An organ was a complex system in an organism which typically performed a vital process which kept the organism alive. Sustaining damage to an organ could cause pain, (TV: The Power of Three, The Tsuranga Conundrum) or even death. (TV: The New World)
A heart was an organ used to pump blood through the body.[source needed] Time Lords had two hearts. (TV: Spearhead from Space, et al)
A stomach was an organ in the digestive tract use by many species[which?] to digest food [source needed] and by others[which?] as a form of communication. (TV: Flatline)
Time Lords possessed an organ called an exo-spleen. (TV: The Tsuranga Conundrum)
A lung was an organ primarily used for respiration. (TV: Smith and Jones) Though some species[which?] could use them aggressively. (TV: Boom Town)