Pulmonology Quicklinks

The lungs are the essential organs for the uptake of oxygen into the blood and the removal of carbon dioxide. They are situated on the left and right side of the thorax and occupy most of the thoracic cavity. Each lung is covered by pleura, a closed invaginated sac. The visceral (pulmonary) pleura adheres to the lung surface, following the interlobar fissures and ensheathing each pulmonary lobe. The parietal pleura is a continuation of visceral pleura at the hila to cover the mediastinal organs.¹

The right lung is divided into superior, middle, and inferior lobes by the oblique and horizontal fissures, and the left lung is divided into superior and inferior lobes by its oblique fissure. The secondary pulmonary lobules are the smallest macroscopic subsection of the peripheral lung. Within each secondary lobule, there are approximately 6 terminal bronchioles, terminating with an acinus for gas exchange.¹

The acinus is the functional unit for respiration, consisting of alveolar ducts, alveolar sacs, and alveoli.² The alveoli are pouches for gas exchange, and their thin walls facilitate this process. The alveolar epithelial cells (pneumocytes) form the alveolar epithelium. The type I pneumocytes are simple squamous epithelial cells and form over 90% of the alveolar area. The type I pneumocytes do not divide; therefore, if damaged, type II pneumocytes will proliferate and differentiate to replace the damaged type I pneumocytes.¹

When type II pneumocytes are unable to proliferate and replace damaged type I pneumocytes, epithelial damage and subsequent destruction of the alveolar basal membrane can occur. The current pathogenesis theory suggests that when the basement membrane is damaged, it initiates the infiltration of fibroblasts, creating an intra-alveolar bud that contains collagen and macrophage-lined surface. Subsequently, the bud becomes lined by regenerating epithelial cells. Fibroblasts then migrate into the luminal region through the damaged alveolar basement membrane, and this area is incorporated into the alveolar wall during re-epithelialization, ultimately forming fibrotic tissue in the alveoli.¹