Temperature Is a Major Factor Affecting Tissue Function

Because body function is the result of chemical and physical processes that are sensitive to changes in temperature, animals use a variety of strategies to regulate the temperature of their tissues. If body temperature is allowed to decrease too much below its normal value of about 38° C (~100° F), metabolic processes are slowed to such an extent that body functions are severely impaired. Below 34° C (93.2° F), animals become unable to regulate their own temperature, and at 27° to 29° C (80.6° to 84.2° F), cardiac fibrillation and death occur. At the other extreme, an increase in temperature to 45° C (113° F) can cause fatal brain lesions.

Homeotherms and Poikilotherms Use Different Strategies to Regulate Body Temperature

Fish, reptiles, and amphibians are called cold-blooded animals, or poikilotherms, because their body temperature varies with the temperature of the environment. However, this does not mean that these animals have no control over their body temperature. They use behavioral methods to prevent major changes in their temperature. For example, the lizard basks on a sun-baked rock to increase its temperature early in the morning and hides beneath the rock later in the day to prevent overheating. Veterinarians are sometimes asked to advise on the management of captive poikilotherms; it is important to remind owners to provide supplemental heat if they want their animals to be active at the cooler times of the year.

Mammals and birds are homeotherms; they maintain a constant body temperature in the presence of considerable changes in environmental temperature. Although the maintenance of a constant temperature allows mammals to live in a wide variety of environments and to remain active during the cold times of the year, it is not without cost. Homeotherms must maintain a high metabolic rate just to provide the heat necessary to maintain body temperature. This requires a high energy intake and therefore almost constant foraging for food. Poikilotherms require much less energy and are better able to survive times of food shortage. Because most veterinarians are primarily concerned with mammals and birds, this chapter focuses on the maintenance of a normal body temperature by homeotherms.

Body Temperature Depends on the Balance Between Heat Input and Heat Output

Heat inputs to the body are from metabolism and from external sources (Figure 53-1). When food energy is ingested, heat is produced at all stages of the metabolic process. Eventually, all food energy is converted into heat, which is dissipated into the environment and radiated into space. Heat production by the body is related to metabolic rate. A basal metabolic rate is necessary to maintain the function of cells. During exercise, metabolic heat production can increase more than tenfold. If this heat is not dissipated to the environment, body temperature can increase to dangerous levels. Furthermore, this increase in body temperature increases metabolic rate, which further increases heat production.

Animals gain heat from the environment when ambient temperature exceeds body temperature and when they are exposed to radiant heat sources. The latter occurs when an animal is exposed to sunlight or is placed close to solid objects that are warmer than its body temperature.

Heat is lost to the environment by radiation from the body surface to a cooler object; by convection as the surrounding air or water is warmed by the body; by evaporation of respiratory secretions, sweat, or saliva; and by conduction to cooler surfaces with which the animal is in contact. A small amount of heat is also lost with urine and feces.

Many of the metabolic heat sources, such as the liver, heart, and limb muscles, are remote from the skin, which is the site of heat loss. Therefore, it is necessary to transfer heat among these sites. Body tissues are poor conductors, so heat is transferred mainly by convection in the circulation.

Shivering Produces Heat by Muscle Contraction

Shivering is one method of increasing the metabolic production of heat. Antagonistic groups of limb muscles are activated so that they produce no useful work. The chemical energy used in shivering is transferred to the body core as heat. If necessary, shivering can go on for many hours and can double heat production. Short bursts of shivering can even increase heat production up to fourfold.

Nonshivering Thermogenesis Is an Increase in Basal Metabolic Rate, Caused Especially by the Oxidation of Brown Fat, to Produce Heat

When animals are chronically exposed to cold, they develop the ability to increase metabolic heat production without shivering (nonshivering thermogenesis). This increase in metabolism is mediated through an increase in secretion of thyroid hormones and the calorigenic effects of catecholamines on lipids. Table 53-1 shows that fat metabolism is an effective way to produce heat. Brown fat is a specialized vascular, mitochondria-rich fat that is used to generate heat. It is widely distributed within the body of all mammals but is especially prevalent in small mammals where, combined with white fat it forms both subcutaneous and trunk depots. Cold stress–induced neural release of norepinephrine from the primarily adrenergic nerve supply activates β₃-adrenergic receptors on adipocytes which increases fat metabolism to produce heat that is distributed around the body through the bloodstream. Furthermore, under cold stress white adipocytes can be converted to the brown adipocytes necessary for heat generation.

Countercurrent Heat Exchange Mechanisms Are Used Both to Conserve and to Lose Heat

When the environmental temperature is high, the blood perfusing the skin vascular beds returns to the body core through superficial veins from which heat is lost to the skin and air. Under cold conditions, limb blood flow returns to the core through deep veins that accompany arteries (Figure 53-4). Heat is transferred by countercurrent exchange from the warm arterial blood to the cooler venous blood and thereby returned to the core of the body.

A similar countercurrent exchange of heat occurs in a carotid rete in sheep and some other ungulates. In this system the carotid artery forms a rete bathed in a sinus of venous blood that has drained the nasal cavity. The colder venous blood from the nose cools the arterial blood supplying the brain and protects the temperature of the brain. This mechanism becomes important during exercise, when the increase in ventilation aids in cooling the blood that drains from the nose. As a result, the arterial blood carrying heat from the exercising muscles is cooled before it enters the brain.

Some mammals, including humans and horses, do not possess a carotid rete and must rely on other thermoregulatory mechanisms to cool their brains during exercise. In the horse the guttural pouches may serve as such a mechanism. The guttural pouches contain air that is cooler than the arterial blood carried in the internal carotid artery. Because anatomically these guttural pouches surround the internal carotid arteries, heat is transferred from the blood to the air in the guttural pouches, thus protecting the brain from hyperthermia (Figure 53-5). In addition, the intracranial cavernous venous sinuses may assist in cooling the horse’s brain during exercise. This mechanism is thought to function in the same manner as the carotid rete, but less efficiently.