Hydrotherapy relies on a number of factors which all influence the outcome of the therapy;

TEMPERATURE.
The cool temperature numbs the area acting as a topical analgesic. The permeability of the blood vessel walls is decreased reducing the amount of fluid that accumulates in the injury site. At a cellular level, the metabolic response of collagnase (a hormone which leads to the degradation of tendon/ligament tissue) is severely inhibited, thereby reducing hypoxic injury.

SALINITY.
The make up of the salt concentration in the water has an important effect on the tissue response. Seawater contains varying amounts of the common salt (sodium chloride), averaging at 25g/L and magnesium chloride 1g/L. The spa bath contains higher salt levels than seawater, producing an increased drawing action, and allowing excess fluids to be taken away from the damaged tissue. This in turn causes a reduction in swelling.

MASSAGE.
The action created by the air jets stimulates an increase in circulation. This ensures that a continous supply of lymphocyte cells are supplied to the affected area which help to accelerate the healing process.

OXYGENATION.
Another effect of the air jets is to significantly enhance the level of dissolved oxygen in the water. Chilled aerated water (between 2-4 °C) hold about 8-11 ppm of dissolved oxygen. The increased availability of oxygen to the damaged tissues is thought to have a major effect on the rate of recovery and allows the horses to relax more than in still cold water.