The blood circulation in mammals and birds, that is, in warm-blooded animals, consists of two completely separate circles: small and large. The small circle begins in the right ventricle of the heart and ends in the left atrium. A small circle passes only through the lungs, where the blood is enriched with oxygen. The large circle begins in the left ventricle of the heart and carries oxygen-rich blood to all parts of the body, to all organs. In both circles, blood circulation occurs exclusively due to heart contractions.
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