The parasympathetic and sympathetic nervous system both innervate the lungs. These control the dilation and constriction of the airways. The parasympathetic nervous system results in bronchoconstriction, while the sympathetic nervous system causes bronchodilation. The autonomic nervous system also helps regulate the cough reflex and the regulation of the oxygen and carbon dioxide levels in the lungs. The lining of the lungs is called the pleura. There is a right and left pleural lining that cover the lungs as the visceral pleura. This is the layer that is closest to the lungs and lines the fissures of the lobes of the lungs. The parietal pleura is a reflection of the visceral pleura and lines the inner aspect of the thoracic wall. The space between these two linings is called the pleural cavity. The pleurae form two major functions. First, they produce pleural fluid, which is secreted by mesothelial cells (on both layers) and lubricates the space between the two layers. This reduces friction between the two layers. It also creates “surface tension” that keeps the two layers in contact with one another. The pleurae also prevent the spread of infection by separating the different organs in the chest cavity.
PULMONARY VENTILATION Pulmonary ventilation is also referred to as the “act of breathing” or the movement of air into and out of the lungs. There are three pressures involved in the act of breathing: the atmospheric pressure, the alveolar pressure, and the intrapleural pressure (the pressure inside the pleural cavity). The act of breathing depends on the relationship between these pressures. Atmospheric pressure is the amount of force exerted by gases in the air around the body. This is expressed in millimeters of mercury. Air pressure is about 760 mm Hg, which is also called “one atmosphere.” This is actually the air pressure that exists in the environment at sea level. When it comes to pulmonary ventilation, a pressure is said to be “negative” if it is less than atmospheric pressure and “positive” if it is greater than atmospheric pressure. This means that at one atmosphere or the air pressure outside, the pressure is considered to be “zero.” The intra-alveolar pressure is the pressure of air within the alveoli, which changes during the different phases of breathing. This pressure eventually equalizes with the atmospheric pressure 281
