There are a number of physical responses that occur in the a mammal's body when it is exposed to heat. It is important to not only understand what thermoregulation is, but the physiological and/or anatomical thermoregulatory responses that allow sustained exercise in horses.
Thermoregulation is the control of body temperature within certain limits even when the surrounding temperature is very different. This enables the body to function effectively and is known as maintaining homeostasis, which is a dynamic state of stability between an animal's internal environment and its external environment.
A relatively constant body temperature is necessary for the efficient functioning of the complicated brain of higher animals. Extreme temperatures alter biological molecules and disrupt body functions resulting in illness such as hyperthermia or hypothermia, which if not treated can lead to death. Mechanisms have subsequently evolved in mammals to enable body temperatures to stay within certain limits.
All mammals are endothermic meaning they maintain and regulate their own body temperature. Mammals and birds maintain a constant body temperature which is usually above the environmental temperature, known as homeothermic.
Adapting to the Environment
Mammals live in a number of widespread environments around the world, forcing them to face daily and seasonal fluctuations in temperatures. Some mammals live in harsh environments, such as arctic or tropical regions, and must withstand extreme cold and heat. In order to maintain its correct body temperature, a mammal must be able to produce and conserve body heat in colder temperatures, as well as dissipate excess body heat in warmer temperatures. Some mammals have adapted to their environment by increasing their surface area in the extremities, such as large ears on the Zebou cattle.
Animals that are exposed to the cold have heavier organs, and their skin color is dependent upon the amount of radiation they are exposed to. In colder climates, fat under the skin provides mammals with necessary insulation. Due the to surface to volume ratio, a large animal has the advantage over smaller animals since less skin is exposed to the elements.
Surviving the Heat
While fat is necessary in colder temperatures, it is also crucial for mammals living in warmer climates. The Zebra cattle deposit fat deep in the body to aid greater heat tolerance, while camels deposit fat in their humps.
In warmer climates, excess body heat can accumulate and cause life-threatening problems for a mammal, making it important for the body to dissipate heat. This is accomplished when circulation near the skin's surface releases heat into the environment, and when moisture from sweat glands or respiratory surfaces evaporates and cools the mammal. In dry regions where water loss is dangerous for mammals, evaporative cooling is less effective, forcing the mammals to seek cover during the hotter daylight hours, and resume activities at night. Smaller animals have an advantage over larger animals because the larger surface to volume ratio allows for greater heat loss.
A Delicate Balance
A mammal's body temperature results from a balance between production and loss of heat. In this balance, heat is constantly produced and lost, and production and loss of heat will be equal, resulting in the temperature remaining constant.
The hypothalamus is responsible for the regulation of a mammal's body temperature. This temperature control requires sensors, a control center and effectors. The two types of sensors which respond to hot and cold are found throughout the body in the skin, body core, and brain. The control center is in the hypothalamus of the brain, and acts as a thermostat which has a temperature set point. The effectors produce more heat (increased metabolic rate, shivering, brown fat metabolism), and change heat loss (blood vessel dilation or constrictions, erection of hair, curling up, sweating).
The skin is primary organ for removal of metabolic heat by cooling the body through the sudoriferous (sweat) glands. There are two types of sweat glands- the apocrine, which secrete pheromones, and the eccrine, which are tubular, secrete sweat, and are found over the entire body of most farm animals. Approximately 90% of body heat is lost through the skin, and if the body temperature is too high, the skin can dilate blood vessels, increasing blood flow by 150 times. In cold temperatures, the skin constricts blood vessels in order to reduce heat loss.
Heat loss is by radiation, conduction, convection and sweating. Sweating can be used to lose enormous amounts of heat, as the sweat glands which are activated by the sympathetic nervous system release secretions on the skin surface. If the ambient temperature is higher than the body temperature, sweating is the only way heat can be lost.
Physiological and Behavioral Mechanisms
There are a number of physiological mechanisms involved in temperature regulation. These mechanisms include dry heat loss, radiation, and convection, which are dependent on environmental factors, evaporation, excretion and panting.
Behavioral mechanisms are concerned with the heat gains and losses in animals and include convection, evaporation, and radiation.
Types of Thermoregulation
There are two types of thermoregulation that are used by animals- physiological regulation and behavioral regulation.
In physiological regulation, an organism changes its physiology to regulate body temperature, such as sweating to cool the body and shaking to create heat and warm the body.
In behavioral regulation, an organism changes its behavior to changes in body temperature, such as finding shade when the body gets too hot in order to cool down.
Dangerous of Heat
If there is a failure of temperature regulation in hot regions, an animal can suffer from heat exhaustion and heat stroke. Heat regulation failure can be triggered by loss of fluids, since animals can lose up to 1.5 liters of water an hour as sweat. If the fluid is not replaced, the blood pressure will fall, heat regulation failure will occur and there will be a rise in body temperature.
Heat exhaustion is characterized by wet, cool skin from sweating and is treated by replacing lost water and salt. Heat stroke is characterized by dry, hot skin and is treated by rapid cooling of the body. Heat stroke is a failure of the sweating mechanism, is very dangerous and can be life-threatening. The body temperature rises so fast during a heat stroke that brain damage and death may result if the body is not cooled quickly.
Heat Stress on Horses
Heat stress occurs when heat production exceeds evaporative capacity of the environment or evaporative mechanisms become impaired due to great loss of body fluid and reduced blood volume, making it necessary to cool the entire body.
A horse that is suffering from heat stress may be distressed, tired and unwilling to continue exercising, lethargic, blow hard continuously, or stand without showing interest in surroundings or grazing.
Since heat stress can lead to heat stroke, anyone exercising a horse should know the warning signs of heat stress as a means of preventing the life-threatening condition. It is crucial to provide extra care and attention to a horse being exercised on hot days, since there is a significant increase in the amount of heat produced by working muscles. Heat production can increase as much as 50% during intense exercise as compare with heat production when the horse is at rest. A horse increases its sweating rate to move more blood to the capillaries under the skin and breaths much harder in an effort to release this build-up of heat. The most commonly observed signs of 'heat stress' are profuse sweating, rapid breathing, and a rapid heart rate. Some horses have a condition leaving them little or no ability to produce sweat, and are referred to as 'Anhydrotic'. Since heat loss is mainly dependent on sweating and its evaporation, anhydrotic horses are prime candidates for heat stress.
If a horse begins to exhibit the signs and symptoms of heat stress, a vet should be called, the horse should be moved to a shady area with ventilation, the legs and lower body should be sprayed with cool water to lower the internal temperature, and in critical conditions, ice packs or cold water soaked towels should be placed on legs and other areas that exhibit large veins on the horse. If a horse is showing signs of heat stress, it should not be given large quantities of water, since a 'hot' horse has the chance to colic.
To determine marginal water loss in a horse, a pinch test is performed. When a section of skin on the neck or shoulder is pinched, the skin will recoil immediately in a normally hydrated horse, however skin recoil will be delayed in a dehydrated horse.
Fluid Loss and Electrolytes
Sweat not only contains water, but salts which when broken down into their chemical components are known as electrolytes, such as sodium, potassium, chloride, magnesium and calcium. These components each carry an electrical charge which allows electrolytes to govern the transfer of water through cell membranes into or out of the cells. If a horse sweated out these electrolytes in the same percentage in…