Lecture 14: Temperature Regulation

Question 1

List factors that determine heart rate of heat production

Answer 1

• Heat is a metabolic byproduct resulting form the inefficiency of the various metabolic pathways
  
  • BMR
  • Muscle activity (i.e. shivering)
  • Thyroxin
  • Norepinephrine and epinephrine
  • Increased cellular chemical activity
  • Extra metabolism for digestion, absorption, and food storage

Question 2

List factors that determine rate of heat loss

Answer 2

• How rapidly heat can be conducted from body core to skin
• How rapidly heat can be transferred from skin to surroundings
  
  • skin and subQ especially act as heat insulators
  • Continuous venous plexus in subQ is upplied 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
  • about 8x increase 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 temperature
• (note that a small amount of heat is transferred by respiratory system)

Question 3

List mechanisms to increase body temperature

Answer 3

• Skin vasoconstriction
• Piloerection
• Increase in thermogenesis
  
  • shivering
    
    • most potent mechanism for increasing heat production
  • metabolic pathway
  • thyroxin secretion
    
    • activates uncoupling protein

Question 4

List and describe mechanisms through which heat is lost from skin

Answer 4

• Radiation (60%)
  
  • loss in the form of infrared heat rays
  • Radiated by all objects at absolute zero
  • If temperature of body is greater than ambient tmperature, more heat is radiated from the body than to the body
• Conduction (3% to objects and 15% to air)
  
  • Kinetic energy of the molecules of the skin is transferred to the air if the air is colder than the skin
• Convection
  
  • Removal of heat from the body by convection air currents
• Evaporation (22%)
• Equation:
  
  • 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

• Low velocity wind has a cooling effect proportional to the square root of the wind velocity
• Water has a specific heat several thousand times as great as that of air
  
  • not that the rate of heat loss in water is usually many times greater than the rate of heat loss in air
• For each gram of water that evaporates form the body surface, 0.58 Calories of heat lost
• Insensible perspiration
  
  • occurs at a rate of 600 to 700 ml/day
  • Causes a continual heat loss at a rate of 16 to 19 Calories/day

Question 5

Compare concentrations of constituents of sweat when flow of precursor through the duct is low (slight stimulation of glands) versus rapid (strong stimulation)

Answer 5

• The composition of the precursor secretion is similar to that of plasma, except that it does not contain plasma proteins
  
  • concentration of sodium= 142mEq/L
  • Concentration of chloride= 104 mEq/L
• as this precursor solution flows through the duct portion of the fland, it is modified by the reabsorption of most of the sodium and chloride ions
  
  • the degree of reabsorption depends on the rate of sweating
• When sweat glands are stimulated only slightly, the precursor fluid passes through the duct slowly. In this instance, essentially all the sodium and chloride ions are reabsorbed, and the concentration of each falls to as low as 5 mEq/L. This lowers the osmotic pressur e of the sweat fluid that most of the water is also reabsorbed. Therefore at low rates of sweating, such constituents as urea, lactic acid, and potassium ions are usually very concentrated
• When sweat glands are strongly stimulated by the sympathetic nervous system
  
  • large amounts of precursor secretion are formed
  • The duct may reabsorb only slightly more than half the sodium chloride
    
    • The concentrationss of sodium and chloride ions are then a maximum of about 50 to 60 mEq/L, slightly less than half the concentrations in plasma.
    • little water is reabsorbed
    • the other dissolved constituents of sweat are only moderately increased in concentration
      
      • urea: about twice that as in plasma
      • lactic acid: about 4 times that as in plasma
      • Potassium: about 1.2 times that as in plasma

Question 6

What effect does aldosterone have on sweat composition

Answer 6

• increased secretion of aldosterone by the adrenocortical glands decreases the concentration of sodium chloride in the sweat, which allows progressively better conservation of body salt

Question 7

Describe the anterior pre-optic areas of the hypothalamus and their role in controlling body temperature

Answer 7

• The anterior hypothalamic pre-optic area and pre-opic area are principal area of the brain that affect body temperature
• Anterior pre-optic area
  
  • Contains both heat-sensitive and cold-sensitive neurons
  • Heat sensitive neurons:
    
    • Increase firing rate 2-10x in response to a 10 degree increase in body temperature
  • Cold-sensitive neurons
    
    • Increase firing rate when temperature falls
• Pre-optic area
  
  • 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
• Mechanisms to reduce body heat
  
  • vasodilation of skin blood vessels
    
    • caused by inhibition of sympathetic centers in posterior hypothalamus that cause vasoconstriction
  • Sweating
  • Decrease in heat production

Question 8

• Where is the primary motor area for shivering located?
• what is the relationship of the primary motor area of shivering with the anterior hypothalamic preoptic area?
• under what conditions is this area activated ?

Answer 8

• The primary motor arear for shivering is located in
  
  • Dorsomedial portion of posterior hypothalamus
• what is the Dorsomedial portion of posterior hypothalamus’ relationship with the anterior hypothalamic preoptic area?
  
  • Normally inhibited by signals form heat center in anterior hypothalamic preoptic area
• Under what conditions is this area activated?
  
  • excited by cold signals from skin and spinal cord

Question 9

Define chemical thermogenesis and explain how it is related to epinephrine/nonepinephrine and brown fat

Answer 9

• Chemical Thermogenesis
  
  • Increase in rate of cellular metabolism
  • Due to sympathetic stimulation (or norepinephrine in blood)
• How is chemical thermogenesis related to epinephrine/norepinephrine?
  
  • Uncouples oxidative phosphorylation
• How is chemical thermogenesis related to brown fat?
  
  • Degree of thermogenesis is directly related to amount of brown fat
• Describe brown fat distribution in humans
  
  • Interscapular space in infants

Question 10

Malignant hyperthermia

Answer 10

• is a syndrome in which heat production is far greater than the heat dissipation.
• It is 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. It is triggered by anesthetics

Question 11

Low velocity wind has a cooling effect proportional to the

Answer 11

square root of the wind velocity

Question 12

Water has a specific heat several ____ times as great as that of air

Answer 12

thousand (note that the rate of heat loss in water is usually many times greater than the rate of heat loss in air)

Question 13

For each gram of water that evaporates from the body surface, ____ Calories of heat lost

Answer 13

0.58 Calories

Question 14

Insensible perspiration occurs at a rate of ___ to ___ ml/day

Answer 14

• 600 to 700 ml/day
• Causes a continual heat loss at a rate of 16 to 19 Calories/day

Question 15

How is thyroxine ouput related to body temperature and the anterior hypothalmic-preoptic area

Answer 15

• Cooling the anterior hypothalamic-preoptic area increases production of thyrotropin-releasing hormone by the hypothalamus
  
  • this hormone is carried by way of the hypothalamic portal veins to the anterior pituitary gland, where it stimulates secretion of thyroid stimulating hormone
    
    • Thyroid-stimulating hormone in turn stimulates increased ouput of thyroxine by the thyroid gland
      
      • thyroxine activates uncoupling protein and increases the rate of cellular metabolism thourghout the body (chemical thermogenesis) (note that this process requires several weeks)

Question 16

How does core temperature (37.1 C (98.8F)) relate to heat loss and heat production

Answer 16

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

Question 17

What is the “set-point” of the temperature control mechanism?

Answer 17

• Level at which sweating begins or shivering begins in order to return to critical core body temperature
  
  • Temperature sensors in skin and hypothalamus read core temperature and relay to the anterior hypothalamus
  • anterior hypothalamus compares recorded temperature with set-point temperature:
    
    • core temperature < set point
      
      • posterior hypothalamus activates heat-generating mechanisms
    • Core temperature > set point
      
      • Anterior hypothalamus activates heat loss mechanisms

Question 18

What is the feedback gain (and how is it calculated) of the temperature control system and how does it compare to that of other biological control systems?

Answer 18

• feedback gain
  
  • is a measure of the effectiveness of a control system
• calculation
  
  • (change in environmental temperature/change in body core temperature) - 1.0 = (28/1) - 1.0 = 27
  • This is extremely high gain for a biological control system

Question 19

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

Answer 19

• Skin Temperature
  
  • when skin temp is high, sweating begins at a lower hypothalamic temperature than when the skin temp is low.
  • Ex.
    
    • Skin temp 33 —> Set point equals 36.7
    • Skin temp 29 –> set point equals 37.4
  • (note the amount of sweating decreases as skin temperature falls)

Question 20

Define “fever” and explain how pyrogens relate to the hypothalamic thermostat set point

Answer 20

• Fever
  
  • Body temperature above the usual range of normal
• What are pyrogens and how do they relate to the set point of the hypothalamic thermostat?
  
  • Pyrogens increase set point temperature by increasing production IL-1 in phagocytic cells
    
    • IL-1 causes anterior pituitary to produce prostaglandins

Question 21

• Explain how the following affect fever:
  
  • IL-1
  • Prostaglandins
  • Aspirin
  • Arachidonic Acid

Answer 21

• IL-1
  
  • Causes anterior pituitary to produce prostaglandins
• Prostaglandins
  
  • increase set point temperature (fever)
• Aspirin
  
  • Decreases set-point temperature by inhibiting cyclooxygenase which results in decrease in production of prostaglandins
• Arachidonic acid
  
  • converted into prostaglandins

Question 22

Under what conditions is heat stroke likely to occur

Answer 22

• Occurs when body temperature increases to point of tissue damage (range of 105 to 108 degrees F)
• Normal response (sweating) is impaired and core temperature continues to increase

Question 23

• What are the symptoms of heat stroke?
• How are they related to circualtory shock?

Answer 23

• Symptoms of Heat stroke
  
  • dizziness
  • abdominal distress, sometimes accompanied by vomiting
  • sometimes delirium
  • eventually loss of consciousness if the body temperature is not soon decreased
• These symptoms are often exacerbated by a degreee of circulatory shock brought on by excessive loss of fluid and electrolytes in the sweat

Question 24

List and describe characteristics of the febrile condition

Answer 24

• chills
  
  • caused because set point is suddenly raised and the body temperature is now less than the set point (even though it is likely higher than normal)
• Crisis, or “flush”
  
  • pyrogen is removed, the set point of the hypothalamic temeprature controller is reduced to a lower value but the body temp is higher thus you have sweating and the dudden development of hot skin because of vasodilation everywhere.

Question 25

At what temperature point is ability of the hypothalamus to regulate temperature lost

Answer 25

• below 85 degrees F

Question 26

What factors contribute to the hypothalamus loss of ability to regulate temperature

Answer 26

• Rate of chemical heat production in each cell is depressed almost twofold for each 10 degree F the body temperature decreases
• Sleepiness develops (later followed by coma), which depresses the activity of the central nervous system heat control mechanisms and prevents shivering

Question 27