The respiratory system draws air into the lungs. Oxygen diffuses through the walls of the alveoli and across the walls of the capillaries where it enteres the bloodstream. The oxygen molecules are picked up by red blood cells, which are pumped through the blood vessels by the action of the heart. As the red blood cells carrying oxygen pass through capillaries near the cells, oxygen diffuses into the cells.
The respiratory system collaborates with the circulatory system to deliver oxygen and remove metabolic waste. It also aids in blood pH regulation. Respiration is the process through which oxygen and carbon dioxide are exchanged between the atmosphere and the body's cells.
