I've addressed a similar subject on my blog before, not too long ago, actually. Many Doctor Who fans are interested in the biology of the Time Lord, but not as many seem to know that there are several hints as to just how different humans are from Gallifreyans internally, despite being identical externally.
Everybody knows that one of the Doctor's defining characteristics, besides travelling through time and space, is his two hearts. But many people overlook the implications of something like that. Such as:
Circulatory System: Time Lords possess bivalved hearts with two distinct beats, as well as two pulses. The fibrillation of a Gallifreyan heart is about 300 and tends to beat at 60 bpm, meaning any human doctor would diagnose them with arrhythmia. While the heartbeat is normal for a human, a fibrillation in the heart, specifically an atrial one, means the top chambers of the heart quiver rapidly, beating abnormally, and not pumping blood properly, classified as a supraventricular tachycardia, most commonly caused by high blood pressure. The double pulse of a Gallifreyan is generally considered to be 170, which is also not unusual for humans, however, this is unhealthy as a resting rate, and only is usual for humanity during exercise. The beats of the hearts are more forceful, making it difficult for a stethoscope to pinpoint the exact location, as the electrical activity is greater than that of a human's. It is generally accepted that the location of the hearts are not right next to each other, however, and that one is slightly lower than the other, as seen in this diagram.
Whereas many fans place the hearts directly next to one another, sometimes attached, like a set of lungs, it is confirmed in the show, that the image above is correct. The series in not decisive on whether or not Gallifreyans are born with one or two hearts, however, as some sources say that even their first incarnation has the dual vascular system, and others say that it forms after the first regeneration. "The Doctor's Daughter" is one such case that suggests Gallifreyans are born with dual, as Jenny does indeed have two hearts, but this can possibly be thrown out by the fact that she is artificially created from the Doctor's DNA alone and is not produced how a normal Time Lord (or Time Lady, rather) would be. It is however, confirmed in all forms that a Time Lord can function with one heart, though for how long is an inconsistency, and when he is forced to, the Doctor appears much weaker and suffering from an immense amount of pain. Total cardiac arrest would require CPR to both hearts as shown in "Smith and Jones" and other episodes show that a Time Lord can gain conscious control of the hearts to temporarily slow them down a significant amount and even stopping it for several hours in order to feign death. Heart transplants are also a thing on Gallifrey and the donor and receiver would be linked, meaning it would be quite difficult to kill the donor, without the receiver's life also ending, however the specifics of this, are not really gone into. The telepathic nature of certain Time Lord abilities, however, may hint at a similar connection. With an unusual heart(s), comes unusual blood, as well. Gallifreyan blood is not only a different color, being darker and more reddish-orange. It seems that the blood could have something to do with the regeneration of a Time Lord, as it is described as having regenerative properties and temporal platelets which allow the Gallifreyan to recover from injuries faster than humans, and some that humans would not be able to recover from without permanent damage or without the assistance of a medical professional. The species possesses multiple blood types, as stated by Romana II, but it does not seem to be of the human variety, such as A and B. The chemical make up is also slightly different than that of human blood and carries a different aroma.
Respiratory System: Time Lords are able to go much longer periods without oxygen because as opposed to a set of two lungs, they have a series of pulmonary tubes similar to the lymphatic system and they can store oxygen, yet they can be affected by altitude illness. They can avoid strangulation, drowning, and the intake of poisonous gases due to a respiratory bypass system and can enter a state of trance in order to reduce the need for oxygen. Similarly, Time Lords will never have the joy of changing their voices to a higher, more hilarious pitch via helium,
Brain: Anyone who has seen the revival of Doctor Who, dubbed by fans as "New Who", may remember Donna Noble, one of the tenth Doctor's companions, as (SPOILERS) having housed the consciousness of a Time Lord briefly, and then being forced to forget the Doctor and all the adventures she shared with him in "Journey's End". She contained all of the Doctor's knowledge, which essentially overloaded her mind, which sounds accurate when you consider how insanely smart he is, but also brings up an interesting point: a Time Lord's brain is able to hold more information than that of a human, which makes sense when you consider the insane life span that a Gallifreyan is capable of having. If we look into the old series, it is indeed confirmed that the brain of a Time Lord is not only larger, but is also incredibly complex and capable of short-term eidetic memory. Their minds also allow multiple telepathic abilities such as knowledge/memory sharing, mind-reading, memory erasure, influence of others' dreams, and ability to put somebody to sleep. Time Lords are connected to one another telepathically and could hold mental conversations with one another in this way.
Integumentary system: A Time Lord's skin is more durable than that of a human because of extra subdural and subcutaneous layers, allowing it to quickly reject foreign objects.
DNA: Their DNA is formed in a triple-helix formation.
Digestive: Time Lord's have two livers. This could explain an excess of energy, possible high metabolism, and could be involved with the process of regeneration, or at the very least, recovering from injuries far faster than humans are able.
Other Organs: There are several organs possessed by Gallifreyans which are completely foreign to humans, however, this is never further gotten into.
Skeletal System: The Gallifreyan skeletal system seems nearly identical to a human's save for possessing two extra ribs, 26 as opposed to 24.
Body Temperature: The Doctor's body temperature is fifteen degrees Celsius (59 F), and can drop to temperatures below freezing. However, it does not seem to affect their external body temperature, as any human who touched him would likely comment on his skin being cold to the touch. It is not ever explained how he can outwardly feel the same temperature as a human with such a low body temp. To put this into perspective, any human with that temperature is legally dead with the ability to be revived, and it is obtained by being in the extreme cold for usually several hours. We can presume that other Time Lords have similar body temperatures.