Horses can regulate their temperature and maintain it at a relatively constant level that is different to the temperature of the environment. The most active organs produce the most heat; this includes the muscles and the liver along with the digestive system.
The control of a horse’s temperature is an example of homeostasis; this means it has receptors, comparators, effectors and neural elements linking it together to keep it at a steady state. Mammals are able to manipulate heat exchange between themselves and the environment in four possible ways. Conduction happens when the horse is in contact with another solid object such as a rug or a numnah. Convection occurs when the horse sends its blood to the periphery of its body and heat is lost to the surrounding air, this cools the blood down, this blood then moves to the core of the body and cools the horse down. Radiation is due to heat loss from the horse into the atmosphere. And evaporation occurs when the horse sweats.
The receptors which monitor temperature are called thermoreceptors; there are two types of these detecting either hot or cold. There are thermoreceptors in the skin which detect changes in environmental temperature and ones in the hypothalamus region of the brain which detect changes in internal temperature.
During exercise basal metabolic rate increases ten to twenty times. This produces a large amount of heat that must be lost to maintain normal body temperature. One way to lose heat is to divert blood to the periphery of the body; this is done by vasodilation, and will increase the heat loss from the blood through the skin. This mechanism is controlled by the autonomous nervous system. Vasodilation increases conduction, convection and radiation and indirectly helps with the evaporation of sweat.
Another way in which heat is lost is through sweating by the evaporation of water. Sweating is the main means of getting rid of heat in the horse. As body core temperature increases so does the sweat production. Sweat is converted from a liquid to a vapour when it reaches the skin; heat is lost in this process. A sweat gland is made of a coiled tube which opens on the skins surface; they are triggered by a raised temperature and adrenaline. Horses are able to sweat everywhere apart from the legs; however they sweat more in some areas such as the ears and the neck. Sweating is a continuous process and is only visible on the skin when the rate of sweating exceeds the rate of evaporation. If sweating occurs for a long period of time dehydration can occur. Most clothing adds resistance to the evaporation of sweat, however there are some effective materials that can be used that promote wicking or free movement away of water vapour. This is why horses often where a thin rug made out of this sort of material when they have been heavily worked as it will help them to cool off by wicking away the moisture. Sweat contains substantial amounts of calcium, potassium, bicarbonate, sodium chloride and magnesium. When fluid and electrolyte loss are severe the horse may lose the ability to sweat even in a high temperature, this is heat stroke.
Image from ehow.com
The thermal stress put on the horses in the hot weather may result in heat stress and exhaustion in some horses. Aggressive cooling of horses is important for the ones that had competed in hot conditions to bring their temperature back down to normal. Profuse sweating may have also led to performance limiting dehydration.
After exercise moving air from fans can help to cool them down by promoting evaporation. Ice cold water can be sponged over the horse, and scraped off. There is little danger off the horse being cooled too rapidly as long as the temperature does not drop too far. To prevent this horses may be blanketed whilst being walked around after having the water put on them. When competing in hot temperatures it is not unusual for a horse to have a rectal temperature of 41 ᵒC, but it should be 39 ᵒC within 30 to 40 minutes.
Training acts as a form of heat acclimatisation. Heat stroke is a major problem in hot climates where horses are being exercised. I may do another post on these two things.
P.S. I apologise for the larrrrge spacings between paragraphs on this post. Not too sure what happened and I couldn't correct it, but think it was from copying and pasting information from word that I was using before editing it!