The Hero Healthstat determines how many hit points a hero will have. Every class has a different base health amount, with the Apprentice having the lowest. You lose hit points every time a monster hits you.
The way your hit points increase depends on your hero's class and Hero Health level. This increment is called scale of progression. Each class has a scale and an equation to calculate this progression.
Below follows the equations for each class. These equations are perfect. All these formulas were discovered by Putziorion in Stat Progression Graphs and reproduced here in a more simplified and understandable version, but still precise.