For that, you need a Medkit. Those will give back health to your limbs, and at the same time may stop a bloodloss, or a fracture. Careful! The AI-2 medikit you get at the beginning doesn't stop bloodloss, so you might want to consider bringing along bandages. However, if your limb is black, you can't add health points to it anymore. This is where Painkillers, Morphine and other meds come in place. These will allow you to keep running even if your legs are blacked, stop coughing if your stomach is dead... They are super useful, but a bit expensive at the beginning. You can find a lot of them in Med Bags along the map. Consider picking them up, you'll need them.