Regulation of Body Temperature Ass. Prof. Dr. ADEL AMRAN DEPARTMENT OF PHYSIOLOGY
objectives heat gain heat loss discuss the mechanisms by which the body gains or loses heat in a variety of healthy or pathological settings. Why is regulation of body temperature so important? How is body heat produced? How does the body lose heat when heat production is excessive? How does the skin help regulate body temperature? What are the dangers of hypothermia?
the regulation of body temperature is vitally important because even slight shifts can disrupt the rates of metabolic reactions. Normally, the temperature of deeper body parts remains close to a set point of 37 C (98.6 F). The maintenance of a stable temperature requires that the amount of heat the body loses be balanced by the amount it produces. The skin plays a key role in the homeostatic mechanism that regulates body temperature
Humans are endotherms : meaning that they generate their own internal body heat and do not rely on the energy of sunlight to warm the body. Human are homeotherms: maintain their body temperatures within very narrow limits despite wide fluctuations in ambient temperature
normal human body temperature core body temperature: Shell body temperature Blood serves as the major agent of heat exchange between the core and shell the rectum typically has a temperature about 0.4 C (0.7 F) higher than the mouth and is a better indicator of core temperature. Internal temperature is not constant Variation in women is a higher temperature during the second half of the menstrual cycle due to the effects of the hormone progesterone Not all regions of the body, therefore, have the same temperature
Mechanisms of Heat Loss or Gain
Radiation is the loss of heat in the form of infrared waves (thermal energy). Any object that is warmer than objects in its environment for example, a radiator and (usually) the Body will transfer heat to those objects Conduction : the loss or gain of heat; transfers heat from a warmer object to a cooler one when the two are in direct contact with each other. For example, when you step into a hot tub, some of the heat of the water transfers to your skin, and warm buttocks transfer heat to the seat of a chair by conduction. Convection is the process whereby conductive heat loss or gain is aided by movement of the air or water next to the body. Evaporation: of water from the skin and membranes lining the respiratory tract is the other major process causing loss of body heat.
Control of Heat Production Changes in muscle activity constitute the major control of heat production for temperature regulation. Shivering Constriction of cutaneous blood vessels. chronic cold exposure induces an increase in metabolic rate nonshivering thermogenesis increased adrenal secretion of epinephrine, increased thyroid hormone secretion, and increased sympathetic activity to brown adipose Tissue in infant humans
Control of Heat Loss by Radiation and Conduction three behavioral mechanisms for altering heat loss by radiation and conduction are - changes in surface area (Curling up into a ball, hunching the shoulders) - changes in clothing - choice of surroundings (moving from a shady spot into the sunlight)
Control of Heat Loss by Evaporation Even in the absence of sweating, there is loss of water by diffusion through the skin during expiration. Sweating - - Dilation of cutaneous blood vessels - Enhanced sweating
Temperature Acclimatization Temperature acclimatization to heat is achieved by an earlier onset of sweating, an increased volume of sweat, and a decreased salt concentration of the sweat. This adaptation, which minimizes the loss of sodium ions from the body via sweat, is due to increased secretion of the adrenal cortex hormone aldosterone.
Problems in Temperature Regulation The body s temperature-regulating mechanism does not always operate satisfactorily, and the consequences may be dangerous. Because body temperature regulation depends on evaporation of sweat from the skin s surface and because high humidity hinders evaporation, athletes are advised to slow down their activities on hot, humid days. They should also stay out of the sunlight whenever possible and drink enough fluids to avoid dehydration. Such precautions can prevent the symptoms of heat exhaustion, which include fatigue, dizziness, headache, muscle cramps, and nausea.
Hypothermia Hypothermia, or lowered body temperature, can result from prolonged exposure to cold or as part of an illness. Hypothermia begins with shivering, a feeling of coldness but if not treated: progresses to mental confusion, lethargy, loss of reflexes and consciousness, and, eventually, a shutting down of major organs. fatal respiratory failure or heart arrhythmia may result. Hypothermia can be prevented by dressing appropriately and staying active in the cold. A person suffering from hypothermia must be warmed gradually so that respiratory and cardiovascular functioning remain stable.
Fever and Hyperthermia Fever is due to a resetting of the temperature set point so that heat production is increased and heat loss is decreased in order to increase body temperature to the new set point and keep it there. The stimulus is endogenous pyrogen, in the form of interleukin 1 and other peptides. The hyperthermia of exercise is due to the increased heat produced by the muscles, and it is partially offset by skin vasodilation infection cancer allergic reactions CNS injuries.
Aspirin reduces fever by inhibiting this prostaglandin synthesis
Homeostatic Imbalance Clinical Application