# Cold Physiology > Cold physiology for MBBS Physiology: shell-core partitioning, shivering and brown fat, hunting reaction, hypothermia ECG and rewarming care. - Canonical URL: https://prepelephant.com/topics/mbbs/physiology/cold-physiology - Exam / course: MBBS · Subject: Physiology - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Cold Physiology", PrepElephant, https://prepelephant.com/topics/mbbs/physiology/cold-physiology ## Direct answer Shivering can multiply metabolic heat production up to about four or five times basal within minutes, and it is only the louder half of the cold defence: non-shivering thermogenesis in brown adipose tissue, driven by sympathetic noradrenaline on beta-3 receptors and uncoupling protein 1, adds up to roughly double basal production in neonates (who barely shiver) and a smaller but measurable fraction in adults, whose brown fat is detectable on positron-emission scanning. Cold exposure first sacrifices the shell — cutaneous vasoconstriction mediated by sympathetic noradrenaline thickens the insulating shell of cool tissue around a defended core — and when even that fails, core temperature drifts down: mild hypothermia at 32-35°C with maximal shivering and tachycardia, moderate at 28-32°C with shivering ceasing around 30°C, bradycardia and the Osborn J wave, and severe below 28°C with a myocardium poised to fibrillate. Fingers in cold water at 5-10°C illustrate the cold-induced vasodilatation or Lewis hunting reaction — cyclic flushing that protects against frostbite — and whole-body cold produces cold diuresis as central volume shifts suppress vasopressin. ## What you must remember - **Shell and core:** the body defends a core of about 37°C while tolerating large shell temperature gradients; vasoconstriction can thicken the shell several-fold, quantified as the insulating tissue cylinder in thermal physiology. - **Shivering ceiling:** asynchronous muscle contractions raising metabolic rate up to about four to five times basal, with no external work done — but it fatigues, and it consumes glycogen and oxygen. - **Non-shivering thermogenesis:** brown adipose tissue uncouples oxidation from phosphorylation through UCP1 (thermogenin), leaking protons to generate heat; abundant in the neonate (interscapular, nape, mediastinal, perirenal depots) and activated in adults by cold. - **Cold-induced vasodilatation:** periodic reopening of digital vessels in near-freezing water (the hunting reaction), alternating with vasoconstriction in 5-10 minute cycles — protective in the cold-immersion worker, absent in the frostbitten. - **Hypothermia staging with numbers:** 32-35°C shivering, tachycardia; 28-32°C shivering stops near 30°C, pupils dilate, J waves appear; below 28°C ventricular fibrillation risk peaks and pulse may be imperceptible — check temperature and a long rhythm strip before calling death. - **Cold diuresis and immersion:** peripheral vasoconstriction shifts blood centrally, natriuresis and diuresis follow with relative vasopressin suppression; cold water immersion additionally triggers the diving reflex and rapidly strips heat (water conducts heat about 25 times faster than air). - **Therapeutic hypothermia:** targeted temperature management at 32-36°C after cardiac arrest improves neurological outcome — physiology turned into protocol, with shivering controlled by sedation. ## A mountain rescue, worked through Trekkers carry down a colleague found confused at 3,900 metres; his rectal temperature reads 30.8°C and the monitor shows a sinus bradycardia of 34 with positive Osborn waves. Stage the physiology before the protocol: he is moderately hypothermic, no longer shivering, so internal heat generation has stalled and must be replaced externally — remove wet clothing, insulate, warm humidified oxygen and warmed intravenous fluids, handling gently because the cold myocardium is irritable and rough movement has triggered fibrillation in case series. If he arrests, resuscitate until rewarming or clear futility, since cold brains tolerate ischaemia far better than warm ones — nobody is dead until warm and dead. Contrast trench foot: hours of wet cold above freezing caused vasoconstriction and hunting-reaction cycles that failed, ending in endothelial injury without ice crystals — managed by rapid rewarming in 37-39°C water, never by rubbing. ## Where students slip First, "alcohol warms you" is the folk claim the physiology exam dismantles: ethanol dilates cutaneous vessels, accelerating heat loss while the drinker feels warmer — a trap question with a one-line answer. Second, students place shivering as the neonate's defence; neonates shiver poorly and rely on brown fat, which is why cold stress in a newborn shows as hypoglycaemia, acidosis and oxygen consumption doubled or tripled rather than shivering. Third, the J wave is attributed to hypocalcaemia or ischaemia; in the cold context it is the Osborn wave of hypothermia, appearing usually below 32°C. Finally, the temperature hierarchy is muddled: the hypothalamic (core) temperature governs the response, and skin cold receptors drive the initial vasoconstriction — so a patient can be hypothermic while feeling warm, the classical exam vignette of the elderly person found indoors after a stroke. ## Frequently asked questions ### How does shivering generate heat? Rapid asynchronous muscle contraction converts metabolic energy almost entirely to heat, raising production up to four to five times basal — external movement prevented so no work is done. ### What is non-shivering thermogenesis and where does it occur? Heat production by uncoupling protein 1 in brown adipose tissue, activated by sympathetic noradrenaline; dominant in neonates and detectable in adult supraclavicular fat. ### What is the Lewis hunting reaction? Cyclic cold-induced vasodilatation in fingers exposed to near-freezing water, periodically rewarming tissue to prevent frostbite. ### Which ECG sign suggests hypothermia below about 32°C? The Osborn J wave, a positive deflection at the junction of the QRS complex and ST segment, alongside bradycardia and conduction slowing. ### Why must hypothermic patients be handled gently? The cold myocardium is electrically irritable below roughly 28-30°C, and mechanical stimulation can precipitate ventricular fibrillation that is very difficult to convert until rewarming.