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Thermoregulation summary
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Lecture notes for thermoregulation
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Thermoregulation lecture summaryThe body functions optimally around 37 degrees Celsius. There are external sources (e.g. sun heat)
and internal sources of heat that keep our body warm. Internal sources are the result of chemical
processes, like cellular respiration in the mitochondria and breaking large stored molecules like fat
down to smaller ones.
We lose heat where we are exposed to the outside world and this process is faster in bodies (or
body parts) with larger surface area-to-volume ratio. That explains why extremities th cool down
quickly and why a person with a high BMI will cool down slower.
The body’s homeostatic mechanism that keeps our body temperature constant around the set point
of around 37°C. This process is called thermoregulation. In many ways it works the same as the
room thermostat that can bi-directionally control temperature to maintain a stable room
temperature with the help of a heater and an air conditioner. Thermo-receptors are the sensors
that measure the actual body temperature of the periphery (thermo-sensitive afferents from the
skin) and of the core (thermo-sensors in the hypothalamus). Normally this set point is 37°C degrees.
Thermo-receptors send the information about the actual temperature through sensory afferents to
the hypothalamus of the brain, which is the control centre that compares actual temperature with
the set point temperature. If it deviates, it will use the autonomic nervous system to activate
‘effectors’ that can change the temperature in the opposite direction of the deviation (negative
feedback) until it is back at the set norm. If the temperature is higher than the set point, the
hypothalamus will cause vaso-dilation of the skin that brings warm blood to the surface where it can
radiate out and will cause sweating that extracts heat by evaporation. If the temperature is lower
than the set point (hypothermia), the hypothalamus will cause vaso-constriction in the skin, keeping
the warm blood away from the surface (to prevent loss of heat by radiation), will cause shivering to
produce more heat by muscle contractions and if that is not enough will activate non-shivering
thermogenesis in brown fat. In addition temperature deviations will trigger behavioural adaptations
to either increase heat loss (find shade, stripping, cool drinks, slow down) or warm up (move to
heater, wrapping up, hot drinks, star jumps).
If a person has a fever (pyrexia) because of an infection or tissue trauma, the body produces
pyrogens that increase the temperature set point of the hypothalamus. The thermoregulation
processes will now increase the body temperature by shivering and vasoconstriction, until the new
set point is reached. During this period the actual temperature may be higher than 37, but is lower
than the elevated set point, so the person will feel cold and has the chills. When the fever breaks
(inflammation process that fights infection and allows wound healing stops) the set point returns to
37, but the body is still warmer and therefore the body sweats and vaso-dilates to cool down, while
the person feels hot.
Hyperthermia is the condition that the body overheats, not because the set-point is higher, like in
fever, but because the thermoregulation mechanisms fail, usually due to internal and external heat
sources that outstrip the cooling effectors. If this goes too far the brain including the hypothalamus
can’t work properly and fails to activate the cooling effectors effectively, spiralling out of control.
Immediate external cooling is necessary to prevent organ failure.
In neonates there is a serious risk of hypothermia, because there thermoregulation control
mechanism is not fully developed, they lack the ability to shiver strong enough and they have a high
surface area-to-volume ratio. Luckily they have a large proportion of brown fat that can produce
heat through non-shivering thermogenesis.
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