by Dr. Kerry Ridgway
The single most important means the horse has for getting rid of the enormous heat load generated during exercise is evaporation, accounting for about 65% of the heat dissipation. Sweat is evaporated off of the skin surface and cools the horse. The lungs account for about another 25%. This capacity of the respiratory tract in dissipating heat from the body becomes very important under conditions of high humidity and high temperature when evaporation conditions are not favourable.
Sweating can only occur at a significant level when the horse is not dehydrated and has plenty of fluids in its body with which to produce sweat. When the horse is dehydrated, the lungs play an even more important role. Even so, the dehydrated horse is severely handicapped in ability to rid the body of any excess heat.
Let’s complicate this process one more step and take a look at what is happening to the circulation. The blood that the horse has available to circulate is what gets the nutrients and oxygen to the individual cells and takes away the waste products of that cellular metabolism, i.e. the wastes, toxins, and heat. It gets rid of much of the waste through the kidneys. In order to get rid of the heat, the circulation to the skin is greatly increased and brought closer to the surface. The surface veins dilate so they can bring even more of the hot blood to the surface for cooling. This is all well and good up to a point; but suddenly we have a strong and literally heated competition going on for who gets the blood — the skin for cooling or the muscles and organs for metabolism and work required to uphold the requested level of performance.
Add dehydration to the picture and there is even less circulating fluid volume available to accomplish the demands for both metabolism and heat dispersion. At this point, the skin loses out and sweating rapidly diminishes, even though the need for heat dissipation remains critical. If work continues, the animal experiences heat exhaustion, or heat stroke and the likelihood of death.
Earlier it is mentioned that a horse trotting at 17–18 km/h (11.2 mph) loses about 12.5 litres of sweat per hour in order to manage its heat load. This fluid isn’t just water — it contains a lot of salt. These salts, when they are broken down into their chemical components, are referred to as electrolytes. These are typically groups of different salts that contain such electrolytes as sodium, potassium, chloride, magnesium and calcium. Each one of these individual components or electrolytes carries an electrical charge (when they are considered or discussed in terms of charged particles, they are referred to as ions). By means of these electrical charges, electrolytes govern the transfer of water through cell membranes into or out of the cells. Thus, they function in getting the nutrients in and the waste products out. They are responsible for getting nerves to fire and muscles to contract. Essentially all of the physiological actions in the body require electrolytes. And importantly, they need to be present in the fluids in the appropriate amounts for these biochemical reactions to proceed in an orderly manner.
If the horse sweated out these electrolytes in the same percentages in its sweat as in its circulating fluids, the body’s concentrations of these electrolytes would remain in balance even though there would exist a depletion of both total fluids and electrolytes. However, this balance is not maintained. The horse’s sweat is more concentrated in electrolytes than the electrolytes in its circulating fluids. This is referred to as being hypertonic. Human sweat, in contrast, is hypotonic, i.e. containing a lesser percentage or concentration of electrolytes than in the circulating fluids. This difference between the two species is a compensating mechanism for the poorer ratio of body surface to mass in the horse.
If we don’t provide at least a minimum electrolyte replacement, horses show up with such medical conditions as metabolic alkalosis, inefficient transport of oxygen and energy substrates, poor tissue perfusion, thumps, muscle spasms, exertional rhabdomyolysis, cardiac arrhythmias, gastrointestinal stasis, anhydrosis, kidney impairment, and poor recoveries. (Actually, poor heart and respiratory recovery is one of the key signs that can lead you to recognize the problems associated with the task of accomplishing thermoregulation.) The point is, most of these problems mentioned stem from the resulting dehydration and electrolyte imbalance. It is nothing short of amazing how much loss and imbalance the horse can endure not only without dying, but often they even continue at a credible performance level. The real question becomes, how much better could the horse do if it were in a state of ideal electrolyte and fluid balance? How many of the horses who fade in the last third, or have prolonged recoveries after the event, could be winners if their electrolytes and fluids were balanced and at appropriate levels?