Alveoli

The alveoli were small sacs located in the lungs, where gas exchange occurred between the blood and the air.

In 2372, while undergoing hyper-evolution, Tom Paris's alveoli mutated to the point that they stopped processing oxygen. 

In 2373, The Doctor reported that the pulmonary system of a dead Borg drone he was examining exhibited evidence of extensive alveolar damage. In 2374, B'Elanna Torres's alveoli briefly stopped processing oxygen after she was mutated by covert Srivani medical experiments. 

Also that year, Seven of Nine used Borg nanoprobes to resurrect Neelix, which caused the alveoli in his lung to quickly regenerate. 

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.''