AHS 2013 Digestion and Metabolism
Sarah Harney
6 — Thermoregulation
• Energy Balance
– Basal metabolic rate (BMR)
• How is body temperature regulated?
• The hypothalamus as thermostat
• What happens during fever and heat stress?
(Reading pp 606-608, 616-620)
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Energy Balance Equation
Energy in = Energy utilization + Output
(First law of Thermodynamics – energy conservation)
• Energy homeostasis is maintained by balancing energy
intake (from nutrients) with the sum of what is used + what
is eliminated
• Energy not utilized/stored is released as heat
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AHS 2013 Digestion and Metabolism
Sarah Harney
Energy Utilization
• Energy is required for heat production and
work (ATP-dependent processes)
- Mechanical work – movement e.g. Muscle contraction
- Chemical work – synthesis of proteins etc.
- Transport work – movement of molecules across membranes
• Energy usage:
Heat production
ATP Synthesis
60 %
40 %
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Basal Metabolic Rate
• Metabolic rate = Energy expenditure/unit of time
(kilocalories/hr)
• Basal metabolic rate (BMR)
– Rate of energy expenditure in awake state, at rest,
fasting for 12 hrs (no absorptive processes), at
comfortable ambient temperature
• Estimated by measuring O2 uptake, as nutrient
oxidation correlates with 02 consumption
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AHS 2013 Digestion and Metabolism
Sarah Harney
Energy Balance
Energy Intake
Energy Output
Nutrients
Basal metabolism
Thermogenesis
Physical activity
• Energy Intake = Energy Output
Neutral energy balance
• Energy Intake > Energy Output
Positive energy balance
• Energy Intake < Energy Output
Negative energy balance
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Thermoregulation
• Body temperature is homeostatically maintained at
optimal level for cellular metabolism
• Normal body temperature is 37 °C
• Diurnal variation – lower in morning, higher in
evening
• Varies during menstrual cycle - ~ 0.5 °C higher during
luteal phase (from ovulation on)
• Lower with aging
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AHS 2013 Digestion and Metabolism
Sarah Harney
Core Body Temperature
•Body temperature is only
maintained within a central core
(internal organs, CNS),
surrounded by an outer shell
(skin and subcutaneous fat)
Cold
Warm
• Temperature varies in the outer
shell to regulate a constant core
temperature
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Heat Transfer
Heat transfer follows a thermal
gradient from warm to cool
Heat loss occurs by 4 mechanisms:
• Radiation – thermal energy
transferred in the form of
electromagnetic waves
• Conduction – transfer of thermal
energy by direct contact
• Convection - via movement of air
or liquid, warm air next to skin is
insulating
• Evaporation - water loss, sweating
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AHS 2013 Digestion and Metabolism
Sarah Harney
Thermoregulation in the
Thermoneutral Zone
• The thermoneutral zone is a narrow range of ambient
temperature (25 – 30 ° C) where regulation of blood flow
to the skin is sufficient to maintain body temperature
• Outside the thermoneutral zone other mechanisms
mediate heat production or heat loss
Skin
vasoSkin
Shivering constriction vasodilation
25
30
Sweating
Ambient temperature
( ° C)
Thermoneutral
Zone
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Skin Vasodilation and
Vasoconstriction
• Capillary loops close to surface
of skin
• Blood flow regulated by
arteriole diameter
• Low body temperature
increases sympathetic activity
vasoconstriction
• High body temperature
reduces sympathetic activity
vasodilation
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AHS 2013 Digestion and Metabolism
Sarah Harney
Heat production
Shivering thermogenesis
• Regulation of heat production by skeletal muscle
• Hypothalamic signal to skeletal muscle (via motor neurons)
increases muscle tone
• Shivering is due to oscillating, rapid contractions (~10-20 per
s) of skeletal muscle
• No work is done, effective heat production – X 2-5 times
increase
• (Opposite occurs when temperature is high – muscle tone is
decreased to reduce voluntary activity)
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Heat production
Non-shivering (Chemical) thermogenesis
• Newborn babies lack the ability to shiver
but have a special type of adipose tissue –
brown adipose tissue or brown fat
• Brown fat contains a large number of
mitochondria
• In brown fat, uncoupling of the electron
transport chain from oxidative
phosphorylation results in loss of energy as
heat, instead of being used for ATP
synthesis
• Role of brown fat in heat generation in
adults is not clear
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AHS 2013 Digestion and Metabolism
Sarah Harney
Heat Loss
• Mechanisms for heat loss are vasodilation and sweating
• Sweat is excreted by the eccrine sweat glands (all over
esp. hands, feet, forehead) and the apocrine glands (arm
pits and genital regions)
• Sympathetic activity increases sweat production (fight or
flight response)
• Heat loss is via evaporation (ineffective in high humidity
i.e. if sweat drips instead of evaporating)
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Thermoregulation — detectors
• CNS thermoreceptors in the hypothalamus
detect core temperature(tc)
Hypothalamus
• Major regulator of
homeostasis
• Produces hormones, regulates
pituitary hormones
• Regulates emotional responses
and behaviours related to sexual
arousal
• Regulates eating and drinking
• Controls body temperature
• Regulates circadian rhythms
and consciousness
• Peripheral thermoreceptors detect skin
temperature
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AHS 2013 Digestion and Metabolism
Sarah Harney
Hypothalamic Regulation of
Temperature
• Hypothalamic thermoreceptors
detect a change from the set
point, 37 °C
• Responses are mediated by the
effectors – skeletal muscle,
smooth muscle in skin arterioles
and the sweat glands
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Fever
• Elevated body temperature due to
infection/inflammation
• Release of endogenous pyrogens , triggering
release of prostaglandins that raise the
hypothalamic set point
• If set point is raised to 39 °C, heat production is
activated i.e. shivering
• When set point is restored to normal, heat loss
mechanisms are activated i.e. skin vasodilation
and sweating
• Fever is believed to have a role in optimising
immune cell function
• Paracetamol, aspirin etc. reduce fever by
inhibiting prostaglandin synthesis
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AHS 2013 Digestion and Metabolism
Sarah Harney
Heat Stress – Heat Exhaustion
• Excessive sweating + skin vasodilation redistributes
blood flow, lowering blood volume and blood pressure
• Results in weakness, dizziness and fainting
• Due to overactivity of heat loss mechanisms
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Heat Stress – Heat Stroke
• Heat exhaustion can progress to heat stroke
• Characterised by breakdown in temperature regulatory
mechanisms – leading to delirium, loss of consciousness
• Loss of hypothalamic regulation – no sweating so
temperature continues to increase
• Increased core temperature accelerates all metabolic activity,
thus further increasing heat generation
• Convulsions occur at ~41 °C, temperatures above 43.3 °C are
fatal
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AHS 2013 Digestion and Metabolism
Sarah Harney
Cold Stress – Hypothermia
• Hypothermia is better tolerated than hyperthermia
• Low temperature slows metabolic processes, this affects CNS
– causing disorientation, loss of judgement, fatigue
• Can lead to depression of cardiovascular and respiratory
activity
• Induced mild hypothermia (32 -34 °C) is used during cardiac
surgery or following cardiac arrest and in babies who have
suffered perinatal hypoxia
• Mild hypothermia is neuroprotective, due to slowing of
metabolism and prevention of oxidative stress
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