Ah, health is the most vital features in every video games you played. As you can see health is measured in entities like you and the NPCs. Right here, I will talk about how healths is measured in video games.

1. General
The General is the bread and butter of the video games. You see it, you can understand it. Healths are measured in bars or percentages. Usually, the health will be colour coded in games. Such green is full and red is critical condition. If your health is lower enough, the game will make a warning like heartbeats, red screens or annoying beeping sound, that the game tells you need to get healing or you will die.

2. Expressions
Old games like Wolfenstein 3D, Quake and Doom have facial expressions that can tell the players if the players are healthy, smiling face means the players are fine. The bloody and sad face shows that player is low of health. Expressions also counted on a physical appearance of objects such as a vehicle, they smoking if they are low in health, they set on fire when their health is empty, giving players a second to run away before it explodes.

3. Separate Life Bars
This one is unique. These health bars are divided into several parts, like bodies. Usually when you get hurt in one specific body. It can cause negative effects. For example, in Deus Ex, when your feet are in red, your character moves very slowly. If your arms are red, the player will have higher recoil when firing guns. As long as your torso or head is green or yellow, You'll be OK. But when your red is head, it will be fatal if players getting shot on the head one more time. In this case, players need to use medical kits to heal at least one of the specific body parts.

4. Collected Items
Sonic The Hedgehog is the most special one. That game features rings as a health bar. If you get hit when you have at least 1 item only, you'll be fine. Getting hit without it, and you're done for.

5. Red Bloody Screens
Expect to see some of First-Person-Shooter games did this. When you taking too many bullets, the sound becomes ringing, player characters breathing faster, screens become red and jelly. The best option is to hide and wait for the screen to become clear again.

So that's it. The health systems in the game are becoming a backbone of any video game creations. You can do whatever you want with the health systems when designing video games. You can just type "Your life-o-meter" and that's it.