It's like the dogfight player, minus the sprint.
Basically, If the player's health is above 75(76 or more), it regenerates to 100, slowly.
If it's above 50(51 or more, you can see where this is going), it regens to 75.
If it's above 25(guess), it regenerates to 50.
If it's above 0, it regenerates to 25.
So you can survive for a little while without a medic, but to stay in tiptop shape, you need one.
Maybe RTB prefs to configure stages.