Bandages
When you're injured you can't run or hold your breath, to heal from the injured state you're going to need a bandage, if you have a bandage simply hold the "F" key and a short animation will play before you're healed, if the chase you don't have any more bandages try to look for a medic as they often carry extra bandages to share with the team.
Tuna Cans and Tablets
When you're low health your change of dying are significantly higher shown by your screen become grey, to heal yourself you can use one of two items you can usually find the inventory of any soldiers, "Tuna Cans" and "Tablets" eating either of this iteams will restore your health at some extend. Tuna Cans restore about 40% of your health, while Tables restore about 90% of your health.