... brings more blood to the area where it is applied.

... reduces muscle spasm, which makes it useful when muscles are tight.

When muscles work, chemical side products are produced that need to be eliminated. When exercise is very intense, there may not be enough blood flow to eliminate all the chemicals. It is the accumulation of chemicals (i.e. lactic acid) that cause muscle ache. Because the blood supply helps eliminate these chemicals, use heat to help sore muscles after exercise.

... reduces blood in the area where it is applied.

... will reduce bleeding, and also numb the hurt area to help ease the pain.

When an injury or inflammation such as tendinitis or bursitis occurs, tissues are damaged and bleeding occurs. Heat will increase blood flow and increase the bleeding. Cold will reduce blood flow to the area and reduce bleeding. Cold also numbs the affected area, which can reduce pain.