The heart and blood vessels form a closed system through which blood keeps circulating because of the pumping action of the heart. However, blood actually circulates in two different loops within this closed system. The two loops are called the pulmonary circulation and the systemic circulation. In both loops, blood passes through the heart. As blood circulates throughout the body, it travels first through one loop and then the other loop, over and over again.     The pulmonary circulation is the shorter loop of the cardiovascular system. It carries blood between the heart and lungs.... Show more

The heart and blood vessels form a closed system through which blood keeps circulating because of the pumping action of the heart. However, blood actually circulates in two different loops within this closed system. The two loops are called the pulmonary circulation and the systemic circulation. In both loops, blood passes through the heart. As blood circulates throughout the body, it travels first through one loop and then the other loop, over and over again.    
The pulmonary circulation is the shorter loop of the cardiovascular system. It carries blood between the heart and lungs. Oxygen-poor blood flows from the heart to the lungs. In the lungs, the blood absorbs oxygen and releases carbon dioxide. Then the oxygen-rich blood returns to the heart.    
The systemic circulation is the longer loop of the cardiovascular system. It carries blood between the heart and the rest of the body. Oxygen-rich blood flows from the heart to cells throughout the body. As it passes body cells, the blood releases oxygen and absorbs carbon dioxide. Then the oxygen-poor blood returns to the heart.    

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