Body temperature and anaesthesia.

The velocity of enzyme catalysed reactions increases with temperature to a maximum beyond which enzyme activity decreases because of protein denaturation. Many biological reactions have a 10 °C temperature coefficient (Qi0) of 2; that is, their rate of reaction doubles for a 10 °C temperature increase [62]. In mammals, protein denaturation begins at about 42 °C and internal temperatures at or above this value can be tolerated only briefly. At the other end of the scale, ice crystals form in mammalian cells at — 1 °C and physical disruption of the cells occurs [82]. In intact animals, vital organ function becomes critically impaired at internal temperatures much higher than freezing; for example, consciousness is lost in man at 30 °C and death often occurs at temperatures less than 25 °C as a result of ventricular fibrillation. Accurate maintenance of the internal temperature at a value close to the optimum for enzymatic activity bestows many benefits, including a constant high rate of metabolism, rapid nervous conduction and muscular contraction, decreased viscosity of blood and, perhaps most importantly, freedom from the constraints of environmental conditions. Homeothermy Homeothermy is denned by The Thermal Physiology Commission of the International Union of Physiological Sciences as "a pattern of temperature regulation in which the cyclic variation in core temperature, either nychthermally or seasonally, is maintained within arbitrary limits of ± 2 °C despite much larger variations in ambient temperature" [25]. Only mammals and birds conform to this definition; however, all other vertebrates exhibit some degree of thermal regulation, mediated usually by behavioural responses [20]. Homeothermy involves sensing body temperature and appropriately driving the mechanisms controlling heat loss and gain so as to maintain a normal value for temperature. Thus thermoregulation is a closed-loop system.