According to that most reliable (!) source, wikipedia: “Although Time Lords resemble humans, their physiology differs in some key respects. For example, like other members of his race, the Doctor has two hearts (binary vascular system). The Doctor’s two hearts normally beat at 170 beats per minute.”
Rose knows the Doctor has two hearts.
“What else has he got two of?” Jackie asks Rose at the start of series 2. The new Doctor (Tennant) lies in his pyjamas, regenerating, oblivious to the alien threat of the Sycorax looming over London.
Cassandra discovers the Doctor’s binary vascular system when she briefly inhabits his body: “Two hearts! Oh baby, I’m beating out a samba!”
Martha, on the other hand says nothing when she finds them. She examines him during rounds at the hospital, just before their appointment with the Judoon on the moon! Again, pyjamas are involved.
Donna discovers that the Doctor’s daughter has two hearts, just like her “father”. Her two hearts both (apparently) stop beating, at the hands of the General Cobb on the planet Messaline.
And unlike the Doctor, the “Doctor-Donna” has but one heart (the result of a rather unique human-Time Lord metacrisis). This human Doctor can give his one heart fully to Rose and return with her to the parallel universe. They will grow old together.
And I believe the companions of Doctor Eleven only find out about his strange physiology when River takes a moment to explain it to them.
The two heart, binary vascular system, seems a very an alien concept. Yet it is a very attractive one. Who wouldn’t want two of the organ that sustains your life? It would be kind of like having a generator backup for when the electricity goes off. But might I propose this Time Lord feature is actually quite a precise analogy for our Christian, human existence on planet earth? We already have two hearts.
The prophet Ezekiel announced God’s intentions (to give us ‘two hearts’) to those Jews taken off to captivity in Babylon around 590BC.
“And I will give you a new heart, and I will put a new spirit in you. I will take out your stony, stubborn heart and give you a tender, responsive heart. And I will put my Spirit in you so that you will follow my decrees and be careful to obey my regulations.” (Ezekiel 36:26-27)
We Christians, walking around with a NEW heart within us, are living fulfillment of God’s promise through Ezekiel.
Besides our physical ‘heart’ now we also have a “spiritual” heart that is responsive to God, powered by the Holy Spirit He poured into us when we believed. If you add them up: two hearts! We live because our physical heart beats; we love because God’s heart beats in us.
And this side of heaven both will continue to beat.
So perhaps it is not that humans are a little like Time Lords afterall.
Rather, Time Lords are more than a little like Christians.
Much more could be said.
Will save that for another day.