Reduce bleeding and swelling
By cooling the surface of the skin and the underlying tissues, ice causes the narrowing of blood vessels, a process known as vasoconstriction.
This vasoconstriction leads to a decrease in the amount of blood being delivered to the area and subsequently lessens the amount of swelling.
After a number of minutes,the blood vessels re-open (dilate) allowing blood to return to the area. This phase is followed by another period of vasoconstriction- this process of vasoconstriction followed by dilation is known as the Hunting Response.
Although blood still flows into the area the amount of swelling is significantly less than if ice is not applied. This decreased swelling or edema, allows more movement in the muscle and so lessens the functional loss associated with the injury.
The swelling associated with the inflammatory response also causes a pressure increase in the tissue and this leads to the area becoming more painful. This pain is intensified by certain chemicals that are released into the blood when the tissue is damaged- hence vasoconstriction from applying ice also decreases pain.