Pure blood enter through the left atrium in heart and send to lung then it gets o2 from alveoli the through blood vessels it circulate around the body and enters the lungs takes co2 thenafter releasing it enters the heart in right atrium and purified 
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the circulation in the heart of humans considered as transportation system in human beings which is THE CARDIO-VASCULAR SYSTEM . 
  the heart pumps the blood around the tissues through  a vasculature . 
    in humans heart is made up 4 chambers and valves with  a septum .
                    In which 2 are atria and 2 ventricles .
        The work of atrium is to collect the blood which are coming from different body tissues and it pushes toward ventricles and ventricles receives the blood from atrium and pumps it powerfully towards the tissues .
    The oxygenated blood to different body parts through aorta then it branches to form arteries that carries blood  to tissues and terminate into capillaries and it helps to exchange the oxygen to carbon dioxide and it joins to veins and from veins from the body parts join to form superior and inferior vena cava it pour the blood into RA and it squeezes blood to RV and then it pumps it via pulmonary arteries . 

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