The term “normothermic” describes the state where the human body maintains a normal core temperature. This balanced thermal state is a fundamental aspect of human physiology, allowing the body’s complex systems to function as intended. Without this precise regulation, cellular processes and organ systems would struggle to perform their necessary roles.
The Normothermic Range
A common belief suggests that 98.6°F (37°C) represents the single normal human body temperature. However, this is a misconception; normal body temperature actually falls within a narrow range, typically 36.5–37.5 °C (97.7–99.5 °F) for core measurements.
An individual’s temperature fluctuates throughout the day, influenced by their circadian rhythm. Body temperature is lowest in the early morning, around 4 a.m., and peaks in the late afternoon or early evening, between 4 and 6 p.m. These daily changes can be around 0.5 to 1.0 °C (0.9 to 1.8 °F). Factors like age, physical activity, and the measurement site (oral, rectal, or axillary) can also cause slight differences in readings.
The Body’s Thermostat
The body possesses a system called thermoregulation that maintains this stable internal temperature. At the center of this system is the hypothalamus, a small region in the brain that acts much like a thermostat. This control center receives signals from temperature-sensitive nerve endings, known as thermoreceptors, located both on the skin and deep within the body’s core.
When the body needs to cool down, the hypothalamus initiates responses. Blood vessels near the skin surface widen, a process called vasodilation, which increases blood flow to the skin and allows excess heat to radiate away. Sweat glands also activate, releasing moisture onto the skin. As this sweat evaporates, it draws heat away from the body, providing a cooling effect.
Conversely, when the body detects a drop in temperature, the hypothalamus triggers mechanisms to generate and conserve heat. Blood vessels under the skin narrow through vasoconstriction, reducing blood flow to the surface and minimizing heat loss. Muscles may also shiver involuntarily, generating heat through rapid contractions. Non-shivering thermogenesis involving brown fat tissue and hormonal influences from the thyroid gland also contribute to heat production.
Deviations from Normothermia
Despite the body’s thermoregulatory system, it can be overwhelmed, leading to deviations from normothermia. Two conditions arise from such imbalances: hypothermia and hyperthermia. Hypothermia occurs when the core body temperature drops below 35°C (95°F), indicating that the body is losing heat faster than it can produce it.
This condition results from prolonged exposure to cold environments, immersion in cold water, or being in wet and windy conditions without adequate protection. Symptoms can include shivering, confusion, and weakness, progressing to severe states if not addressed. In contrast, hyperthermia describes an uncontrolled rise in body temperature, exceeding 37.8°C (100°F), or 40°C (104°F) in severe cases like heatstroke.
Hyperthermia results from prolonged exposure to high environmental temperatures, high humidity, or intense physical exertion. It is distinct from a fever, which is a regulated increase in the body’s temperature set point, as an immune response to fight infection or inflammation. During hyperthermia, the body’s internal thermostat remains set at a normal temperature, but its cooling mechanisms fail to cope with the external heat load or internal heat production.
Medical Significance of Maintaining Normothermia
Maintaining normothermia is important in medical settings, especially during surgical procedures. The practice of perioperative normothermia involves keeping a patient’s body temperature within the normal range before, during, and after surgery. This practice directly counters inadvertent perioperative hypothermia (IPH), defined as a core temperature dropping below 36°C (96.8°F).
Inadvertent hypothermia during surgery poses risks to patient recovery and outcomes. Even a mild temperature drop can increase the likelihood of surgical site infections, as reduced blood flow and impaired immune function compromise the body’s defenses. It can also lead to increased blood loss and a greater need for blood transfusions due to negative effects on blood clotting.
IPH can also delay a patient’s recovery from anesthesia, prolong their post-operative recovery, and increase the length of their hospital stay. To mitigate these risks, medical teams employ strategies, including prewarming patients before anesthesia, using warming blankets during surgery, and administering warmed intravenous fluids. These interventions help ensure optimal conditions for healing and recovery.