Blood vessels widen when the body becomes too warm. This automatic physiological response is called vasodilation, meaning the widening of blood vessels. Vasodilation is the primary mechanism the body uses to dissipate excess heat and maintain a stable internal temperature, a state known as thermoregulation or homeostasis. This widening allows for an increase in blood flow near the skin’s surface, preparing the body to shed heat into the surrounding environment.
The Body’s Cooling Strategy
When the core body temperature begins to rise, the circulatory system shifts into a heat-dumping mode. The widening of peripheral blood vessels, particularly the small arterioles and capillaries beneath the skin, increases blood flow to these areas. This increased flow acts like a built-in radiator, delivering heat energy from the warmer core of the body to the cooler outer surface.
The skin serves as the boundary layer where heat exchange with the environment occurs. Once the warm blood arrives at the surface, heat transfers away from the body through several physical processes. Radiation is the transfer of heat in the form of infrared waves, while convection is the transfer of heat to moving air passing over the skin. Vasodilation enhances this heat loss by creating a warmer skin temperature, increasing the thermal gradient between the skin and the air.
This mechanism works most effectively in conjunction with sweating, though the two are distinct processes. The increased blood flow provides the necessary heat to convert liquid sweat into vapor, which is the most effective form of cooling through evaporation. Without sufficient blood flow to the skin, the heat required for sweat evaporation would not be readily available, making the cooling process less efficient.
How Thermal Signals Control Blood Flow
The entire process of vasodilation is orchestrated by the brain, which acts as the body’s thermostat. Temperature sensors located throughout the body, including the skin and core, constantly monitor thermal conditions. These sensors relay information to the hypothalamus, a region of the brain that serves as the central control center for thermoregulation.
When the hypothalamus detects that the core temperature has exceeded the optimal set point, it activates the heat-dissipating mechanisms. The anterior region of the hypothalamus, known as the heat loss center, sends signals to the cardiovascular system. This signaling is managed through the autonomic nervous system, which controls involuntary body functions.
The signal for vasodilation is primarily achieved not by activating a “vasodilator” nerve, but by reducing the activity of sympathetic nerves that usually keep peripheral blood vessels slightly constricted. The smooth muscle tissue surrounding the small arteries and arterioles is held in a semi-constricted state by a constant background signal from the sympathetic nervous system. When the hypothalamus reduces this nerve traffic, the smooth muscles relax, causing the blood vessels to widen and blood flow to surge to the skin.
This relaxation of the muscle tissue, called vasorelaxation, is often chemically mediated by substances released from the endothelial cells lining the vessels, such as nitric oxide. The combination of reduced sympathetic nerve signaling and the action of local chemical messengers ensures a rapid widening of the vessels. The result is an increase in skin blood flow, which can rise up to seven to eight liters per minute during heat stress, significantly increasing the body’s capacity to shed heat.
Vasodilation Versus Vasoconstriction
The body’s circulatory system is designed for dynamic temperature control, relying on two opposite processes to maintain a constant core temperature. Vasodilation is the response to warmth, while its opposite, vasoconstriction, is the response to cold.
Vasoconstriction involves the narrowing of the peripheral blood vessels, serving the opposite purpose of vasodilation. When the body is cold, the hypothalamus increases sympathetic nerve activity to the skin vessels, causing the surrounding smooth muscles to contract. This contraction reduces the vessel diameter, decreasing the amount of warm blood flowing to the skin’s surface.
By shunting blood away from the periphery and toward the internal organs, vasoconstriction reduces heat loss to the environment. This action conserves the body’s heat and helps maintain the temperature of the core. The pale appearance of skin in a cold environment is a visible sign of this process, contrasting with the flushed, reddened skin associated with vasodilation when the body is hot.
Limits of the System
While vasodilation is an effective cooling mechanism, it comes with trade-offs and limitations. When the peripheral blood vessels widen, the total volume of the circulatory system increases, causing blood pressure to drop. The heart must work harder and faster to maintain adequate blood pressure and ensure blood continues to circulate to the brain and other organs.
This drop in blood pressure, known as hypotension, can become problematic if vasodilation is too extensive or prolonged. In extreme heat or intense exercise, the body’s demand to cool itself can conflict with the need to maintain blood pressure. This potentially leads to symptoms like dizziness or fainting, medically termed syncope. The body attempts to compensate by reducing blood flow to less essential areas like the digestive tract, but this is not always enough.
If the environmental temperature is too high, or humidity prevents sweat from evaporating, the vasodilation response can be overwhelmed. When the body cannot dissipate heat fast enough, the core temperature continues to climb, leading to heat exhaustion. Untreated, this can progress to heat stroke, a condition where the core temperature rises above 40 degrees Celsius and the thermoregulatory system begins to fail. Warning signs that the system is reaching its limit include confusion, aggressive behavior, an increased heart rate, and an eventual lack of sweating.