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Physio - E2 - Lecture Slides > L-5 Temp Regulation > Flashcards

L-5 Temp Regulation Flashcards

1
Q

what is heat and what determines heat production?

A

heat is a metabolic byproduct resulting from the inefficiency of the various metabolic pathways.

  • BMR
  • muscle activity (shivering)
  • thyroxin
  • norepinephrine and epinephrine
  • incr cellular chemical activity
  • extra metabolism for digestion
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2
Q

describe malignant hyperthermia

A

its a syndrome

heat production is far greater than heat dissipation.

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3
Q

whats it due to/

A

probably due to genetic abnormalities in the ryanodine receptors in skeletal muscle which leads to excess release of sarcoplasmic calcium ion, leading to prolonged excitation-contraction coupling.

its triggered by anesthetics

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4
Q

what are factors that determine rate of heat loss

A
  • how rapidly heat can be conducted from body core to skin
  • how rapidly heat can be transferred frmo skin to surroundings:
  • skin and subQ especially act as heat insulators
  • continuous venous plexus in subQ is supplied by inflow of blood from capillaries from dermis
  • rate of blood flow into the plexus can be as great as 30% of total cardiac output

*note that a small amount of heat is transferred by respiratory system

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5
Q

elaborate on how rapidly heat can be transferred from skin to surroundings:

A
  • about an 8x incr in conductance between fully vasoconstricted state to fully vasodilated state
  • heat conduction to skin is controlled by: degree of vasoconstriction of arterioles and the arteriovenous anastomoses that supply blood to the venous plexus of the skin
  • vasoconstriction is controlled almost entirely by sympathetic system in responses to core temperature and environmental temp
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6
Q

what are the 3 mechanisms of heat loss from skin surface

A

radiation

conduction

convection

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7
Q

define radiation

A
  • loss in the form of infrared heat rays
  • radiated by all objects not at absolute zero
  • if temperature of body is greater than ambient temperature, more heat is radiated from the body than to the body.
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8
Q

define conduction

A
  • kinetic energy of the molecules of the skin is transferred to the air if the air is colder than the skin
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9
Q

define convection

A
  • removal of heat from the body by convection air currents
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10
Q

what is the heat storage equation

A

S = (M - Wk) +/- (C+K+R) - E

S= heat storage
M= metabolism
Wk= external work
C= convection
K= conduction
R= radiation
E= evaporative heat transfer
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11
Q

elaborate on mechanisms of heat loss from skin surface

A
  • low velocity wind has a cooling effect proportional to the square of the wind velocity
  • water has a specific heat several thousand times as great as that of air: note that the rate of heat loss in water is usually many times greater than the rate of heat loss in air (scuba diving)
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12
Q

how many Calories of heat is lost per gram of water that evaporates from the body?

A

0.58 Calories

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13
Q

define insensible perspiration:

A

type of heat loss youre unaware of

  • occurs at a rate of 600 to 700 ml/day
  • causes a continual heat loss at a rate of 16 to 19 Calories/day
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14
Q

heat loss by percentages:

A
  • radiation: 60%
  • conduction to objects: 3%
  • conduction to air: 15%
  • evaporation: 22%

*note: heated air must be moved away from skin for continued heat loss to occur thru conduction to air

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15
Q

heat TRANSFER: radiation

A
  • refers to thermal energy transferred to objects in the external environment
  • amount transferred depends on temperature difference and ability of object to absorb energy
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16
Q

heat transfer: conduction

A
  • transfer of energy from one body to another when they are in close contact
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17
Q

convection

A
  • heat is transferred between two objects by air or water (fluid)
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18
Q

evaporation

A

heat is dissipated by the use of thermal energy to cause a change from fluid to gas

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19
Q

describe the stimulating factors for sweating

A
  • stimulation of anterior hypothalamus-pre-optic area in the brain electrically or by excess heat
  • cholinergic sympathetic nerve fibers (muscarinic)
  • circulating epinephrine and norepinephrine
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20
Q

what is the precursor to secretion?

A
  • composition similar to that for plasma without proteins
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21
Q

what effect does aldosterone have on sweat composition?

A

decrease in sweat rate and incr in sweat sodium concentration

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22
Q

describe the strong stimulation of sweat glands

A
  • large amounts of precursor secretion are formed
  • ducts reabsorb only about half the sodium chloride
  • concentrations of sodium and chloride ions are about 50 to 60 mEq/L
  • Little water is reabsorbed
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23
Q

how much sweat per hour does an unacclimatized person normally produce?

A

1 liter or less

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24
Q

what happens when a person is exposed to hot weather for 1-6 weeks?

A

they may produce 2-3 liters/hour, increasing the heat removal by a factor of 10; due to changes in internal sweat gland cells

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25
Q

why is regulation of core temp essential?

A

because most of the metabolic processes necessary for life are strongly tempt-dependent

range: 55F-130F

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26
Q

what are the principal areas of the brain that affect body temperature?

A
  • anterior hypothalamic nuclei

- medial pre-optic area

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27
Q

describe the anterior pre-optic area

A

contains warm-sensitive, cold-sensitive and temp insensitive neurons

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28
Q

describe warm sensitive neurons

A
  • increase firing rate 2-10x in response to a 10C incr in body temp
29
Q

describe cold sensitive neurons

A
  • increase firing rate when temp falls (below 37 degree)

- comprise about 5% of neuronal pool in APO area byt are more prevalent in the posterior hypothalamic nucleus

30
Q

what is the pre-optic anterior hypothalamus also referred to?

A

as the heat dissipation center

31
Q

what about the posterior hypothalamus?

A

heat generation/conservation center

32
Q

why is this?

A

because of the higher concentration of warm-sensitive neurons in the POA-hypothalamus that promote heat loss;

and

the higher concentration of cold-sensitive neurons in the posterior hypothalamus that promote heat generation

33
Q

T/F: warm sensitive neurons have membrane receptors that are sensitive to changes in brain and blood temps above 37 degrees?

A

true

34
Q

T/F: cold-sensitive neurons have intrinsic temp-sens receptors

A

False

35
Q

fun fact between hypothalamus and body temp

A

disinhibition due to decrease in discharge of warm-sensitive neurons results in increase in discharge of cold-sensitive neurons which are driven by tonic inputs from temperature insensitive neurons

36
Q

which neurons are the most numerous of the neurons in the POA hypothalamus?

A

temperature-insensitive (60%)

37
Q

what is the temperature set point essentially a function of?

A

the activity of the warm-sensitive neurons of the pre-optic anterior hypothalamus

38
Q

what are some mechanisms to reduce body heat?

A
  • action by pro-optic anterior hypothalamus
  • vasodilation of skin blood vessels
  • sweating
  • decrease in heat production (due to inhibition of shivering and thermogenesis)
39
Q

describe action by POA hypothalamus

A
  • heating this area immediately causes:
  • dilation of skin blood vessels over the entire body
  • profuse sweating over the entire body
  • inhibition of excess heat production
40
Q

how is vasodilation caused?

A

caused by inhibition of sympathetic centers in posterior hypothalamus that cause vasoconstriction

41
Q

what are the mechanisms that increase body heat?

A
  • skin vasoconstriction
  • piloerection
  • incr in thermogenesis: (shivering; metabolic pathways; thyroxin secretion)
42
Q

what is responsible for shivering?

A
  • dorsomedial portion of the posterior hypothalamus
43
Q

what inhibits or excites this area?

A
  • normally inhibited by signals from heat center in anterior hypothalamic preoptic area
  • excited by cold signals from skin and spinal cord
44
Q

describe shivering

A
  • the most potent mechanism for increasing heat production
  • orchestrated by the posterior hypothalamus
  • when activated, transmits signals into lateral columns of spinal cord to anterior motor neurons: alpha motor neurons and gamma motor neurons are activated
45
Q

what increases muscle tone of skeletal muscles throughout body?

A

nonrhythmical signals

46
Q

when does shivering begin?

A

when tone rises above a certain critical level

- may involve feedback oscillation of muscle spindle stretch reflex mechanisms

47
Q

define chemical thermogenesis

A
  • incr in rate of cellular metabolism

- due to sympathetic stimulation (or norepinephrine in blood)

48
Q

how is chemical thermogenesis related to epinephrine/ norepinephrine?

A
  • uncouples oxidative phosphorylatoin
49
Q

how is chemical thermogenesis related to brown fat?

A
  • degree of thermogenesis is directly related to amount of brown fat
50
Q

describe brown fat distribution in humans

A
  • interscapular space in infants
51
Q

what effect does incr thyroxine output have on cellular metabolism?

A

activates uncoupling protein
- a mitochondrial inner membrane protein that can dissipate the proton gradient before it can be used to provide the energy gradient for oxidative phosphorylation –> energy is used to generate heat

52
Q

how is thyroxine output related to body temperature and the anterior hypothalamic-preoptic area?

A

textboook***

53
Q

what is the critical body core temperature?

A

37.1 C (98.8F)

54
Q

how does this core temp relate to heat loss and heat production?

A

heat loss is greater at temperatures above this temperature and heat production is greater at temps below this temp

55
Q

what is the “set-point” of the temp control mechanism?

A
  • level at which sweating begins or shivering begins in order to return to critical core body temp
56
Q

describe what happens

A
  • temperature sensors in skin and hypothalamus read core temp and relay to the anterior hypothalamus
  • anterior hypothalamus compares recorded temp with set-point temp and:
  • core temp < set point: posterior hypoth activates heat=generating mechanisms; vise verse with anterior hypoth and heat loss mech
57
Q

what is the feedback gain of the temp control system and how does it compare to that of other biological control systems?

A

27

(change in environment temp/ change in body core temp) - 1.0 = (28/1)-1 = 27

58
Q

what are the physiological mechanisms that alter the critical set point?

A
  • primarily skin temperature changes
59
Q

define fever

A

body temperature above the usual range of normal

60
Q

what are pyrogens and how do they relate to the set point of the hypothalamic thermostat?

A
  • pyrogens incr set point temp by incr’ing production of IL-1, TNF, IL-6, and INF in phagocytic cells
  • IL-1 causes anterior pituitary to produce prostaglandins
61
Q

how does IL-1 and prostaglandins effect fever?

A
  • increases set point temp
62
Q

how does aspirin effect fever?

A

decreases set-point temp by inhibiting cyclooxygenase, which results in decrease in production of prostaglandins

63
Q

under what conditions is heat stroke likely to occur?

A

occurs when body temp increases to point of tissue damage

normal response (sweating) is impaired and core temp continues to incr

64
Q

what are the symptoms of heat stroke? how are they related to circulatory shock?

A

TEXTBOOK

65
Q

at what temperature point is the ability of the hypothalamus to regulate temp lost?

A

TEXTBOOK

66
Q

what factors contribute to this loss of ability to regulate body temp?

A

TEXTBOOK

67
Q

describe heat exhaustion

A
  • caused by excessive sweating

- blood volume and arterial blood pressure decreases, resulting in fainting

68
Q

describe hyperthermia

A

caused in susceptible individuals by inhaling anesthetics

  • characterized by massive increase in oxygen consumption and heat production by skeletal muscle –> rapid rise in body temperature
69
Q

describe hypothermia

A

ambient temperature is so low that heat-generating mechanisms cannot maintain core temperature near set point.

Physio - E2 - Lecture Slides (8 decks)

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