The respiratory system is the system responsible for gaseous exchange in animals. In humans, this is the mouth, trachea, bronchial tubes, which lead into the lungs, which contain the alveoli, where oxygen is exchanged for carbon dioxide. The circulatory system is the system in the body that is responsible for pumping blood to every cell in the body. This would include the heart, all the blood vessels, which are the arteries, veins, and capillaries, and the blood. Consider the blood the "super-highway" for transportation in the body. If it has to get there, it will be transported by the blood. The oxygen breathed into the lungs attaches to a red blood cells heme molecule and is transported to a cell, where it is used in the process known as cellular respiration to create energy for that cell. Carbon dioxide is a waste product formed as a result of this process, and is taken back to the lungs to be exhaled by the blood. Oxygen in, carbon dioxide out, courtesy of the respiratory system. The respiratory and circulatory systems work in tandem to provide gaseous exchange for the body.