How did all of the Doctor's regenerate?

Answer 1

First Doctor- After his first encounter with the (original) Cybermen, the Doctor grows increasingly frail, due to old age, to the point of collapsing on the Tardis floor, where his body (and for this regeneration only, his clothes) changes.

Second Doctor- Having been on the run from his own planet since the inception of the series, The Doctor needs the Time Lords help when he discovers fellow members of his race using illegal Time Scopes to try and create the perfect intergalactic army, even though he knows this will mean his own capture. At his trial, he is sentenced to exile on 20th Century Earth and forced to regenerate.

Third Doctor- After a battle with giant Spiders on Metebelis 3, the Doctor becomes poisoned with radiation. He manages to make it back to Earth in the Tardis, but then collapses and regenerates in UNIT's science lab. (NOTE- this is the first time the process is called 'regeneration' in the series)

Fourth Doctor- The Master and the Doctor battle on top of a high tower, leaving the Doctor dangling from a cable. He slips to Earth, and while prone on the floor, the mysterious Watcher (who is seen at various points during the story) appears and merges with the Doctor to become...

Fifth Doctor- The Doctor and companion Peri have been poisoned by a fatal chemical. They manage to get the antidote and escape to the Tardis, but the Doctor spills half of it en route. So he gives it to Peri and, after an inner struggle, he regenerates on the Tardis floor.

Sixth Doctor- Renegade Time Lady, the Rani, causes the Tardis to crash. While his companion, Mel, is unharmed, it's enough to trigger the Doctor's regeneration process. (NOTE- actor Colin Baker had fallen out with the BBC, so refused to come back to film a regeneration scene. So the whole thing is a cobbled together, unemotional scene, with eventual Seventh Doctor, Sylvester McCoy doing it wearing a blond wig)

Seventh Doctor- The Master has been executed by the Daleks, and the Doctor is charged with taking the remains back. However, the Master isn't as dead as was thought, and appears as a fluid, snake like figure, causing the Tardis to crash land in San Fransisco, New Years Eve 1999. Once there, the Doctor is shot by a street gang as soon as he opens the Tardis door. He regenerates in hospital, after being pronounced dead.

Eighth Doctor- Unknown, but it is assumed by fans that it was possibly injury during the Great Time War between the Time Lords and the Daleks, in which many races suffered across the whole of Space-Time

Ninth Doctor- His companion, Rose, absorbs energy from the heart of the Tardis, and will die. So the Doctor kisses her, enabling him to absorb the energy into himself. A short time later, he regenerates.

Tenth Doctor- After a battle with the Master and the returning Time Lords, the Doctor finds former companion Donna's grandad, Wilfred, trapped in a booth that will soon be flooded with radiation. The Doctor makes a sacrifice and switches the power flow so he will take the whole dose. The Doctor then revisits some former companions, until the radiation finally takes it's toll and he regenerates in the Tardis, just above the Earth.