The circulatory system and respiratory system work closely together within the body. Oxygen, which is an essential part of the metabolic process of nearly all cells, is gathered through the respiratory system and transported through the bodies of complex organisms, such as humans, through the circulatory system. These two systems also work together to eliminate carbon dioxide, which is a metabolic waste product. In humans, these two systems are connected in the lungs, where the exchange of oxygen and carbon dioxide occurs. These systems are controlled by structures deep within the brain and are largely unconscious processes.
In humans, the path of oxygen through the circulatory system and respiratory system begins with inhalation. When a person inhales, the diaphragm contracts, pulling air into the lungs. The air moves through a series of tubes that lead from the nose and mouth into the lungs. Once air has reached the lungs, it moves into small, specialized structures, known as alveoli, which are surrounded by capillaries.
The alveoli and capillaries in the lungs are the point at which the circulatory and respiratory system meet. When air comes into contact with capillaries, the oxygen in the air diffuses through the capillary walls. This diffusion happens because there is more oxygen in the lungs and less in the surrounding blood. Once oxygen molecules have moved into the blood, they bind to sites on the red blood cells and are carried through the body.
Oxygenated blood moves from the lungs to the heart. Once it reaches the heart, it is pumped into the rest of the body through a powerful contraction. The oxygen-rich blood moves through a series of vessels, some of them large enough for a great volume of blood to move through and some of them so small that red blood cells are able to come into contact with the cells of the body individually. When red blood cells come into contact with other cells, they give oxygen molecules to these cells so that they can metabolize energy; in turn, molecules of carbon dioxide are removed.
Red blood cells then take this carbon dioxide through a different system of blood vessels back to the heart. Once they reach the heart again, they are pumped over to the lungs, where they again come into contact with the alveoli. Here, the circulatory system and respiratory system interact once more when red blood cells release unneeded carbon dioxide back into the lungs as a waste product. When a person exhales, this marks the end of the cycle.