Among its many functions, the lung has two major ones: it must harvest oxygen to fuel aerobic respiration and it must vent acid-forming carbon dioxide. This chapter will offer a brief overview of how the lung fulfills these functions. It will also discuss some of the mechanisms through which adequate oxygenation can fail. A secure understanding of these principles allows an insight into the way in which mechanical ventilation strategies can be altered in order to enhance oxygenation and carbon dioxide clearance.

Functional anatomy of the lung

The airways

During inspiration, air is drawn into the oropharynx through either the mouth or the nasal airway. Nasal breathing is preferred, as it is associated with enhanced particle removal (by nasal hairs and mucus-laden turbinates) and humidification. However, this route is associated with a fall in pharyngeal pressure. Just as Ohm's law dictates that voltage is the product of current and resistance, so pharyngeal pressure is the product of gas flow and pharyngeal resistance. A ‘fat apron’ around the pharynx because of obesity may lead to increased pharyngeal compliance, and thus increase the risk of dynamic pharyngeal collapse in such patients. In adults, when pharyngeal flows exceed 30 to 40 litres per minute, the work of breathing becomes high and the fall in pharyngeal pressure too great for the adequate intake of air: the mouth then becomes the preferred route for breathing.