The Diaphragm

37.4 When the diaphragm contracts, it becomes flatter and drops, and increases the volume of the thoracic cavity (Applegate, 2000, 313). The parietal and visceral layers of the pleura adhere to each other because of an attraction between the water molecules in the serous fluid between the two layers. This increases the volume of the thoracic cavity and causes the lungs to expand their volume. The intrapleural pressure is less than the pressure within the lungs, which also contributes to lung expansion. During labored breathing, the external intercostal muscles and other muscles of respiration work with the diaphragm to increase the volume of the thoracic cavity.

37.5 The physiological advantage of a substantial increase in chest circumference during forced inspiration is that more air enters the lungs and therefore enables a higher rate of oxygenation of the blood.

37.6 An opening in the chest cavity, known as a pneumothorax, causes the pressure in the pleural space to rise above that in the lungs (it is normally lower), and the lung collapses partially or completely (Berkow, Beers and Fletcher, 1997, 208).

37.7 A small pneumothorax usually requires no treatment, and the air is usually absorbed within a few days and causes no breathing problems (Berkow, Beers and Fletcher, 1997, 209). In a larger pneumothorax, the air may take from two to four weeks to be absorbed, but it can be removed more quickly by the placement of a chest tube into the pneumothora

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