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Regulation of Body Temperature

Regulation of Body Temperature. Role of the Hypothalamus. Normal body temperature. Core temperature Temperature of deep tissues mouth 98.6 o F ( ±1 o F) 37 o C (± 0.6 o C) 1 o F higher in rectum 1 o F less in axilla Skin temperature Varies with temperature of surroundings.

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Regulation of Body Temperature

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  1. Regulation of Body Temperature Role of the Hypothalamus

  2. Normal body temperature • Core temperature • Temperature of deep tissues • mouth • 98.6o F (±1o F) • 37o C (± 0.6o C) • 1o F higher in rectum • 1o F less in axilla • Skin temperature • Varies with temperature of surroundings

  3. Body core tempe-rature

  4. Body temperature is balance between Heat Production and Heat Loss

  5. Heat Production Mainly By-product of metabolism • BMR • Extra metabolism • Muscle activity • Shivering • Thyroxine, Growth Hormone, testosterone • Sympathetic stimulation • ↑ chemical activity in cells due to ↑temperature • Thermogenic effect of food

  6. Heat Loss

  7. Heat Loss • Radiation (Infrared Heat waves) = 60% • Evaporation =22% • Skin Sweat glands Diffusion • Respiration • Conduction • To air (Convection)= 15% • To objects = 3% • Urination • Defecation

  8. Skin circulation

  9. continuous venous plexus • supplied by inflow of blood from the skin capillaries • In the exposed areas of the body—plexus directly from the small arteries through highly muscular • arteriovenous anastomoses

  10. A high rate of skin flow causes heat to be conducted from the core of the body to the skin with great efficiency • reduction in the rate of skin flow can decrease the heat conduction from the core to very little

  11. Effect of changes in body temperature on heat conductance from the body to the skin surface

  12. Regulation of body temperatureTemperature regulatory centers in Hypothalamus

  13. Primary Steps in Regulation of body temperature • Detection of temperature by • Anterior Hypothalamic and Preoptic nuclei • Receptors in • Skin • Deep body tissues Types of receptors • Cold receptors • Warmth receptors • Integration POSTERIOR Hypothalamus • Effects/ RESPONSE

  14. Detection of temperature • Anterior Hypothalamic- Preoptic area • Thermostatic body temperature control centre • Large number of Heat-sensitive neurons • One third Cold-sensitive neurons

  15. The heat-sensitive neurons increase their firing rate 2- to 10-fold in response to a 10°C increase in body temperature • The cold-sensitive neurons---increase their firing rate when the body temperature falls

  16. Hypothalamic-preoptic area ----thermostatic body temperature control Heating preoptic area results in • profuse sweating from skin • Cutaneous blood vessels ----dilate • body loses heat---temperature toward the normal level. • any excess body heat production is inhibited.

  17. Detection of temperature • Skin • Deep body tissues • Spinal cord • Abdominal viscera • Great veins in upper abdomen and thorax • Types of receptors • Cold receptors(10 times as many) • Warmth receptors • peripheral detection of temperature mainly concerns detecting cool and cold instead of warm temperatures

  18. molecular mechanisms -- not well understood----- transient receptor potential family of cation channels Increase in the temperature of the body occurs by • shivering, with a resultant increase in the rate of body heat production • inhibiting sweating • skin vasoconstriction to reduce loss of body heat from the skin

  19. Integration of temperature senses • POSTERIOR Hypothalamus • Integration of • Peripheral temperature sensory signals and • Central temperature sensory signals

  20. Effects/ RESPONSE • Temperature-Decreasing Mechanisms When the Body is Too Hot • Temperature-Increasing Mechanisms When the Body is Too Cold Regulatory mechanisms of body continually attempt to bring the body temperature at Set-Point

  21. Temperature-Decreasing Mechanisms When the Body is Too Hot • Vasodilation of skin blood vessels Mechanism Inhibition of sympathetic centers in posterior hypothalamus • Sweating Especially important in hot weather (Stimulation of cholinergic sympathetic nerve fibers) • ↓ Heat production • Behavioral control • Psychic sensation of heat comfort • Appropriate environmental adjustments 5. Insensible Perspiration

  22. Temperature-Decreasing Mechanisms When the Body Is Too Hot • Vasodilation of skin blood vessels-- skin blood vessels become intensely dilated-- caused by inhibition of the sympathetic centers in the posterior hypothalamus -- vasoconstriction. Full vasodilation --- increase the rate of heat transfer eightfold

  23. Balance between heat loss and production maintains body core temperature Set Point

  24. 2.Sweating : increased body temperature causes sweating ( blue curve) ---sharp increase -- when the body core temperature rises above the critical level of 37°C (98.6°F)

  25. Sweat gland innervated by an acetylcholine-secreting sympathetic nerve

  26. 3. Decrease in heat production ----as shivering and chemical thermogenesis--- are strongly inhibited

  27. Temperature-Increasing Mechanisms When the Body Is Too Cold 1. Skin vasoconstriction throughout the body ---- caused by stimulation of the posterior hypothalamic sympathetic centers

  28. 2. Piloerection (hairs “standing on end”) • Mechanism • stimulation of sympathetic nerves causes contraction of arrector pili muscle • Not important in humans

  29. 3. Increase in thermogenesis (heat production). --- promoting shivering ---sympathetic excitation of heat production ---- thyroxine secretion

  30. primary motor center for shivering--- (posterior hypothalamus) • normally inhibited by signals from the heat center in the anterior hypothalamic-preoptic area • excited by cold signals from the skin and spinal cord • Signals are transmitted through anterior motor neurons to cause increase in muscle tone .

  31. Shivering center becomes activated when the body temperature falls • the sudden increase in “heat production” (red curve) when temp falls

  32. b. Sympathetic chemical excitation of Heat production increase the rate of cellular metabolism • Uncoupling of oxidative phosphorylation • Excessive foodstuffs oxidized to release heat but ATP not formed

  33. c. Increased Thyroxineproduction • Cooling the anterior hypothalamic-preoptic area --- increases production of TRH ---- hypothalamic portal veins to the anterior pituitary gland--- TSH The increased thyroxine causes chemical thermogenesis • activates uncoupling protein • increases the rate of cellular metabolism

  34. “Set-Point” for temperature control • At set-point (normal body temperature) Heat loss = Heat production

  35. Balance between heat loss and production maintains body core temperature Set Point

  36. Skin Temperature determines the set point level at which sweating begins Effect of changes in the internal head temperature on the rate of evaporative heat loss from the body

  37. Skin temperature determines the set point level at which shivering begins Effect of changes in the internal head temperature on the rate of heat production by the body

  38. Temperature-Increasing Mechanisms When the Body is Too Cold 4. Behavioral control • Psychic sensation of cold comfort

  39. Abnormalities of Body temperature regulation • Hyperthermia • Fever • Heatstroke • Hypothermia • Frostbite • Artificial hypothermia

  40. Fever • Definition: • A body temperature above the usual range of normal • Mechanism • Effect on temperature-regulating centers • Toxic substances • Abnormalities in brain • Causes • Febrile illnesses • Heat stroke • Hard exercise • Brain lesions

  41. Fever caused by Toxic substances • Toxic substances that cause fever are called PYROGENS • PYROGENS are released from bacterial cell membrane • Many proteins • Protein breakdown products • Lipopolysaccharides • PYROGENS cause the SET-POINT of the Hypothalamus to Rise • Mechanism:

  42. Mechanism of action of PYROGENS Fever Bacteria +Bacteria breakdown products Aspirin Formation of Prostaglandin Phagocytosis Hypothalamus Products of digestion Interleukin-I

  43. Fever caused by Brain Lesions • Lesions that affect Hypothalamic-temperature regulatory areas

  44. Characteristics of febrile conditions • Chills • Set-point suddenly rises • Body temperature less than set-point, although more than normal • No feeling of hot and cold when body temperature= set-point • Crisis (Flush) • Set-point starts decreasing (pyrogens removed naturally or by medicines) • Body temperature more than set-point, and more than normal

  45. Chills and Crisis

  46. Heat Stroke • When body temperature rises above 105oF(40.56oC) (in hot weather; more likely in hot humid weather): Heat Stroke occurs • At or above environmental temperature of 94oF(34.44oC) if there is more humidity • May not occur in temperature of 130oF(54.44oC) in weather is dry • If heavy work, may occur at 85oF(29.44oC)-90oF(32.22oC)

  47. Symptoms/ Effects of Heat Stroke • Dizziness • Abdominal distress • Vomiting • Delirium • Circulatory shock (Loss of fluid and electrolytes in sweat) • Loss of consciousness

  48. Harmful effects of high temperature • Local hemorrhages • Degeneration • Brain Damage (Irreversible damage to neurons) • Liver (Failure) • Kidneys (Failure) • Death

  49. Body temperature under different conditions

  50. Treatment of Heat Stroke • Emergency • Cold water bath ?(Shivering) • Sponge or spray cooling

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