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Basic Control Mechanisms of Thermoregulatory Process in Livestock Essay

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Introduction

Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different (Wikipedia, 2012). Thermoregulation could also be referred to as the mechanisms and control systems used by the body to balance thermal inputs and thermal losses so as to maintain its core temperature nearly constant (Monique, 2002). This process is one aspect of homeostasis: a dynamic state of stability between an animal’s internal environment and its external environment (the study of such processes in zoology has been called ecophysiology or physiological ecology) (Wikipedia, 2012).

If the body is unable to maintain a normal temperature and it increases significantly above normal, a condition known as hyperthermia occurs and any prolonged exposure (longer than a few hours) at this temperature without control mechanisms to bring it back to normal is tantamount to death of the animal.

The opposite condition, when body temperature decreases below normal levels, is known as hypothermia. Most body heat is generated in the deep organs, especially the liver, brain, and heart, and in contraction of skeletal muscles (Guyton and Hall, 2006).

Animals and humans have been able to adapt to a great diversity of climates, including hot humid and hot arid. High temperatures pose serious stresses for the animal body, placing it in great danger of injury or even death. For animals, adaptation to varying climatic conditions includes both physiological mechanisms as a byproduct of evolution, and the conscious development of cultural adaptations (Harrison et al., 1988; Weiss and Mann, 1985). There are four avenues of heat loss: convection, conduction, radiation, and evaporation (Wikipedia, 2012).

If skin temperature is greater than that of the surroundings, the body can lose heat by radiation and conduction. But if the temperature of the surroundings is greater than that of the skin, the body actually gains heat by radiation and conduction. In such conditions, the only means by which the body can rid itself of heat is by evaporation. So when the surrounding temperature is higher than the skin temperature, anything that prevents adequate evaporation will cause the internal body temperature to rise (Guyton, 2006). During sports activities, evaporation becomes the main avenue of heat loss (Wilmore et al., 1999).

Humidity affects thermoregulation by limiting sweat evaporation and thus heat loss (Guyton and Arthur, 1976). The skin assists in homeostasis (keeping different aspects of the body constant e.g. temperature). It does this by reacting differently to hot and cold conditions so that the inner body temperature remains more or less constant. Vasodilation and sweating are the primary modes by which humans attempt to lose excess body heat. The brain creates much heat through the countless reactions which occur. Even the process of thought creates heat. The head has a complex system of blood vessels, which keeps the brain from overheating by bringing blood to the thin skin on the head, allowing heat to escape. The effectiveness of these methods is influenced by the character of the climate and the degree to which the individual is acclimatized.

Classification of Animals by Thermal Characteristics

Based on thermal characteristics, animals could be classified into four broad groups: • Endotherms: These are animals that create most of their heat via metabolic processes, and are colloquially referred to as warm-blooded. Most mammals and humans belong to this group. • Ectotherms: These are animals that use external sources of temperature to regulate their body temperatures. They are colloquially referred to as coldblooded despite the fact that body temperatures often stay within the same temperature ranges as warm-blooded animals. Examples of animals that belong to this group are fish, amphibians and reptiles.

• Homeotherms: These are animals or organisms with stable body temperature which is independent of the temperature of the surrounding environment. Most endothermic organisms are homeothermic, like mammals. Although, fish are ectotherms because all of their heat comes from the surrounding water. However, most are homeotherms because their temperature is very stable. • Poikiotherms: These are animals or organisms with variable body temperature. The body temperature varies according to the temperature of the surrounding environment. Animals with facultative endothermy are often poikilothermic, meaning their temperature can vary considerably. Examples of poikilotherms include amphibians, reptiles and fish.

The Hypothalamus as a Thermoregulatory Centre

Thermoregulation in both ectotherms and endotherms is controlled mainly by the preoptic area of the anterior hypothalamus (Romanovsky, 2007). In general, the posterior hypothalamus controls responses to cold, and the anterior hypothalamus

controls responses to heat (Martha, 2010), hence, both serving as thermoregulatory centres. This area receives input from temperature receptors in the skin and mucous membranes (peripheral thermoreceptors) and from internal structures (central thermoreceptors), which include the hypothalamus itself (Dominika, 1998). The temperature sensory signals from the preoptic area and those from the periphery are combined in the posterior hypothalamus to control the heat producing and conserving reactions of the body. The hypothalamic thermostat works in conjunction with other hypothalamic, autonomic and higher nervous thermoregulatory centers to keep the core temperature constant.

Some of these thermoregulatory responses are involuntary, mediated by the autonomic nervous system, some are neurohormonal and others are semi-voluntary or voluntary behavioral responses (Dominika, 1998). The brain receives signals regarding body temperature from the nerves in the skin and the blood. These signals go to the hypothalamus, which coordinates thermoregulation in the body. Both sets of information are needed so that the body can make appropriate adjustments. The thermoregulatory centre sends impulses to several different effectors to adjust body temperature. The signals from the hypothalamus control the sympathetic nervous system, which affects vasoconstriction, metabolism, shivering, sweating, and hormonal controls over temperature to bring the increased or decreased temperature back to normal (Wikipedia,2012) Figure 1: Schematic representation of the thermoregulatory centre role in thermoregulation (IHW, 2006)

Feedback Control Mechanism for Thermoregulation in Hot Condition When the surroundings are hot or when the animal body is vigorously exercising, the following could be described as the summary of the feedback control system responsible for regulating the temperature back to normal: • As the body core temperature starts to rise, the increase in temperature is detected by heat receptors in the body. • • These receptors send signals to the hypothermic thermostat. The thermostat inhibits the adrenergic activity of the sympathetic nervous system and stimulates the vasomotor system to dilate the capillaries underlying the skin. • Arteriolar vasodilation occurs. The smooth muscle walls of the arterioles relax allowing increased blood flow through the artery. This redirects blood into the superficial capillaries in the skin increasing heat loss by convection, conduction and radiation. • If the heat is sufficiently intense, the cholinergic sympathetic fibers, which innervate sweat glands release ACh, stimulating sweat.

• The eccrine sweat glands under the skin secrete sweat (a fluid containing mostly water with some dissolved ions) which travels up the sweat duct, through the sweat pore and onto the surface of the skin. This causes heat loss via evaporative cooling; however, a lot of essential water is lost (Wikipedia, 2012). • The hairs on the skin lay flat, preventing heat from being trapped by the layer of still air between the hairs. This is caused by tiny muscles under the surface of the skin called arrector pili muscles relaxing so that their attached hair follicles are not erect. These flat hairs increase the flow of air next to the skin increasing heat loss by convection. • Behavioral responses to heat, such as lethargy, resting, lying down with limbs spread out or wallowing in pool of water or mud, decreases heat production and increases heat loss • As the animal body gets cooler, the hypothalamic receptors detect this and diminish the heat loss prevention responses. • The body core temperature returns to normal.

It should be noted that most animals can’t sweat efficiently. Cats and dogs have sweat glands only on the pads of their feet. Horses and humans are two of the few animals capable of sweating. Many animals pant rather than sweat because the lungs have a

large surface area and are highly vascularised. Air is inhaled, cooling the surface of the lungs and is then exhaled losing heat and some water vapour.

Figure 2: Schematic representation of thermoregulatory process in the body of an animal (IHW, 2006)

Feedback Control Mechanism for Thermoregulation in Cold Condition When the surroundings are cold or when the animal body is resting, the following could be described as the summary of the feedback control system responsible for regulating the temperature back to normal: • As the body core temperature starts to drop, this is detected by cold receptors in the body. • These receptors send signals to both the hypothalamic thermostat and higher cortical centres in the CNS.

• The activation of the sympathetic centre results in several response which slow down the activity of the sweat glands. • This lowers the production of sweat and it decreases the evaporation of sweat, which reduces heat loss by evaporation. • The muscles under the surface of the skin called arrector pili muscles (attached to an individual hair follicle) contract (piloerection), lifting the hair follicle upright. This makes the hairs stand on end which acts as an insulating layer, trapping heat. • The hypothalamus also signals the vasomotor system to constrict the capillaries underlying the skin. • Arterioles carrying blood to superficial capillaries under the surface of the skin can shrink (constrict), thereby rerouting blood away from the skin and towards

the warmer core of the body. This prevents blood from losing heat to the surroundings and also prevents the core temperature dropping further. This process is called vasoconstriction. It is impossible to prevent all heat loss from the blood, only to reduce it. In extremely cold conditions excessive vasoconstriction leads to numbness and pale skin. Frostbite only occurs when water within the cells begins to freeze, this destroys the cell causing damage (Wikipedia, 2012). This reduces heat loss by conduction, radiation, and convection. • A Shivering Center in the hypothalamus is also activated which activates the brainstem motor centers to initiate involuntary contraction of skeletal muscles causing shivering.

This increases heat production as respiration is an exothermic reaction in muscle cells. Shivering is more effective than exercise at producing heat because the animal remains still. This means that less heat is lost to the environment via convection. There are two types of shivering: low intensity and high intensity. During low intensity shivering animals shiver constantly at a low level for months during cold conditions. During high intensity shivering animals shiver violently for a relatively short time. Both processes consume energy although high intensity shivering uses glucose as a fuel source and low intensity tends to use fats. This is a primary reason why animals store up food in the winter (Wikipedia, 2012). • There is also epinephrine secretion from adrenal medulla that increases thermogenesis.

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