Gas exchange occurs via simple diffusion in unicellular organisms. However, complex multicellular organisms require a respiratory system for effective gas exchange, which is integrated with the circulatory system to carry gases to and from the tissues. In humans, this system consists of the airways, the lungs, and the respiratory muscles that control the movement of air in and out of the body.
Respiration serves as a means for the body to exchange gases with the atmosphere via blood. Each inhalation transports clean oxygenated air into the lungs, where molecules of oxygen diffuse into the blood. Carbon dioxide molecules simultaneously diffuse from the blood and are transported out of the lungs during exhalation.
