Cardiopulmonary system

The cardiopulmonary system consisted of the heart and lungs. It coordinated the circulation of the blood and its oxygen content. According to Icheb and Mezoti, the Borg had strengthened cardiopulmonary systems. 

The Tri-ox compound was designed to strengthen the cardiopulmonary system. In 2374, Doctor Bashir prescribed, in vain, 15 ccs every 4 hours of the compound to Captain Lisa Cusak, whom was experiencing the effects of hypoxia and carbon dioxide poisoning. This was due to her prolonged exposure to the atmosphere of an Class L planet. 

In 2151, Phlox questioned Elizabeth Cutler about why she thought that the heart was the source of emotion, since it was "nothing more than a very efficient pump." She responded by saying "You know, you may know about our cardiopulmonary system, but you have a lot to learn about the Human heart."

In 2374, the entire crew of USS Voyager (excluding Seven of Nine and The Doctor) was put into stasis in order to traverse a Mutara-class nebula. During this time, their cardiopulmonary system was slowed. 

The following year, when a creature attached itself to B'Elanna Torres, The Doctor could not remove the creature without causing irreparable damage to Torres' organs. Janeway suggested beaming it out, but this was deemed impossible to do without also removing Torres' cardiopulmonary system along with it.