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Chapter 17 Energy Balance and Temperature Regulation Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Energy Balance • First law of thermodynamics – Energy cannot be created or destroyed • Energy input-output balance • Energy input – Energy in ingested food • Energy output – External work • Energy expended when skeletal muscles are contracted to move external objects or to move body in relation to the environment – Internal work • All other forms of biological energy expenditure that do not accomplish mechanical work outside the body – Skeletal muscle activity used for purposes other than external work (postural maintenance contractions, shivering) – All the energy-expending activities that go on continuously just to sustain life Chapter 17 Energy Balance and Temperature Regulation Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Energy Conversion • Energy from nutrients that is not used energize work – Transformed into thermal energy or heat • Only about 25% of chemical energy in foods is harnessed to do biological work • Remainder is converted to heat – Much of this heat is used to maintain body temperatures Chapter 17 Energy Balance and Temperature Regulation Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Metabolic Rate Metabolic rate = energy expenditure/unit of time • Basal metabolic rate (BMR) – Minimal waking rate of internal energy expenditure – Measured under following conditions • Person should be at physical rest • Person should be at mental rest – Minimizes skeletal muscle tone – Prevent rise in epinephrine • Measurement should e performed at a comfortable room temperature – Shivering can greatly increase heat production • Person should not have eaten any food within 12 hours before BMR determination – Avoid diet-induced thermogenesis Chapter 17 Energy Balance and Temperature Regulation Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Metabolic Rate • Measurement – Direct calorimetry • Not practical – Calorimeter chamber is expensive and takes up a lot of space – Indirect calorimetry • Measures person’s O2 uptake per unit of time • Factors influencing BMR – Thyroid hormone • Primary determinant of BMR – Epinephrine • Increases BMR Chapter 17 Energy Balance and Temperature Regulation Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Energy Balance • Three possible states of energy balance – Neutral energy balance • Energy input = energy output • Body weight remains constant – Positive energy balance • Energy input is greater than energy output • Energy not used is stored primarily as adipose • Body weight increases – Negative energy balance • Energy input is less than energy output • Body must use stored energy to supply energy needs • Body weight decreases Chapter 17 Energy Balance and Temperature Regulation Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Control of Food Intake and Energy Balance • Food intake – Primarily controlled by hypothalamus • Appetite center – Signals give rise to hunger and promote eating • Satiety center – Signals lead to sensation of fullness and suppress eating • Arcuate nucleus of hypothalamus – Contains two clusters of appetite regulating neurons • Neurons that secrete neuropeptide Y (NPY) – Increases appetite and food intake • Neurons that secrete melanocortins – Suppress appetite and food intake Chapter 17 Energy Balance and Temperature Regulation Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Control of Food Intake and Energy Balance • Adipocytes – Secrete hormone leptin • One of the most important adipokines • Reduces appetite and decreases food consumption – Inhibits NPY-secreting neurons – Stimulates melanocortins-secreting neurons • Insulin – Hormone secreted by pancreas in response to rise in glucose concentration • Ghrelin – Hunger hormone – Appetite stimulator produced by stomach and regulated by feeding status – Stimulates the hypothalamic NPY-secreting neurons Chapter 17 Energy Balance and Temperature Regulation Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Control of Food Intake and Energy Balance • PYY3-36 – Produced by small and large intestines – At lowest level before meal – Rises during meals and signals satiety – Believer to be an important mealtime terminator • Lateral hypothalamus area (LHA) – Secretes orexins • Strong stimulators of food intake • Paraventricular nucleus (PVN) – Releases neuropeptides that decrease food intake Chapter 17 Energy Balance and Temperature Regulation Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Control of Food Intake and Energy Balance • Nucleus tractus solitarius (NTS) – In brain stem – Serves as satiety center – Plays key role in short-term control of meals • Psychological and environmental factors can also influence food intake above and beyond internal signals that control feeding behavior Chapter 17 Energy Balance and Temperature Regulation Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Obesity • Excessive fat content in adipose tissue stores • Possible causes of obesity – Disturbances in leptin signaling pathway – Lack of exercise – Differences in the “fidget factor” – Differences in extracting energy from food – Hereditary tendencies – Development of an excessive number of fat cells as a result of overfeeding – Existence of certain endocrine disorders such as hypothyroidism – An abundance of convenient, highly palatable, energydense, relatively inexpensive foods – Emotional disturbances in which overeating replaces other gratifications – A possible virus link Chapter 17 Energy Balance and Temperature Regulation Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Temperature Regulation • Changes in body temperature in either direction alter cell activity – Increase • Speeds up cellular chemical reactions • Overheating more serious than cooling – Nerve malfunction, irreversible protein denaturation – Internal body temperature 106°F → convulsions – 110°F → upper limit compatible with life – Decrease • Slows down cellular reactions • Pronounced, prolonged fall in body temperature slows metabolism to fatal level Chapter 17 Energy Balance and Temperature Regulation Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Temperature Regulation • Temperature varies from organ to organ • Internal core temperature – Abdominal and thoracic organs, CNS, skeletal muscles – Tissues function best at relatively constant temperature of about 100°F – Subject to precise regulation • Outer shell – Consists of skin and subcutaneous fat – Skin temperature varies between 68°F and 104°F without damage Chapter 17 Energy Balance and Temperature Regulation Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Temperature Regulation • Sites for monitoring body temperature – Oral and axillary temperatures are comparable – Rectal temperature averages about 1° higher – Eardrum scan • Normal variations in core temperature – Because of innate biological rhythm, core temperature normally varies about 1.8°F during the day – Monthly rhythm in core temperature in connection with female menstrual cycle – Core temperature increases during exercise – Core temperature may vary slightly with exposure to extremes of temperature Chapter 17 Energy Balance and Temperature Regulation Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Maintenance of Core Temperature • Heat input must balance heat output to maintain stable core temperature • Heat input – Heat gain from external environment – Internal heat production • Heat output – Heat loss from exposed body surfaces to the external environment • If core temperature ↓ heat production is increased • If core temperature ↑ heat loss is increased and heat production is reduced Chapter 17 Energy Balance and Temperature Regulation Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Heat Exchange • Four mechanisms of heat transfer – Radiation – Conduction – Convection – Evaporation Chapter 17 Energy Balance and Temperature Regulation Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Heat Exchange • Radiation – Emission of heat energy from a surface in form of electromagnetic waves (heat waves) – Human body both emits and absorbs radiant energy – Net heat transfer always from warmer objects to cooler objects – Humans lose almost half of their heat energy through radiation Chapter 17 Energy Balance and Temperature Regulation Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Heat Exchange • Conduction – Transfer of heat between objects of differing temperatures that are in direct contact – Heat moves from warmer to cooler object • Transferred from molecule to molecule – Rate of heat transfer depends on temperature difference between touching objects – Only small percentage of total heat exchange takes place by conduction alone Chapter 17 Energy Balance and Temperature Regulation Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Heat Exchange • Convection – Transfer of heat energy by air (or water) currents – Combines with conduction to dissipate heat from body – Convection currents • Help carry heat away from body Chapter 17 Energy Balance and Temperature Regulation Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Heat Exchange • Evaporation – Heat required for evaporation is absorbed from skin – Occurs from • Passive processes – Surface of skin – Linings of respiratory airways • Active process – Sweating » Under sympathetic nervous control – Sweat » Dilute salt solution actively extruded to surface of skin by sweat glands » Relative humidity – most important factor determining extent of sweat evaporation Chapter 17 Energy Balance and Temperature Regulation Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Hypothalamus • Serves as body’s thermostat • Integrates thermosensory inputs • Thermoreceptors – Central thermoreceptors • Located in hypothalamus, CNS, and abdominal organs • Monitors core temperature – Peripheral thermoreceptors • Monitor skin temperature throughout body and transmit information to hypothalamus • Centers in hypothalamus for temperature regulation – Posterior region • Activated by cold – Anterior region • Activated by warmth Chapter 17 Energy Balance and Temperature Regulation Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Thermogenesis • Low core temperature – Shivering • Primary involuntary means of increasing heat production – Motivated behaviors to generate heat • Jumping up and down, hand clapping • High core temperature – Reduced muscle tone – Curtain voluntary movement Chapter 17 Energy Balance and Temperature Regulation Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Thermogenesis • Nonshivering thermogenesis – Important in newborns because they lack ability to shiver • Brown fat • Heat loss – Adjusted by blood flow through skin • Heat is eliminated when skin vessels dilate • Heat is conserved when skin vessels constrict Chapter 17 Energy Balance and Temperature Regulation Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Coordinated Adjustments in Response to Cold or Heat Exposure Chapter 17 Energy Balance and Temperature Regulation Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Thermogenesis • Thermoneutral zone – Control of core body temperature by skin vasomotor activity when environmental temperature is between the upper 60s and mid 80s – Below this zone, shivering occurs – Above this zone, sweating is dominant factor Chapter 17 Energy Balance and Temperature Regulation Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Fever • Elevation in body temperature as result of infection or inflammation • Hypothalamic thermostat is “reset” at elevated temperature • Hyperthermia – Elevation in body temperature above normal range – Can occur unrelated to infection – Causes • Exercise-induced • Pathological Chapter 17 Energy Balance and Temperature Regulation Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning