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Alveoli: The bronchioles terminate in small dilated airsacs, known as alveoli, which are lined with a network of capillaries. In these alveoli the exchange of oxygen and carbon dioxide takes place between air and blood. The total surface area of the alveoli is around fifty square metres, which is approximately twenty times the surface area of the body. In fact, the alveolar surface of the lungs presents the vastest area from which interceptive impulses travel to the brain, particularly when the lungs are stretched to their fullest extent.
Pleura: The lungs are surrounded by a double membrane, known as pleura, which provides a lubricating surface between the lungs and inner chest cavity during respiratory movements. The pleura consist of a closed sac of serous membrane (one for each lung) which contains a small amount of serous fluid. This sac forms two layers: one adheres to the lung and the other to the wall of the thoracic cavity.
The two layers of pleura, with serous fluid between them, behave in the same way as two pieces of glass separated by a thin film of water. They glide over each other easily, but can be pulled apart only with difficulty, because of the surface tension between the membranes and the fluid. If either layer of pleura is punctured, the underlying lung collapses due to its inherent property of elastic recoil.
Pulmonary arteries and veins: The pulmonary artery originates in the heart and divides into two branches, conveying deoxygenated blood to each lung. Within the lungs each pulmonary artery divides into many branches, which eventually end in a dense capillary network around the walls of the alveoli. The walls of the alveoli and those of the capillaries each consist of only one layer of flattened epithelial cells. The exchange of gases between air in the alveoli and blood in the capillaries takes place across these two very fine membranes. The pulmonary capillaries join up, eventually becoming two pulmonary veins in each lung. They leave the lungs at the hilum and convey oxygenated blood to the left atrium of the heart. The innumerable blood capillaries and blood vessels in the lungs are supported by connective tissue.
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