored up! The veins (return vessels) meet in the vena cava. The blood then reaches the right ventricle through and opening called the auricle. The blood continues its long and complicated journey towards the lungs which are near the heart. On the way out of the lungs, the blood, cleared of the carbon monoxide, is loaded with oxygen. It enters the left ventricle by the left auricle and the circuit starts again.
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