Temperature

Abstract

Abstract Humans are homeothermic, that is, in the face of considerable variation in the temperature of the environment, the core body temperature is kept comparatively constant. The enzyme reactions that preserve homeostasis operate most favourably when body temperature is maintained within a narrow range. Normal core body temperature follows a circadian pattern, varying by 0.5 to 0.7°C, being highest in the evening and lowest at approximately 06:00 h; the morning oral temperature averages at 36.7°C, but in 95% of young adults this value lies between 36.3 and 37.1°C. Therefore, core body temperature values outside this range may be normal. In addition, the luteal phase or latter half of the menstrual cycle is associated with an increase of 0.3 to 0.5°C in basal body temperature. Maintenance of a normal body temperature depends on equilibrium between the production of heat and heat loss. Cell metabolism, muscular activity, and food ingestion are the major sources of heat. Heat is lost through the processes of radiation, conduction, convection, and evaporation. The control centre for temperature regulation is the hypothalamus, which integrates afferent information from peripheral and central thermoreceptors to primarily produce appropriate autonomic and behavioural responses. The principal effectors triggered by cold are vasoconstriction, particularly of arteriovenous shunts in fingers and toes, and shivering. Those triggered by heat are sweating and active cutaneous vasodilatation. These mechanisms maintain tight control of core body temperature around a set point, for example, 37°C. When heat loss exceeds heat production, body temperature falls: when the threshold temperature, say 36.8°C, is breached, vasoconstriction occurs; shivering may not commence though until 35.5°C is reached. The threshold for sweating may typically be 37.1°C. The difference between the thresholds for vasoconstriction and sweating is defined as the inter-threshold range and is approximately 0.2 to 0.4°C.