Energy Balance and Temperature Regulation 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 Indirect 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 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, energy-dense, 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