The human body maintains its internal temperature within a narrow range, typically centered around 37 degrees Celsius (98.6 degrees Fahrenheit). This thermal window is necessary because cellular processes, particularly those involving enzymes, function optimally only at this specific temperature. The coordinated process of balancing heat production and heat loss to keep the body in this state is called thermoregulation. Even a small deviation can slow down metabolic reactions or cause proteins to lose their functional shape.
The Central Regulator: Setting the Body’s Thermostat
The control center for thermoregulation resides deep within the brain, specifically in the preoptic area of the hypothalamus. This region acts as the body’s thermostat, constantly monitoring and comparing the current temperature to a predetermined “set point.”
The hypothalamus integrates thermal information from two distinct sets of sensors. Central thermoreceptors monitor the temperature of the core organs and the blood flowing through the brain. Peripheral thermoreceptors, located primarily in the skin, sense the external temperature and relay that data back.
This dual-sensing mechanism ensures the body reacts to internal changes and anticipates potential thermal shifts based on the environment. When these signals indicate a deviation from the set point, the hypothalamus sends signals to effector organs, initiating an appropriate response.
Cooling Down: How the Body Dissipates Heat
When the core temperature exceeds the set point, the body increases heat loss through the skin surface. This is primarily achieved through vasodilation, where the smooth muscles in the arterioles near the skin relax, widening the blood vessels.
This increases blood flow to the skin surface. The warmer blood circulating near the surface allows heat to escape into the environment through radiation and convection.
This increased blood flow also supplies the eccrine sweat glands, which are the main effectors of evaporative cooling. These glands secrete fluid onto the skin surface, and the subsequent evaporation carries heat away from the body.
Sweating is the most effective heat-loss mechanism, but high humidity can impair the body’s cooling ability by slowing evaporation. The hypothalamus also sends signals that reduce the overall metabolic rate, slowing down internal heat generation.
Warming Up: Generating and Conserving Core Heat
When the core temperature falls below the set point, the body initiates two strategies: conserving existing heat and generating new heat. The immediate response is vasoconstriction, the opposite of vasodilation, which involves the narrowing of blood vessels near the skin. By constricting these vessels, the body shunts warm blood away from the surface and directs it toward the internal organs, minimizing heat loss to the environment.
The second strategy involves involuntary, rapid muscle contractions known as shivering. This is an effective form of thermogenesis, or heat generation. The mechanical work of these contractions requires the breakdown of adenosine triphosphate (ATP), converting the released energy into heat.
A more subtle mechanism is non-shivering thermogenesis, which increases the metabolic rate of certain tissues. Hormones like epinephrine and thyroid hormones stimulate cells to increase their energy expenditure.
Brown adipose tissue (BAT), particularly in infants, can be activated by the sympathetic nervous system. BAT contains specialized mitochondria that uncouple energy production from ATP synthesis, causing energy to be released directly as heat. Piloerection, causing “goosebumps,” is a vestigial response that is functionally negligible in humans.