Jagex said they changed Hitpoints to Constitution so that when new players started and were training combat, enemies had a chance at hitter lower than 1/10th of your total hitpoints. Before the update if somebody with 10 Hp were fighting a level 2 man for example, the man would be able to hit a 1 or a 0. Now they can hit 0-10, giving the person a higher chance of surviving. They also said that update was so they could have a constitution requirement for quests. I don't see how this could work with prayer. :-?