Final answer:
The hypothalamus is triggered to raise the set point in pyrexia, or fever, when chemicals called endogenous pyrogens are released into the blood. These pyrogens circulate to the hypothalamus and reset the thermostat, increasing the body's temperature as a defense mechanism against bacterial infections.
Step-by-step explanation:
The hypothalamus maintains the set point for body temperature through reflexes that cause vasodilation and sweating when the body is too warm, or vasoconstriction and shivering when the body is too cold. When a bacterium is destroyed by phagocytic leukocytes, chemicals called endogenous pyrogens are released into the blood. These pyrogens circulate to the hypothalamus and reset the thermostat. This allows the body's temperature to increase in what is commonly called a fever. An increase in body temperature causes iron to be conserved, which reduces a nutrient needed by bacteria. An increase in body heat also increases the activity of the animal's enzymes and protective cells while inhibiting the enzymes and activity of the invading microorganisms. Finally, heat itself may also kill the pathogen.
A fever that was once thought to be a complication of an infection is now understood to be a normal defense mechanism. In pyrexia or fever, the chemical triggers that cause the hypothalamus to raise the body's temperature set point are endogenous pyrogens. These substances are typically released into the bloodstream when phagocytic leukocytes destroy bacteria. The pyrogens then circulate to the hypothalamus and reset the thermostat, leading to an increased body temperature. This elevation in temperature serves as a defense mechanism by conserving iron, enhancing the activity of protective cells and enzymes, and may directly inhibit or kill pathogens.