Thermoregulation in normal and extreme environments Flashcards
Many of the metabolic processes
essential to life produce ?
HEAT.
To maintain a constant body
temperature this HEAT must be
lost to the environment at a rate of?
that equals its production.
heat gain = heat loss
4 ways of Heat Gain
Conduction e.g. touching a hot object Convection e.g. from a fan heater Metabolism e.g. basal, digestion, exercise Radiation e.g. from the sun
4 ways of Heat Loss
Conduction e.g. touching a cold object Convection e.g. blood flow to skin Evaporation e.g. sweat Radiation e.g. skin to environment
rate of heat production is proportional to
metabolic rate
• Heat is
a spontaneous flow of energy from one object to
another caused by a difference in temperature between the
two objects.
Temperature is not heat
• Heat exists only as
energy in transit from one object to
another.
• When heat energy is added to a system, it is stored as
kinetic
energy of the atoms and molecules making up the system.
The Heat-Balance Equation
[metabolism (energy gained)– work (energy lost) ] - (heat loss) = storage of heat (Hs)
Storage heat formula:
Hstorage = ctissue .x massbody .x ΔTempbody
c= specific heat capacity
is the amount of heat energy required to raise the
temperature of 1kg mass by 1 Kelvin
1. Conduction • Heat energy is transferred through a solid, liquid or gas by direct \_\_\_\_\_ • Under normal circumstances, heat gain or loss by conduction is \_\_\_\_\_ • Depends on the thermal \_\_\_\_\_\_\_of the materials in contact
contact
minimal
conductivity
- Convection
• Convection transfers heat by
fluid movement driven by a
__________
Transfer of heat from skin to fluid (air or H2O) warms the fluid,
thereby reducing its ______; it rises and is replaced by cooler
fluid.
• Air has a low specific heat capacity. It warms rapidly, establishing ______-currents.
_______impedes convection
temperature gradient
density
convection
Clothing
Conduction eqn
Hcond. = κcond.A.(T2 – T1)
Heat transfer is dependent on thermal conductivity and the temp.
difference between the two objects
Convection eqn
Hconv. = κconv.A.(T2 – T1) .
κconv. Is the convection coefficient W m-2 K-1
A = surface area
T = temperature of object 1 and 2
Higher wind speeds ______convective heat loss
and the ‘_____’ temperature is lower than the
‘ambient/air’ temperature
increase
apparent
- Evaporation
•Heat is continually transferred to
the environment as water is
vapourized from the ______
passages & the ____surface.
Evaporative heat transfer is dependent on the __________________ between the solution and the environment
(Note: partial pressure is dependent on temperature)
respiratory
skin
water vapor pressure gradient
Evaporation eqn
Hevap. = ĸevap.A.(P2 – P1)
Where:
• ĸevaporation = evaporation coefficient (W m-2 kPa-1)
• A = surface area available for heat exchange
• P = partial pressure of the evaporating fluid (usually H2O)
at a specified temperature (kPa)
.
The body has 2-4 million ____ ______throughout the surface.
• These eccrine glands are controlled by _______nerves &
secrete large quantities of weakly saline solution (hypotonic 0.2 – 0.4% NaCl).
• Cooling effect occurs as sweat evaporates.
sweat glands
cholinergic
4. Radiation: • Radiation is the transfer of \_\_\_\_\_\_\_energy by means of \_\_\_\_\_\_\_\_waves (infrared). • Radiation heat transfer does NOT require a?
thermal
electromagnetic
material medium
4 physiological modes of thermoregulation (effectors):
- metabolism
- vasomotor regulation (blood flow)
- sweating
- shivering
2 main Thermal regions
Core temperature - tightly maintained • Shell or skin temperature - highly variable • The core expands in a hot environment and contracts in a cold environment. • Changes to the shell vary the thickness of the ‘insulation’
• Mean body temperature MBT eqn
MBT = 0.64Tcore + 0.36Tshell
In females, core
temperature fluctuates with
the ?
menstrual cycle:
• hormone levels
• endometrial thickness
• ovulation
Where is Heat Produced?
At Rest:
During Exercise:
primarily brain, heart,
liver and kidneys
primarily skeletal
muscles
Where is Heat Produced?
Overwhelmingly through the skin, via: Radiation Conduction Convection & Evaporation
At normal ambient temperatures 50 – 65 % of the heat of metabolism is lost by ___, with most of the
remainder lost by evaporation.
In a cold environment, _______is the major source of heat loss (blood flow to the skin ↓s to prevent this)
radiation
convection
Insulation of the shell
Qualitative Variation:
Quantitative Variation:
(vary the medium) • Fat (κcond = 1/3) • Feathers • Fur/Hair • Behavioural (clothing)
(vary the thickness) • “Winter fat” • Piloerection (air-trapping) • Variable blood-flow to the skin (vasodilation & vasoconstriction)
Thermoregulatory Control Feedback System Receptors Controller (Integrator) Effectors
R- SKIN, BRAIN C- Hypothalamus E-1 Metabolism 2. Vasomotor 3. Sweating 4. Shivering
Temperature sensors - receptors Unlike many Negative Feedback Systems, there are two distinct types of peripheral (skin) sensors:
Warm Receptors & Cold Receptors. Afferents from these receptors project to the pre-optic hypothalamus
Process of regulation of Heat transfer
Peripheral (skin) thermoreceptors, core thermoreceptors • Input signal compared with ‘set point’ • Effectors: shivering, vasomotor, sweat • Activate/deactivate heat transfer
Metabolism Effector location
-Brown adipose
tissue, Mainly in newborns
Vasomotor Effector location
Blood vessels
Vasoconstriction at skin,
vasodilation at core = heat
retention
Sweat Effector location
Increased sweat leads to
evaporative heat loss in dry
environments
Shivering Effector location
Increases metabolic heat
production
Piloerection Effector location
Traps a layer of air between skin
and hair = insulation
Metabolic heat gain: brown adipose tissue ‘brown fat’ high density of ? situated close to ?
• Newborn humans & hibernating animals (possibly also sig. adult humans)
• High density of mitochondria for high level of metabolic activity
• Situated close to blood vessels so that heat produced by metabolism of
fatty acids can be quickly distributed to the rest of the body
How is heat transferred
within the body?
Conduction
Advection/Convection
Conduction speed?
Advection/Convection speed?
Conduction: SLOW
Advection/Convection: FAST
Heat transfer through vasodilation
To remove heat produced by metabolism,
______is the primary mode of heat loss
convection
vasodiation
Hyperthermia of exercise
At the onset of exercise:
heat gain > heat loss = ↑ Tcore The hypothalamic integrator: Tcore ≠ Tset = “error” Neural output to activate heat loss via skin blood flow and sweating When heat loss = heat gain storage of heat decreases to zero But the elevated Tcore persists as long as exercise is maintained.
Heat stroke – a medical emergency
• Occurs when the thermoregulatory system fails and core
temperature increases to 41°C or above
• Excessive vasodilation at skin causes drop in blood pressure (&
decreased brain perfusion) – confusion, loss of consciousness
• Can be rapid in onset
Heat stroke – a medical emergency
• Treatment
is to sponge with tepid water (evaporative cooling) \+ a fan (convective cooling) • Only place ice packs over skin where large vessels are near surface (neck, under arms, groin
Fever Hyperthermia
• Set point is raised (different from other types of hyperthermia)
Fever Hyperthermia
• Caused by
cytokines from the immune system crossing the
blood-brain barrier which increases Tset
• Brain sends neural output to increase heat gain/retention to
increase Tcore to new higher Tset
Therapeutic hypothermia
• Lowering core temp. can protect the brain from reperfusion
damage post-Stroke or cardiac arrest
• ↓ metabolism, reactive oxygen species, cell death, glutamate
Heat exchange with the environment is used to regulate body temperature via:
– Conduction
– Convection
– Radiation
– Evaporation
The body produces heat from
metabolism
_____varies between people, throughout the day and with hormone level in
females
Tcore
Thermoregulatory control system involves
receptors, the controller/integrator and
the effectors
Fever hyperthermia is initiated by
an increase in Tset
• Exercise hyperthermia involves a maintained increase in ?
Tcore
