Thermoregulation

Dr. Harold Helbock

Thermoregulation

• Homeotherms use the:• Environment• Vascular and metabolic

adaptations • Shivering and sweating

Cold-blooded animals thermoregulate by

changing environmental exposure.

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

5oC environment No less than 23oC

No less than 33oC

Large Body surface ( A) Body Volume

• Infants have:

• 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

Thin body shell ( k)

Thermo-neutral zone: temperature range of minimal metabolic rate.(Normal body temperature: 37oCLower 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.

• 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

Brown fat mechanism is known as nonshivering thermogenesis

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.