Thermoregulation Flashcards
Temperature for hyperthermia
> 40 deg C
Temp for hypothermia
< 35
What reactions included in metabolism
chemical reacitons which sustain life
Catabolism
breakdown of molecules into smaller components
to release energy
Example of catabolism
protein –> amino acids
polysacchardies –> glucose
how is energy lost in catabolism?
lost as heat and ATP
what is ATP
a unit of energy storage
Anabolism
building up molecules
requires energy: making ATP from ADP
How is anabolism regulated?
Anabolic steroids, regulated by hormones
BMR
basal metabolic rate: the basic rate of breakdown of molecules to release energy
what processes make up the rate of breakdown of molecules in BMR?
catabolism (brekdown of molecules)
anabolism (build up of molecules)
maintenance of an ionic gradient
manufacturing of secretions and hormones
nerve impulses
minimum muscle activity
what is average BMR?
72kcal/h
How is BMR normalized?
divided by average body surface area = 1.8 m^ 2
Formula describing metabolism and the first law of thermodynamics
heat change in the body = change in internal energy (metabolism) - work done by the body
formula for internal energy
catabolic rate (rate of breakdown of molecules to release ATP)- measure from oxygen consumption
formula for change in heat added to the body
rate of heat production - measure from human calorimetry
formula for rate of change of work done by body
power delivered by body - measure from treadmill
what are the smaller effects changing heat added to the body across individuals?
weight loss/gain, excretion
What metabolic rates are included in the basal metabolism?
basal heat released from liver, brain, skeletal muscle and others
how much warmer are metabolically active organs?
1 deg C
How much of the energy in food is stored as ATP
50% lost as heat in production of ATP
45% stored as ATP
formula for energy released as heat production
temperature change x mass x specific heat capacity
what is the avg specific heat capacity of the body?
3500
Q = heat production at BMR = 80 W
body mass = 75kg
specific heat capacity of the body = 3500
rate of temperature increase?
80 W = 80 J / s
in 1 hour total heat energy released = 80 x 3600s
rate of temperature increase as deg C / hour
80 x 3600 / 75 x 3500 = 1 deg C / hour
What is conduction?
a form of heat loss
direct transfer of heat energy between vibrating molecules in contact
What is fourier’s law?
the rate of heat transfer is proportional to temperature gradient and area through which the heat flows is perpendicular to the gradient
mathematical formula for the rate of heat transfer in conduction
- constant of thermal conductivity x crosssectional surface area x temperature gradient (degrees C / metre )
2 most important methods of conduction?
direct conduction to objects
conduction within the body
Direct conduction to objects
generally a small effect, contributing to 3% of the total heat loss. Most of the body is insulated by clothes so direct conduction doesn’t occur very frequently
how to decide which thermal conductivity constant to use for 2 directly conducting objects ?
use the k for the object that is generating heat
Conduction within the body
minimal as the rate of temperature change with respect to distance tends to 0 – the temperature difference between 2 neighbouring areas is very small
How is heat mainly transferred in the body?
by blood
How to calculate the heat loss from conduction within the body?
Model the body as a series of geometric shapes
In practice, use finite element modeling
Convection
bulk movement of heated fluids
Newton’s Law of Cooling
the rate of heat loss of a body is proportional to the difference in temperatures between the body and its surroundings
compare conduction and convection
more complex than conduction
what does the rate of convection depend on?
dimensions, properties of fluids involved (density, viscosity, specific heat capacity, velocity), temperature, conduction of heat to surface
2 types of convection
free
forced
Free convection
Warm body causes temperature and desntiy gradients in stationary fluid
Forced convection
Relative movement between body and fluid
No sharp boundary between free and forced convection
Depends on many effects
Radiation
Any object with temperature T radiates according to the Stefan-Boltzmann Law
Maximum wavelength of radiation emitted by skin
9000nm
Type of radiation from human body
infrared
emissivity of human body at normal wavelengths
0.97
Evaporation methods
sweat secretion, sweat secretion (to moist air, forced to moist air), respiration
factors affecting the latent heat of evaporation
temperature and pressure
How does pressure affect the rate of evaporation?
higher pressure gradient, higher rate of evaporation
Diffusion of water through the skin
water leaks passively through skin
insensible water loss
independent of exercise and sweating
what does the rate of water loss depend on
the difference between the partial pressure of water at the skinand the partial pressure of water in the ambient air
What condition for sweating to occur?
activity > BMR
how does sweat lead to energy loss?
sweat is turned to vapour
evaporation process requires energy
Assumption in calculations for heat lost in sweat secretion
no sweat is lost as liquid, and all the sweat evaporates freely into dry air
Sweat secretion to moist air compared to dry air
less efficient
Partial pressure
pressure that one gas in a mixture of gases would have if it alone occupied the volume
Forced sweat secretion to moist air
Sweating often takes place in moving air
It has the same effect as convection
It is the dominant source of heat loss during exercise
Factors affecting rate of sweat secretion to moist air
partial pressure due to water at surface - partial pressure due to water at ambient pressure
calculate rate of respiration
include calculations in heat loss due to heating air and due to evaporation from process of inhalation and exhalation
Counter current mechanism
to minimise a gradient
Condition: ambient temperature is cold, hot blood goes to skin and is cooled leading to significant hat loss
as the blood leaves the core (warm) it mixes with blood that is coming back from the skin (cold)
the process is where heat exchange happens across neighbouring vessels continually, in which the blood flow directions are opposite (going away from core vs. going to core)
How is core temperature measured ?
peripheral and core thermoreceptors. Integrated and regulated by preoptic region of hypothalamus
2 types of mechanisms for core temperature control
linear control
on-off control
how does the hypothalamus work to control temperature
sets a desired reference temperature and can feed-forward to counter expected changes in core temperature
Type of feedback that metabolic rate experiences
positive
Types of temperature receptors
warm and cold receptors
firing rates of warm vs cold receptors
firing rate increases with temperature in warm receptors
firing rate decreases with temperature increase in cold receptors
Thermoregulation in the neonate
Full-term newborn can thermoregulate if kept dressed in a warm room, pre-term cannot
why do newborns have lower ability to thermoregulation?
immature skin –> increased water loss
big surface area : volume ratio
immature vasculature –> limited vasocontriction to draw blood away from the surface of the skin
little fat –> poor heat insulation
unable to shiver due to less muscle
how are premature babies protected from poor thermoregulation?
heated incubator
wrapped in plastic
wear a hat
radiant heater
heated mattress
neural thermal environment
no energy required to be expended to maintain body temperature, so more energy available to grow
examples of neutral thermal environment
premature babies incubator, womb
brown fat in term babies
metabolically active fat which contains mitochondria
where is brown fat found
term babies, hibernating animals
cold-blooded vs warm blooded animals
variable vs stably maintained body temperature
body temperature varied externally vs by varying rate of metabolism
metabolism is maintained at high vs metabolism varied for reasons other than thermoregulation e.g reproduction
advantages of cold blood
lower energy requirements
disadvantages of cold blood
lower efficiency
cannot engage in high-energy activities
Types of response to hot conditions
fever, hyperthermia, heat exposure, heatstroke
when does fever occur?
when the set-point is increased by the hypothalamus and normal regulatory mechanisms maintain body temperature at this higher level to destroy pathogens
Hyperthermia
increased body temperature due to failure of autoregulation due to high ambient temperature, side-effect of metabolism increasing drugs
heat exposure
leads to hot flushed skin heavy sweating, dizziness, fatiugue, mental confusion
heatstroke
body temperature is above 40
failure in homeostasis
autoregulation fails
metabolism increases with temperature increase
leading to positive feedback
Areas in a burn
zone of coagulation
zone of stasis
zone of hyperaemia
Zone of coagulation
center of the burn
maximum damage due to proteins denatured
irreversible
Zone of stasis
Reduced perfusion
damaged but not destroyed cells
Zone of hyperaemia
minimal cell injury
Adjuvant hyperthermia therapy
physiological hyperthermia
adjuvant hyperthermia
physiological hyperthermia
mild heating for long time periods
leads to increased metabolism
adjuvant hyperthermia
treat cancer
tumour cells have altered surface so lower ability to thermoregulate
increases the radiosensitivity of the tumour cells to chemotherapy
Ablative hyperthermia therapy
High intensity focussed ultrasound
destroys cells in few mm cubed tissue to denature the protein and allow coagulation
Formula for net heat exchange assuming all heat exchange is radiated
net heat exchange = conductivity x emissivity of the source x effective area x temperature change
what is the specific heat capacity of water?
4200
what is the emissivity of human skin
0.97
what is the conductivity of skin
8
