How the Body Regulates Temperature

Body temperature regulation — thermoregulation — is one of the most vital homeostatic functions in the human body. The enzymes that drive every metabolic reaction in your cells operate optimally within a narrow temperature range: roughly 36.5–37.5°C (97.7–99.5°F). Even small sustained deviations from this range impair cellular function; large deviations are rapidly lethal. Understanding how the body maintains this precise thermal equilibrium — and what happens when it fails — is essential for understanding fever, heat illness, hypothermia, and general physiology.

Body Temperature: Basics

Core temperature vs. shell temperature: "Body temperature" refers to core temperature — the temperature of the deep body compartment (brain, thoracic and abdominal organs). Core temperature is tightly regulated. The shell (skin and extremities) has a much wider and more variable temperature range, serving as the body's thermal buffer and exchange surface.

Normal core temperature: Approximately 36.5–37.5°C (97.7–99.5°F) when measured orally. Rectal temperature is approximately 0.5°C higher than oral; axillary temperature is approximately 0.5°C lower. Core temperature varies naturally by approximately 0.5°C throughout the day (lowest in early morning, highest in late afternoon) — the circadian thermal rhythm, driven by the suprachiasmatic nucleus.