Just as there are mechanisms for regulating blood sugar and blood pressure, the body regulates the body temperature and water.

Heat exchange occurs through a number of processes. Most of the heat for maintenance of body temperature comes from metabolism. Conduction is the transfer of heat due to physical contact. Heat transfer also occurs through the fluid medium which the animal moves or convection. Normally this fluid is air. If the air is cooler than the surface temperature, then convection will favor heat loss. If warmer than surface temperature, then it will favor heat gain. Objects also exchange heat by electromagnetic radiation. During the day, in summer, sky and surface temperatures can be well above surface temperature of the horse. That means a net energy transfer into the horse. If the horse is black, it will absorb more in the visible wavelengths and will have a greater heat load. At night, in winter, the net radiative exchange will be from the horse to the environment.

For an animal to remain in energy balance, heat gains must = heat losses:

(Metabolism - Evaporative water loss) = ± Conduction ± Convection ± Radiation

Fluid regulation:

The body also regulates water. Most tissue is water, 60-70%. This water is divided into extracellular and intracellular components. Extracellular means outside of the cells and includes the blood. When water is lost, it is lost first from the extracellular space and then from intracellular spaces. Loss of too much fluid compromises cardiovascular function and thermoregulatory capabilities and leads to a loss of performance. A loss of 3-5% of body weight in fluids can alter performance. A loss of 10% is critical. For a 1100 lb (500 kg) horse, that is 50 kg. Since tissue is mostly water, 50 kg is equivalent to 50 L. For our purposes, 1 L is about 1 quart.

Water is also important in maintaining body temperature. Evaporation is very effective in removing excess body heat and regulation body temperature. For a horse to maintain body temperature of 101 F at an air temperature of 80 F during a walk, about 1 L of water is evaporated per hour. At the canter, this could be 20 L per hour. Most animals evaporate water primarily from the respiratory tract. Horses, like humans, have an additional evaporative means--sweating. Sweat gland literally pump water onto the surface of the skin and these glands show regional differences in number and sensitivity. There is an extremely high number of glands in the neck region. The mechanism by which water is moved onto the skin utilizes pumps which move solids such as sodium, chloride, and even protein onto the skin surface. Water follows.

Sweat glands in the horse are under the control of the nervous system. Nervous horses will sweat and sweating can be induced by injection of drugs that mimic the nervous system, such as adrenaline.

Exercise produces nervous responses which initiate the sweating process. Additionally there is an increased skin blood flow. This serves to increase heat loss through both radiative, convective (if the air is cooler) and evaporative mechanisms. Increased insulation will hamper this exchanges (hence the idea of clipping).

Water is regulated as are the dissolved solids. Most of these are minerals such sodium, chloride and potassium. The sweating process, which actively uses these components can produce a loss. Proper rehydration requires not only replacement of fluid, but also electrolytes. Equine sweat consists of high concentrations of chloride, sodium, and potassium. In addition, other components of the sweat act to move water out. These include waste products such as urea and lactic acid, but in the horse, protein as well. In fact the lather of the horse is produced by the action of protein and water. Earlier concerns about the amount of protein lost by this process have not been substantiated. Animals eating a balanced diet will have adequate amounts of protein to replace that lost during sweating.

If proper hydration is not maintained, that means there is less fluid to be moved by the cardiovascular system. This could mean a loss in blood pressure. In response, the heart works harder. For example, a horse working with a heart of 160 and normal hydration would have to increase its heart rate to 175 under moderate hydration and even up to 190 during severe dehydration.

The amount of water lost and the rate depend upon a number of factors. Clearly the nature of the exercise event (intensity and duration) have profound impact. A training racehorse may lose 5-10 L while an endurance horse may have to replace 25-30 L (or more, maximal rates can be at 12 L/hour). The effectiveness of sweating relies upon the ability of the water to evaporate. High humidity decreases the effectiveness of sweating. High ambient temperatures place a strain on the system by requiring an increased amount of water. Both of these components, humidity and high temperature, can be used in a practical fashion in a number called the Heat Index. The heat index is equal to the temperature (in degrees F) plus the humidity (in %). When the HI is under 130, normal heat loss is not usually impaired. At 150, post exercise treatments to cool of the animal should be employed. These include removing the saddle (there are a large number of sweat glands on the back and their effectiveness is decreased by covering them with a saddle), sponging with water, and if necessary, giving an enema. Water can be cold (45 F) and very effective. Studies in 1995 indicated no evidence of tying up using water this cold. The addition of cold towels should be avoided!. Although initially cooling, they very quickly become an insulator.

At values above 180, the horses cooling mechanism is relatively ineffective and supplemental cooling regimes must be used. The use of fans and misters, a long honored tradition in Palm Springs, was brought to a high level in Atlanta. During exercise, an increase in body temperature is normal--is mandatory, muscles function better at elevated temperatures. Normal body temperature is 37-38.5 c = 99-101.5 F [average, 38=100.5] Temperatures above 104 after intense exercise are not uncommon. However, if the animal is unable to dissipate this, the increased heat load can lead to exhaustion and stroke.

At temperatures of 106 and above, normal physiological systems start to fail to regulate. Not only is the CV system not able to keep up with demands of exercise, but it can't keep up with heat loss demands. HR are grossly elevated (as noted above) and lactates are increasing. CNS function is decreased. This can lead to collapse, convulsions, and death. Clearly aggressive means are needed to replace vital fluids and decreased the body temperature.

Anhydrosis means without sweat. This condition can be progressive or acute and has been seen predominantly in horses brought from cool climates (England) to hot-humid climates. The pathology of this disease has not been well established, and treatments, therefore are generally palliative. Long term treatment involves taking the horses back to cool climates.

Another thermoregulatory concern is post exercise. If the animal has been working in a hot, but dry environment, sweat may not be evident due to the low humidity. If the horse is put away immediately, sweating starts out of the owners eye sight, and a post exercise chill may be attendant.

Fluid replacement: