Answer
CTQ 22.2a
Each lung is in a separate pleural cavity. The pleural cavity is a space enclosed by a double layer of serosa. The parietal layer covers the adjacent thoracic wall and the upper surface of the diaphragm . It passes upward medially around the heart , forming the lateral wall of the mediastinal cavity. The visceral layer of the pleural covers and closely invests the external surface of the lung. external surface, dipping into the fissures of the organ. . The space between the two layers is filled with pleural fluid which acts as a lubricant. The pleural fluid resists separation of the pleural layers; as a result, when the thoracic cavity changes volume, the lungs expand or recoil as needed for adjustment to the volume changes.
Work Step by Step
The lungs are kept expanded by trans pulmonary pressure. This is the difference between the alveolar pressure ( Ppu and the intrapleural pressure within the pleural cavity(Pip). The intrapulmonary pressure(Ppul) is the same as atmospheric pressure in the environment, that is , 760 mmHg at sea level. The intrapleural pressure is negative that is, it is lower than atmospleric( pulmonary) pressure( 760-4 =756 mmHg). The transpulmonary pressure is the intrapulmonary pressure (Ppul) minus the intrapleural pressure(Pip). This transnspulmonary pressure is normally negative ( about -4mmHg). Any attempt to equalize the the intrapulmonary pressure and the intrapleural pressure will cause the lung to collapse. The knife wound which apparently also pierced the pleural cavity abolished the intrapulmonary pressure on that side and resulted in lung collapse.
