A healthy resting adult sits quietly in a room at 21 degrees C with normal air movement. Which mechanism accounts for the GREATEST proportion of his total heat loss?

• A Evaporation of insensible water
• B Radiation to surrounding objects ✓
• C Conduction to the chair he is sitting on
• D Convection by room air currents

Explanation

At rest in a thermoneutral environment, radiation accounts for about 60 percent of total heat loss, because all objects warmer than absolute zero emit infrared rays and nude skin exchanges heat radiatively with walls and furniture. Evaporation accounts for roughly 20 percent, mostly insensible losses from skin and lungs. Convection contributes about 15 percent and conduction is minimal because air is a poor conductor and contact surfaces are small. Only when ambient temperature approaches skin temperature does evaporation become dominant.

Reference: Guyton and Hall Textbook of Medical Physiology, 14th ed.

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Written and medically reviewed by the StethoPrep medical team.