To the left of the health bar, there are 3 notches that fill up sometimes. What do these do? They seem to go up and down depending on what is going on. I have no idea what this does.
These contribute toward integrity upgrades. Once you fill all 3 notches, you will get a health upgrade. You fill notches by picking up the health items around the levels. Some will restore health and if your health is full, a normal health vial will give you a notch toward an upgrade. There are dedicated upgrades around the maps too that can give between 1 and 2 points toward a health upgrade.