The Lungs
The lungs are paired, cone-shaped organs that occupy the thoracic cavity except for the mediastinum, a central area that contains the primary bronchi, the heart, and other organs. The right lung has three lobes, and the left lung has two lobes, allowing room for the heart whose apex points left. A lobe is further divided into lobules, and each lobule has a bronchiole serving many alveoli. The apex is the superior narrow portion of a lung, and the base is the inferior broad portion that curves to fit the dome-shaped diaphragm, the muscle that separates the thoracic cavity from the abdominal cavity. The lateral surfaces of the lungs follow the contours of the ribs in the thoracic cavity.
Each lung is enclosed by a double layer of serous membrane called the pleura. The visceral pleura adheres to the surface of the lung, while the parietal pleura lines the thoracic cavity. The pleura produces a lubricating serous fluid that allows its two layers to slide against one another. Surface tension is the tendency for water molecules to cling to each other (due to hydrogen bonding between the molecules) and to form a droplet. Surface tension holds the two pleural layers together when the lungs recoil during expiration.
The Alveoli
With each inhalation, air passes by way of the bronchial tree to the alveoli. An alveolar sac is made up of simple squamous epithelium surrounded by blood capillaries. Gas exchange occurs between the air in the alveoli and the blood in the capillaries Oxygen diffuses across the alveolar and capillary walls to enter the bloodstream, while carbon dioxide diffuses from the blood across these walls to enter the alveoli.
The alveoli must stay open to receive the inhaled air if gas exchange is to occur. Gas exchange takes place across moist cellular membranes, and yet the surface tension of water lining the alveoli is capable of causing them to close up. The alveoli are lined with a surfactant, a film of lipoprotein that lowers the surface tension and prevents them from closing. The lungs collapse in some newborn babies, especially premature infants, who lack this film. The condition, called infant respiratory distress syndrome, is now treatable by surfactant replacement therapy.