Organ (anatomy)

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. Time Lords had two hearts. (TV: Spearhead from Space, et al.)

A stomach was an organ in the digestive tract use by many species to digest food and by others as a form of communication. (TV: Flatline)

Eyes were used for sight. While some species, such as the Time Lords and humans, had two eyes, (TV: An Unearthly Child, et al.) some Silurians had three eyes. Their third eye could be used for telepathic purposes or as a weapon. (TV: Doctor Who and the Silurians, PROSE: Blood Heat, AUDIO: Bloodtide)

Time Lords possessed an organ called an ectospleen. (TV: The Tsuranga Conundrum)

A lung was an organ primarily used for respiration. (TV: Smith and Jones) Though some species could use them aggressively. (TV: Boom Town)

In the 51st century, the Clockwork Droids of the SS Madame de Pompadour used their human crew's organs to replace ship parts. Rose Tyler and Mickey Smith found a heart wired up to machinery, and an eyeball incorporated into a security camera. Their ultimate objective was to seize the brain of Madame de Pompadour in 18th century France, when she was of the same age of their spacecraft. The Droids were stopped by the Tenth Doctor. (TV: The Girl in the Fireplace)