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Lecture Notes

Thermoregulation 

  1. Process of thermoregulation
    1. reflects the ability of an organism to balance heat production and heat loss from the body
    2. types of body temperature: core body and surface
      1. core body temperature (CBT)
        1. temperature of the deep tissues of the body
          1. e.g., the cranium and thoracic, abdominal, and pelvic cavities
        2. fluctuates very little in healthy adults
          1. critical range, or set point, is from 36.7° C (98° F) to 37° C (98.6° F)
      2. surface temperature (ST)
        1. temperature of the surface of the body
          1. e.g., skin, subcutaneous tissues, and fat
        2. fluctuates widely in healthy adults
          1. ranges from 20° C (68° F) to 40° C (104° F)
    3. normal ranges of body temperature
      1. infant
        1. 36.1 - 37.7° C (97 - 100° F)
      2. child
        1. 37 - 37.6° C (98.6 - 99.6° ) F
      3. adult
        1. 37 - 37.6° C (98.6 - 99.6° )
      4. older adult
        1. 36 - 36.9° C (96.9 - 98.3° F)
    4. control of body temperature occurs through balancing heat production and heat loss
      1. methods of heat production by the body
        1. physiologic mechanisms (mediated by the sympathetic portion of the autonomic nervous system [ANS])
          1. metabolism
          2. muscular activity
          3. increased sympathetic nervous stimulation
          4. increased thyroxine secretion
          5. fever
          6. increased appetite
        2. voluntary mechanisms
          1. adding clothing ("bundling-up")
          2. increasing physical activity ("keep moving")
          3. decreasing the amount of skin surface available for heat loss ("curling up in a ball")
          4. moving to a warmer environment
      2. methods of heat loss from the body
        1. physiologic mechanisms (mediated by the parasympathetic portion of the ANS)
          1. radiation
            1. transfer of heat from one surface to another without contact between the two surfaces, mostly in the form of infrared rays
              1. e.g., 60% of total heat lost from the body of a nude person at room temperature is the result of radiation of heat from the person into the room
          2. conduction
            1. transfer of heat from one surface to another through direct contact between the two surfaces
            2. two types of conduction: to objects or air
              1. conduction to objects
                1. e.g., 3% of total heat lost from the body of a nude person sitting in a chair at room temperature is the result of conduction of heat from the person to the chair
              2. conduction to air
                1. e.g., 15% of total heat lost from the body of a nude person at room temperature is the result of conduction of heat from the person to the air
          3. convection
            1. dissipation of heat by air currents
              1. e.g., 15% of total heat lost from the body of a nude person at room temperature is the result of convection of heat away from the person after it has been conducted from the person to the air
          4. evaporation
            1. dissipation of heat by transformation of water to a gas
              1. e.g., 22% of total heat lost from the body of a nude person at normal room temperature is the result of vaporization of moisture from mucus membranes, the mouth, and/or the skin; called insensible heat loss
          5. decreased appetite
        2. behavioral mechanisms
          1. removing clothing ("dressing for warm weather")
          2. decreasing physical activity ("slow-down")
          3. increasing the amount of skin surface available for heat loss ("stretch out")
          4. moving to a cooler environment
    5. control of body temperature through regulation of heat loss and production occurs as the result of the integrative function of the hypothalamus
      1. depends on three factors
        1. thermal regulators
        2. a central integrator
        3. effectors
      2. thermal regulators
        1. sensory receptors for cold and warmth
        2. two types:
          1. peripheral tissue thermal receptors
            1. located in the skin
            2. send information regarding the external environment to the central integrator
          2. deep body tissue thermal receptors
            1. located in the spinal cord, abdominal viscera, and in an around great veins
            2. send information regarding the internal environment to the central integrator
        3. both types have for more cold than warmth receptors
          1. 10 cold: 1 warmth
          2. more concerned with preventing hypotherma
      3. central integrator
        1. hypothalamus
          1. a structure located in the diencephalon of the brainstem; called the "body’s thermostat"
        2. posterior hypothalamus
          1. receives input from peripheral and deep tissue thermal receptors that the CBT is below the critical set point as a result of:
            1. increased heat production and/or
            2. decreased heat loss
          1. stimulated effectors to return the CBT tot he critical set point by:
            1. increasing heat production and/or
            2. decreasing heat loss
        1. preoptic area of the anterior hypothalamus
          1. receives input from peripheral and deep tissue thermal receptors that the CBT is above the critical set point as a result of:
            1. increased heat production and/or
            2. decreased heat loss
          1. stimulates effectors to return the CBT to the critical set point by:
            1. decreasing heat production and/or
            2. increasing heat loss
      1. effectors
        1. blood vessels
        2. sweat glands
        3. skeletal muscle
  1. Factors influencing thermoregulation
    1. developmental state
      1. e.g., an elderly individual, whose aging process has resulted in loss of subcutaneous tissue and fat and, consequently, the insulating effect of these substances, may experience alterations in thermoregulation
    2. diurnal variations (circadian rhythms)
      1. e.g., a healthy individual, whose entrainment to 24-hour cycles in the environment has resulted in fluctuations of CBT by as much as 1.0° C (1.8° F) throughout the day, may experience alterations in thermoregulation
    3. hormones
      1. e.g., a healthy woman, whose menstrual cycles result in the monthly secretion of a high level of progesterone during ovulation which raises CBT by as much as 0.3° C (0.6° F) to 0.5° C (1.0° F), may experience alterations in thermoregulation
    4. stress
      1. e.g., an individual who is stressed, which results in stimulation of the sympathetic portion of the ANS and, consequently, secretion of epinephrine and norepineprhine which increases basal metabolic rate and heat production, may experience alterations in thermoregulation
    5. environment
      1. e.g., an individual living in the Artic, which has resulted in exposure to a very cold environmental temperatures for an extended period of time, may experience alterations in thermoregulation
      2. e.g., an individual living in the tropics, which has resulted in exposure to a very high environmental temperatures for an extended period of time, may experience alterations in thermoregulation
    6. nervous system impairment
      1. e.g., an individual with quadriplegia, which has resulted in poikliothermia (taking on the ambient environmental temperature as CBT) due to the inability to vasoconstrict, shiver, or sweat, may experience alterations in thermoregulation
      2. e.g., an individual with a head injury, which has resulted in damage to the hypothalamus, may have experience alterations in thermoregulation
    7. genetic
      1. e.g., an individual with a genetic predisposition for malignant hyperthermia, which has resulted in a dangerously elevated temperature upon administration of general anesthetic agents, may experience alterations in thermoregulation
    8. circulatory impairment
      1. e.g., an individual with peripheral vascular disease (PVD), which has resulted in the inability to constrict or dilate blood vessels in the periphery normally, may experience alterations in thermoregulation
    9. integumentary impairment
      1. e.g., an individual who is severely burned, which has resulted in extensive damage to and loss of large areas of skin and blood vessels, may experience alterations in thermoregulation
    10. infection
      1. e.g., an individual who has an infection caused by bacteria, virus, fungi, and/or other microorganisms, which has resulted in the release of endogenous pyrogens, may experience alterations in thermoregulation
    11. exercise
      1. e.g., an individual who exercises vigorously, which has resulted in an increased muscular activity, metabolic rate and heat production, may experience alterations in thermoregulation
    12. altered cognitive states
      1. e.g., an individual who is confused, which has resulted in an inability to interpret the ambient environmental temperature and the need to dress accordingly, may experience alterations in thermoregulation
    13. altered nutrition
      1. e.g., an individual who is extremely thin, which has resulted in extensive loss of subcutaneous tissue and fat and, consequently, the insulating effect of these substances, may experience alterations in thermoregulation
  2. Common alteration in thermoregulation: fever (pyrexia)
    1. definition
      1. the state in which an individual’s CBT is elevated above his/her normal range
      2. severity of fever
        1. low-grade fever
          1. a fever between 37.1° C to 38.2° C (98.8° F to 100.6° F)
        2. high-grade fever
          1. a fever between 38.3° C to 40.4° C (100.9° F to 104.7° F)
        3. hyperpyrexia
          1. a fever over 40.5° C (104.9° F)
    2. types of fever
      1. constant fever
        1. rise in temperature above normal that remains consistently high with little fluctuation
      2. intermittent fever
        1. rises or spikes in temperature above normal at some point during a 24-hour period
          1. usually late-afternoon or evening
      3. remittent fever
        1. rise in temperature that is always above normal during a 24-hour period, but the amount of elevation above normal fluctuates
          1. e.g., more than 2° C (3.6° F)
      4. relapsing fever
        1. rises in temperature lasting for several days, alternating with several days of normal temperature
    3. phases of fever
      1. onset (cold or chill) phase
        1. occurs when the body’s heat-producing mechanisms are attempting to increase the CBT to a higher set point due to a development of a factor which causes a high temperature (e.g., an infection)
        2. clinical signs of the onset (cold or chill) phase include the following:
          1. increased heart rate
          2. increased rate and depth of respiration
          3. shivering due to increased skeletal muscle tension and contractions
          4. pallid, cold skin due to vasoconstriction
          5. complaints of feeling cold
          6. cyanotic nail beds due to vasoconstriction
          7. "gooseflesh" appearance of the skin due to contraction of the arectores pilorium muscles
          8. cessation of sweating
          9. rise in CBT
      2. course (fever) phase
        1. occurs when the body’s heat-producing mechanisms have reached the new, higher set point
        2. clinical signs of the course (fever) phase include the following:
          1. absence of chills
          2. skin that feels warm
          3. feelings of neither hot nor cold
          4. increased pulse and respiratory rates
          5. increased thirst
          6. mild to severe dehydration
          7. simple drowsiness, restlessness, or delirium and convulsions due to irritation of nerve cells
          8. herpetic lesions of the mouth
          9. loss of appetite (if the fever is prolonged)
          10. malaise, muscle weakness, and aching muscles due to protein catabolism
      3. defervescence (flush or crisis) phase
        1. occurs when the body’s heat-loss mechanisms are attempting to decrease the CBT to a new, lower set point due to sudden removal of the cause of the high temperature (e.g., an infection)
        2. clinical signs of the defervescence (flush or crisis) phase include the following:
          1. skin that appears flushed and feels warm
          2. sweating
          3. decreased shivering
          4. possible dehydration
      4. common interventions for fever
        1. administer antipyretics as ordered
          1. decreases set-point to a lower level
        2. administer antibiotics as ordered
          1. destroys microorganisms which have increased set-point to a higher level
        3. administer oxygen
          1. 7% increase in oxygen consumption for each degree in rise of temperature
        4. apply hypothermia blanket
          1. body heat is transferred to the blanket (conduction)
        5. increase caloric intake
          1. increased need for calories due to increased basal metabolic rate
        6. increase cool fluid intake to 2,000 to 3,000 milliliters a day
        7. measure intake and output
        8. maintain prescribed intravenous fluids
        9. reduce physical activity
        10. apply lubricant to dried lips
        11. keep mucus membranes moist
        12. administer a tepid sponge bath to increase heat loss through conduction
        13. administer an alcohol sponge bath to increase heat loss through evaporation
        14. increase air circulation to increase heat loss through convection
        15. provide dry clothing and bed linens to increase heat loss through conduction
  3. Common alteration in thermoregulation: hypothermia
    1. the state in which an individual’s body temperature is reduced below normal range
    2. two types: induced and artificial
      1. induced hypothermia
        1. deliberately lowering of the CBT to a range of 30° C to 32° C (86° F to 89.6° F) to decrease the need for oxygen by the body tissues, metabolic rate, blood loss and, consequently, preserve vital organs during surgery
          1. e.g., cardiac or neurologic surgery
      2. accidental hypothermia
        1. unintentional exposure to a cold environment or immersion in cold water
    3. clinical signs of hypothermia
      1. all cases of hypothermia
        1. reduction of body temperature below normal range
      2. at a CBT of 35° C (95° F):
        1. increased respirations, poor judgment, shivering
      3. at CBT of 35° C to 34° C (95° F to 93.2° F):
        1. bradycardia or tachycardia, myocardial irritability/dysrhythmias, muscle rigidity, shivering, lethargy/confusion, decreased coordination
      4. at CBT of 34° C to 30° C (93.2° F to 86° F):
        1. bradycardia, hypoventilation, generalized rigidity, metabolic acidosis, coma
      5. at CBT of 30° C (86° F):
        1. no apparent vital signs, heart rate unresponsive to drug therapy, coma, cyanotic, dilated pupils, areflexic, no shivering, appearance of death
    4. common interventions for hypothermia
      1. remove the client from the cold
      2. apply blankets
      3. hyperthermia blankets
      4. warmed intravenous solutions
      5. remove wet clothing and keep dry
      6. keep environment warm
      7. apply layers of clothing to trap air between them to act as insulation
      8. warm gradually to prevent vasodilitation which can lead to shock
      9. in severe cases, extracorporeal rewarming through hemodialysis or use of cardiopulmonary bypass may be needed

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This page was last modified on 6/1/01