In response to the Honorable Mr. Willikers' query: Yes, I have studied the subject. No, I'm not a medical student or practitioner; I'm a 28 year lawman interested in defensive shooting and the efficiency of such shooting.
I have a couple books on anatomy and a full color, large (larger than the typical book, anyway) set of anatomical drawings. I have looked and read on the subject a lot.
Best place to shoot is the UPPER center of the torso, on a three dimensional perspective. Lower center torso tends to be less vital components; painful, but not (reliably) incapacitating.
Historically, there have been some officer involved shootings - one with the criminal attacker being the driver of an automobile and the officer standing by the driver's window, in classic 'car stop' position - where the officer reverted to his range training and shot at the front of the attacker's shirt. The officer survived and prevailed in the encounter, but the attacker's wounds were all 'slashing' wounds across the front of the chest. Not really 'stopping' wounds.
Instead of shooting DIRECTLY at the attacker, he shot at where the "X" ring would have been. From the front.
As a defensive shooter, one must change one's approach in some regards. The three dimensional concept is critical, as is ignoring the posture of the attacker.
AND, I want to address a limiting thought expressed in this thread.
"In a defense encounter, one cannot aim at specific targets on the body."
I submit one cannot do much of anything one does not accept as possible. In other words, convincing one's self of one's inability does not increase ability.
Instead, I encourage all to practice shooting - even at two dimensional targets, but with differing postures - to the point where one can reliable hit various targets to a boring degree. Yes, I agree training isn't the same as actual self-defense, but the higher one's [honest] ability, the less one is inclined to panic.
Convincing one's self all one can do is to fire wildly and hope for the best is not conducive to success.
Again, upper center torso area. Practice a lot.