It helps to think of Hit Points not as a quantity of "how much damage can I take exactly" but "how much will this hurt?"
1st level fighter = 10 HP.
20th level fighter = 120 HP. (for the sake of argument).
Both guys get hit by an arrow doing 4 points of damage. 1st level guy goes "OW! There goes 40% of my HP!" while 20th level guy shrugs at the 3% loss and shrugs. The arrow did, basically, less damage to the experienced fighter. Imagine they both had 100 HP. The 1st level fighter would have taken 40 from the arrow, the 20th level fighter would have taken 3. Why? Because like someone said, HP are not a measure of how many pints of blood or mystical "life energy" you can lose before dying, it represents how good you are at rolling with the damage, reducing the blow, and getting mostly out of the way. To the 1st level fighter, the arrow is a major injury. To the 20th level fighter, it's a flesh wound.
AC, on the other hand, is about not getting hit at all. If the 20th level fighter wants to be simply harder to hit, then he raises his Dex, which adds to his AC. Makes sense, the nimble guy's harder to hit than the slower one. In armor, you can hit them but have the armor deflect the blow (when the attack roll beats the touch AC, but not the actual AC), which again makes sense. For ranged touch attacks, both the nimble guy in leathers and the nimble guy in chainmail are at the same risk of being zapped (if their stats are equal), because the armor provides no protection.
A high AC can partially compensate for low HP, to reflect the guy who just seems to be in the way of arrows a lot, so he layers on the armor. In the same way, high HP can compensate for low AC, to reflect the character who takes numerous "flesh wounds" but they don't hamper him as much.
edited to provide following example
Here we have two fighters. Private Pyle, with 40 HP and in a giant armor suit that gives him AC 19. Then we have General Hannibal, with 160 HP and tiny leather armor that gives him an AC of 13. Imagine both soldiers are standing still, with an unskilled commoner (no attack bonus) chucking a brick (average damage, 2 points) at them nonstop. Yes, both fighters have a WIS of 3 to be volunteering for this experiment.
The commoner hits Pyle hard enough to damage him with one out of every ten throws (a 19 or a 20). To do 40 HP worth of damage, it will take 200 rounds (20 minutes) of a brick barrage to KO Private Pyle.
Then the commoner starts in on General Hannibal. He hits him four out of every ten times (13-20). Hannibal's tough, though. The hits don't bother him as much, because he can roll with the blows (while the actual damage from each brick is the same, each one does less comparative damage to Hannibal than it does to Pyle). To do all 160 points of damage, the commoner has to throw for... 200 rounds (20 minutes). He hits Hannibal more often, but each hit counts less, because Hannibal's used to this kind of abuse.
If you were to put Hannibal in the big armor suit, it'd take that commoner forever and a day (80 minutes, to be exact) to KO Hannibal, because most of the bricks that WOULD have hit him before now just bounce off the armor. Of those that get through and would have hurt Private Pyle pretty significantly, General Hannibal just laughs and toughs it out. He's a gristly old cuss, ain't he?
~M.
