The application of cold triggers three basic reactions. Firstly, at a cellular level, the metabolic response of the cells is reduced, so the cells need less oxygen to function and thereby suffer less hypoxic injury. Secondly, the permeability of the blood vessel walls is decreased, thus reducing the amount of fluid that accumulates in the injured area. And thirdly, the cold numbs the area to a certain degree, acting as a topical analgesic.
One of the best things about cold is that it does not override the beneficial effects of the healing process. Please note that the application of cold should not be overdone. The overuse of ice, for example, can cause tissue damage.
The salt concentration of the water has an impact on the healing process. Higher salt concentrations will draw away more of the fluids that accumulate around an injury. Additionally, the proportion of various salts has been found to allow injuries to heal more quickly.
The greater the depth of the water, the greater the physical pressure exerted on the tissues. This, again, aids the dispersal of accumulated fluids.
Aeration of the water has a two fold effect: it increases the dissolved oxygen level of the water and subjects the soft tissue to a gentle massaging action. It is thought that the increased oxygen level aids the healing process much as hyperbaric chambers do for humans, while the massage effect influences dispersal of fluids. In addition, horses tend to relax more than in still cold water as is evidenced by reduced heart rates.