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An alveolus (plural: alveoli, from Latin alveolus, "little cavity") is an anatomical structure that has the form of a hollow cavity. Found in the lung parenchyma, the pulmonary alveoli are the terminal ends of the respiratory tree, which outcrop from either alveolar sacs or alveolar ducts, which are both sites of gas exchange with the blood as well. Alveoli are particular to mammalian lungs. Different structures are involved in gas exchange in other vertebrates. The alveolar membrane is the gas-exchange surface. Carbon dioxide rich blood is pumped from the rest of the body into the alveolar blood vessels where it through diffusion releases its carbon dioxide and absorbs oxygen. In an adult lung, there are about 300 to 400 million alveoli. The average diameter of an alveolus is about 200 to 300 microns. The alveoli consist of collagen and elastic fibers. These are lined with epithelial cells and the pores between them are called Kohn. During exhalation, the elastic connective tissues which compose the space between alveoli of the lungs come into function. Three different major cells such as squamous alveolar cells, squamous epithelial cells (Type 1) and great alveolar cells (Type 2) compose an alveolus. The structure of an alveolar wall consists of squamous alveolar cells. The squamous epithelial cells make up the capillaries whose function is diffusion of gases. Finally, the great alveolar cells secrete surfactant that helps on reduction of surface tension of water and also helps in separation of membranes and increasing the exchange of gases. The great alveolar cells help in repairing damaged endothelium of the alveolus. The alveolus also consists of macrophage cells which help in destroying any foreign matter like bacteria and has functions related to the immune system.