Exam 3 - Thermoregulation Flashcards
what is a calorie?
energy capable of raising 1 g of water from 0 to 1 degree C
what is the heat production in humans at rest?
about 1 Kcal/hour/kg
at what level are increases in temperature damaging and potentially lethal?
increase 4-5C (7-9F)… so >105F
what are the two sources of insensible evaporation?
- loss due to saturation of expired air
- diffusive loss from epidermis via the stratum conium
(25% of total normal caloric intake)
what is sensible evaporation?
sweating due to cholinergic sympathetic stimulation of sweat glands
what happens if air temperature > skin temperature and the air is saturated?
sweat cannot evaporate, so it drips off without removing heat
what is conduction?
heat can be lost or gained by contact of the body with other conductive fluids, mostly air or water
Heat_conductive = AcKc(Tskin - Tambient)
what does physiological thermoregulatory compensation involve?
- usually involves changing Tskin
- also can reduce Ac (curling up)
why is there far greater danger of hypothermia in water than air?
because the K value for water is 25x larger than that for air
what is convection?
bulk movement of conductive fluid as a function of temperatures differences within the fluid
what is the equation for radiation?
Heat_radiation = ArKr(Tskin - Tradiator)
describe the normal percent breakdown for heat loss
85% via conduction/convection/radiation
15% via insensible evaporation
describe the directionality of the different mechanisms of heat loss
- radiative/conductive/convective = bi-directional
- evaporative = always results in heat loss
what temperatures can you take that are good measures of core temperature?
- pulmonary artery catheter is best
- oral
- rectal
- tympanic membrane
what dominates heat exchange in superficial tissues?
conduction b/w capillary blood (Tc) and the normally cooler skin surface (Tskin)
what changes occur during hypothermia?
vasoconstriction to reduce blood flow near skin - lowers Tskin relative to Tambient, thus reducing further heat loss
what changes occur during hyperthermia?
vasodilation to get rid of excess heat via the skin - increases Tskin relative to Tambient to enhance heat loss
-BUT this decreases blood flow to core (CNS)
cutaneous and core thermoreceptors
cutaneous - all over skin; hot and cold
core - pre-optic area of hypothalamus + spinal cord; respond to direct heating or cooling of these areas
what are the 3 major thermoregulatory states that serve to maintain core temperature?
- vasomotor regulation (changes in skin blood flow)
- metabolic regulation (only when max vasoconstriction is ineffective -> shivering or voluntary exercise)
- combined sudomotor + vasomotor regulation (sweating + active vasodilation)
what happens when you have a fever?
hypothalamic temp set-point is reset at a higher temp -> elevated body temp
what are pyrogens? types?
chemicals that cause a febrile response
- exogenous: heat-stable, high MW polysaccharides from G(-) bacteria
- endogenous: heat-labile proteins from monocytes, macrophages, Kupffer cells in response to bacterial pyrogens
what is the most important endogenous pyrogen?
IL-1B:
- produces fever by direct action on hypothalamus
- activates immune system
- neutrophil release from marrow
- increased Ab production
TNF is also an important pyrogen/cytokine
what can mediate the function of IL-1B?
- activated by PGE2 (prostaglandin E2)
- aspirin inhibits PGE2 synthesis, which in turn inhibits IL-1B
what causes hyperthermia (heat stroke)?
an exogenously applied head-load from over-exertion, high Tambient, or both (also some endocrine disorders, neural insults)
what is the autonomic response to hyperthermia?
increase in subcutaneous blood flow to increase heat loss
- blood flow to skin can increase to 60% of CO
- apical regions: inhibition of tonic vasoconstriction
- non-apical: more due to active vasodilation
describe classical heatstroke
- most commonly due to excessive heat + humidity -> increased body temp + ineffective sweating
- can get dehydration, which will accelerate hyperthermia
- also drop BP with the vasodilatory drop in subQ resistance
- MUST treat with electrolyte fluid replacement
describe exertional heatstroke
heavy exercise in heat + humidity compromises the efficiency of both temp and BP regulation
-can also get intense muscular fatigue
how do you manage heatstroke?
- physical cooling
- if non-pathological, remove the exogenous heat-load
- anti-pyretics are not useful
what is hypothermia defined as?
core cooling to <92F
- failure of hypothalamic-based compensatory mechanisms
- general CNS depression
- unaided recovery is impossible
