Temperature Regulation

Annotations:

• Humans are homeothermic. Maintain body temperature at a narrow range due to the need of a stable environment for enzymes to function. Basically, the temperature is stable for most parts especially for the rectal temperature. Oral temperatures are susceptible to fluctuations fue to people drinking different temperature stuff or such.

1. Variations in Core Temperatures

   Annotations:

   • Core temperatures tend to be stable, but it also follows a few fluctuations.
   1. Circadian fluctuations

      Annotations:

      • Day and night cycles, a normal human with normal sleeping patterns will have the coldest core temperature at around 36.2deg.  
   2. Phase of Menstrual Cycles

      Annotations:

      • Women generally have lower temperature range in the follicular phase than the luteal phase.
   3. Age

      Annotations:

      • Children tend to have higher body core temperatures.
   4. Activity

      Annotations:

      • Exercise tends to generate higher body heat.
   5. Emotion/Excitement

      Annotations:

      • Excited people have higher metabolic activity. Hence, generate more heat.
   6. Constitutional Hyperthermia

      Annotations:

      • Adults that have original basal temperature above the normal range.
2. Core and Shell

   Annotations:

   • Body is divided into warm inner core and a cool outer shell.  Skin temperatures varies more than the core temperature. The shell has a variable size depending on the temperature of the environment. Heat produced within the body is generated in the deep organs and skeletal muscles during exercise. It is transferred to the skin t be lost to the surroundings.
   1. Cool Subject

      Annotations:

      • Skin blood floor is low. Convection is low. Core-to-skin transfer dominated by conduction. Conduction is decreased as the shell is large.
   2. Warm Subject

      Annotations:

      • Skin blood flow is high. Convection is increased. Conduction increased as the shell is thin. Fat has little effect on convection.
3. Heat Balance

   Annotations:

   • Body temperature is determined by the balance between heat production and heat loss.
   1. Heat Production

      Annotations:

      • Measures using oxygen consumption. For a basal man 70kg. Each litre of oxygen consumed releases 4.82kcal.
      1. Processes that Contribute to Basal Metabolic Rate (BMR)

         Annotations:

         • Minimum energy required for the body to survive. It is measured under standard condition. Such as, fasting for 12 hours as there are thermic effect on food.  Well rested body Awake and air temperature of 25degrees.
         1. Male vs Female

            Annotations:

            • Male produce more BMR as they have more muscle hence are more  metabolically active. Females on the other hand have more fat and hence do not need to have a higher BMR.
      2. Muscle Contraction

         Annotations:

         • Accounts for 90% of the heat production in exercise.
      3. Thermic Effect of Food

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         • Up to 10 to 30 percent increase in heat production. Obligatory energy expenditure that assimilation of food.
      4. Hormones

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         • Catecholamines: Short term Thyroid Hormones: Long term
      5. Brown Fat

         Annotations:

         • One of the 2 kinds of adipose tissues. Brown fat is important for chemical thermogenesis. Locates at the sternum, nape of neck, and around kidneys. High density of mitochondria hence the fat looks brown. Activated by, cold stress and trigger increase in adrenaline and thyroid stimulating hormone (TSH). TSH generates thyroxine (T4) and adrenaline increases peripheral conversion of T4 to T3
         1. Thermogenesis

            Annotations:

            • T4 activated by adrenaline to T3. T3 up-regulates uncoupling proteins (UCP) or thermogenin. Leads to oxidative phosphorylation. Instead of making ATP it makes heat.
   2. Heat Loss

      Annotations:

      • Note that conduction, convection and radiation can be a route of heat gain. 70% Radiation and Conduction 27% Vaporisation of Sweat 2% Respiration 1% Urine and Faeces
      1. Conduction

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         • Also known as direct heat loss Heat exchange between objects at different temperatures which are in direct contact with each other. Thermal energy is transferred by collision of molecules. Generally small heat loss
      2. Convection

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         • Transport of heat by movement of heated substance such as air or liquid. Greatly increases heat loss by conduction. Heat loss by convection depends on few factors such as the temperature between skin and the ambient fluid.  Area of skin in contact Velocity of the fluid Type of fluid (water/air)
         1. Insulator Zone

            Annotations:

            • Like a blanket or a feather/fur on animals. Bind the air close to the skin. Reduce heat loss to the environment. Zone could be increased by fluffing of feathers or erection of hairs (erector pili)
         2. Water vs Air

            Annotations:

            • Water have higher specific heat capacity. It also have higher conductivity. At moderate temperatures, the difference of heat loss of water is much greater to air in the same temperature. Difference is negligible in the cold.
      3. Radiation

         Annotations:

         • Transfer of heat by infrared EM radiation. All objects not at absolute zero emits infrared rays. Naked person lose about 60% of the total heat loss by radiation.
      4. Evaporation

         Annotations:

         • Vaporization of water occur in the mouth and respiratory tract and skin. 1g of water removes 0.58kcal of heat. Latent heat of evaporation. ONLY mode of heat loss when the surrounding environment is greater than the internal.
         1. Insensible

            Annotations:

            • Unaware Such as breathing in and out (moisture) Water that is transfused thru the skin
         2. Sweating
      5. Urine and Faeces
   3. Equation: M= E + R + C + K + W +S

      Annotations:

      • M: Metabolic rate/heat production E: Evaporation R: Radiation C: Conduction K: Convection (konvection?) W: Heat loss thru work S: Rate of heat storage If it is heat gain, it will present itself as a negative value.
4. Regulation of Body Temperature

   Annotations:

   • Core temp can be maintained for several hours in dry air over a range of 15 to  50 deg Thermoneutral zone (body does best) is 27 - 31 deg.
   1. Set Points

      Annotations:

      • A base line. The body will try to follow the baseline.  Regulatory centre will be in the hypothalamic regulating centres Anterior Hypothalamic Area + preoptic area controls the area that regulate heat. Posterior Hypothalamic Area regulate actions such as shivering.
      1. Afferents
         1. Temperature sensitive cells in hypothalamus

            Annotations:

            • 75% heat sensitive, 25% cold sensitive. Nerves fire more according to the conditions. Main function is to stop hyperthermia. Deals with Core Heat
         2. Deep body temperature receptors

            Annotations:

            • Spinal cord, abdominal viscera and great veins. Mainly detect cold Prevents hypothermia
         3. Cutaneous Temperature Receptors

            Annotations:

            • Skin 10 Times as many cold receptors than warmth. Deals with preventing hypothermia
      2. Efferents
         1. Cold Activated
            1. Increase heat production

               Annotations:

               • Shivering: Controlled by dorsomedial of posterior hypothalamus, send impulses to anterior motor neurones to increase muscle tones. In bursts. 4 -5 fold in heat production Hunger: Thermic effect of food. Increased activity Sympathetic activation and secretion of catecholamines: Thermogenesis of brown fat Thyroxine secretion
            2. Decrease Heat loss

               Annotations:

               • Cutaneous Vasoconstriction Curling up Piloerection
         2. Heat Activated
            1. Increased heat loss

               Annotations:

               • Cutaneous Vasodilation: Inhibits sympathetic centres which cause vasoconstriction.  Sweating: Rate increases sharply after 37 deg Increased Respiration: Like dog panting.
               1. Sweat Glands

                  Annotations:

                  • Flow rate determines how much sodium concentration in sweat. High flow: NaCl not absorbed by body. Hence higher conc. Low flow: Vice versa
            2. Decreased heat production

               Annotations:

               • Anorexia: Stop/decreased intake of food Apathy and Inertia: Lack of motivation to move Inhibition of shivering and chemical thermogenesis
5. Examples
   1. Heat Stroke Victims

      Annotations:

      • Exercise in heat is particularly dangerous as blood goes to exercising muscles and skin. Hence decreased venous return and stroke volume and increased heart rate. Decreased cooling and increased in body temperature. and hence could lead to heat stroke.
      1. Heat Acclimatization

         Annotations:

         • Train in hot environment. Body increased in tolerance to heat. Features: Lowered heart rate, rectal temperature/ core temperature. Shows higher sweating rate. Increased in conc of NaCl. Lowered threshold for cutaneous vasodilation and increased in plasma volume. Lowered urinary sodium losses.
   2. Fever

      Annotations:

      • Body temperature above basal range. Indication of diseases Shivering and vasoconstriction to increase the temperature, then sweating and vasodilation to decrease it.
      1. Pyrogens

         Annotations:

         • Substances that cause the heat changes. Caused by white blood cells creating cytokines at the preoptic area of the hypothalamus, prostaglandins to raise set points. Aspirin inhibits prostaglandins and hence lower fever.