What is the functional unit of lungs?

The functional unit of the lungs is the alveolus (plural: alveoli). These are tiny, balloon-like air sacs located at the ends of the bronchial tubes where the actual gas exchange occurs. Each lung contains millions of alveoli, providing a massive surface area (about the size of a tennis court!) for oxygen to pass into the bloodstream and carbon dioxide to be removed. The alveoli are surrounded by capillaries, and their thin walls allow for efficient diffusion of gases, making them perfectly designed for their respiratory function.

To be more precise, while the alveolus is the primary site of gas exchange, the pulmonary acinus is sometimes considered the functional unit. This includes the respiratory bronchioles, alveolar ducts, and alveoli themselves. However, most textbooks and medical references point to the alveolus as the fundamental functional unit because it's where the essential work of the lungs - gas exchange - actually takes place. The incredible design with millions of these microscopic sacs maximizes surface area within the limited space of our chest cavity.