The document discusses body temperature regulation and fever. It describes how the hypothalamus acts as the body's temperature control center, detecting core and skin temperatures and initiating mechanisms to increase or decrease body heat such as vasodilation, sweating, shivering and changing metabolic rate. Fever occurs when pyrogens directly or indirectly reset the hypothalamus set point to a higher temperature, causing the body to actively generate heat through mechanisms like increased prostaglandin production. The body raises its temperature during fever through vasoconstriction which reduces heat loss and increased heat production through shivering and metabolic changes.

1. Body Temperature Regulation, and Fever
-Dr. Dipti Magan

2. Core Temperature - Skin Temperature
• Core Temperature - temperature of deep tissues
is relatively constant, varying less than 1 degree F
under normal conditions
• Skin Temperature - varies widely, depending on
temperature of environment

3. Constancy of Internal Body Temperature

4. How is Body Temperature Detected?
• Hypothalamus temperature control center
- Preoptic area of anterior hypothalamus
- Heat sensitive and cold sensitive neurons
• Skin and deep body temperature receptors
- Mainly detect cold and cool temperatures
- Function to prevent hypothermia
• Role of posterior hypothalamus
- Receives input from anterior hypothalamus and
peripheral temp receptors to elicit mainly heat
producing and heat conserving reactions

5. Mechanisms of Heat Loss
• Radiation -
- Infrared heat waves
- Effectiveness depends on heat of surrounding objects.
• Conduction/Convection -
- Heat transfer to layer of air at body surface
- Conduction is enhanced by convective air flow
• Evaporation -
- Insensible evaporation
- Evaporation of sweat (under physiologic control)

6. Physics of Heat Loss From the Body

7. How is heat lost from the body when the
environmental temperature is greater than the
body temperature?
Question

8. Mechanism of Sweat Secretion
• Sweating caused by stimulation of anterior
hypothalamus-preoptic area (AHPA)
• Nerve impulses from AHPA lead to
stimulation of cholinergic nerve fibers
that innervate sweat gland
• Recall that cholinergic nerve fibers secrete
acetylcholine but run in sympathetic nerves
along with adrenergic fibers
• Sweat glands can also be stimulated by
circulating epinephrine or norepinephrine.
Adrenergic fibers do not actually
innervate sweat glands
• Low flow: Na, Cl, and water are mostly absorbed.
Urea, lactic acid, K are very concentrated
• High Flow: Na and Cl concn. are ~half of that of plasma
Lactic acid and K still greater than plasma

9. • Unacclimatized person:
– one liter sweat/hour
– 15-30 grams NaCl lost per day
• Acclimatized person:
– 2-3 liters sweat/hour (caused by a morphological change in sweat
gland)
– 3-5 grams NaCl lost per day (caused by increased secretion of
aldosterone)
Acclimatization of Sweating Mechanism

10. Skin circulation
Sympathetic control of Skin Blood Flow

11. How effective is the skin radiator system?
8-fold difference in
heat conduction
between maximum
dilation and maximum
constriction

12. Temperature Decreasing Mechanisms
• Vasodilation - transfers heat to skin
- Inhibition of sympathetic centers in posterior hypothalamus
that cause vasoconstriction
• Sweating - evaporative heat loss
- Stimulation of preoptic area of hypothalamus stimulates
sweating via sympathetic cholinergic fibers
- Circulating Epi and NE during exercise
• Decreased heat production
- shivering and chemical thermogenesis are inhibited

13. Temperature Increasing Mechanisms
• Vasoconstriction -
- Impedes heat transfer to skin
- Stimulation of sympathetic centers in posterior
hypothalamus
• Piloerection - usually not important in humans
• Increased heat production -
- Shivering
- Sympathetic excitation of heat production
- Thyroxine secretion

14. Hypothalamic Stimulation of Shivering
• Primary motor center for shivering (in hypothalamus)
– Normally inhibited by signals from heat center in anterior
hypothalamic-preoptic area
– Excited by cold signals from skin and spinal cord
– Leads to… stimulation of anterior motor neurons which increase
muscle tone
– Shivering occurs when muscle tone is sufficiently high – may involve
feedback oscillation of muscle spindle stretch reflex mechanism
Can raise body heat production by 4-5 fold

15. Importance of Thermoregulation
 The mechanisms thermoregulation are all designed to
return the body to homeostasis or a state of
equilibrium
 This process helps in controlling the loss or gain of
heat and maintaining of an optimum temperature
range by an organism

16. Regulation of Core Body Temperature

17. Fever - Resetting the Set-point T°
• Pyrogens can directly reset set-point -
- Bacteria - lipopolysaccharides are pyrogens
- Pyrogens from degenerating tissues
• Pyrogens can indirectly reset set-point -
- Interleukin-1 released from phagocytes following
phagocytosis of blood-borne pyrogens
- IL-1 raises set-point by increasing prostaglandin
production (mainly E2)

18. Pathogenesis of Fever

20. What is the mechanism by which the body raises the temperature to cause a fever?

21. Time Course of Fever

22. Heat Stroke
• Heat stroke occurs when body temperature rises above
106-108F
- Malfunction of preoptic temp control center - sweating
ceases
- Rising body temperature increases metabolism (*Q10
effect) which generates more heat (vicious cycle)
• Ice bath and ice cold enemas, alcohol wipe down
- Body temperature can go too low
- Body temperature must be externally controlled
*Q10 is a measure of the temperature sensitivity of an enzymatic reaction rate or a
physiological process due to an increase by 10°C
Discontinuities are indicating physiological perturbations

23. References
1. Guyton & Hall. Textbook of Medical Physiology, 11th
Edition, 2005
2. John E. Hall. Textbook of Medical Physiology, 13th Edition,
2016
3. Kim E. Barrett et al. Ganong's Review of Medical Physiology,
26th Edition, 2019
4. Hutchinson’s Clinical Methods. 24th Edition.