The human heart is sometimes called a double pump because it drives blood along two separate pathway in the circulatory system.
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Human heart is a  double pump because it pumps blood towards two destinations, the lungs and to different parts of the body or the pulmonic circulation and the systemic circulation. the pulmonic is done by the right side where blood is pump towards the lungs to collect oxygen and release carbon dioxide while the systemic is done by the left side that pumps blood towards different parts of the body to distribute oxygen and collect carbon dioxide