Exam 3 - Thermoregulation

Question 1

what is a calorie?

Answer 1

energy capable of raising 1 g of water from 0 to 1 degree C

Question 2

what is the heat production in humans at rest?

Answer 2

about 1 Kcal/hour/kg

Question 3

at what level are increases in temperature damaging and potentially lethal?

Answer 3

increase 4-5C (7-9F)… so >105F

Question 4

what are the two sources of insensible evaporation?

Answer 4

• loss due to saturation of expired air
• diffusive loss from epidermis via the stratum conium

(25% of total normal caloric intake)

Question 5

what is sensible evaporation?

Answer 5

sweating due to cholinergic sympathetic stimulation of sweat glands

Question 6

what happens if air temperature > skin temperature and the air is saturated?

Answer 6

sweat cannot evaporate, so it drips off without removing heat

Question 7

what is conduction?

Answer 7

heat can be lost or gained by contact of the body with other conductive fluids, mostly air or water

Heat_conductive = AcKc(Tskin - Tambient)

Question 8

what does physiological thermoregulatory compensation involve?

Answer 8

• usually involves changing Tskin

- also can reduce Ac (curling up)

Question 9

why is there far greater danger of hypothermia in water than air?

Answer 9

because the K value for water is 25x larger than that for air

Question 10

what is convection?

Answer 10

bulk movement of conductive fluid as a function of temperatures differences within the fluid

Question 11

what is the equation for radiation?

Answer 11

Heat_radiation = ArKr(Tskin - Tradiator)

Question 12

describe the normal percent breakdown for heat loss

Answer 12

85% via conduction/convection/radiation

15% via insensible evaporation

Question 13

describe the directionality of the different mechanisms of heat loss

Answer 13

• radiative/conductive/convective = bi-directional

- evaporative = always results in heat loss

Question 14

what temperatures can you take that are good measures of core temperature?

Answer 14

• pulmonary artery catheter is best
• oral
• rectal
• tympanic membrane

Question 15

what dominates heat exchange in superficial tissues?

Answer 15

conduction b/w capillary blood (Tc) and the normally cooler skin surface (Tskin)

Question 16

what changes occur during hypothermia?

Answer 16

vasoconstriction to reduce blood flow near skin - lowers Tskin relative to Tambient, thus reducing further heat loss

Question 17

what changes occur during hyperthermia?

Answer 17

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)

Question 18

cutaneous and core thermoreceptors

Answer 18

cutaneous - all over skin; hot and cold

core - pre-optic area of hypothalamus + spinal cord; respond to direct heating or cooling of these areas

Question 19

what are the 3 major thermoregulatory states that serve to maintain core temperature?

Answer 19

• 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)

Question 20

what happens when you have a fever?

Answer 20

hypothalamic temp set-point is reset at a higher temp -> elevated body temp

Question 21

what are pyrogens? types?

Answer 21

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

Question 22

what is the most important endogenous pyrogen?

Answer 22

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

Question 23

what can mediate the function of IL-1B?

Answer 23

• activated by PGE2 (prostaglandin E2)

- aspirin inhibits PGE2 synthesis, which in turn inhibits IL-1B

Question 24

what causes hyperthermia (heat stroke)?

Answer 24

an exogenously applied head-load from over-exertion, high Tambient, or both (also some endocrine disorders, neural insults)

Question 25

what is the autonomic response to hyperthermia?

Answer 25

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

Question 26

describe classical heatstroke

Answer 26

• 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

Question 27

describe exertional heatstroke

Answer 27

heavy exercise in heat + humidity compromises the efficiency of both temp and BP regulation
-can also get intense muscular fatigue

Question 28

how do you manage heatstroke?

Answer 28

• physical cooling
• if non-pathological, remove the exogenous heat-load
• anti-pyretics are not useful

Question 29

what is hypothermia defined as?

Answer 29

core cooling to <92F

• failure of hypothalamic-based compensatory mechanisms
• general CNS depression
• unaided recovery is impossible