Physiology · Temperature, exercise and metabolism
Hypothermia and cold injury !
- Mechanism
- Exam priority: medium
If the body loses heat faster than it makes it, core temperature falls below 35 °C and everything slows down.
Updated 2026-11-01
Also called: Frostbite, Trench foot
See it in 3D: Skin
What it is
Hypothermia is a core temperature <35 °C. At first the body shivers and constricts vessels; as it cools further shivering stops and metabolism, heart and brain slow down. Local cold injury (frostbite) damages tissue through ice crystals and ischemia. In Colombia it happens in the páramos and high mountains, after immersion, and in older or homeless people.
How it works, step by step
- Heat lost by conduction (water: ~25 times more than air), convection and radiation exceeds production.
- The hypothalamus constricts skin vessels and triggers shivering; core temperature is defended at first.
- Below ≈32 °C shivering fails and metabolism falls ≈6–7% per °C.
- Cardiac conduction and the CNS slow: bradycardia, confusion and then coma.
- In frostbite, extracellular water freezes and dehydrates cells, and reperfusion causes thrombosis and edema.
Stages by core temperature
| Parameter | Value | Note |
|---|---|---|
| Mild | 32–35 °C | vigorous shivering, tachycardia, clumsiness |
| Moderate | 28–32 °C | shivering stops, confusion, bradycardia |
| Severe | <28 °C | coma, fixed pupils, risk of ventricular fibrillation |
Approximate values for a healthy adult at rest; each laboratory sets its own ranges.
Clinical notes
- Standard thermometers do not read below ~34 °C: core measurement is needed (esophageal, rectal or bladder).
- Raynaud phenomenon: exaggerated digital vasospasm in the cold (white, blue, red); it is not systemic hypothermia.
- Severe hypothyroidism (myxedema coma) can present with hypothermia.
- In frostbite, rewarming and then refreezing tissue worsens the damage.
Test yourself
Think of the answer, then tap the question to see it.
Why does a hypothermic patient stop shivering as they get worse?
Below ≈32 °C the hypothalamus and muscle slow down and fuel runs out; shivering stops and temperature falls faster.
Why do people with hypothermia pass a lot of urine?
Peripheral vasoconstriction shifts blood centrally; volume receptors cut ADH and the cold tubule responds less: cold diuresis.
Why does immersion in cold water cool far faster than air at the same temperature?
Water conducts heat about 25 times better and holds more heat; convection in moving water removes even more.
See it in 3D
The structures of Durgical's 3D anatomy where it happens.
Related topics
Related conditions
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: Hypothermia: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Frostbite: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); 24.6 Energy and Heat Balance (OpenStax (Rice University), 2022); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 74). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).
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Representative model, for educational purposes only.