Mechanisms of Breathing

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.

The intra-alveolar pressure is the pressure of air in the alveoli. It is also called the intrapulmonary pressure. It is always equalized with the atmospheric pressure. The transpulmonary pressure is the pressure between the lungs and the pleural space. It is about 4 millimeters of mercury different from the atmospheric pressure. The intrapleural pressure is the pressure between the parietal and visceral pleura. It is negative 4 millimeters compared to the intra-alveolar pressure.

There are forces within the thorax that lead to the negative intrapleural pressure. One of these is the elasticity of the lungs themselves. This pulls the lungs away from the thoracic wall. The surface tension of the fluid in the alveolus creates an inward pull on the lung tissue. The opposing force in the lungs is the surface tension in the pleural cavity, which pulls the lungs outward. This depends on the having the right amount of fluid in the intrapleural space. Finally, the elasticity of the chest wall opposes the inward pull. The outward pull is greater than the inward pull so as to keep the lungs expanded.

Breathing depends on the contraction and relaxation of the different muscle fibers of the thorax and diaphragm, with the lungs being passive in the breathing process. The negative intrapleural pressure keeps the lungs adherent to the chest wall when breathing happens. It’s the muscle movements that allow the air to enter or leave the lungs. One of the main things that affect airflow is the resistance generated by the diameter of the airways.

The thoracic wall compliance is also a factor. This compliance is the ability of the thoracic wall to stretch under pressure. The thoracic cavity needs to expand in order for inspiration to occur and the chest wall compliance plays a big role. Things like obesity can affect thoracic wall compliance, which affects the amount of air that can get into or outside the lungs.

The basic driving force behind pulmonary ventilation is the flow of air down a pressure gradient. Air will flow into the lungs due to pressure differences between the atmospheric pressure and the intra-alveolar pressure. The air flows out for the same reason—an increased intra-alveolar pressure versus the atmospheric pressure.

There are two main events that happen in pulmonary ventilation. The first is inspiration and the second is expiration. One respiratory cycle is inspiration plus expiration. There