The circulatory system's main organ is the heart whereas the respiratory system's main organ is the lungs
Circulatory system pumps blood throughout the body. It consists of the heart, blood, the blood vessels (arteries, veins, and capillaries) and lymph nodes.
The respiratory system takes in oxygen and expels carbon dioxide. It consists of the nose, pharynx, larynx, trachea, bronchi, bronchioles, alveoli (air sacs, where the gas exchange occurs) in the lungs.