Medical brain notes 44

  Normal Body Temperatures Core Temperature and Skin Temperature.  The tempera-ture of the deep tissues of the body—the “core” of the body—remains very constant, within ±1°F (±0.6°C), day in and day out, except when a person develops a febrile illness. Indeed, a nude person can be exposed to temperatures as low as 55°F or as high as 130°F in  dry  air and still maintain an almost constant core temperature. The mechanisms for regulating body temperature represent a beautifully designed control system.             The  skin temperature,  in contrast to the  core temperature,  rises and falls with the temperature of the surroundings. The skin temperature is the im-portant temperature when we refer to the skin’s ability to lose heat to the surroundings. Normal Core Temperature.  No single core temperature can be considered normal,because measurements in many healthy people have shown a  ...

  Body Temperature Is Controlled by Balancing Heat Production Against Heat Loss When the rate of heat production in the body is greater than the rate at which heat is being lost, heat builds up in the body and the body temperature rises. Conversely, when heat loss is greater, both body heat and body temperature decrease. Heat Production Heat production is a principal by-product of metabolism. We discuss the different factors that determine the rate of heat production, called the  metabolic rate of the body.  The most impor-tant of these factors are listed again here: (1) basal rate of metabolism of all the cells of the body; (2) extra rate of metabolism caused by muscle activity, including muscle contractions caused by shivering; (3) extra metabolism caused by the effect of thyroxine (and, to a less extent, other hormones, such as growth hormone and testosterone) on the cells; (4) extra metabolism caused by the effect of epinephrine, nor-epinephrine, and sympathetic stimu...

  Regulation of Body Temperature-Role of the Hypothalamus Figure 73–6 shows what happens to the body “core” temperature of a nude person after a few hours’ expo-sure to  dry  air ranging from 30° to 160°F. The precise dimensions of this curve depend on the wind move-ment of the air, the amount of moisture in the air, and even the nature of the surroundings. In general, a nude person in dry air between 55° and 130°F is capable of maintaining a normal body core temperature some-where between 97° and 100°F. The temperature of the body is regulated almost entirely by nervous feedback mechanisms, and almost all these operate through temperature-regulatingcenters  located in the  hypothalamus.  For these feed-back mechanisms to operate, there must also be temperature detectors to determine when the body temperature becomes either too high or too low. Role of the Anterior Hypothalamic-Preoptic Area in Thermostatic Detection of Temperature Experiments have been per...

  Neuronal Effector Mechanisms That Decrease or Increase Body Temperature When the hypothalamic temperature centers detect that the body temperature is either too high or too low, they institute appropriate temperature-decreasing or temperature-increasing procedures. The reader is probably familiar with most of these from personal experience, but special features are the following. Temperature-Decreasing Mechanisms When the Body Is Too Hot The temperature control system uses three important mechanisms to reduce body heat when the body tem-perature becomes too great: 1.   Vasodilation of skin blood vessels.  In almost allareas of the body, the skin blood vessels become intensely dilated. This is caused by inhibition of the sympathetic centers in the posterior hypothalamus that cause vasoconstriction. Full vasodilation can increase the rate of heat transfer to the skin as much as eightfold.   2.   Sweating.  The effect of increased bodytemperature to cause sw...

  Concept of a “Set-Point” for Temperature Control In the example of Figure 73–7, it is clear that at a crit-ical body core temperature of about 37.1°C (98.8∞F), drastic changes occur in the rates of both heat loss and heat production. At temperatures above this level, the rate of heat loss is greater than that of heat produc-tion, so the body temperature falls and approaches the 37.1°C level. At temperatures below this level, the rate of heat production is greater than that of heat loss, so the body temperature rises and again approaches the 37.1°C level. This crucial temperature level is called the “set-point” of the temperature control mechanism. That is, all the temperature control mechanisms con-tinually attempt to bring the body temperature back to this set-point level. Feedback Gain for Body Temperature Control.  Let us recall the discussion of feedback gain of control systems. Feedback gain is a measure of the effectiveness of a control system. In the case of body temp...

  Behavioral Control of Body Temperature Aside from the subconscious mechanisms for body temperature control, the body has another temperature-control mechanism that is even more potent. This is  behavioral control of temperature,  which  can be explained as follows:   Whenever the internal body temperature becomes too high, signals from the temperature-controlling areas in the brain give the person a psychic sensation of being overheated. Con-versely, whenever the body becomes too cold, signals from the skin and probably also from some deep body receptors elicit the feeling of cold discomfort. There-fore, the person makes appropriate environmental adjustments to re-establish comfort, such as moving into a heated room or wearing well-insulated clothing in freezing weather. This is a much more powerful system of body temperature control than most physiologists have acknowledged in the past. Indeed, this is the only really effective mechanism to prevent body heat ...

  Abnormalities of BodyTemperature Regulation Fever Fever, which means a body temperature above the usual range of normal, can be caused by abnormalities in the brain itself or by toxic substances that affect the temperature-regulating centers. Some causes of fever(and also of subnormal body temperatures) are pre-sented in Figure 73–10.They include bacterial diseases,  brain tumors, and environmental conditions that may terminate in heatstroke. Resetting the Hypothalamic Temperature- Regulating Center in Febrile Diseases— Effect of Pyrogens Many proteins, breakdown products of proteins, and certain other substances, especially lipopolysaccharide toxins released from bacterial cell membranes, can cause the set-point of the hypothalamic thermostat to rise. Substances that cause this effect are called  pyro-gens.  Pyrogens released from toxic bacteria or thosereleased from degenerating body tissues cause fever during disease conditions. When the set-point of the hypotha...