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.
Pleural membranes prevent friction between the lungs and the body wall.
Inspiration and expiration Caused by movements of the intercostal muscles, the ribs, the diaphragm and, sometimes, the abdominal muscles. The diaphragm resembles a strong drumskin that divides the thoracic and abdominal cavities, but it is thickened by muscle where it joins the body wall. In a dressed carcass, the muscular part of the diaphragm remains as a flap of muscle running diagonally across the inside of the ribcage. Protein may be isolated as a powder or texturized to form fibers. Lungs also may be processed to isolate heparin, an anticoagulant for medical use.