Alveoli are the fundamental structural unit of gaseous exchange within the lung. Alveoli are found with increasing distance distally within the respiratory tree:
In the adult, there are 120-160 million alveoli per lung which are closely packed together and separated only by the alveolar septa. Each alveolus is adapted specially for gaseous exchange between its luminal air and the capillaries within its wall. The alveolar wall or septum is extremely thin and is composed of two layers of respiratory epithelium of neighbouring alveoli which are only separated by the network of capillaries.
Surfactant is largely produced by the type II pneumocytes within the alveolar walls. It has a role in preventing the collapse of alveoli.
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