Temperature Regulation

Your body is like a finely tuned engine — and like all engines, it works best within a narrow temperature range. Normal core body temperature is about 37°C (98.6°F), though it varies slightly throughout the day (lower in the morning, higher in the afternoon) and between individuals. Why does temperature matter so much? Because almost everything that keeps you alive depends on enzymes, and enzymes are exquisitely sensitive to temperature. At 37°C, human enzymes work at or near their optimum rate. A few degrees above or below, and they start to malfunction. - At 40°C (104°F): enzyme activity becomes erratic. You feel very unwell. Brain function is affected. - At 41–42°C: serious risk of brain damage. Heat stroke. - At 43°C+: proteins begin to denature (unfold permanently). Fatal. - At 35°C (hypothermia): enzyme activity slows dramatically. Heart rhythm becomes unstable. - At 28–30°C: the heart may stop (ventricular fibrillation). Humans are endotherms (warm-blooded): We generate our own body heat through metabolism. Every chemical reaction in your cells releases some heat as a byproduct. Your basal metabolic rate (BMR) — the energy your body uses at rest just to keep you alive — generates a significant amount of heat continuously. The challenge is not generating heat — it is balancing heat generation against heat loss to maintain a stable temperature despite changes in the environment (hot summer, cold winter) and activity level (sleeping vs running).

The body constantly gains and loses heat through physical processes. Understanding these helps explain how to treat heat stroke and hypothermia — and why certain situations are dangerous. How the body gains heat: - Metabolism — the biggest source. Every cell produces heat as a byproduct of chemical reactions. Muscle activity amplifies this enormously — vigorous exercise can increase heat production 10–20 times above resting rate. - Radiation from a warm environment (e.g. direct sunlight) - Conduction from hot surfaces in contact with the skin - Convection from hot air currents How the body loses heat: - Radiation — heat radiates from warm skin to the cooler environment (no contact needed). Accounts for about 60% of heat loss at rest in a cool room. - Conduction — heat passes from the body to cooler objects in contact with it. Cold water conducts heat away ~25 times faster than air at the same temperature — why cold water immersion causes hypothermia far faster than cold air. - Convection — cool air moving over the skin carries heat away. Wind increases convective heat loss significantly (the "wind chill" effect). - Evaporation — sweat evaporating from the skin surface. The most powerful active cooling mechanism. Each gram of sweat that evaporates carries away about 2.4 kJ of heat. A person exercising in heat can sweat 1–2 litres per hour. Important: evaporation only works in relatively dry air — in humid conditions, sweat cannot evaporate → cooling is inefficient → heat stroke risk rises dramatically. Why wet and windy conditions cause hypothermia faster: Wet clothing conducts heat away from the body much faster than dry clothing. Add wind (increased convection) and the body loses heat faster than it can generate it — even at temperatures well above freezing. Hikers caught in summer rain in the mountains can develop hypothermia in minutes.

A fever is different from heat stroke. In heat stroke, the body loses control of its temperature — the cooling mechanisms are overwhelmed. In a fever, the hypothalamus deliberately raises its set point to a higher temperature. This is a controlled, intentional response. Why the hypothalamus raises its set point: When pathogens (bacteria, viruses) infect the body, the immune system responds by releasing chemical signals called pyrogens (from the Greek for "fire makers"). The most important pyrogens are cytokines — particularly interleukin-1 (IL-1), interleukin-6 (IL-6), and TNF-alpha, produced by macrophages. These cytokines travel in the blood to the hypothalamus and act on it to raise the set point. The actual mechanism: Pyrogens stimulate the hypothalamus to produce prostaglandin E2 (PGE2). PGE2 is the direct chemical that tells the hypothalamus to raise its set point. This is why paracetamol and NSAIDs (like ibuprofen) reduce fever — they inhibit the enzymes (COX enzymes) that make prostaglandins, including PGE2. What happens when the set point rises: The body behaves as if it is cold (even though it is warmer than normal) until it reaches the new set point: - Shivering and vasoconstriction — this is why you feel cold and shiver at the start of a fever, even though your temperature is rising. You feel "chills." This is the body actively generating heat to reach the new, higher set point. - Once the new set point is reached, sweating and vasodilation take over to maintain it. - When the infection is cleared and pyrogens fall, the set point returns to 37°C → you feel hot and sweat heavily as your body now cools back down. Is fever helpful or harmful? Moderate fever (up to about 39°C) appears to be beneficial — it speeds up immune cell activity, makes the environment less hospitable for many bacteria, and may slow viral replication. Very high fevers (above 40–41°C) are dangerous — especially for children, who can have febrile convulsions (seizures triggered by rapid temperature rise).

Heat Stroke: When the body temperature rises above 40°C and the cooling mechanisms are overwhelmed (or stop working), heat stroke occurs. Unlike heat exhaustion (where the person is still sweating and compensating), classic heat stroke involves: - Core temperature above 40°C - Absence of sweating (in classic heat stroke) — the cooling system has failed - Altered mental state — confusion, slurred speech, seizures - This is a life-threatening emergency Treatment: Rapid cooling by any means available — cool water immersion is the most effective. Remove clothing. Ice packs to the armpits, groin, and neck (areas where large blood vessels are close to the surface). Call emergency services immediately. Risk factors: Very old or very young people are most vulnerable (impaired thermoregulation). Heavy exercise in hot, humid conditions. Certain drugs that impair sweating (anticholinergics, some antipsychotics). Dehydration. Hypothermia: When core body temperature falls below 35°C, hypothermia occurs. Classified as mild (32–35°C), moderate (28–32°C), or severe (below 28°C). - Mild: Shivering, confusion, impaired coordination, pale skin - Moderate: Shivering stops (a dangerous sign — the body has run out of energy to shiver), severe confusion, muscle rigidity - Severe: Unconscious, very slow pulse, risk of cardiac arrest Treatment: Gentle rewarming — active external rewarming (warm blankets, heating pads) for mild cases. Severe hypothermia requires internal rewarming (warmed IV fluids, warmed humidified oxygen). Handle patients gently — cold hearts are electrically unstable and jarring can trigger fatal arrhythmias. Therapeutic hypothermia: Deliberately lowering body temperature (to 32–36°C) is used medically after cardiac arrest — cooling the brain reduces metabolic demand and can prevent brain damage during recovery. A remarkable example of medicine using the body's physiology for treatment.