The respiratory membrane allows gases to be exchanged between the pulmonary capillaries, or blood vessels, and the respiratory units of the lungs, which consist of bronchioles, alveolar ducts, atria and alveoli, according to John E. Hall in the Guyton and Hall Textbook of Medical Physiology. This exchange transports oxygen from the alveoli into red blood cells and carbon dioxide from blood cells into the alveoli.
Certain factors affect the diffusion rate, including the membrane's thickness and surface area and the concentration gradient, notes Hall. An exchange of gases supplies oxygen to the cells and removes carbon dioxide, a waste product that is exhaled by the lungs. The respiratory membrane can be thickened by fluid in the lungs, which forces gases to diffuse through the fluid and the membrane.
Pulmonary diseases that result in fibrosis or scarring of the lungs can also cause the membrane to become thicker. The surface area of the lung is decreased by the removal of a lung or by emphysema, causing the alveoli to coalesce, thus destroying alveolar walls. The loss of alveolar walls decreases the surface area of the respiratory membrane, which impedes the exchange of gases and hinders the ability of the lungs to expel air, explains The McGraw-Hill Human Physiology textbook. The respiratory membrane is coated with mucus, a clear, slippery substance that protects tissues from dust, bacteria and viruses.