Hot and cold temperatures affect the body in opposite ways, and they both have effects that can be used therapeutically. Heat generally makes a person's blood vessels expand, and this increases blood flow to a given area. Cold causes blood vessels to contract, and this decreases blood flow and metabolism. Over the long term, heat can potentially help with the healing process in many situations, but in the short term, cold provides immediate symptom relief by slowing down the body’s injury-recovery process.
Cold therapy has the added benefit of decreasing pain by causing the area to become numb. This makes ice useful for any kind of injury that causes pain near the surface of the skin. It’s also the reason that ice is frequently used as a remedy for teething babies.
One of the advantages of cold therapy is the potential to reduce bruising. This happens because bruises are essentially created due to bleeding beneath the surface of the skin. The blood pools up after an injury and gradually turns black, causing a discoloration. If an ice pack is applied to the area of an injury early enough, it will usually reduce the blood flow to the area, thereby decreasing the bleeding beneath the skin and lessening the size of the bruise.
People have used cold therapy for many different kinds of injuries, from minor bruises to broken bones. It’s useful for any kind of impact injury, and it can also help with joint problems. Cold therapy can potentially be used to decrease bleeding in an emergency situation if someone has suffered a severe wound, although it normally wouldn’t be sufficient by itself to stop any dangerous blood loss.