Pneumocytes are a type of cell that lines the air sacs, or alveoli, of the lungs. The alveoli are the site of gas exchange in the body, which refers to the process in which oxygen is drawn from inhaled air into the bloodstream, and carbon dioxide (CO2) is removed from the blood and exhaled. Pneumocytes mainly act by supporting the functions of the alveoli, and can be separated into two subtypes: type I and type II.
Type I pneumocytes are long and thin cells that are flattened over a large area, and therefore account for approximately 95 percent of the alveolar surface area even though they represent only about 40 percent of the actual cells. These cells form the alveolar wall structure, allow the exchange of oxygen and carbon dioxide in the alveoli, and help control the movement of fluid between the interstitium and the airspace. The thinness of these cells makes them particularly susceptible to either mechanical or toxic injury, and they lack mitotic potential and are therefore unable to regenerate.
The second type of alveolar cell, type II pneumocytes, are small, plump cells that account for approximately 3 percent of the alveolar surface and represent about 60 percent of the actual cells. Unlike type I cells, these cells do have mitotic potential, which allows them to proliferate and to differentiate into the crucial type I cells, replacing them after injury. An important function of type II cells is the production and secretion of pulmonary surfactant — a fluid that coats the air sacs and reduces the amount of work required by the lungs for breathing.
Pulmonary surfactant is a protein and phospholipid mixture that completely coats the alveoli and contributes to the elasticity of the lungs. The two major functions of pulmonary surfactant is to increase the gas exchange capability of the lungs, and to reduce the surface tension of the alveoli, which decreases the amount of effort required to inflate the lungs during inspiration. If the lungs have an insufficiency of this fluid, they may be prone to partial or total collapse — also known as atelectasis.
Dust cells, or alveolar macrophages, are a phagocytic type of cell located near the pneumocytes. They are positioned at a major boundary between the body and the outside world — an area where the host is particularly vulnerable to invading pathogens, toxins, and foreign substances — and host defense is their primary function. Dust cells normally respond to foreign substances by engulfing and digesting them; however, in the case of a threat that is too large to be controlled by phagocytosis alone, these cells are also capable of releasing an array of proinflammatory cytokines to call on the body’s larger immune response.