If you are low on health and there is not any Med Stations nearby, look for the flying bug-like creatures that are found throughout the game. Walk up to one and press X. Riddick will eat it, and by doing so will gain health.

Your health is divided up into squares. When you are damaged, the square will start to minimize. At any time you can recharge the last square to the right that is not fully depleted by waiting awhile. This can be done at any time, but it only
recharges your current square. This works faster while crouching.