Transcript 17.Energy balance-Temp 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
• 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