Millions of cilia beat in a coordinated manner so that they can propel a continuous stream of mucus from the lungs to the nasal cavity. Thus, any small particles that have entered the lungs, despite the protective filtering of incoming air by the turbinate bones, can be removed. In mammals, the alveoli are the final blind-ending branches of the air duct system. Beneath the moist epithelium which lines each alveolus is an extensive meshwork of lung capillaries. Oxygen is taken up by the blood in a loose combination with the hemoglobin of red blood cells or erythrocytes.
Carbon dioxide is more soluble and diffuses faster than oxygen.
The ratio of bicarbonate to carbonic acid determines the pH or acidity of the blood. Gaseous exchange does not occur across the walls of the major air ducts that lead into the lungs. Thus, the last fraction of air that is inhaled becomes the first fraction to be exhaled, and the oxygen it contains is not utilized. The rate of respiration is controlled by the medulla oblongata in the posterior part of the brain. When the lungs are removed from the body, slippery pleural membranes may be seen covering both the inner surface of the thoracic cavity and the lung surface.