Thermoregulation
Dr. Harold Helbock
Thermoregulation
Cold-blooded animals
thermoregulate by
changing environmental
exposure.
• Homeotherms use the:
• Environment
• Vascular and metabolic
adaptations
• Shivering and sweating
So what’s your problem?
Heat loss (thermal conductivity) (surface area)
(temperature gradient) or H = kA dt/dx
• Body surface (A)
– Adult is 1.73 M2 or 0.01 M2
per pound.
– Preterm baby is 0.1 M2 or
0.05 M2 per pound.
• Thermal conductivity
– Copper cup vs. coffee cup
• Dt/dx
–
986 vs. -20 / insulation thickness
Warmed chicken
embryos
Thermoregulation
• Human fetus produces 2x heat per kilo
of body weight as the adult. Heat
dissipation problem.
• Heat is lost by placenta.
• Which acts as a heat exchanger.
Maintenance of Body
Temperature
No less than 33oC
5oC environment
No less than 23oC
• Infants have:
Large Body surface ( A)
Body Volume
Thin body shell ( k)
• Therefore, cannot tolerate a heat gradient
(dt/dx).
• To maintain body temperature during a minor
cold stress:
– Infant must produce 5x heat per kilo as an adult
faced with the same stress
Thermo-neutral zone: temperature range of minimal metabolic rate.
(Normal body temperature: 37oC
Lower limit is 26oC for adults, 32oC for full-term infants, and 35oC for
preterm infants.
Preterm infant temperatures drops to temperatures less than
thermo-neutral, despite 2-3x the Oxygen consumption
Involuntary Response to Cold
Stress
• Shivering in adults
• Nonshivering thermogenesis in infants
until 3-6 months of age
Nonshivering Thermogenesis
• Temperature sensors are in
the skin (in newborns
particularly the face), the
spinal cord and the
hypothalamus.
• Temperature information is
processed in the
hypothalamus.
Norepinephrine (NE) is
released in response to
cold stress.
• Result: Vasoconstriction
and increased metabolic
activity.
Nonshivering Thermogenesis
• Vasoconstriction also occurs in infants but the
primary response is increasing heat production
from brown fat metabolism.
Brown fat mechanism is known as
nonshivering thermogenesis
• NE stimulates receptors on brown fat cells;
activates lipase which releases intracellular fatty
acids.
• Conversion of T4 to T3 inside brown fat cells. T3
cellular metabolic rate
Uncoupling protein uncouples mitochondrial oxidative phosphorylation.
H+ gradient
heat rather than ATP
Result: Large increase in heat production and O2 consumption.
In case of hypoxia, temperature
because of
O2
Conduction: heat transfer from
one solid body to another.
Rate of transfer depends on are
of contact and temperature
difference
Convection: heat transfer from
solid to surrounding gas or liquid.
Thin layer of air/liquid in contact
with body warms up acting as an
insulator. Air/water currents
disturb layer and new layer must
be heated. In air, currents can be
produced by thermal movement
or forced air flow.
Evaporation: air passing over body
evaporates water on surface and
removes 0.58 calories of heat for every
gram of H2O removed.
Largest source of heat loss in infants (dry
the baby) and is the mechanism of
cooling by sweating.
Radiation: exchange of infrared (IR)
photons with surroundings.
Heat is gained/lost depending on
temperature gradient. Radiant warmers
used to keep some sick infants warm.
They produce IR energy in a wavelength
that can be absorbed and thereby
transfer heat to child.
Isolettes are made with double walls.
Inside wall stays warm and shields infant
from cold outside wall. Infant does not
radiate to the inside wall.