The Life gauge is the bar at the top of the screen that indicates how healthy an operative is. As they gets injured it gets shorter until it runs out, resulting in a Game Over. The gauge is traditionally replenished by using rations, either manually or by having them equipped as the operative's non-weapon item. In some cases/games, the operative's life gauge will turn orange and they will bleed if considerable damage is taken. This can be stopped with a bandage or by crouching or lying down. Due to the Metal Gear series' focus on stealth, the Life gauge commonly starts at a particularly low maximum length (even on the easiest difficulties, three or four direct attacks will kill a careless operative at a game's outset), which is normally extended following certain in-game events (i.e. the aftermath of boss battles), though MGS3 allows the player some control over its length by extending it following healing from critical condition.
These are gauges that determine how quickly the Health gauge recovers over time.
This gauge is used in MGS3. This is a secondary gauge that has various effects on Naked Snake, the most prominent being that the fuller it is, the quicker Snake's Life gauge recovers after his injuries are treated. It decreases as Snake runs around or if he gets hit by a tranquilizer dart. Snake can increase it by eating food. Life Medicine takes the place of the previous games' Rations as an instantaneous Life recovery item, and is considerably rarer due to the Stamina Gauge's ability to automatically regenerate Life.
The Stamina Gauge is also used in Metal Gear Solid: Portable Ops and Portable Ops Plus as a sort of "fuel" gauge for the currently controlled operative. Due to the squad-based nature of Portable Ops, an operative whose Stamina Gauge runs completely out is merely rendered unusable for the remainder of the mission (or leg of stages in MPO+'s Infinity Mission mode), and is restored to 1 Stamina the next time the Sneaking Unit regroups. Since the Stamina Gauge is not drawn on for Life recovery in MPO/MPO+, recovery items for both the Stamina and Life gauges appear with roughly the same frequency.
This gauge is used in MGS4. Like the Stamina gauge, its status affects regeneration of the Life gauge. The gauge decreases during battle, as the stress of combat affects Old Snake's ability to keep FOXDIE at bay. It increases if Snake gets out of a stressful situation, "uses" a magazine, or hides in the Cardboard Box or Drum Can. During certain cutscenes the gauge increases or decreases during conversations with other characters, either due to Snake suffering from the nanomachines suppressing the FOXDIE or as comic relief.
The Psyche Gauge returns in a simplified format in Metal Gear Solid: Peace Walker, functioning as an operative (or enemy)'s secondary Life Gauge as well as a barometer of the natural regeneration speed of the Life Gauge proper (and vice versa). Psyche damage in Peace Walker generally consists of taking hits from weapons whose primary/secondary effects impair awareness, such as Stun Grenades, tranquilizer rounds, or shockwaves. Depletion of the Psyche Gauge (and thus nonlethal incapacitation of the operative) does not immediately result in a Game Over: instead, players fall unconscious and must wiggle the Analog Stick to recuperate back to 100% Psyche before control is regained.
When under water or exposed to gas, this gauge appears under the life gauge. It decreases over time and, if it runs out, the life gauge decreases in its stead. This gauge is reset when the operative returns to a normal environment.
This gauge first appears in Metal Gear 2: Solid Snake, although prerelease screenshots for the MSX2 version of Metal Gear indicated that the O2 gauge would have been implemented into that game.
The Grip Gauge, starting from MGS2 onward, determines the length of time an operative can hang on an edge or railing. Its maximum length is equal to the current length of the operative's Life gauge - the more damage he has taken, the less time he can hang onto surfaces.
Certain sources of Life Gauge damage will turn the operative's gauge orange, signifying that he is bleeding from the respective wound. If left untreated, not only will the operative face impaired life recovery or slow life drain, but he will leave a trail of bloody footsteps that can readily alert enemies to his location. As stated in the top of the article, bandages are an expedient remedy to openly bleeding wounds.
Use by enemies and bosses
Starting with Metal Gear Solid, a Life Gauge was also used by various bosses to indicate how much health they have left before being beaten. Metal Gear Solid 3 also had a stamina gauge for various bosses, and only for that game despite non-lethal kills being prevalent in Metal Gear Solid 2 as well as other games after its release.