Thermoregulation



Thermoregulation



Key Points



1. Temperature is a major factor affecting tissue function.


2. Homeotherms and poikilotherms use different strategies to regulate body temperature.


3. Body temperature depends on the balance between heat input and heat output.


Heat Production


1. Heat is a byproduct of all metabolic processes.


2. Shivering produces heat by muscle contraction.


3. Nonshivering thermogenesis is an increase in basal metabolic rate, caused especially by the oxidation of brown fat, to produce heat.


Heat Transfer in the Body


1. Because tissues are poor conductors, heat is most effectively transferred in the blood.


2. Countercurrent heat exchange mechanisms are used both to conserve and to lose heat.


Heat Exchange with the Environment


1. Heat loss by convection occurs when the body warms air or water.


2. Heat loss by conduction occurs when the body is in contact with a cooler surface.


3. Heat loss by radiation occurs when infrared radiation emitted by the body is absorbed by cooler objects.


4. Heat loss by evaporation occurs when the water in sweat, saliva, and respiratory secretions is converted into water vapor.


Temperature Regulation


1. Mammals and birds regulate the input and output of heat to maintain body temperature within a narrow limit.


2. Temperature-sensitive receptors are located in the central nervous system, the skin, and some internal organs.


3. Information from central and peripheral heat-sensitive neurons is integrated in the hypothalamus to regulate heat-losing or heat-conserving mechanisms.


Integrated Responses


1. The responses to heat stress are peripheral vasodilation and increased evaporative cooling.


2. The responses to cold stress are peripheral vasoconstriction, piloerection, and increased metabolic heat production by shivering and nonshivering thermogenesis.


3. Fever is an elevation of body temperature that results from an increase in the thermoregulatory set point.


Heat Stroke, Hypothermia, and Frostbite


1. Heat stroke occurs when heat production or input exceeds heat output, so body temperature rises to dangerous levels.


2. Hypothermia occurs when heat output exceeds heat production, so body temperature decreases to dangerous levels.


3. Frostbite occurs when ice crystals form in the tissues of the extremities.





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.



Heat Production


Heat Is a Byproduct of All Metabolic Processes


Table 53-1 shows the amount of heat produced by the metabolism of carbohydrates, fats, and proteins. The basal metabolic rate (BMR) is the rate of energy metabolism measured under minimal stress while the animal is fasting. BMR is greater in homeotherms than in poikilotherms because homeotherms need to generate heat to maintain body temperature. The BMR per kilogram of body weight is greater in smaller than in larger mammals (Figure 53-2). This is necessitated partly by the greater surface/volume ratio of smaller animals. The relatively greater surface area per kilogram body weight of small animals provides a larger area for heat loss.






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.



Heat Transfer in the Body


Because Tissues Are Poor Conductors, Heat Is Most Effectively Transferred in the Blood


Because heat is produced primarily in muscles of the limbs and in the liver and is eliminated through the skin and the respiratory tract, it is necessary to transfer heat around the body. Tissues have poor thermal conductivity; therefore, conduction is not an efficient means of heat redistribution.


The blood perfusing a metabolically active organ collects heat and transfers it to cooler parts of the body by circulatory convection. Redistribution of blood flow can deliver heat preferentially to certain body regions, or it can allow regions to cool when the maintenance of the temperature of the brain and major viscera (core temperature) is threatened.


Under conditions of heat stress, circulatory transfer of heat to the skin can be increased dramatically by two mechanisms. First, the arterioles of skin vascular beds dilate, which results in increased capillary blood flow. Second, arteriovenous anastomoses open in the limbs, ears, and muzzle. These two actions greatly increase the total blood flow to the periphery, and the increased heat delivery increases the temperature of the skin, which facilitates heat loss. Conversely, under cold stress, skin vascular beds vasoconstrict, and arteriovenous anastomoses close; thus skin and limb temperatures decrease. This results in reduced heat loss from the skin and in a gradient of temperatures along the limb (Figure 53-3). Under severe cold stress, the skin temperature of the extremities can approach ambient temperature. Interestingly, the lipids in the limb extremities have a lower melting point than those in the core, so fats do not solidify in extreme cold stress.




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.


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Jul 18, 2016 | Posted by in PHARMACOLOGY, TOXICOLOGY & THERAPEUTICS | Comments Off on Thermoregulation
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