There is a few ways to regain blood. But, the simple method is to eat and drink, and wait. All you need to do is drink until you are full, then eat until it says energized. You will notice that energized is only dark green. Eat a little bit more until it is in a lime green state. Afterward, a little message will appear that says healing. You will soon heal to full health and a newer message will say healthy. That's all you need to do. Just eat and drink enough that energized and hydrated status are lime green.
