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Thermoregulation

Thermoregulation. Thermoregulation. Maintenance of internal body temperature regardless of environmental temperature In utero : Maternal temperature Heat produced dissipated through the placenta High thermal diffusion capacity At birth Ineffective insulating layer .

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Thermoregulation

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  1. Thermoregulation

  2. Thermoregulation • Maintenance of internal body temperature regardless of environmental temperature • In utero: • Maternal temperature • Heat produced dissipated through the placenta • High thermal diffusion capacity • At birth • Ineffective insulating layer

  3. Thermoneutral zone • Range of temperatures within which metabolic rate is at a minimum and temperature regulation can be achieved through nonevaporative physical processes alone • Adults • 26-28° C • Infants • 32-34° C • Premature Infants • Minimum of 35° C

  4. Mechanisms for Heat Production • Physical • Shivering thermogenesis • Important for adult thermogenesis • Chemical • Nonshiveringthermogenesis • Most effective and efficient source of heat production in the neonate

  5. Norepinephrine and Epinephrine • Adult shivering response mediated by epinephrine • Neonate nonshivering response mediated through norepinephrine • Infusion of norepinephrine • Increase in plamsanonesterified fatty acids (NEFA) • Increased oxygen consumption • Rise in body temperature • Norepinephrine plays a large role in a newborn infant’s defense against cold and also has effects on intermediary lipid metabolism

  6. Premature infants and norepinephrine levels • 9 premature infants • 6 of 9 showed increased excretion of norepinephrine • 6 infants: mean temperature fall 2.4° C • 3 infants (no increase): temperature fall 3-5° C • Two weeks later • All 9 infants exhibited increased norepinephrine levels • Mean temperature fall for all infants 0.9° C

  7. Premature infants and norepinephrine levels • Second Study • Norepinephrine levels examined at 12 and 47 days • Difference between ages may suggest : • Norepinephrine response is a major component of infant’s response to cold • Maturation of the infant may parallel the development of thermal stability

  8. Brown Adipose Tissue (BAT) • BAT is main site of nonshiveringthermogenesis in neonate • Human adult: BAT comprises 1% body weight • Infant: BAT comprises 5-7% body weight • BAT contains: • Sympathetic nerve fibers which maintain synaptic contact with cell membrane • These fibers trigger release of norepinephrine • Initiates thermogenesis • Activates lipase

  9. Brown Adipose Tissue (BAT) • BAT cells have central nucleus with fat lobules and mitochondria • Mitochondria has specialized protein • “Uncoupling protein” • Short circuits the electrochemical gradient of respiratory chain • Uncoupling protein discharges the chemical gradient in substrate oxidation and ADP phosphorylation without the generation if ATP • Energy generated by uncoupling oxidative phosphorylation is simply released as heat

  10. Brown Adipose Tissue (BAT) • Postnatal development of respiratory enzymes and uncoupling protein in BAT occurs within the first hours of birth • Accelerated by cold stress through increase in rates of transcription of the uncoupling gene • BAT overall provides up to 2/3 of total heat produced through nonshiveringthermogenesis. • Nonesterifiedfatty acids (NEFA) (which rise in response to an increase in norepinephrine) appear to reflect the increased lipolyticactivity of BAT

  11. NonshiveringThermogenesis • Precocial species • Non-shivering thermogenesis greatest at birth and disappears within a few weeks • Exposure to cold prevents this disappearance • Altricial species • Gradual increases in non-shivering thermogenesis for the first few weeks of life • Infants • Gradual disappearance of brown fat stores within the first year • This correlates with the conversion from non-shivering to shivering thermogenesis when an infant is exposed to the cold

  12. Thermal imbalance • Hyperthermia • Neonates exhibit increased oxygen consumption • Only slightly elevated temperatures • Hypothermia • Constriction of blood vessels in infants • Increase tissue insulation to maximal value by increasing internal temperature gradient • Maximally constricted – tissue insulation is low in comparison to the adult

  13. hypothermia • Increase in metabolic rate in infants • Poses extraordinary challenges, infant already has difficulty in respiration • Challenge of increased oxygen consumption may exceed physiological limits • Protective mechanism in place against the effects of hypoxia due to cold stress • Moderate acute hypoxia shows no effect on minimal oxygen consumption • Reduced metabolic response to cold • Results in reduction of increase in metabolic demand for a hypoxic infant but also makes it more difficult to maintain thermal equilibrium

  14. Hypothermia • Effects on acid-base balance • Drives pH down – more acidic • Drop in pH hypothesized to trigger action of norepinephrine in response to hypothermia • Restrictions to adaptation • Body temperature is maintained as long as heat loss doesn’t exceed capacity for heat production • With continued increasing hypothermia the thermoregulatory drive induced in thermo-integrative area of the central nervous system is eventually reduced

  15. Hypothermic Effects on Passive Transfer of Antibodies • Cold stress delays onset and significantly decreases rate of absorption of immunoglobulins up to 15 hours after first feeding of colostrum • Net absorption of IgG not affected • In dogs hypothermia causes decreased venous outflow from the small intestine, decreased overall intestinal motility, and net reduction in transport of substances from the intestinal lumen into the blood

  16. Hyperthermia and Passive Transfer • Similar mechanism thought to be responsible for delay and decrease in rate of absorption of IgG in calves and other farm species • Absence of an effect on net absorption of colostralimmunoglobulins in calves may be due to the short duration of the cold stress • Cold stressed calves also exhibited muscular weakness and reluctance to stand and nurse • Therefore caused a decrease in total amounts of colostrum ingested and absorbed

  17. Thermoregulation and thriftiness at birth • Time it takes to stand and suckle are tied to ability to maintain temperature in newborn lambs • Heavier lambs, blackface lambs, and single or twin lambs were quicker to stand and suckle from their mothers then lightweight, Suffolk, or triplet lambs • Low birth weight lambs had lower rectal temperatures • Lambs slow to suckle also had lower temperatures that persisted for 3 days

  18. Thermoregulation and Lambs • Energy needed to sustain body temperature supplied from brown adipose tissue (BAT) • Oxidation of BAT for energy accomplished through triiodothyronine (T3) and thyrodine (T4) • Heavier lambs and blackface lambs exhibited higher T3 and T4 levels • Lower weight lambs exhibited lower T3 and T4 levels • Indicates maturity at birth may play role in T3 and T4 levels

  19. Thermoregulation and Lambs • Overall: lighter weight/smaller size and slower ability to suckle resulted in reduced ability of efficient heat regulation • Thermoregulation also believed to be affected by the fat content in colostrum • High fat content provides greater energy supply for thermoregulation • Allows for species differences in thermoregulating ability

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