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Chapter 9

Chapter 9.docx

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University of Toronto Scarborough
Janelle Leboutillier

9 | Temperature Regulation, Thirst, and Hunger Homeostasis - physiological state of equilibrium or balance - set point: value that is defended by regulatory systems, such as core temperature or particular body weight - deviations from bodys ideal values of temperature, fluid levels, or weight rapidly assessed by nervous system once deficit recognized, makes appropriate internal adjustments and motivates behaviour to regain ideal state motivation: process of activating and directing behaviour activates behaviour by generating tension and discomfort in form of drive states (like thirst or hunger) then initiate behaviour to solve specific problem ensure that motivation is specific to getting food, or seeking water to drink, action wont be random - systems maintaining homeostasis isnt full proof regulatory systems help identify problem reaction to this info involves complicated set of psychological and biological processes Regulation of Body Temperature - extreme temperatures limit life through impact on chemical properties of living cells too low = ice crystals form in cells and damage cell membrane too high = proteins necessary for carrying out cell functions become unstable - regardless of where they live, must maintain internal temperature ideal for normal activity of their bodies cells Adaptations to Temperature - endotherm: animal that can use internal methods, such as perspiration or shivering, to maintain body temperature e.g., mammals; endon is greek for within - ectotherm: animal that relies on external methods, such as moving into sun or shade, for maintaining body temperature e.g., amphibians, reptiles, and fish; ektos is greek for outside - maintenance of body temperature influenced by animals surface-to-volume ratio figure 9.3, p.257 surface-to-volume ratios affect temperature regulation small animals have more surface area relative to overall body volume than larger animals = small must use more energy to maintain constant body temperature - features evolved to fit particular environmental niche figure 9.4, p.258 Behavioural Responses to Heat and Cold - ectotherms more dependent on behavioural devices move to right type of environment body position can be adjusted when very warm, stretch out to increase surface area; when cold, curl up to reduce exposed surface area social animals huddle together when cold animals can change weight, colour,a nd composition of fur in response to seasonal changes in temperature humans use dark, heavy clothing to absorb and maintain heat; lighter clothing reflects and dissipate heat - further protection: dens, burrows, nests, and other shelters Endothermic Responses to Heat and Cold - demonstrate variety of automatic internal responses to deviations from temperature set point humans defend temperature set point of 98.6 Fahrenheit (37 degrees Celsius) if drop below that, begin to shiver = muscle twitches produces heat but at cost of high expenditure of energy blood vessels constrict = keep most blood away from surface area (where heat loss is greatest) figure 9.5, p.259 raynauds disease produces extreme reaction to cold if cold persists after shivering, thyroild gland increases release of thyroid hormone higher lvls of this hormone = greater overall metabolic activity = warms body deficits in thyroid activity diagnosed based on lower-than-normal body temperature in human infants and smaller animals sympathetic nervous system responds to cold by stimulating greater metabolic activity in brown fat cells o located primarily in torso (vital organs) o appear brown due to large 3s of mitochondria organelles responsible for energy production very warm temperatures perspiration coos skin through evaporation humans have around 2.5 million sweat glands, lose average of a litre of sweat each day under average conditions dogs who dont perspire much, pant or lick fur cooling by evaporation blood vessels near surface of skin dilate in hot environments = more heat loss (ppl often become red-faced in warm temperatures) Deviations in Human Core Temperature - core temperature = temperature maintained for vital organs w/in head and torso - disturbances in ability to maintain normal core temperature set point can result in hot flashes months or years surrounding menopause likely that changes in estrogen associated w/ menopause lasts seconds to minutes, characterized by sweating, flushing, heart palpitations, and subjective feeling of being very warm - fever: carefully controlled increase in bodys thermal set point that is often helpful in ridding body of disease-causing organisms results from chemical byproducts of bacteria or viruses pyrogens = enter brain and causing brain to increase core temperature set point - hyperthermia (heat stroke): life-threatening condition in which core body temperature increases beyond normal limits in uncontrollable manner occurs when bodys normal compensations cant keep core temperature w/in normal limits results from conditions that limit bodys ability to get rid of excess heat (strenuous physical activity or wearing heavy clothing in hot, humid environments) typically not result of illness, but interacts w/ immune system functioning ppl w/ existing infections = more likely to develop heat stroke during exercise figure 9.6, p.260 claim life of Minnesota Vikings football player korey stringer players surface-to-volume ratios probably made it difficult to dissipate excess heat effectively - hypothermia (low core body temperature): potentially fatal core body temperature below 31 degrees C/87.8 degrees F uncontrollable, intense shivering, slurred speech, pain, and discomfort (below 35 C/95 F) (below 31 C/87.9 F) pupils dilate, behaviour resembles drunkenness, and consciousness is gradually lost deliberately produced is common method to reduce brain damage following cardiac arrest or open heart surgery not sure how it exactly works probably counteracts typical negative reactions to interruption in blood supply to brain Brain Mechanisms for Temperature Regulation - regulation results from structural hierarchy froms pinal cord, and extending through brainstem to hypothalamus sensitivity increases from lower to higher lvls of this hierarchy spinal cord doesnt respond until core temperature is as much as 2-3 degrees away from set point higher lvls = more precise thermostats hypothalamus initiates compensation whenever it deviates as little as 0.01 degrees from ideal set point - figure 9.7, p.261 hypothalamus controls temperature regulation preoptic area (POA): part of the hypothalamus involved in # of regulatory functions along w/ adjacent areas of anterior hypothalamus and septum = coordinate incoming info from thermoreceptors w/ structures that trigger responses to higher core temperatures posterior hypothalamus responsible for initiating responses to cooler core temperatures hypothalamus also sensitive to thermoreceptors w/in itself - POA has three types of neurons 1) warm-sensitive increase firing rate and inhibit cold-sensitive neurons when core temperature increases 2) cold-sensitive increase firing rate and inhibit warm-sensitive neurons when core temperature drops 3) temperature-insensitive retain fairly steady rate of responding under all temperature conditions temperature set points possibly reflect comparison of activity of temperature-sensitive and temperature-insensitive neurons - experiment demonstrated temperature sensors in hypothalamus can override input from skin sensors cool water applied to hypothalamus through surgically inserted micropipette when either skin or hypothalamus temperature is raised rat presses lever to obtain cool puffs of air when hypothalamus is cool, rat still presses lever even if room remains very warm - pyrogens target the hypothalamus blood-brain barrier reltatively weak near POA = pyrogens enter stimulate release of prostaglandin E2 = inhibits firing rate of warm-sensitive neurons = responds by producing and retaining heat
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