regulation of oxygen and carbon dioxide levels are determined by the autonomic nervous system as well. Sensation of the lungs comes from the vagus nerve. The pulmonary plexus is formed by nerves located at the hilum. The nerves follow the bronchi, branching to serve the blood vessels, glands, and muscle fibers that are a part of the lungs. The lining of the lungs involves a serous membrane called the pleura. There are two layers of the pleura. There is a visceral pleura that is directly attached to the lung surface and that goes into the fissures of the different lobes. The parietal pleura is adherent to the chest wall, the diaphragm, and the mediastinum. Between the two layers is the pleural cavity. A few millimeters (10 to 20 milliliters) of pleural fluid is made by the mesothelial layers from both pleural layers, which lubricates the surfaces. The fluid also creates surface tension to maintain the lung position during breathing and is adhesive enough to allow the lungs to enlarge during inspiration. Finally, it acts to prevent movement of pathogens between the different parts of the lungs.
MECHANISMS OF BREATHING Breathing involves changing the intra-alveolar pressure in order to have air enter and leave the lungs. The ability to breathe depends on the pressure of air in the atmosphere and the pressure of air in the lungs. Both inspiration and expiration depend on the differences in pressure between the lungs and the atmosphere. There are several gas laws that apply to breathing. The first is Boyle’s law, which states that, at a constant temperature, the change in volume of a container will change its pressure. As the volume is decreased the pressure is increased. There are three pressures that play a role. The first is the atmospheric pressure, which is the air pressure in the surrounding air. It is equal to 1 atmosphere or 760 millimeters of mercury. For respiration, however, the atmospheric pressure is set at zero, with positive pressures being greater than atmospheric pressure and negative pressures being less than atmospheric pressure.
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