Gary E. Gygax said:
It is quite unreasonable to assume that as a character gains levels of ability in his or her class that a corresponding gain in actual ability to sustain physical damage takes place. It is preposterous to state such an assumption, for if we are to assume that a man is killed by a sword thrust which does 4 hit points of damage, we must similarly assume that a hero could, on the average, withstand five such thrusts before being slain! Why then the increase in hit points? Because these reflect both the actual physical ability of the character to withstand damage -- as indicated by constitution bonuses – and a commensurate increase in such areas as skill in combat and similar life-or-death situations, the “sixth sense” which warns the individual of some otherwise unforeseen events, sheer luck, and the fantastic provisions of magical protections and/or divine protection. Therefore, constitution affects both actual ability to withstand physical punishment hit points (physique) and the immeasurable areas which involve the sixth sense and luck (fitness).
Harkening back to the example of Rasputin, it would be safe to assume that he could withstand physical damage sufficient to have killed any four normal men, i.e. more than 14 hit points. Therefore, let us assume that a character with an 18 constitution will eventually be able to withstand no less than 15 points of actual physical damage before being slain, and that perhaps as many as 23 hit points could constitute the physical makeup of a character. The balance of accrued hit points are those which fall into the non-physical areas already detailed. Furthermore, these actual physical hit points would be spread across a large number of levels, starting from a base score of from an average of 3 to 4 going up to 6 to 8 at 2nd level, 9 to 11 at 3rd, 12 to 14 at 4th, 15 to 17 at 5th, 18 to 20 at 6th and 21 to 23 at 7th level. Note that the above assumes the character is a fighter with an average of 3 hit points per die going to physical ability to withstand punishment and only 1 point of constitution bonus being likewise assigned. Beyond the basic physical damage sustained, hits scored upon a character do not actually do such an amount of physical damage.
Consider a character who is a 10th level fighter with an 18 constitution. This character would have an average of 5½ hit points per die, plus a constitution bonus of 4 hit points, per level, or 95 hit points! Each hit scored upon the character does only a small amount of actual physical harm --- the sword thrust that would have run a first level fighter through the heart merely grazes the character due to the fighter’s exceptional skill, luck, and sixth sense ability which caused movement to avoid the attack at just the right moment. However, having sustained 40 or 50 hit points of damage, our lordly fighter will be covered with a number of nicks, scratches, cuts, and bruises. It will require a long period of rest and recuperation to regain the physical and metaphysical peak of 95 hit points.