Feeling unwell often brings with it a perplexing sensation: a deep chill that seems to penetrate the body, even when a fever is present. This common experience of feeling cold when sick can be confusing, as one might expect warmth to dominate during an elevated body temperature. The body’s intricate systems, however, orchestrate a series of responses designed to combat illness, and this sensation of cold is a direct outcome of these internal adjustments.
How Your Body Regulates Temperature
The human body maintains a remarkably stable internal temperature, typically around 98.6 degrees Fahrenheit (37 degrees Celsius), through a complex regulatory system. A region in the brain, the hypothalamus, functions like a precise thermostat, continuously monitoring and adjusting the body’s heat balance. It orchestrates processes that either generate heat or release it, ensuring the core temperature remains within a narrow, healthy range. For instance, metabolic activities within cells naturally produce heat, while mechanisms like sweating help dissipate excess warmth. This constant regulation is fundamental to maintaining bodily functions, ensuring enzymes and cellular processes operate efficiently across various external conditions.
The Fever Response
When the body encounters invading pathogens, such as bacteria or viruses, the immune system initiates a coordinated defense. Immune cells, recognizing these foreign invaders, release specific chemical messengers known as pyrogens into the bloodstream. These pyrogens, which include substances like interleukin-1, interleukin-6, and tumor necrosis factor-alpha, travel through the circulatory system to the brain. Upon reaching the hypothalamus, these molecules bind to receptors, effectively signaling the brain’s temperature control center to reset its internal thermostat to a higher temperature. This new set point might be several degrees above the normal average of 98.6 degrees Fahrenheit.
This elevation of the body’s internal thermostat is the onset of a fever, a deliberate physiological response to create an environment less favorable for pathogen replication and to enhance certain immune functions. The body, now perceiving itself as “too cold” relative to this new, elevated set point, begins actively working to increase its internal temperature. It interprets the discrepancy between its current temperature and the new target as a deficit, prompting actions to generate and conserve heat. This drive to reach the higher temperature set point is the underlying reason for the sensation of cold and chills often experienced at the beginning of a fever.
Physiological Responses to a Rising Temperature Set Point
Once the hypothalamus has reset its temperature target, the body initiates several physiological responses to reach this new, higher temperature. One significant mechanism is vasoconstriction, where the small blood vessels near the surface of the skin narrow considerably. This constriction reduces blood flow to the skin, thereby minimizing heat loss from the body’s surface to the external environment through processes like convection and radiation. As a result, the skin may feel cool to the touch, and the reduction in warmth reaching the extremities contributes directly to the subjective feeling of being cold, even as the core temperature starts to rise. This redirection of blood flow helps to conserve heat internally, ensuring warmth is retained where it is most needed for organ function.
Simultaneously, the body activates mechanisms to generate more internal heat to meet the new set point. Shivering is a prominent example of this, involving rapid, involuntary contractions and relaxations of skeletal muscles. These muscle movements are highly metabolically active, burning energy and producing a significant amount of heat as a byproduct of muscle activity, effectively warming the body from within. The sensation of shivering, along with the feeling of cold, acts as the body’s signal that it has not yet reached its desired internal temperature. Both vasoconstriction and shivering are coordinated efforts to reach the new, elevated set point established by the fever response.