Body temperature is normally tightly regulated but is less well-controlled in the very young, and impaired by age, disease, injury and alcohol/drugs.
As the body cools, the metabolic rate falls and neural transmission is inhibited [4]. Multiple derangements occur including:
Depressed myocardial contractility
Leftwards shift of the oxygen dissociation curve (leading to reduced availability of oxygen at the tissue level)
Vasoconstriction
Ventilation-perfusion mismatch
Increased blood viscosity
All of these reduce tissue oxygenation [5].