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Body temperature is the balance between heat produced by the body and heat lost to the external environment. The average adult body temperature ranges from 97°F to 99°F. The hypothalamus, a brain structure, maintains this balance through thermoregulation by detecting temperature changes and activating heat production or loss mechanisms to keep core temperature constant despite environmental conditions.

When temperature rises, the anterior hypothalamus stimulates the sympathetic nervous system to open arteriovenous shunts—tiny connections between arterioles and venules beneath the skin. This causes vasodilation, allowing blood to flow to the skin surface where heat escapes through evaporation, radiation, convection, and conduction, effectively cooling the body.

When the posterior hypothalamus detects temperature below the setpoint, it activates heat production through metabolism, thyroid hormones, piloerection (goosebumps), shivering, and exercise. These mechanisms generate heat to maintain core body temperature. Simultaneously, vasoconstriction narrows blood vessels, reducing blood flow to skin and extremities to minimize heat loss.

Core body temperature, measured in deep tissues beneath the skin, typically ranges from 97.0°F to 99.5°F and remains relatively constant. Surface body temperature varies depending on blood circulation to the skin and heat dissipation. Core temperature is measured at tympanic or rectal locations using infrared thermometers, while surface temperature is measured orally, axillary, or on skin using thermometers or thermal imaging.

Heat is produced by metabolic activities in core tissues and transmitted to the skin's surface by circulating blood. At the skin, heat transfers to the environment through radiation, conduction, convection, and evaporation. Arteriovenous shunts directly under the skin can open to allow heat escape or close to retain heat, making blood circulation essential for temperature regulation.

The sympathetic nervous system responds to hypothalamic signals to regulate blood vessel diameter and heat production. When the anterior hypothalamus detects rising temperature, it stimulates the sympathetic nervous system to open arteriovenous shunts and promote vasodilation for heat loss. Conversely, when the posterior hypothalamus detects falling temperature, it signals vasoconstriction to conserve heat.

The body maintains constant core temperature through complex physiological mechanisms controlled by the hypothalamus, which detects even minor temperature variations. When environmental conditions change or physical exertion increases heat production, the anterior and posterior hypothalamus activate appropriate heat loss or conservation mechanisms, ensuring core temperature stability for optimal cellular function.