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
Nonshivering thermogenesis
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 plamsa nonesterified 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
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
Norepinephrine Level
Mean Fall in Rectal Temperature
Day 12
No apparent rise
4.5° C
Day 47
Striking increase in levels
1.5° C

8. Brown Adipose Tissue (BAT)
BAT is main site of nonshivering thermogenesis 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 nonshivering thermogenesis.
Nonesterified fatty acids (NEFA) (which rise in response to an increase in norepinephrine) appear to reflect the increased lipolytic activity of BAT

11. Nonshivering Thermogenesis
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 Hypothermia
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 Restrictions to adaptation
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 colostral immunoglobulins 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