The Mechanics of Breathing

1. Relationships of Pressure
• atmospheric air pressure - 760 mm Hg (at sea level)
• negative air pressure - LESS than 760 mm Hg
• positive air pressure - MORE than 760 mm Hg
• intrapleural pressure - pressure within the pleural "balloon" which surrounds the lung
• intrapulmonary pressure - pressure within the alveoli (tiny sacs) of the lung itself
2. Factors holding lungs AGAINST the thorax wall:
• Surface tension holding the "visceral" and "parietal" pleura together
• Intrapulmonary pressure ALWAYS slightly greater than intrapleural pressure by 4 mm Hg
• Atmospheric pressure acting on the lungs
• atelectasis (collapsed lung) - hole in pleural "balloon" causes equalization of pressure and collapse of the lung
• pneumothorax - abnormal air in the intrapleural space, can lead to collapsed lung
  

3. Factors facilitating lung movement AWAY from thorax wall:
• Elasticity of lungs allows them to assume smallest shape for given pressure conditions
• Fluid film on alveoli allows them to assume smallest shape for given pressure conditions

Category: Physiology Notes