Human physiology under cold exposure

In order to minimize heat loss cold stress induces peripheral vasoconstriction via the sympathetic nervous system. This effect is most pronounced in the extremities. Vasoconstriction does not appear in the head-neck region--a fact of great importance in emergency situations. In order to compensate for heat loss shivering is an early event, where involuntary muscle contractions increase metabolic rate 2-6 fold. Early tachycardia and elevated blood-pressure, followed by progressive bradycardia and lowered pressure are common cardiovascular effects of hypothermia. Death due to ventricular fibrillation or asystole occurs between 28 degrees-25 degrees C. Cold stress causes an osmolal diuresis with sodium and chloride as the main constituents. The natriuresis is of tubular origin and could be due to impaired autoregulation in the kidney and/or depend on the natriuretic polypeptide. The augmented urine flow decreases blood volume, lowers physical working capacity and increases blood viscosity--all negative events in a hazardous situation. Sudden immersion initiates hyperventilation for 1-2 minutes with an increasing risk of drowning. Thereafter ventilation decreases to rates consistent with metabolic requirements. In severe hypothermia carbon dioxide retention causes respiratory and metabolic acidosis. Hypothermia induces progressive depression of mental functions starting with apathy and bizarre behaviour and ending in lethargy and coma often between 30 degrees-28 degrees C. The paradoxal feeling of heat with undressing in agony could depend on cerebral receptor disturbances.