THERMOREGULATION

*Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different. *This process is one aspect of homeostasis ( a dynamic state of stability between an animal’s internal environment and its external environment ).

*If the body is unable to mantain a normal temperature and it increases significantly above normal,a condition known as hyperthermia occurs. *The opposite condition, when body temperature decreases below normal levels, is known as hypothermia.

*Whereas an organism that thermoregulates is one that keeps its core body temperature within certain limits, a thermoconformer is subject to changes in body temperature according to changes in the temperature outside of its body. *It was not until the introduction of thermometers that any exact data on the temperature of animals could be obtained.

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 far, metabolic processes are lowed to such an extent that body function ceases. Conversely, an increase in temperature beyond the normal value can denature proteins and also be fatal.

Homeotherms and Poikilotherms *Fish, reptiles and amphibians are called cold-blooded animals or poikilotherms because their body temperature varies with the temperature of the environment. They use behavioral methods to prevent major changes in their temperature.

* Mammals and birds are homeotherms; they maintain a constant body temperature in the presence of considerable changes in the environmental temperature. *They 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.

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

Heat loss by convection * The amount of heat lost by convection depends on the thermal gradient ( temperature difference ) between the skin of the animal and the fluid overlying the skin; a bigger thermal gradient results in more heat loss. * In natural convection, the warmer air or water rises from the surface of the animal, because it is less dense than the cooler fluid. * In forced convection, cooler fluid is moved over the skin surface by a breeze or current or simply because the limbs and animal are moving.

* The thermal gradient for heat loss can be altered by changes in the skin blood flow and the amount of insulation separating the animal from the environment. * Increasing blood flow to the skin raises skin temperature and, therefore, heat loss, whereas a reduction in skin blood flow reduce heat loss. * Reducing exposed body surface area by curling up in a ball or by huddling with other animals also reduce convective heat loss.

Heat loss by conduction * Occurs when the body is in contact with a cooler surface. Because animals usually do not lie on cool surfaces for long periods, conduction is not usually a major form of heat loss. * There are, however, some situations in which conductive heat loss can lead to hypothermia.

Adult pigs cool themselves by conduction when they wallow in cool mud puddles.

Heat loss by evaporation * Evaporative heat loss occurs continuously by the diffusion of water through the skin and by loss of water vapor from the respiratory tract. This water loss is obligatory but under thermal stress, evaporative cooling can increase greatly, because sweat glands are activated or the animal begins to pant. * Evaporative heat loss becomes increasingly important as the ambient temperature approaches body temperature; it is the only form of heat loss available once ambient temperature exceeds body temperature.The effectiveness of evaporation is reduced as the relative humidity increases and the air becomes more saturated with water vapor.

* Panting is one mode of increasing evaporation from the respiratory tract. Small tidal volumes are moved at rapid frequency over the respiratory dead space. * Panting occurs close to the resonant frequency of the respiratory system, so the work of breathing is minimized and does not add to the heat load. * By ventilating primarily dead space, severe hiperventilation and respiratory alkalosis are avoided. * Mammals vary in the relative importance of different modes of evaporative heat loss. In horses and cattle, sweating is the major form of evaporative heat loss.

Sheep sweat, but panting is also of considerable importance. The dog relies almost totally on panting. Even in small rodents, which neither pant nor sweat, evaporation can be increased by smearing saliva or water on the fur.

All solid objects emit invisible electromagnetic radiation in the infrared range. Warm objects emit on a shorter wavelength and more emissions per unit time than cool objects. When these emissions strike another object, some are absorbed and thus transfer heat. Although all objects emit radiant heat, the net heat transfer is from warm to cool objects. It is important to realize that radiant heat loss can occur even when the animal is surrounded by thermally neutral or warm environment. Heat can be lost from an animal to the uninsulated walls of a building even though the intervening air is warm. Heat loss by emission of infrared radiation and its absorption by cooler objects can be significant

Heat production *Heat is a byproduct of all metabolic processes *The basal metabolic rate is the rate of energy metabolism measured under minimal stress while the animal is fasting. *Basal metabolic rate is greater in homeotherms than in poikilotherms, because the former need to generate heat to maintain body temperature. *The metabolic rate per kilogram of body weight is greater in small than in larger mammals. *The relatively greater surface area of small animals provides a bigger area for heat loss.

Shivering produces heat by muscle contraction *Shivering is one method to increase 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.

* Under warm conditions, the core body temperature extends down into the limbs and close to the skin surface of the animal. * Under cool conditions, vasoconstriction in the peripheral blood vessels results in a gradient of temperatures between the core and the extremities. The core temperature is maintained only in the abdomen, thorax, and brain of the animal. The more peripheral tissues are allowed to cool considerably. Diagrammatic representation of the distribution of temperatures in a pony under warm and cool environmental conditions

CORE : 38ºC Horse before the exercise Horse after the exercise

Fever *Elevation of body temperature, which results from an increase in the thermoregulatory set-point. *A common accompaniment to infectious and some other diseases. *Shivering, peripheral vasoconstriction, piloerection, and huddling behavior are all characteristic of the onset of fever.

¡¡¡¡THANKS!!!!