Physiology · Temperature, exercise and metabolism
Heat exhaustion and heat stroke !
- Mechanism
- Exam priority: medium
In great heat and effort the body loses water and salt; if it can no longer cool down, temperature climbs out of control and damages organs.
Updated 2026-11-01
Also called: Hyperthermia, Heat illness
What it is
Heat illness ranges from cramps to heat stroke. In heat exhaustion the control mechanisms work but drain circulating volume; in heat stroke they fail and core temperature exceeds ≈40 °C with nervous system dysfunction. It is a real risk on Colombia's Caribbean coast, in the Llanos and the Magdalena valley, above all for outdoor workers and athletes.
How it works, step by step
- Environmental and muscle heat load the body; the hypothalamus dilates skin vessels and turns on sweating.
- Sweat (up to 1–2 L/h) removes water and sodium: plasma volume and preload fall.
- To hold blood pressure, cardiac output cannot supply skin and muscle at once: skin flow falls and cooling capacity is lost.
- Core temperature rises; above ≈40–42 °C proteins and membranes are directly damaged.
- Gut ischemia lets endotoxin through; cytokines and endothelial injury lead to multi-organ failure.
Starling forces along a capillary
Arteriolar NFP +4 mmHgVenular NFP −13 mmHgFiltration 0.5Reabsorption 5.0Net absorption 4.5 a.u.
Dehydration: the blood is concentrated (↑ πc) and Pc falls: absorption wins and fluid moves from the interstitium into the plasma (the same happens after a hemorrhage).
Tap a force (Pc, πc, Pif, πif), the lymph or the net pressure value.
- Filtration (fluid out)
- Absorption (fluid in)
- Net filtration pressure
- Normal net pressure
- Lymphatic (returns the excess)
- Edema
The spectrum of heat illness
| Condition | Core temperature | Mind | Mechanism |
|---|---|---|---|
| Heat cramps | Normal | Normal | Sodium lost in sweat |
| Heat syncope | Normal or slightly high | Brief loss of consciousness | Skin vasodilation and venous pooling |
| Heat exhaustion | <40 °C | Normal | Hypovolemia from sweating |
| Heat stroke | >40 °C | Altered | Failed thermoregulation, cell damage |
Clinical notes
- Antipyretics do not work: the set point is normal; temperature falls only with physical cooling.
- Drinking large volumes of plain water during long efforts can cause hyponatremia.
- Acclimatization (10–14 days) expands plasma volume and makes sweating earlier and less salty.
Test yourself
Think of the answer, then tap the question to see it.
Which clinical finding separates heat exhaustion from heat stroke?
Nervous system dysfunction (confusion, seizures, coma) with a core temperature >40 °C defines heat stroke.
Why does dehydration impair the ability to cool down?
With less plasma volume the heart cannot perfuse skin and muscle together; compensatory skin vasoconstriction cuts heat loss and sweating.
Why does a soldier with exertional heat stroke in the Llanos pass dark urine and have a high creatinine?
Rhabdomyolysis releases myoglobin, which injures the renal tubule, and hypovolemia lowers renal perfusion: acute kidney injury.
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: Heat emergencies: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); 24.6 Energy and Heat Balance (OpenStax (Rice University), 2022); Dehydration: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); 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.