Thermoregulation, the body’s process for maintaining a stable internal temperature, is fundamentally controlled by the hypothalamus. This small region in the brain acts as the body’s master thermostat. It keeps the core temperature within a narrow range necessary for proper metabolic function by constantly monitoring and adjusting heat production and heat loss.
The Body’s Central Temperature Regulator
The hypothalamus is a small collection of nuclei located deep within the brain, just above the brainstem. Although it is about the size of an almond, this structure functions as the primary control center, integrating signals from the nervous system and the endocrine system. The preoptic area is the main thermoregulatory center, where the body’s temperature is compared against a target value, or set point. The hypothalamus receives sensory input and coordinates autonomic responses that dictate whether the body conserves or dissipates heat.
Sensing Temperature and Setting the Point
The body maintains its core temperature around a specific set point, typically near 37°C (98.6°F). The hypothalamus monitors temperature through two distinct sets of sensors called thermoreceptors. Central thermoreceptors are positioned within the hypothalamus, spinal cord, and major organs, directly measuring the temperature of the circulating blood and deep tissues.
Peripheral thermoreceptors are situated mainly in the skin, providing the hypothalamus with information about the external environment. This allows the regulatory center to anticipate potential changes in the body’s thermal load. The hypothalamus integrates the signals from both central and peripheral sensors to determine if the core temperature deviates from the established set point, initiating responses to restore thermal balance.
The Body’s Cooling and Heating Responses
When the core temperature rises above the set point, the hypothalamus triggers mechanisms designed to increase heat loss. One primary action is vasodilation, where the blood vessels near the skin’s surface widen. This shunts warm blood toward the periphery, allowing heat to radiate away from the body. Simultaneously, the hypothalamus activates the sympathetic cholinergic fibers that innervate the eccrine sweat glands.
The secretion of sweat onto the skin’s surface provides effective cooling through evaporation. As the water changes from a liquid to a gas, it draws heat energy away from the body, leading to a reduction in core temperature. Conversely, if the core temperature begins to fall below the set point, the hypothalamus initiates heat-conserving responses.
To conserve heat, the sympathetic nervous system causes vasoconstriction, narrowing the blood vessels in the skin. This reduces blood flow to the body’s surface, minimizing heat loss. The body also generates heat through thermogenesis, most noticeably through shivering, which involves rapid, involuntary muscle contractions. Non-shivering thermogenesis increases the metabolic rate, particularly in organs like the liver and brown adipose tissue, to boost internal heat production.
When the Set Point Changes
The most common situation where the body’s thermostat is intentionally altered is during a fever. A fever is a regulated response where the temperature set point is temporarily elevated, not a malfunction of the hypothalamus. This change is caused by pyrogens, substances released by immune cells during an infection.
These pyrogens induce the synthesis of Prostaglandin E2 (PGE2) in the hypothalamus. PGE2 acts on thermoregulatory neurons, effectively “resetting” the thermostat to a higher temperature. The body then perceives its current temperature as too low and initiates heating responses like shivering and vasoconstriction to reach the new set point. Once the pyrogens are cleared, the set point returns to normal, and cooling mechanisms are activated to bring the temperature back down.