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Alveoli are the smallest structures in the respiratory system. The walls of the alveoli are very thin and are composed of a single layer of tissues called epithelial cells and tiny blood vessels called pulmonary capillaries. Alveoli are tiny, balloon-shaped air sacs in your lungs. The alveoli are made up of two different types of cells. The first is Type I pneumocytes. These are the cells responsible for the exchange of oxygen and carbon dioxide. The second is Type II pneumocytes. These cells perform two important functions, they produce surfactant, the fluid inside the alveoli that helps keep the balloon shape from collapsing. And they can also turn into stem cells to repair the alveoli if needed. The function of the alveoli is to move oxygen and carbon dioxide molecules into and out of your bloodstream. The way this happens is when you take a breath in, the air travels down the trachea, through airways called the bronchi and into the lungs. The air is then directed through bronchioles. It then moves through a tiny duct called the alveolar duct and finally enters the alveoli. At this point, the oxygen molecules move through a single layer of lung cells in the alveoli, then through a single cell layer in a capillary to enter the bloodstream. As oxygen moves out of the alveoli, carbon dioxide molecules pass into it. They are then breathed out of the body through the nose or mouth. This is important because the alveoli are the gateway through which oxygen enters the bloodstream, as well as the primary way that the waste product carbon dioxide exits the body keeping the body alive.
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