Lecture 14: Temperature Regulation Flashcards
1
Q
List factors that determine heat production.
A
> BMR > Muscle activity (i.e., shivering) > Thyroxin > Norepinephrine and epinephrine > Increased cellular chemical activity > Extra metabolism for digestion, absorption, and food storage
2
Q
List 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 from skin to surroundings
NOTE: a small amount of heat is transferred by respiratory system.
3
Q
What are the three mechanisms of heat loss from the skin surface?
A
> radiation
conduction
convection
evaporation
4
Q
What is radiation in respect of heat loss from the skin surface?
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
5
Q
What is conduction in respect of heat loss from the skin surface?
A
- kinetic energy of the molecules of the skin is transferred to the air if the air is colder than the skin
6
Q
What is convection in respect of heat loss from the skin surface?
A
- removal of heat from the body by convection air currents
7
Q
What are the characteristics of malignant hyperthermia?
A
> Heat production is far greater than heat dissipation.
Probably de to genetic abnormalities in the tyanodine receptors in skeletal muscle which leads to excess release of sarcoplasmic calcium ion, leading to prolonged excitation-contraction coupling.
It is triggered by anesthetics.
8
Q
Is the rate of heat loss in water many times greater than the rate of heat loss in air?
A
Yes - water has a specific heat several thousand times as great as that of air.
9
Q
How many calories of heat is lost for each gram of water that evaporates from the body surface?
A
0.58 calories of heat is lost
10
Q
What rate does insensible perspiration occur?
A
600 - 700 ml/day
11
Q
What does insensible perspiration cause?
A
A continual heat loss at a rate of 16 - 19 calories/day.
12
Q
What does low velocity wind have cooling effect proportional to?
A
The square root of the wind velocity.
13
Q
What percentage of heat loss is due to evaporation?
A
22%
14
Q
What percentage of heat loss is due to radiation heat waves?
A
60%
15
Q
What percentage of heat loss is due to conduction of air?
A
15%
16
Q
What percentage of heat loss is due to conduction to objects?
A
3%
17
Q
In heat transfer, what is 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.
18
Q
In heat transfer, what is conduction?
A
- Transfer of energy from one body to another when they are in close contact.
19
Q
In heat transfer, what is convection?
A
- Heat is transferred between two objects by air or water (fluid).
20
Q
In heat transfer, what is evaporation?
A
- Heat is dissipated by the use of thermal energy to cause a change from fluid to gas.
21
Q
List 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.
22
Q
How much sweat per hour does an unacclimatized person normally produce?
A
1 liter seat/hour (or less)
23
Q
What is the composition of precursor secretion?
A
Na 142 mEq/L ; Cl 104 mEq/L
24
Q
What happens in strong stimulation of seat glands?
A
- Large amounts of precursor secretion are formed.
- Ducts reabsorb only about half the sodium chloride.
- Concentrations of Na and Cl ions are about 50-60 mEq/L.
- Little water is reabsorbed.
25
People exposed to hot weather for 1-6 weeks may produce how much sweat?
2-3 liters of sweat/hour, increasing heat removal by factor of 10.
** this is due to changes in internal sweat gland cells **
26
What are the principle areas of the brain that affect body temperature?
- anterior hypothalamic pre-optic area
| - pre-optic area
27
what does the anterior pre-optic area contain?
Contains both heat-sensitive and cold-sensitive neurons.
28
What do heat sensitive neurons in the anterior pre-optic area do?
Increase firing rate 2-10x in response to a 10 C increase in body temperature.
29
What do cold-sensitive neurons in the anterior pre-optic area do?
Increase firing rate when temperature falls.
30
What does heating of the pre-optic area immediately cause?
- Dilation of skin blood vessels over the entire body.
- Profuse sweating over the entire body.
- Inhibition of excess heat production.
31
What causes dilation of skin blood vessels over the entire body, profuse sweating over the entire body, and inhibition of excess heat production?
Immediately heating the pre-optic area.
32
What are the mechanisms to reduce body heat?
- Vasodilation of skin blood vessles.
- Sweating
- Decrease in heat production
33
In the vasodilation of skin blood vessels mechanism to reduce body heat, what is it caused by?
Inhibition of sympathetic centers in posterior hypothalamus that cause vasoconstriction.
34
What causes a decrease in heat production in the mechanism to reduce body heat?
Inhibition of shivering and thermogenesis.
35
What are the mechanisms to increase body heat?
- Skin vasoconstriction
- Piloerection
- Increase i thermogenesis
> shivering
> metabolic pathways
> thyroxin secretion
36
Where is the primary motor are for shivering located?
dorsomedial portion of posterior hypothalamus
37
How does the dorsomedial portion of the posterior hypothalamus relate to the anterior hypothalamic preoptic area?
normally inhibited by signals from heat center in anterior hypothalamic preoptic area
38
Under what conditions is the dorsomedial portion of the posterior hypothalamus activated?
excited by cold signals from skin and spinal cord
39
How does the dorsomedial portion of the posterior hypothalamus initiate shivering?
- shivering is the most potent mechanism for increasing heat production
- shivering is orchestrated by 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
- nonrhythmical signals increase muscle tone of skeletal muscles throughout body
- shivering begins when tone rises above a certain critical level
- may involve feedback oscillation of muscle spindle stretch reflex mechanisms
40
What is the definition of chemical thermogenesis?
- Increase in rate of cellular metabolism.
| - Due to sympathetic stimulation (or norepinephrine in blood).
41
How is chemical thermogenesis related to epinephrine/norepinephrine?
uncouples oxidative phosphrylation
42
How is chemical thermogenesis related to brown fat?
Degree of thermogenesis is directly related to amount of brown fat.
43
Describe brown fat distribution in humans?
interscapular space in infants
44
What effect does increased thyroxine output have on cellular metabolism?
- 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.
45
What is the critical body core temperature?
37.1 C (98.8 F)
46
How does this core temperature relate to heat loss and heat production?
Heat loss is greater at temperatures above this temperature and heat production is greater at temperatures below this temperature.
47
What is the "set-point" of the temperature control mechanism?
Level at which sweating begins or shivering begins in order to return to critical core body temperature.
48
What happens if the core temperature is
posterior hypothalamus activates heat-generating mechanisms
49
What happens if the core temperature is > set point?
anterior hypothalamus activates heat loss mechanisms
50
True or False:
| Temperature sensors in the skin and hpothalamus read core temperature and relay to the anterior hypothalamus.
True
51
What does the anterior hypothalamus do one it receives information from the temperature sensors in the skin and the hypothalamus reads the core temperature?
compares recorded temperature with set-point temperature
52
What are the physiological mechanisms that alter the critical set point?
primarily skin temperature changes
**refer to figures 74-8 and 74-9**
53
What is the feedback gain and how is it calculated?
- Feedback gain is a measure of the effectiveness of a control system.
- It is important for the internal core temperature to change as little as possible, even though the environmental temperature might change greatly from day to day or even hour to hour.
= ratio of the change in environmental temperature to the change in body core temperature minus 1.0
(28/1) - 1 =27
54
How does the feedback gain of the temperature control system **to that of other biological control systems?
Extremely high gain (27) for a biological control system.
**baroreceptor arterial pressure control system, by comparison, has a feedback gain of
55
True or False:
| The set-point increases as skin temperature decreases.
True
56
Does the amount of sweating decrease or increase as skin temperature falls?
Decrease
57
What would happen to the hypothalamic set-point if the skin temperature (ST) decreased from 33 C to 29 C?
SP = 36.7 increases to 37.4 C
*SP = hypothalamic set-point*
58
True or False:
| Set point increases as skin temperature increases.
False - set point DECREASES as skin temperature increases.
59
What happens to the hypothalamic set point if the skin temperature rises from 20 C to 31 C?
SP = 37.1 -> 36.5 C
60
How is thyroxine output related to body temperature and the anterior hypothalamic-preoptic area?
- Cooling the anterior hypothalamic-preoptic area also increases production of thyrotropin-releasing hormone by the hypothalamus.
- Thyrotropin-releasing hormone stimulates secretion of thyroid-stimulating hormone (TSH).
- TSH stimulates increases output of thyroxine by thyroid gland.
- This increased thyroxine activates uncoupling protein and increases the rate of cellular metabolism throughout the body, which is yet another mechanism of chemical thermogenesis.
61
What is the definition of fever?
Body temperature above he usual range of normal.
62
What do pyrogens do?
Increase set point temperature by increasing the production of IL-1 in phagocytic cells.
63
What does IL-1 cause?
Causes anterior pituitary to produce prostaglandins.
64
How do IL-1 and prostaglandins effect fever?
Increase the set point temperature.
65
How does aspirin effect fever?
Decreases the set-point temperature by inhibiting cyclooxygenase which results in decrease in production of prostaglandins.
**aspirin is called a antipyretic**
66
Under what conditions is heat stroke likely to occur?
- Occurs when body temperature increases to point of tissue damage.
- Normal response (sweating) is impaired and core temperature continues to increase.
67
What are the symptoms of heat stroke?
- dizziness
- abdominal distress sometimes accompanies by vomiting
- sometimes delirium
- eventually loss of consciousness if the body temperature is not soon decreased
68
How are the symptoms of heat stroke related to circulatory shock?
The heat stroke symptoms are exacerbated by a degree of circulatory shock brought on by excessive loss of fluid and electrolytes in the sweat.
*hyperpyrexia is also exceedingly damagin to the body tissues, especially the brain, and is responsible for many of the effects.*
69
At what temperature point is the ability of the hypothalamus to regulate temperature lost?
Body temperature falls below 85 F, ability of the hypothalamus to regulate temperature is lost.
*greatly impaired even when the body temperature falls below about 94 F*
70
What factors contribute to the loss of ability for the hypothalamus to regulate body temperature?
- Reason for this diminished temperature regulation is that the rate of chemical heat production in each cell is depressed almost twofold for each 10 F decrease in body temperature.
- Also, sleepiness develops, which depresses the activity of the CNS heat control mechanisms and prevents shivering.
71
What causes heat exhaustion?
Caused by excessive sweating.
72
What do patients with heat exhaustion typically faint?
Blood volume and arterial blood pressure decreases due to the excessive sweating, resulting in fainting.
73
What causes malignant hyperthermia in susceptible individuals?
Caused in susceptible individuals by inhalation anesthetics.
74
What is malignant hyperthermia characterized by?
Massive increase in oxygen consumption and heat production by skeletal muscle -> rapid rise in body temperature.
75
What happens in hypothermia?
Ambient temperature is so low that heat-generating mechanisms cannot maintain core temperature near set-point.
76
What happens to an individual if the set-point is suddenly raised from 98.8 F to 103 F?
```
Chills to raise the body temperature to the set-point:
> vasoconstriction
> piloerection
> epinephrine secretion
> shivering
```
77
What happens to an individual if the set-point is suddenly reduced to 98.8 F from 103 F?
Vasodilation and sweating to lower the body temperature to the set-point.
78
How much of an increase in conductance is there between fully vasoconstricted state and fully vasodilated state?
About an 8x increase in conductance between fully vasoconstricted state to fully vasodilaed state.
79
How is the heat conduction to skin controlled?
Degree of vasoconstriction of arterioles and the arteriovenous anastomoses that supply blood to the venous plexus of the skin.
80
How is vasoconstriction controlled, which controls the heat conduction to the skin?
Vasoconstriction is controlled almost entirely by sympathetic system in responses to core temperature and environmental temperature.
81
What acts as heat insulators?
skin and subQ
82
Where does the continuous venous plexus in the subQ get its blood supply from?
Supplied by inflow of blood from capillaries from dermis.
83
How great can the rate of blood flow into the venous plexus in subQ be?
30% of total cardiac output.
84
What are the concentration of constituents of sweat when flow of precursor through the duct is low (slight simulation of glands)?
- essentially all of the Na and Cl ions are reabsorbed, and the concentration of each falls to as low as 5 mEq/L
- this process reduces the osmotic pressure of the seat fluid to such a low level that most of the water is alos reabsorbed, which concentrates most of the other constituents
- Therefore, low rates of sweating, urea, lactic acid, and K ions are usually very concentrated.
85
What are the concentration of constituents of sweat when flow of precursor through the duct is rapid (strong stimulation)?
- large amounts of precursor secretion are formed, and the duct may reabsorb only slightly more than half the Na and Cl; the concentrations of Na and Cl ions are then (in an unacclimatized person) a maximum of about 50-60 mEq/L slightly less than half the concentrations in plasma.
- Sweat flows through the glandular tubules so rapidly that little of the water is reabsorbed.
- Urea is twice that in the plasma
- Lactic acid about 4 times
- K about 1.2 times
**a significant loss of NaCl occurs in the sweat when a person is unacclimatized to heat. Much less electrolyte loss occurs, despite increases sweating capacity, once a person become acclimatized. **
86
What effect does aldosterone have on sweat composition?
Also associated with acclimatization is a further decrease in the concentration of NaCl in the sweat, which allows progressively better conservation of body salt.
most of this effect is caused by increased secretion of aldosterone by the adrenocortical glands, which results from a slight decrease in NaCl concentration in the extracellular fluid and plasma.
