The mod is made in order to give more sense for trips to the bosses.
When you kill a boss, the heart drops out of it, raising your health limit.
It is assumed that you want to continue exploring the world, rather than complete it as quickly as possible.

Leg of Three-Eye (in Pyramid)

Drops the heart at 50 HP.

Dragon (in Dragon Cave)

Drops the heart at 100 HP.

Three-Eye (in Laboratory)

The heart that the boss can drop depends on the number of orbs collected.

• If orbs are collected from 0 to 2 inclusive, the boss drops 25 HP.
• If orbs are collected from 3 to 5 inclusive, the boss drops 50 HP.
• If orbs are collected from 6 to 8 inclusive, the boss drops 75 HP.
• If orbs are collected from 9 to 10 inclusive, the boss drops 100 HP.
• If orbs are collected more than 10, the boss drops 200 HP.

Planned

• drop hearts from chests (5HP, 10HP)
• drop healing hearts from strong mobs (eg big worm)
• buy hearts in the store along with other items (if you're lucky and it spawned)
• spawn hearts in other structures, in which usually go long/difficult/dangerous and which usually nothing useful not give