Which of the following best describes what alveoli are?
Pulmonary alveoli (plural: alveolus) are air sacs in your lungs where gas exchange occurs. Each alveolus has a large surface area compared to its volume and is protected by a thin lining called the pulmonary epithelium. The epithelium consists of two specialized cell types: alveolar type 1 cells, which are elongated and cover over 95% of the alveolar surface to facilitate gas exchange into capillaries; and alveolar type 2 cells that make pulmonary surfactant to reduce surface tension, helping prevent the collapse of the alveoli as you breathe.
Air enters your lungs through two long air passages, the trachea and bronchial tubes. The bronchial tubes then branch into smaller air passages, bronchioles, that end in tiny, clustered, gas-filled air sacs called alveoli. Alveoli are the workhorses of your lungs and take in the oxygen you breathe in and release the carbon dioxide you exhale.
When you inhale, the alveoli stretch out to pick up the oxygen molecules from the air and then they expand as you exhale. This is gas exchange, which allows you to get the energy your body needs to survive.
Once oxygen is in the capillaries, it moves to the red blood cells that carry it throughout your body tissues. At the same time, carbon dioxide molecules move out of the cells into the capillaries and the bloodstream, where it can be pumped to your lungs to be exhaled.