Gallifreyans were outwardly indistinguishable from humans. Although they looked identical to humans and many humanoids, they had a variety of biological differences that set them apart.
The Doctor told Madame Vastra that the Time Lords "became what they did" through prolonged exposure to the Time Vortex. According to the Eleventh Doctor, this took billions of years for the people of Gallifrey and that "[i]t didn't just happen." After being subjected from alterations by Madame Kovarian and her associates, River Song, a human who was conceived inside the Doctor's TARDIS, also developed at least some of these traits before she was born. (TV: A Good Man Goes to War)
Taxonomy
The Gallifreyans were classified by humans as Dominus temporis. (PROSE: The Last Dodo). The Tenth Doctor once stated that Gallifreyans were unrelated to primates, despite sharing many physiological characteristics. (TV: The Doctor's Daughter)
External appearance
Externally, Gallifreyans were usually identical to humans, though they tended to have a mostly European/Caucasian appearance. Hair and eye colour were of any colour found among humans. (PROSE: The Last Dodo) Teeth were indistinguishable from human teeth. (TV: The Gunfighters, Utopia) However, while Romana I was regenerating into Romana II, she took on various appearances varying in height and build, and even one with blue skin. (TV: Destiny of the Daleks) The Ninth Doctor referred to the regeneration process as "dodgy", and indicated that it was possible to regenerate without a head. (TV: The Parting of the Ways)
Resilience
Gallifreyans were, on the whole, extremely tough and resilient. An average Gallifreyan was superior to a human at their peak. (PROSE: Doctor Who and the Terror of the Autons)
Gallifreyans could survive some falls which would shatter the bones of humans. The Tenth Doctor survived a fall from a low flying spaceship through a glass skylight and was able to stand and move afterwards. (TV: The End of Time) However, the fall from a radio telescope dish was enough to cause the Fourth Doctor to regenerate. (TV: Logopolis)
Gallifreyans could survive extreme cold, (TV: The Tomb of the Cybermen, The Seeds of Doom, The Ribos Operation, 42, The Infinite Quest, Planet of the Ood) due to having a "souped-up metabolism". (COMIC: The Betrothal of Sontar) They could also survive extreme heat. (TV: The End of the World) They could even survive the subzero temperatures and extremely low pressure of vacuum for around six minutes, (TV: Four to Doomsday, The Doctor, the Widow and the Wardrobe) and survive electric shocks that would be fatal to humans. (TV: Genesis of the Daleks, Terror of the Zygons, World War Three, Evolution of the Daleks, The Vampires of Venice, The Pandorica Opens) Röntgen radiation affected Gallifreyans so minimally that Gallifreyan children were routinely given radioactive toys in the nursery. They could, at will, absorb very high doses of Röntgen radiation, transform it into a form harmless to humans, and expel it from their bodies. (TV: Smith and Jones) Radiation of other kinds could be fatal, but even then a Gallifreyan could handle much higher doses than a normal human could, and could hold out much longer than even most terrestrial life-forms. (TV: Planet of the Spiders, The End of Time)
Gallifreyans needed less sleep than humans, and could make do with as little as an hour. (TV: The Talons of Weng-Chiang, Mummy on the Orient Express, HOMEVID: Night and the Doctor, PROSE: The Highlanders)
A Gallifreyan who was severely injured without actually needing to regenerate to heal the damage would generally slip into a healing coma, and devote all his or her energy to healing the injury. While in the coma, they would appear to be dead. (TV: Inferno, Planet of the Daleks, PROSE: EarthWorld, Vanishing Point)
Time Lords also seem to have an increased resilience to higher frequencies of sound. (TV: The Christmas Invasion, Partners in Crime)
Gallifreyans could be disabled by a blow to the left shoulder, which possessed a vulnerable nerve cluster. (PROSE: Set Piece)
Lifespan
Even without regeneration, Gallifreyans had considerable lifespans. Within one regeneration, Gallifreyans could live for hundreds of years, yet look much younger than a human of equivalent age. When artificially aged 500 years, the Fourth Doctor looked like an elderly human (TV: The Leisure Hive). During his eighth incarnation, the Doctor spent over a century trapped on Earth (PROSE: The Ancestor Cell to Escape Velocity) and never seemed to physically age during that time. In his eleventh incarnation, the Doctor looked essentially the same for two centuries, (TV: The Impossible Astronaut) though at least three more centuries caused him to age somewhat and six more centuries [source needed] of further time caused him to age into a very old man, near death from old age. (TV: The Time of the Doctor)
However, Gallifreyan children grew at about the same rate as humans of the same age. (TV: The Sound of Drums) After this point, ageing would slow, with the Gallifreyan looking like a teenager for decades. (PROSE: Legacy of the Daleks) The Tenth Doctor referred to himself at ninety years old as being "just a kid". (TV: The Stolen Earth)
The Second Doctor once stated that, barring accidents, his people could "live forever". (TV: The War Games) The Doctor once said that he considered himself at approaching 750 to be middle-aged. (TV: Pyramids of Mars) One Time Lord, Quences, was killed when he was over 7,000 years old. (PROSE: Lungbarrow) Professor Chronotis suffered from senility when over 12,000 years old (PROSE: Shada), and the Eleventh Doctor showed some signs of the same on the brink of his thirteenth and supposedly final death at "over 2000" years old. (TV: The Time of the Doctor, Deep Breath)
Senses
Gallifreyans had all the senses possessed by humans, and to generally superior degrees. Gallifreyans also had extraordinary reflexes and precision timing, literally superhuman. (PROSE: Doctor Who and the Terror of the Autons, TV: The End of the World, The Doctor's Daughter) The Third Doctor stated that their reflexes were ten times a human's. (TV: The Time Monster) Gallifreyans showed great hand-eye coordination and dexterity with a wide variety of tool and weapons. (TV: The Android Invasion, The Pirate Planet) Gallifreyans (in "younger" bodies) were, consequently, very physically able and highly athletic. The Third Doctor was a martial arts master, [source needed] the Fifth Doctor was a skilled cricketer (TV:Black Orchid), and the Eleventh Doctor discovered a natural talent for football. (TV: The Lodger)
Gallifreyan eyes were better at seeing in the dark and could gather and enhance available light. (PROSE: Lucifer Rising) They could notice incredible amount of detail from distances of at least one hundred yards away (TV: The Eleventh Hour) as well as people that were well beyond the human line of sight. (TV: The Hungry Earth)
The Tenth Doctor perceived sounds from the TARDIS, while located several sections away, aboard the airship Valiant. (TV: The Sound of Drums)
Gallifreyans were capable of identification by taste. This included blood, (TV: The Christmas Invasion, PROSE: Bad Therapy) wood type, (TV: Tooth and Claw) plastic composition, (TV: The Idiot's Lantern) age of an object, (TV: The Eleventh Hour) origin of an object (TV: Day of the Moon) or the atmospheric composition. (TV: The Time of Angels, Dinosaurs on a Spaceship)
The Gallifreyan sense of smell was equal to their sense of taste. (PROSE: Wishing Well) They could do a chemical analysis of the air using their sense of smell. (PROSE: Doctor Who and the Carnival of Monsters) On some occasions Time Lords were also able to judge what time period and location they were in by the smell of the air. (TV: The Unicorn and the Wasp, The Time of Angels) The Second Doctor knew the Master had tricked Jamie when he smelt the Master's handkerchief. (PROSE: The Nameless City)
Gallifreyans were better at coping with sudden changes in position than humans and were harder to disorient. (AUDIO: The Forever Trap)
As well as the senses shared with humans, Gallifreyans had further senses, with at least a sixth sense, (PROSE: Wishing Well) although the Doctor tended to keep his sensory input limited to the human norm to make things more interesting and allow himself to be surprised. (PROSE: Eye of Heaven) Gallifreyans had time- and spatial-related senses and physical attributes; they were able to resist fields of slow time, (TV: The Time Monster, Invasion of the Dinosaurs) notice distortions and jumps in time, (TV: Invasion of the Dinosaurs, City of Death, The Lodger) directly perceive the interstellar motions of cosmological bodies or their inhabitants (TV: Rose) — including sensing the "shape" of the world to the extent that they were aware when trapped in pocket dimensions such as Lychburg (PROSE: Heart of TARDIS) — and perceive all possible timelines. (TV: The Parting of the Ways, The Fires of Pompeii) Gallifreyans could sense the presence of others of their own species, with the sense being specific enough to allow identification of one another just by sight, regardless of potential recent regeneration. (TV: The Sound of Drums) Due to their time sensitive nature, Gallifreyans could retain memories of negated (TV:Journey to the Centre of the TARDIS) or alternative (TV: The Name of the Doctor, The Time of the Doctor) timelines.
Internal anatomy
Body temperature
According to Tom Cardwell's bioscan, the Eighth Doctor had a body temperature of fifteen degrees Celsius. (AUDIO: Blood of the Daleks) While recovering from a serious injury, it could drop spontaneously to below-freezing temperatures. (TV: Planet of the Daleks) The Eighth Doctor and Miranda Dawkins could both demonstrate this lower body temperature on a cold night, their breath not visible while other humans' was. (PROSE: Father Time)
The brain
The Time Lord brain was much larger and more complex than the human brain. The size differences effectively ruled out brain transplants from a Gallifreyan to a human. (TV: The Brain of Morbius) They had a form of eidetic memory, able to recall every detail of even the most insignificant moments in time. (TV: The Eleventh Hour) The Timewyrm noticed that on a quantum level, the Seventh Doctor's brain could receive information from possible futures, possibly without even realising it consciously. (PROSE: Timewyrm: Revelation) Time Lords could also separate the hemispheres of their brain, allowing them to multitask easily. (PROSE: Island of Death) The autonomic functions could be artificially supplanted with a special device, allowing the Gallifreyan to think with their autonomic brain. (PROSE: Shada)
Time Lords had an additional brain lobe dedicated to mechanical and other bodily functions, freeing the other lobes for intellectual endeavours. (AUDIO: Spare Parts)
Time Lords displayed the ability of touch-enabled mental manipulation; this manifested itself in a number of different ways, including hypnosis, (TV: Terror of the Autons, The Ribos Operation, Fear Her) mind-reading, (TV: The Girl in the Fireplace) thought sharing, (TV: The End of Time) the ability to relieve dementia, (TV: The Shakespeare Code) putting others to sleep, (TV: Listen) mentally influencing the dreams of others (TV: The Eleventh Hour) and memory erasure. (TV: Journey's End) They could also transfer knowledge quickly to another person by headbutting them. (TV: The Lodger)
Time Lords were telepathically linked to one another and could join the entire Time Lord intelligence as one. (TV: The Invisible Enemy) They could hold telepathic conversations over distances, but this was more difficult. (TV: The Sensorites, The Three Doctors, The Pirate Planet) They could converse with each over the astral plane, although this ability required intense concentration, and an interruption might have fatal consequences for the Time Lord. (TV: The Two Doctors) Their telepathy extended to less intelligent animals. (TV: The Lodger) In ancient times, Gallifreyans who were capable of blocking out the telepathic thoughts of other Gallifreyans were called Individuals. They usually had red-gold hair and often went on to become Young Heroes. (PROSE: Cat's Cradle: Time's Crucible)
They were highly resistant against, if not immune to, other forms of mind control. (TV: The War Machines, The Green Death) However, they were vulnerable to more powerful forms of mind control, such as the Morpho, (TV: "The Velvet Web") the Great Vampires (TV: State of Decay) or Sutekh (TV: Pyramids of Mars) employed.
Skeletal structure
Gallifreyan skeletal structure was reported to have been identical to human skeletal structure, with one exception. In one account, the Gallifreyan rib cage had 26 ribs, two more than humans. (PROSE: Blood Heat) Other sources suggested that they had 24 ribs. (TV: Spearhead from Space)
Circulatory system
Gallifreyan blood was different from human blood, darker and with a slight orange tinge to it. (TV: The Two Doctors, PROSE: The Left-Handed Hummingbird) The differences were readily apparent under a microscope. (TV: Spearhead from Space, The Invisible Enemy, Doctor Who) It was highly adaptive, with regenerative properties. (PROSE: The Art of Destruction) Gallifreyan blood had temporal platelets, allowing for a more "ad-hoc" recovery from injury not serious enough to require regeneration. (AUDIO: The Bride of Peladon) It didn't have the same A and B types as human blood (i.e., the A and B antibodies were not present), but different blood types still existed with Romana II noting that her blood was not compatible with the Fourth Doctor when he was badly injured by vampires, requiring him to receive a transfusion from his future self. (PROSE The Eight Doctors) The extra-cellular matrix was incompatible with Earth-based life and the structure of haemoglobin was different. (PROSE: The Art of Destruction, TV: The Invisible Enemy, PROSE: Placebo Effect) The Tenth Doctor explained to Christina that fried bread would clog arteries, but he had "tiny things" inside his blood that "sluice[d] it out again." (PROSE: Keeping up with the Joneses)
Binary vascular system
Gallifreyans naturally born before the time of Rassilon and Pythia had two hearts. (PROSE: Cat's Cradle: Time's Crucible, Lungbarrow)
Accounts differed as to the vascular systems of Gallifreyans in their first incarnation. Some accounts indicate that Gallifreyans normally had two hearts even in their first body. (AUDIO: Frostfire, TV: The Doctor's Daughter) Other accounts indicated that Gallifreyans Loomed after the Pythia's curse of sterility originally emerged with a single heart and gained their second only after their first regeneration, (PROSE: The Man in the Velvet Mask) while those of Newblood houses sprang from the Looms with two hearts from the outset. (PROSE: Christmas on a Rational Planet)
A Gallifreyan could survive with only one heart, should the other be punctured, destroyed or otherwise stopped, though he or she would be weakened greatly. (PROSE: Managra, The Adventuress of Henrietta Street, TV: The Shakespeare Code, The Power of Three) A Gallifreyan in total cardiac arrest would need CPR on both hearts. (TV: Smith and Jones) Regeneration would become impossible if both hearts were burnt out simultaneously. (TV: Forest of the Dead)
Gallifreyans could, with training, gain conscious control over their hearts, enabling them to stop their hearts and feign death, (TV: Destiny of the Daleks) and could slow them down significantly. (TV: Terror of the Zygons, PROSE: Wishing Well) A dual cardiovascular system being more efficient, Gallifreyans in a healing coma could have a resting pulse rate of as few as ten beats per minute. (TV: Spearhead from Space) The two heartbeats could be mistaken by human cardiologists such as Dr Grace Holloway as "fibrillating badly". (TV: Doctor Who) Two pulses could be detected in the wrist. (TV: The Five Doctors, Time and the Rani, AUDIO: Primeval)
Cybermen from Pete's World could detect the presence of a Gallifreyan binary vascular system, and stated that it was an unknown upgrade that would require further analysis. (TV: The Age of Steel)
Respiratory system
In order to use their two hearts, Gallifreyans had an advanced respiratory system. Instead of large lungs, they had a series of pulmonary tubes parallel to the lymphatic system. This made them positively buoyant, allowing them to swim with ease. (PROSE: Island of Death) They could survive longer without much oxygen, past the point where a human would be unconscious; (TV: The Ark in Space, The Two Doctors, Smith and Jones, Mummy on the Orient Express, PROSE: Legacy) the Fifth Doctor explicitly claimed to be able to store oxygen for several minutes. (TV: The Caves of Androzani) Despite this, Gallifreyans (especially frailer individuals) could be affected by altitude sickness. (PROSE: Marco Polo)
Like most aspects of Gallifreyan physiology, the respiratory system was largely under conscious control. Gallifreyans could, at will, enter a trance state which reduced the need for oxygen. (TV: Terror of the Zygons, Four to Doomsday) Gallifreyans also had a respiratory bypass system, which allowed them to survive strangulation, (TV: Pyramids of Mars, PROSE: Human Nature) avoid breathing poison gas, (TV: The Two Doctors) and stave off drowning. (PROSE: Killing Ground)
Gallifreyans were immune to the effect of helium gas, which could make human voices sound high-pitched and squeaky. (TV: The Robots of Death)
Other organs
The skin of a Gallifreyan had more subdural and subcutaneous layers than a human, adding considerably to its durability and damage resistance. Gallifreyan skin tended to reject foreign objects such as subcutaneous implants or tags relatively quickly. (PROSE: Burning Heart) Implants that went below the subcutaneous layers, such as those that bonded with cartilage, would be retained. (TV: Day of the Moon)
Gallifreyan physiology was described by a human medic as having "four of everything he should have two of and two of everything else." (PROSE: Burning Heart) The Gallifreyans had two livers. (PROSE: Halflife) They also had several organs which had no analogue in humans. (PROSE: Placebo Effect)
Due to internal incompatibilities, Gallifreyans were not suitable for certain forms of Cyber-conversion. (TV: Closing Time)
Gallifreyans also had DNA in a triple-helix formation. (PROSE: The Crystal Bucephalus)
Biochemistry
On the whole, Gallifreyan biochemistry seemed quite similar to human biochemistry: they could eat anything humans could and could breathe on Earth. (PROSE: The Left-Handed Hummingbird)
Gallifreyan biochemistry, like many other aspects of Gallifreyan physiology, was subject to the conscious control of the individual. For example, a Gallifreyan was able to counteract the negative properties of Psilocybin by modifying the chemicals into something harmless, though the techniques were not foolproof and drug overdoses were still possible. (PROSE: The Left-Handed Hummingbird) Similarly, though Gallifreyans were susceptible to the intoxicating effects of alcohol, they could easily shrug off the effects when they needed to. (PROSE: The Quantum Archangel) As with humans, however, excessive consumption could produce a hangover. (AUDIO: Slipback) Ginger beer would reduce their ability to tolerate alcohol. (AUDIO: The Kingmaker)
Some human medicines seemed to work on Gallifreyans as well as humans. Anaesthetic gases of the type commonly administered before surgery, however, did not work well on Gallifreyans; a much greater quantity of anaesthetic could be administered, and even then it might not work completely. Anaesthetic could however almost destroy the process of regeneration. (TV: Doctor Who) Other sleep-inducing drugs might work on a Gallifreyan, but would take roughly twice as long as with humans, even if the Time Lord in question took double the dosage that the humans had. (TV: The Androids of Tara) Aspirin was toxic to Gallifreyans; a single dose could kill one. (TV: The Mind of Evil, PROSE: The Left-Handed Hummingbird) It was said to either fatally stop platelet aggregation, (AUDIO: The Condemned) cause massive allergic, pulmonary and cerebral embolism, (PROSE: Burning Heart) or interfere with the hormone receptor intermediaries. (PROSE: The Taking of Planet 5) Death could be prevented by entering a deep coma which was outwardly mistakable for death, (PROSE: Burning Heart) or by ingesting chocolate. (AUDIO: The Condemned)
Gallifreyans were as susceptible as humans to the devastating effect of spectrox poisoning; spectrox toxaemia proceeded at roughly the same rate in both species, though a Gallifreyan, being tougher, would not succumb as quickly. (TV: The Caves of Androzani) They could counteract cyanide poisoning, given a number of ingredients including ginger beer, something salty (but not salt), protein and a physical or mental shock, but only if done very soon after the cyanide is ingested. (TV: The Unicorn and the Wasp)
Mustard gas would kill a Gallifreyan as much as a human, though Gallifreyans were far more susceptible to aid for mustard gas. (AUDIO: The Great War)
During a decontamination attempt by Silurians intended for humans, the Eleventh Doctor claimed that removing the germs would remove half of the things keeping him alive. (TV: Cold Blood)
Time Lords did not need to brush their teeth like humans because their bodies did not allow for the same type of tooth decay. However, the Tenth Doctor enjoyed brushing his teeth because he liked the feel of the minty froth on his tongue. (PROSE: Keeping up with the Joneses)
Like humans, Time Lords were capable of having allergic reactions to specific substances. The Fifth Doctor stated that he was allergic to certain gases in the Praxis series, and that he could eat the celery stalk on his left lapel in order to counteract the reaction. [disputed statement] (TV: The Caves of Androzani) Rosemary could also make the Fifth Doctor sneeze when he tasted it in his drink. (TV: Castrovalva)