As stated in DMG, in regards to Hit Points:
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).
Hit points are mostly non-physical in nature. The increase of Hit Points (HP) come from advancing in level. Level advancement comes from defeating monster, treasure, magical items, and from second edition from class skills (page 70, DMG – April 1995 printing). Class skills arise from the performance of certain skill that are specifically attributable to that character and his class, i.e a thief climbing walls, but not a magic user; a priest casting a spell in furtherance of his ethos and not just any spell, etc. Thus, HP come from adventuring and class skills.
To determine each classes’ proportion of Class skill EXP and Adventure EXP, it must determine what percentage of class skills is proportional to the total experience points. In essence, how much of the class is related to the non-physical (meta) HP. A fighter would have a higher proportion meta-HP than a magic user. The best measuring tool to determine is percentage is the classes’ Hit Dice.
Thus, here is the rule: each class needs to gain a percentage of experience points from adventuring equal (compared to experience points from class skill) to his Class Hit Dice die in order to gain full allotment of his HP
Magic User: d4: 40% Adventuring EXP; 60% Class skill EXP
Thief d6: 60% Adventuring EXP; 40% Class skill EXP
Cleric d8: 80% Adventuring EXP; 20% Class skill EXP
Fighter d10: 100% Adventuring EXP; a fighter gains all is from defeating monsters, gaining treasure and magical items.
Thus, a Cleric that needs 1,500 EXP to go from 1st level to 2nd level, he needs 1,200 EXP from adventuring and 300 from class skill EXP.