Physiology of the Heart and Lungs:
The used (oxygen-poor) blood in the main blood vessels (veins) enters the heart from the right atrium. Oxygen has been removed from the used blood, by your body's organs and tissues. The used blood then travels to the right ventricle, which in turn pumps the blood from the heart to the lungs. When you are breathing air in, the oxygen passes from your lungs to the blood vessels where it is added to your blood. Carbon dioxide, a waste product, is passed from the used blood, to the lungs and is removed from your body when you are breathing out. The oxygen-rich blood enters the heart from the lungs to the left atrium. The left atrium pumps the oxygen-rich blood, to the left ventricle which then pumps through the aorta (main blood vessel), to the rest of the body.
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