As you may have noticed, when low on health, occasionally if enough damage is done in one go you'll get taken into a slow-motion 'critical' state, where you're supposed to patch Snake up by prodding a button. This actually has ramifications outside any given mission, as firstly if you leave the mission in low health, you'll notice Snake has a blood drip going into his arm, but if you manage to exfiltrate whilst in this critical state, there'll be an intravenous drip keeping him alive.