Entering combat will result in enemies shooting at Snake. If Snake is hit, he will take damage and this is represented by the burn marks that appear around the outside of the screen. Too much damage will have our hero die and force a checkpoint re-set. As we don't want this to happen, taking cover to avoid incoming fire, breaking line of sight and hiding from enemies will allow the damage to regenerate slowly over time.
If Snake takes too much damage, it becomes critical and you will be required to use a medical spray ('Y' or 'Triangle') when an spray icon pops up in the centre of the screen. Doing so will restore health, but take too long and snake will bleed out and head back to the previous save point.