Alveoli
The alveoli were small sacs located in the lungs, where gas exchange occurred between the blood and the air.
In 2151, Doctor Phlox noted that a Suliban's alveoli clusters had been modified to allow for easier processing of different atmospheres. (ENT: "Broken Bow")
In 2372, while undergoing hyper-evolution, Tom Paris's alveoli mutated to the point that they stopped processing oxygen. (VOY: "Threshold")
In 2373, The Doctor reported that the pulmonary system of a dead Borg drone he was examining exhibited evidence of extensive alveolar damage. (VOY: "Unity")
In 2374, B'Elanna Torres's alveoli briefly stopped processing oxygen after she was mutated by covert Srivani medical experiments. (VOY: "Scientific Method")
In an alternate timeline, The Doctor examined Kathryn Janeway after she breathed in gas seeping into the critically damaged USS Voyager from a class 9 nebula. He found that the alveoli in her lungs had been chemically burned. She insisted that The Doctor treat her with trioxin so she could continue to work on repairing Voyager. (VOY: "Year of Hell, Part II")
Also that year, Seven of Nine used Borg nanoprobes to resurrect Neelix, which caused the alveoli in his lung to quickly regenerate. (VOY: "Mortal Coil")