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Saturday, June 21, 2008

Physiology of Fever

source:Physiology text book for 2nd year,Mansoura University

What is fever?
Fever is elevation of body temperature due to a resetting of hypothalamic thermostat.

What is hypothalamic thermostat?

It is the main regulator of body temperature. It is located in the hypothalamus. It adjust core temperature at set point (36.6 c -37.2 c) by controlling heat loss and heat production.

A person with a fever still regulates body temperature in respose to heat or cold but at higher set point (above 37.2 c).

What cause fever?

The most common cause of fever is infection, but physical trauma and stress can also induce fever.

Pathophysiology of thermostat resetting

Chemical messengers collectively called endogenous pyrogens (EP) are release from macrophages as well as other type of cell in the presence of infection or other fever producing stimuli.

EP circulates in the blood and reaches the brain to act upon the thermoreceptors in the hypothalamus thus altering their input to integrating center.This resets the thermostat at a new higher level.The immediate mechanism of the resetting is thr local synthesis and release of prostaglandins within the hypothalamus.

How aspirin act on reducing fever?

Aspirin always taken to reduce fever.Aspirin reduces fever by inhibiting prostaglandins synthesis thus re-resetting of the hypothalamic thermostat into its normal set point.

Body Temperature

Normal body temperature is 36.6 c to 37.2 c with absolute limit of life 20 c - 42 c.

How to measure body temperature?

Body temperature is measured by measuring body core temperature by one of three method.
  1. Oral method - by measuring temperature through the mouth.This method is simple and accurate.In some condition, oral method is prevented (eg : newly born & young children, during convulsion, during coma and in mouth breathers).
  2. Axillary method - by measuring temperature through axilla.It is used when nasal passages are blocked as in common cold and in unconsiousness.Axilla temperature is less than oral temperature by 0.5 c.
  3. Rectal method - by measuring body temperature through rectum.It is used in infants and comatosed patients.Is is higher than oral temperature by 0.5 c.

What are benefit of fever?

  • It increases production of antibodies.
  • It prevents growth of certain microorganism
  • It prevents growth of certai tumours