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WHAT IS MEAN BODY TEMPERATURE? • Very different for different animals WHY?

WHAT IS MEAN BODY TEMPERATURE? • Very different for different animals WHY? Regional differences - core body temperature Hottest region - lumen of stomach + upper part small intestine - distinctly hotter than aortic blood

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WHAT IS MEAN BODY TEMPERATURE? • Very different for different animals WHY?

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  1. WHAT IS MEAN BODY TEMPERATURE? • Very different for different animals WHY? Regional differences - core body temperature Hottest region - lumen of stomach + upper part small intestine - distinctly hotter than aortic blood (Grayson, 1983) - not due - digestive or absorptive functions - nor to metabolism

  2. Estimates of body temperature (steady-state conditions) Tb = (a1 x Tcore) + (a2 x Tskin) a factors = empirically determined contributions of core & shell to mean body temperature Some estimates : 9 : 1 and 6 : 4 Varies with thermal environment and investigator •Tb = 0.67 Tre + 0.33 Tskin Which thermal sensors are important?

  3. How do you determine mean surface temperature? Humans: Tskin = 0.07Tfeet + 0.32Tlegs + 0.18Tchest + 0.17Tback + 0.14Tarms + 0.05Thand + 0.07Thead Periphery - skin, mucous surfaces (mouth, nose) Core - spinal cord, hypothalamus, medulla oblongata, + organs Difficult - determine by experiment - which are important sites

  4. Chicken - exposed cold Ta - shivers before change in deep body temperature Suggests - heat production - controlled by thermal receptors in skin. Kuhnen and Jessen - 1988 - goat Change - skin temperature - alone - alters MR

  5. Populations of Temperature Sensitive Structures • (skin, spinal cord, hypothalamus) • Warm sensors - activity increases with local temperature increase • No discharge at skin temperature below 30 C • Receptors discharge with increase temperature • from 30 >> 35 C • • But little sustained activity • Further increase - skin temperature >> increased sustained • discharge that changes with temperature • Peak activity - 45-47 C • Activity ceases at higher temperature

  6. Cold Sensors - activity decreases over same temperature range • Most active at 15 - 35 C & peak at 20 - 30 C • =========================================================== • PARADOXICAL RESPONSE • Most cold receptors >> no discharge above 35 C • But may get discharge near noxious level (43 - 47 C) • (ex. sudden chill - stepping into hot shower) • Also paradoxical warm receptor response - BUT not as common

  7. Proportions - cold & warm sensors - vary with site In general - more warm sensors - hypothalamus more cold sensors - cutaneous regions Exceptions - scrotal region - high concentration warm sensors Sheep & rats - local heating scrotum >> powerful heat loss effector activity If sustained >>> decrease in body temperature Heating scrotum - ram - above 36 C >> panting & decrease core body temperature (~ 2 C) Similiar effects - heating mammary skin - ewe

  8. General facts about skin temperature Not homogenous - large difference in skin temperature in different parts of the body May account for regional differences by averaging surface area •BUT - density of skin thermoreceptors are different in different areas Larger weight to face and less to extremities •Face - approx. 20% of total skin signal

  9. Skin temperature - represents interaction between convective heat exchange to surface by blood + removal of heat (geometry, external insulation, ambient temperature) Virtually all skin areas - including trunk - constrict at cold Ta (Exception = human forehead - no vasoconstriction atcold Ta)

  10. CENTRAL RECEPTORS Under normal conditions - trunk sites do not vary more than 0.5°C Arterial temperature (e.g., carotid) - contain core temperature sensitive sensors Hypothalamus - contains many neurons and thermoregulatory circuits Preoptic Anterior Hypothalamus (POAH) still a black box

  11. During heat stress and exercise - may maintain brain temperature below trunk In panting carnivores - blood supply passes through carotid rete A net-like structure - base of brain Numerous small arteries embedded in cavernous sinus Rete = heat exchanger

  12. •Cooled venous blood - evaporating surfaces (e.g., nasal mucosa) •arterial blood entering brain •running antelope - brain temperature ~3°C below carotid temperature •Also goat and sheep •maintenance of lower brain temperature also found in species without carotid rete (rabbit, squirrel, monkey, man) •Nonpanting animals - cool venous blood - sweating face Site of exchange with arterial blood - unknown

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