As we know Snake will get covered in blood as he sustains injuries in the game. Kojima has shared some small new details on the way the blood differs in this game from how it’s generally used in games (translation once again by JunkerHQ).
Blood on Snake doesn’t reset after missions or side ops. It differs from how blood is conventionally used in games. Gradually Snake’s entire body will get bloody from combat. Also, the IV is for recovering from serious injuries.
If you want to wash away the blood you’ll have to stand in the rain or take a shower at Mother Base.
Source: JunkerHQ, Kojima Twitter