Constitution

Hitpoints also known as "health", common abbreviation "HP", represent the amount of hits you can take before dying.

Most RPG's have a hitpoints scale, and the question of wounds is often brought up, as you fight as if uninjured until your last.

In combat, the hitpoints of the combatants appear as a bar over their heads, with all green being 100% healthy, and all red being a trip back to Lumbridge. They can be checked at any other time, by looking at your statistics view (the bar grap icon). This is a proportional scale, and does not tell you the total or remaining hitpoints of an opponent, unless you can relate it to the level of hits you are scoring - eg. you hit them for 10, and the green drops by 1/4 - they have 40 HP and you're beating up on them.

Increasing hitpoints total
Any form of melee combat training gives a quarter of the monster experience (normally around 5 times the monster's hitpoints) to your hitpoints statistic, and the remainder to your chosen combat style, or equally to all three styles if fighting in the "controlled" style avaliable with some weapons.

The division of monster experience in other combat classes is unclear. In magic, the magic experience is granted for using spells, successful or not, while in ranged, it does depend on the monster hitpoints.

Recovering hitpoints
While you recover slowly if not under attack, eating food recovers an amount of hitpoints dependent on the level of food.

The Monks at the Monastery will also heal you if asked, 10 hitpoints each time.

The "rapid heal" prayer is supposed to double the rate of recovery, but twice slow is still slow. It's poor value for the prayer point usage, unless you are close to a prayer altar to restore your prayer points.