Temparature Regulation Flashcards
An individual is sitting still in a bath tub filled with water. His body (with exception of his head and
neck) is under the water. The water temperature is lower than his body surface temperature. Assuming
there is absolutely no movement of the water surrounding his body, by which mechanism will the heat
most likely be transferred from his skin to the water?
A. Conduction
B. Convection
C. Radiation
D. Evaporation
Answer: A
Explanation: Conduction is the transfer of heat between objects which are in direct contact. In this
example, the skin is in direct contact with the non-moving water. Since the water is at a lower
temperature than the skin surface, there will be net conductive heat loss / transfer from skin to the
water. Convection is the transfer of heat resulting from the movement of a liquid or a gas – since the
water is not moving in this example, convective heat loss from the skin will be minimal. Radiation is heat
transfer in the form of electromagnetic waves between solid objects not in direct contact. Evaporation
of fluid from the surface if the skin requires a water vapor pressure gradient, with evaporation rate
becoming negligible at ambient humidity > 75%. Since water is effectively 100% humid, evaporative heat
loss will be negligible in this example.
Which human thermoregulatory response is most likely controlled by cholinergic sympathetic fibers?
A. Brown fat thermogenesis
B. Constriction of cutaneous blood vessels
C. Shivering thermogenesis
D. Sweat secretion
E. Thermoregulatory behavior
Answer: D
Explanation: Sympathetic cholinergic fibers innervate sweat glands and stimulation of these glands
(with ACh) leads to an increase in sweat production mediated via muscarinic M3 receptors. Brown fat
thermogenesis(non-shivering thermogenesis) is mediated by sympathetic (norepinephrine) stimulation
of beta-3 adrenoceptors on BAT. Constriction of cutaneous blood vessels is mediated via sympathetic
(norepinephrine) stimulation of alpha-1 adrenoceptors on vascular smooth muscle cells.
Shivering is stimulated by somatic nervous system cholinergic innervation of skeletal muscle. Here ACh
release at the neuromuscular junction sets in motion a chain of events leading to skeletal muscle
contraction. Control of thermoregulatory behavior (putting on extra clothes and moving around more
when cold, switching on a fan and moving away from direct heat source when hot) involve higher brain
centers.
Which of the following is most likely to occur during the rising phase of fever? A. Fall in core body temperature B. Increased skin blood flow C. Increased sweat secretion D. Stimulation of shivering E. Perception of being warm
Answer: D
Explanation: During the rising phase of fever, the hypothalamic temperature set point (Tset) is greater
than the body temperature (Tb). This the hypothalamus “thinks” that Tb is too low and stimulates
thermoregulatory mechanisms to elevate Tb. This includes cutaneous vasoconstriction, decreased skin
blood flow, decreased heat loss, decreased sweating and increased shivering.
A 15-year old girl falls through the ice while skating on an outdoorpond. She is removed from the
water within 1 minute, but dry clothing is not available. The emergency medical team arrive after 20
minutes. At this point, the girl is shivering uncontrollably and she is very agitated. Her skin is pale and
cold to touch. Her heart rate is 80 bpm, blood pressure is 130/100 mmHg and ear (tympanic)
temperature is 34°C. Given this presentation, what is the most likely diagnosis?
A. Mild hypothermia
B. Moderate hypothermia
C. Severe hypothermia
D. Frostbite
E. Artificial hypothermia
F. Patient presentation is within normal limits
Answer: A
Explanation: This patient has mild hypothermia characterized by body temperature in the 32-35°C
range, shivering, tachycardia, peripheral vasoconstriction, increased blood pressure and agitated state.
Immersion in cold water is the most common cause of accidental hypothermia. The convective heat
transfer coefficient in water is 100 times that of air – this means that the body can lose heat to water at
a rate 100x that to air such that body temperature when immersed in water can fall very quickly. For
this reason, if immersed in cold water it is best to stay as still as possible in order to reduce convective
heat loss. Moderate hypothermia is characterized by body temperature 28-32°C, bradycardia, decreased
respiration rate, eventually leading to unconsciousness while maintaining pupillary reflexes. Severe
hypothermia is characterized by body temperature <28°C, coma, lack of pupillary reflexes and asystole.
Frostbite, cold injury at the extremities, can occur with prolonged cold exposure. Artificial hypothermia
is induced in controlled situations (e.g. prior to surgery).
Pyrogenic (fever-inducing) pathogens cause which of the following effects?
A. Decreased production of interleukin-1 (IL-1)
B. Decreased hypothalamic temperature set point (Tset)
C. Shivering
D. Vasodilation of blood vessels in the skin
Answer: C
Explanation: Pyrogenic pathogens stimulate release interleukin-1 and other cytokines from immune
cells which act to increase production of prostaglandins, ultimately raising the hypothalamic
temperature set-point. The hypothalamus now “thinks” that the body temperature is too low (because
the core body temperature is lower than the new set-point temperature) and initiates mechanisms for
generating heat –shivering, vasoconstriction, and shunting blood away from the venous plexus near the
skin surface.
At an ambient temperature (Ta) of 32°C (89.6°F), core body temperature (Tb) is most likely
maintained by which of the following mechanisms?
A. A decrease in heat conduction from core to skin
B. A decrease in sweating
C. An increase in cutaneous blood flow
D. An increase in non-shivering thermogenesis
E. An increase in skeletal muscle heat production
Answer: C
Explanation: Remember that the body is always producing heat and therefore must continuously
dissipate heat. It achieves this by increasing blood flow to the skin (increased cutaneous blood flow),
from where heat can be transferred to the environment. All of the other options in this answer will
effectively increase heat storage and increase Tb.
