Temperature Regulation

1. Temperature Regulation

2. Why temperature regulation?

3. Why temperature regulation? • All organisms are limited by their ability to survive in different temperatures • Some, like reptiles and amphibia are poikilothermic • Others like humans are homeothermic

4. Core temperature The core temperature of the human body is 37°C ± 0.6°C The core of the human body includes the organs of the thorax, abdomen and the head This is where the vital organs are located Their enzyme systems must operate in optimum conditions The periphery of the body can withstand some deviation from the core temperature

5. Core Temperature

6. Mechanisms of Heat Transfer • 1) Radiation, especially infrared • 2) Conduction: heat transfer by direct contact • conductors – high conductance • insulators – low conductance (high resistance) •  Convection:  conduction due to movement of a liquid or gas

7. 3) Evaporation • 0.58 Cal required to evaporate 1g of H2O from the surface of the skin. • heat of vaporization = 0.54 Cal • When environmental temperature > body temperature, evaporation is the only mechanism to cool the body.

8. Body Skin Environment Basal metabolic rate Muscle activity Shivering Vasomotor Sweating Piloerection Conduction Convection Radiation Evaporation Balance

9. Reference Set-point hypothesis Response Controlled system Feedback So how is it regulated?

10. Balance hypothesis Response warm cool Feedback Controlled system Feedback

11. Inputs Outputs Central thermosensors Neural sweating shivering vasoconstriction vasodilation warm warm cool cool Anterior Hypothalamus Peripheral thermosensors Hormonal adrenaline TRH

12. Central Control of Body Temperature • Controlled Variable • temperature of body core • Set Point • 37oC • Receptors • central thermoreceptors: in hypothalamus of brain • peripheral thermoreceptors: in skin (mostly affect behavior, very little role in regulating core temperature) • Control Center • hypothalamus • Effectors • dermal blood vessels • skeletal muscle • endocrine glands • brown fat • sweat glands

13. Hypothalamic Control of Body Temperature • When core body temperature drops below the set point, the following mechanisms are activated to increase body temperature: •  heat loss vasoconstriction [piloerection]  heat production shivering muscle contraction generates heat hormone secretion e.g., thyroxine, epinephrine (adrenaline) activation of brown fat e.g., in human infants mitochondria generate heat instead of ATP

14. Hypothalamic Control of Body Temperature • When core body temperature rises above the set point, the following mechanisms are activated to decrease body temperature: •  heat loss vasodilation (inhibit vasoconstriction) sweating  heat production  voluntary activity  fuel intake  thyroxine secretion

15. Muscles of skin arteriole walls relax Sweat glands increase secretion Core body temperature >37°C nerves Hypothalamus Thermoreceptors Muscles reduce activity NEGATIVE FEEDBACK Blood temperature Thermoreceptors Body loses heat Return to 37°C

16. Muscles of skin arteriole walls constrict nerves Sweat glands decrease secretion Core body temperature <37°C Hypothalamus Thermoreceptors Muscles shivering nerves NEGATIVE FEEDBACK Blood temperature Thermoreceptors Body loses less heat Return to 37°C Body gains heat

17. Babies • Babies have an extra mechanism (cant shiver) • Brown fat. • Suprascapular deposits • Rich in Iron containing mitochondria

18. 37.4 Postovulatory Rectal temp OC 36.8 Preovulatory 36.2 12 18 24 6 12 Time And the menstrual cycle

19. Fever and hyperthermia An elevation of body temperature that exceeds the normal daily variation and occurs in conjunction with an increase in the hypothalamic set point----fever Fever due to a disturbance of thermal regulatory control----hyperthermia

20. Development of fever Pyrogenic activator Macrophage, monocyte, lymphocyte, etc. Endogenous pyrogen Thermoregulatory Centre Mediators in regulation of body temperature Rise in set point fever

21. Pyrogenic activator A substance that can manufacture and release endogeneous pyrogen which produces a fever. 1．Exogenous pyrogen bacteria virus other microorganism 2. internal metabolic product antigen-antibody complex steroid inflammatory substance

22. Fever • monocytes and phagocytes release endogenous pyrogen (Interleukin-1, IL-1). • The anterior hypothalamus is sensitive to IL-1 • Hypothalamic sensitivity to temperature is altered.

23. So… • The body temperature then becomes regulated at a new, higher level. • There is some evidence that the raised body temperature enables the fight against the infection.

24. But... • Every 1OC rise in temperature increases basal metabolic rate and oxygen consumption by about 14%, • In acute infection, the ability to mobilise fat stores is inhibited.

25. Consequently • Skeletal muscle is broken down and the amino acids are used in gluconeogenesis. • This can be debilitating.

26. And even worse • In addition to the increased demand for energy • Temperatures (above 42OC) damage nerve cells • impair thermoregulation • have more serious consequences.