A circulatory system in which blood travels twice through the heart in one complete cycle of the body is called double circulation. In the human circulatory system, the pathway of blood from the heart to the rest of the body and back to the heart is called systemic circulation , and the pathway of blood from the heart to the lungs and back to the heart is called pulmonary circulation. These two types of circulation taken together make double circulation. The double circulation is necessary to supply oxygenated blood to the whole body (except lungs), and then to get deoxygenated blood reoxygenated in the lungs. (For detailed description of double circulation in human beings, see page 61 of this book).
