Temperature 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
A. Conduction- touching
B. Convection- movement of molecules
C. Radiation- rays/waves
D. Evaporation- vaporization from liquid to gas
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
D: 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.
A. Brown fat thermogenesis (non-shivering thermogenesis) is mediated by sympathetic (norepinephrine) stimulation of beta-3 adrenoceptors on BAT.
B. Constriction of cutaneous blood vessels is mediated via sympathetic (norepinephrine) stimulation of alpha-1 adrenoceptors on vascular smooth muscle cells.
C. 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.
E. 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
D: During the rising phase of fever, the hypothalamic temperature set point (Tset) > body temperature (Tb). –> 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.
Plateau phase of fever
Tb = Tset
no shivering, warm skin, increased heart and respiratory rate, drowsiness, weakness, thirst, mucle pain
Falling phase of fever
Tb > Tset
cutaneous blood flow increases, starts sweating, feeling hot, sweating, skin red and clammy
A 15-year old girl falls through the ice while skating on an outdoor pond. 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 (tym panic) 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
A: 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.
B. Moderate hypothermia is characterized by body temperature 28-32°C, bradycardia, decreased respirationrate, eventually leading to unconsciousness while maintaining pupillary reflexes.
C. Severe hypothermia is characterized by body temperature <28°C, coma, lack of pupillary reflexes and asystole.
D. Frostbite, cold injury at the extremities, can occur with prolonged cold exposure.
E. Artificial hypothermia is induced in controlled situations (e.g. prior to surgery/pre-op).
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
C: Pyrogenic pathogens stimulate release interleukin-1 and other cytokines from immune cells which act to increase production of prostaglandins, ultimately raising the hypo thalamic 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.
Exogenous pyrogens (endotoxins, pathogens) induce endogenous pyrogens (eg cytokines IL1, IL6, TNFa) that do not have blood-brain barrier to trigger prostaglandins PGE2 from endothelial cells. PGE2 diffuse to hypothalamus and increase Tset –> onset of fever, increase heart-rate, increase energy metabolism (fatigue, joint pain, headache, possible disorientation and seizures)
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
C: 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.
