What is the function of the pleura? How do differences in alveolar and atmospheric pressure contribute to ventilation? What is a pnemothorax? What is the mechanism contributing to the pathology? Solution Pleura is the outer covering on the lungs.These are two membranous strcutures called visceral pleura(inner) and parietal pleura(outer) that cover the lungs. The function of pleura is to enclose the space called pleural space that contains pleural fluid which helps in easy and frictionless movement of lungs during expansion and contraction inside the ribcage. Alveloar pressure is the pressure of air inside the lungs. When inspiration starts, lungs expand in the thoracic cavity. This expansion increases the volume of the lungs that causes expansion of air too inside the lungs. The expanded air has low pressure , so the pressure of air decreases slighlty in the lungs whereas the pressure outside(atmospheric pressure) is higher. Air moves from high pressure to low pressure so it starts moving from atmosphere to respiratory pathway to alveoli. During Expiration, Lungs contract, volume inside the lungs decreases, alveolar pressure increases. This pressure difference between lungs and atmposhere is sufficient to cause outward movement of air. Pneumothorax is the condition in which the air collects outside the lungs in the chest/thoracic cavity.This condition can arise due to a disease, injury to lung that causes damage to pleura or even sometimes in absence of anysuch condition (Spontaneous pneumothorax). Pathophysiology of pneumothorax is not very clearly known. It has been noticed during this condition the emphysema like changes occur iin alveoli, necrosis of alveolar tissue,formation of air filled lesions under the pleural surface.