Blood is an abstract score that represents vitality and health. If you have ever played any sort of RPG, then Blood can be thought of as “hit points.”
When you are wounded, your Blood score will drop. When you rest or take healing medicines, your Blood score will rise. You begin this game of survival with a Blood score of 10. This is your maximum Blood; your Blood cannot rise above this amount. For example, if your Blood has dropped to 6, and you heal 7 Blood, then your Blood increases to 10. However, as you rise in levels, your maximum Blood score will increase.
If your Blood ever reaches zero, then you have died, and the game of survival is lost.