Topic 67 - Heat balance regulation

Question 1

Words to include

Answer 1

Defense against cold

• Heat conservation
  
  • Diminishing heat-loss
• Muscular activity ↑
  
  • Tonic (isometric) or shivering ↑
• Chemical (non-shivering) thermogenesis
• Brown adipose tissue (compared to white adipose tissue):
  
  • Foamy
  • intensive blood supply
  • Mitochindria
• Heat production in brown adipose:
  
  • Cold
  • Sympathetic activation (epinephrine, norepinephrine)
  • Futile cycle
  • cAMP production
  • Lipase activity ↑
  • FFA level ↑
    
    • Transport to other organs
      
      • Oxidation
    • Oxidation in mitochondria
• Sympathetic excitation
  
  • Adrenaline / noradrenaline effect
  • Thyoxine effect
    
    • Uncoupling of oxidative phosphorylation by thermogenins
• Calorigenic effect of thyroid hormones
  
  • Heat production prolonged effect
  • BMR
  • Number of mitochondria
  • Amount of thermogenin
  • Activity of Na⁺/K⁺ pump
• Brown adipose tissue
  
  • Foamy
  • Fine IC granular content
  • Intensive blood supply
  • Many mitochondria
    
    • High humber of cyochrome-oxidase
  • α and β-adrenergic receptors on their surface
• Morpho-functional changes against cold
  
  • Thickening of fur and subcutaneous tissue
  • Secretion of calorigenic hormones ↑:
    
    • Thyroid hormones
    • Glucocorticoids
    • Metabolic rate ↑
  • Superficial vasoconstriction (a/v shunt)

Defense against heat

• Humid heat loos: evaporation
  
  • Persipiratio sensibilis
    
    • Sweating
  • Persipiratio insensibilis
    
    • Diffusion from surface of alveoli, skin and oral mucous membranes
    • Panting
• Panting:
  
  • Evaporation (water loss)
  • Advantage: Influence degree of heat loss
  • Disadvantage: Alkalosis and consume energy
• Behaviroal defense reactions
• Dry heat loss
• Conduction
  
  • Direct contact with surrounding objects
• Convetion
  
  • Air/water touching body
• Morpho-functional changes against heat:
  
  • Thickness and density of insulating layers ↓
  • Heat dissipating mechanisms
  • Vasodilation

Circulation and heat balance

• Hypothalamic center
  
  • Control body temperature
• Circulation in heat:
  
  • Superficial skin layer
    
    • Precapillary sphincters open
  • Deep system:
    
    • Major veins contract
    • Superficial veins dilate
• Circulation in cold:
  
  • Superficial skin layer
    
    • Precapillary sphincters close
  • Deep system:
    
    • Major veins dilate
    • Superficial veins contract

Specialized structures

• Defense against major heat loss through limbs
  
  • Looping heat exchange
    
    • Warm blood flowing towards limbs can get cool
      
      • Ø temperature gradient among body and environment
• Counter current heat exchange
  
  • In hot
    
    • In the leg of birds (rete mirabile)
  • In cold
    
    • In head (rete of carotis)
    • In testicles (rete testis)
• Heat tolerant species
  
  • Sheep
  • Cooling of brain and CNS

Central thermoregulation

• Hypothalamic center
  
  • Thermostat
  • Set value of core temperature
  • Effector system
    
    • Maintain body temperature near to hypothalamic set-point
• Receptors
  
  • Peripheral
  • In organs
• Afferentation
• Hypothalamic termostat
• Efferentation
  
  • Somatic
  • Vegetative
  • Hormonal
• Hypothermia
  
  • Below controllable range
  • Metabolism ↓
  • Heat / respiratory frequency ↓
  • Circulatory failure
    
    • Hypercapnia
      
      • Too much CO₂ in blood
    • Hypoxia
      
      • Too little dissolved CO₂
  • Disorders of:
    
    • Digestion
    • Absorption
    • Kidney functions
• Fainting
• Ventricular fibrillations
• Hyperthermia
  
  • Insufficient operation of heat dissipating mechanism
  • Metabolism ↓
  • Corruption of circulation → overheating shock
  • Hypercapnia
  • Hypervolemia
  • Loss of salts and water
  • Muscle seizure
  • Fever

1. Fever
2. Fever is a regulated hyperthermia
   
   • Induced by:
     
     • Infections
     • Inflammations
     • Tissue destruction
3. Exogenous pyrogens get into organism
4. Macrophage activation
5. Formation of interleukin-1 & 6
6. Direct stimulation of hypothalamic heat center by local prostaglandin (PGE1) release

• Pyrogens
  
  • Endotoxins of Gram-negative bacteria
  • Heat stable polysaccharides
  • Membrane derbis
  • Lipopolysaccharide fragments

1. Pyrogens set hypothalamic set point to a higher temperature
2. Incude heat production and heat conserving processes
   
   1. Shivering
3. Temperature stabilizes at a higher value
4. Removing pyrogens → set point drops back to previous normal value
   
   1. Sweating (cooling mechanism starts to work)

Acclimatization

• Adaption
  
  • Shift to lower/higher critical temperature
    
    • Shift of thermoneutral zone
• Morpho-functional changes in heat:
  
  • Thickness and density of insulating layers ↓
  • Dominance of heat dissipating mechanisms
    
    • Activation of sweat glands
    • Activity of respiratory muscles ↑
    • Superficial vasodilation of skin

Question 2

Topics to include in the essay

Answer 2

1. Maintaining constant body temperature
   
   • Defense against cold
     
     • Sympathetic excitation
     • Calorigenic effect of thyorid hormones
     • Brown adipose tissue
     • Morpho-functional changes against cold
     • Hypothermia
   • Defense against heat
     
     • Panting
     • Morpho-functional changes against heat
     • Hyperthermia
2. Circulation and heat balance
   
   • Circulation in heat
   • Circulation in cold
3. Specialized structure
   
   • Limbs (defense against heat-loss)
   • Heat tolerant species (sheep)
4. Central thermoregulation
5. The fever
6. Acclimatization

Question 3

Defense against cold

Answer 3

1. Heat conservation: diminishing heat-loss
   
   1. Animals living in groups cuddle up to each other diminishing the surface are of heat loss
2. Muscle activity ↑
   
   1. ​Tonicity (isometric) or shivering ↑
3. Chemical (non-shivring) thermogenesis
   
   1. ​If the two above-mentioned forms are not able to maintain the constant body temperature

​Sympathetic excitation

• Effect of adrenaline / noradrenaline
• Long term thyroxine effects
  
  • (Uncoupling of oxidative phosphorylation by thermogenin)

Calorigenic effect of thyroid hormones

• Prolonged effect on heat production
• Basic metabolic rate (BMR)
• Number of mitochondria
• Increase energy consumption and heat production
• Amount of thermogenin
• Activity of Na⁺/K⁺ pump

Brown adipose tissue

• Foamy
• Fine IC granular content
• Intensive blood supply
• Many mitochondria
• High number of cytochrome-oxidase
• α and β-adrenergic receptors on their surface

Hypothermia

• Body temperature decreases below the controllabele range
• Metabolism ↓
• Heart / respiratory frequency ↓
• Circulation failute
  
  • ​Hypercapina (too much CO2 in blood)
  • Hypoxia (too little dissolved CO2)
• Disorders of:
  
  • Digestion
  • Absorption
  • Kidney functions
• Fainting
• Ventricular fibrillations

Morpho-functional changes

• Thickening of fur coat and subcutaneous adipose tissue
• Calorigenic hormones increases secretion:
  
  • Thyroid hormones ↑
  • Glucocorticoids ↑
  • Metabolic rate ↑
• Superficial vasoconstrictor mechanism of sikn (a/v shunt)

Figure: heat production in brown adipose

Question 4

Defense against heat

Answer 4

• ​​Humid heat-loss: evaporation:
  
  1. ​Perspiratio insensibilis
     
     • ​Panting by diffusion from the alveoli, skin and oral mucous membranes
  2. Perspiratio sensibilis
     
     • ​Sweating
• Panting:
  
  • ​Advantage: animal can actively influence the degree of heat-loss
  • Disadvantage: excessive panting can cause alkalosis and consumes energy

Hyperthermia:

• Insufficient operantion of heat dissipating mechanisms:
  
  • Metabolism ↓
  • Corruption of circulation → overheating shock
  • Hypercapnia
  • Hypeovolemia
  • Water and salt loss
  • Muscle sizure

Morpho-functional changes:

• Thickness and density of isulation layers ↓
• Activation of sweat glands
  
  • Aldosterone effect
• Respiratory muscles ↑
• Superficial vasodilation of skin

Question 5

Circulation and heat balance

Answer 5

• Thermal conductivity of tissues is lower, therefore are heat transported between the organs, inner body parts and the body surface
• Thermoneutral zone​
• Circulation in heat:
  
  • Superficial skin layer:
    
    • Precapillary sphincters open
  • Deep system:
    
    • Major veins contract
    • Superficial veins dilate
• Circulation in cold:
  
  • Superficial skin layer:
    
    • Precapillary sphincters close
  • Deep system:
    
    • Major veins dilate
    • Superficial veins contract

Question 6

Specialized structures in different species

Answer 6

• Defense against major heat loss through limbs:
  
  • Looping heat exchange
  • Warm blood flowing towards the limbs can get cool to a degree that close to the ground there is hardly any temperature gradient among the body and the environment
  • Polar animals that are exposed to extremely low temperatures has a much higher proportion of unsaturated fatty acids in the cells of the limbs compared to other body parts
• Heat tolerant species (sheep) have special circulatory units
  
  • Ensuring cooling of brain and CNS
    
    1. ​​Plexus of nasal cavity: cools blood by panting
    2. Cooled venous blood gets to ring of Willsius
    3. Blood of ascending a. carotis gets cooled by heat exchang before reaching the brain

Question 7

Central thermoregulation

Answer 7

• The hypothalamic center as a thermostat determines a certain set point value for the core temperature
• The effector systmes are used to maintain body temperature near to the hypothalamic set-point

Question 8

The fever

Answer 8

• Pyrogens:
  
  • ​Endotoxins of Gram-negative bacteria
  • Heat stable polysaccharides
  • Membrane derbis
  • Lipopolysaccharide fragments

1. At the beginning of fever process the pyrogenic substance sets the hypothalamic set point to a higher temperature
2. This induces heat production and heat conserving processes (shivering)
3. As the current value reaches the set point the fever temperature becomes stabilized at a higher value
4. After removing the pyrogenic substance the set point drops back to the previous normal value:
   
   • Cooling mechanisms start to work (sweating) and the fever becomes solved
5. Feves is regulated hyperthermia
   
   1. ​Influenced by:
      
      1. Infection
      2. Inflammations
      3. Tissue destruction
6. Exogenous pyrognes get into the organism
7. Macrophage activation
8. Formation of:
   
   1. Interleukin-1
   2. Interleukin-6
9. Direct stimulation of hypothalamic heat center by local prostaglandin (PGE1) release
10. Hypothalamic regulation

Question 9

Acclimatization

Answer 9

• Adaptation can shift the lower or higher critical temperature (shift of the thermoneutral zone) to higher or lower levels