Lecture 5: Heat energy and body temperature control

Normal body temperature

37ºC

Specific heat capacity

Amount of energy to increase 1kg of substance by 1 degrees celsius/kelvin

Core Temperature

Temperature of vital organs in thoracic/abdominal cavities and brain

Shell temperature

Temperature of peripheral parts of the body (e.g. skin)

Radiation

Heat transfer through space

Directionality of radiation

Two-directional

How much of body heat is lost by radiation?

60%

Evaporation

Transfer of heat from evaporation of water in lungs and skin

Directionality of evaporation

Unidirectional: Heat loss only

Convection

Heat transfer through the movement of liquid/gas

Directionality of convection

Two-directional

Conduction

Heat transfer between objects in direct contact down temperature gradient

Directionality of conduction

Two-directional

Heat transfer between core and shell

Through cutaneous blood flow (convection)

Vasoconstriction

Cutaneous blood vessels constrict in response to cold → less blood flow to peripheral shell

Vasodilation

Cutaneous blood vessels dilate in response to heat → more blood flow to peripheral shell

Heat transfer between body shell and environment

Radiation, evaporation, convection, conduction

Peripheral thermoreceptors

Monitor temperature in the skin

Central thermoreceptors

In the hypothalamus, abdominal visceral organs, spinal cord, great veins, monitor core temperature

Hypothalamus

The thermoregulatory center in the brain

Body response to heat

Vasodilation of blood vessels near the skin, sweat glands produce sweat that evaporate off skin

Body response to cold

Vasoconstriction of blood vessels near the skin, skeletal muscle contraction → shivering, prevent sweat secretion

Pyrogens

Chemical substances causing fever

Exogenous pyrogens

Bacterial endotoxins (e.g. lipopolysaccharides) and microbial products

Endogenous pyrogens

Pyrogenic cytokines released by immune system

Mechanism of Fever

Pyrogens promote prostaglandin E (PGE2) production in hypothalamus

PGE2

Raises the set-point of hypothalamic temperature-regulating center to a higher level

Purpose of Fever

Elevated body temperature suppresses growth of pathogens

Antipyretic drugs

Non-steroidal anti-inflammatory drugs that lower set-point back to normal

Examples of antipyretic drugs

Acetaminophen, paracetamol