Regulation of body temperature

From Iusmphysiology

• started here on 04/08/11.

Contents 1 Regulation of body temperature 1.1 Objectives 1.2 The range of body temperature 1.3 Heat production and loss 1.3.1 Metabolism as a heat producing process 1.4 Body temperature measurements 1.5 Heat transfer 1.6 Evaporative heat loss 1.7 Heat dissipation 1.8 Active regulation of heat transfer 1.9 Active regulation of body temperature 1.10 Pathologic temperature regulation 1.10.1 Temperature elevation in fever versus exercise 1.11 Environmental temperature acclimatization 1.12 Measuring BMR

Regulation of body temperature

Objectives

• The student will be able to describe the balance between metabolic heat production and heat loss.
• The student will be able to describe the 5 mechanisms whereby body heat is lost.
• The student will be able to describe how heat is transferred to the environment.
• The student will understand the factors that affect body temperature and how the body regulates temperature in narrow limits, despite these factors.
• The student will be able to describe how acclimatization to environmental temperature is achieved.
• The student will be able to describe metabolic rate and list the factors affecting it.

The range of body temperature

• At rest, the normal rectal temperature range is 36-38 C.
• During exercise the normal rectal temperature is 38-41 C.
• Heatstroke and brain lesions cause rectal temperatures 41-44 C.
• Temperature regulation is said to be impaired when temperatures are below 36 or above 41.

• Different areas of the body are maintained at different temperatures.
  • Warmest -> coolest: rectum, head, trunk, skin, hands, feet.

• Body temperature fluctuates, following a circadian (per day) rhythm.
  • Note that hyper or hypo thyroidism shifts the curve up or down, respectively, but does not change the rhythm of fluctuation.

Heat production and loss

• There are a handful of variables that effect the production and loss of heat.

• Production: basal metabolism, muscular activity, thyroxine effects, epinephrine effects, temperature effects.
• Loss: radiation, evaporation, conduction-convection

• When production is elevated over loss, one becomes hyperthermic; conversely, when loss is elevated over production, one becomes hypothermic.

Metabolism as a heat producing process

• Energy exists in food as chemical energy.
• Our convert whole food into a pool of metabolic chemical energy, then as chemical energy within cells, and then as work.
  • At each step, heat is generated.
  • Work comes in several forms: osmotic, mechanical, electrical, chemical.
• The human body uses about 20% of the chemical energy in food for work.
• Note that a Calorie (big C) is the same as a kcal; that is, one Calorie is 1000 calories.
• We define metabolic rate as the amount of food energy converted per unit time.
  • Metabolic rate could also be defined as the total daily energy expenditure.

• There are many factors that affect the metabolic rate (total daily energy expenditure): exercise, thermogenic effect of food, hormones, body size, age, gender, disease, and ambient temperature.
  • Exercise: the more exercise, the more calories needed each day; can range up to 4800 cal / day for hard physical labor.

**Thermogenic effect of food: we generate heat by processing food so 

• • Hormones: catecholamines and thyroid hormones can be used to regulate metabolism
    • Catecholamines increases glycogen breakdown
    • Thyroid hormones increase heat by increasing metabolism of glucose and fat.
  • Body size: the metabolic rate is proportional to the surface area of the animal due to loss of heat at the surface area.
    • Large animals generally have slower metabolisms because they lower surface-area-to-volume ratios and proportionally less heat loss at the surface.
  • Age / gender: younger and males generally have higher metabolisms
    • Younger pts are synthesizing more products so must have higher metabolisms.

***We don't know what the subcutaneous fat in females is all about.

• • Disease: metabolic rate is often increased to inhibit infections
    • A new metabolic rate is set and thus a fever occurs.
  • Ambient temperature: cold environments can lead to elevated metabolic rate
    • A new set point is set by elevating the thyroid hormones.

Body temperature measurements

• The body is said to have a homeothermic core that is kept a constant temperture.
• The body also has a poikolothermic shell; poikilothermic means "changes with the environment".
  • The poikilothermic shell does vary with the environment yet is maintained by the core source of heat.
• We measure body temperature in three ways: the core temperature, the mean skin temperature, and the mean body temperature.
  • The mean body temperature is a composite of the core temperature and the mean skin temperature.

• The core body temperature:
  • Core body temperature is normally 37.5 C.
  • Core body temperature can be measured rectally (RT), orally, or axillary-ly.
  • The core temperature has a normal range and is measurement dependent.

What does "measurement dependent" mean?

• The mean skin temperature (MSC):
  • The mean skin temperature is a composite of four separate measurements: chest, arm, thigh, and calf.
  • The MST has a normal variance and is dependent on the muscle mass beneath the location of measurement.
  • The equation for calculating the MSC combines the four measurements with coefficients.
  • MST = 0.3 (chest + arm) + 0.2 (thigh + calf)
  • MST = 0.3 (32 + 36) + 0.2 (34 + 31)
  • MST = 20 + 13
  • MST = 33C

• The mean body temperature (MBT):
  • The mean body temperature is a compsite of the core body temperature and the mean skin temperature (MSC).
  • MBT = 0.3 (MST) + 0.7 (RT)
  • MBT = 0.3 (33) + 0.7 (37.5)
  • MBT = 35.9

Heat transfer

• Heat transfers from the core to the poikilothermic shell via convection much more effectively than conduction.
  • The circulatory system moves heat from the core to the shell via convection.
  • Note that conduction is not efficient through tissue.
    • The only exception is the vasculature which is actually a very good conductor.

Evaporative heat loss

• We lose heat through evaporation: both sensible (sweating) and insensible.
  • Sensible evaporative heat loss (sweating) can lose up to 1500 mL / hour in extreme conditions.
  • Insensible evaporative heat loss generally costs about 600 mL / day.
    • Note that diffusion provides the constant source of water at the skin surface to be evaporated off.
• It takes about 0.58 Kcal of energy to cause each gram of H20 to evaporate.

Heat dissipation

• Recall that the metabolic rate (M) is the total production of energy from intake (diet).
• We can define the "rate of heat storage (S)" as the metabolic rate (M) minus all the sources of heat loss.
• Source of heat loss include radiative heat loss (R), convective heat loss (C), and evaporative heat loss (R).
• So we say: S = M - R - C - E

What about conduction? and aren't evaporative and convective some what dependent?

Active regulation of heat transfer

• Recall that the skin has cold and warm receptor fibers.
• Feedback by these receptors can help the brain regulate heat transfer.
  • Warm fibers fire more and more rapidly beginning at 30 C (and ending at 46ish).
  • Cold fibers fire more and more rapidly beginning at 43 C (and decreasing again at 27ish).

Active regulation of body temperature

• Peripheral and central thermoreceptors provide information to the CNS which integrates the information at the hypothalamus.
  • The feedback provided by the thermoreceptors is negative feedback.

• Peripheral and central thermoreceptors:
  • Peripheral cold and warm thermoreceptors increase their firing rate upon appropriate sensation.
    • We have 10 times as many cold receptors as we do warmth receptors.
  • Central core cold and warm receptors are found in the viscera and send input to the brain.
  • The central and peripheral thermoreceptors report back to a single controller.

• CNS integration of feedback:
  • The hypothalamus is the main controller and maintains a "metabolic set point" and "skin set point".
    • The hypothalamus sends efferent signals to the skin, sweat glands, and muscle upon integration of afferent signals from central and peripheral thermoreceptors.
  • The CNS responds to an increase in "warm" feedback: vasodilation and sweating.
    • This is called the antirise response.
  • The CNS responds to an increase in "cold" feedback: vasoconstriction and shivering.
    • This is called the antidrop response.

Pathologic temperature regulation

• Fever:
  • Fever occurs as a mechanism to fight infections.
  • Pyrogens released by the immune system can cause an elevation in the hypothalamic set point.
    • Pyrogens can include cytokines or bacterial products.
  • Once the pyrogen is gone, the set point returns to normal.

See what he says on slide 15.

• Hyperthermia:
  • Hyperthermia is an upregulated temperature increase; that is, an abnormal storage of heat in the body core.
  • Hyperthermia is most often caused by prolonged exposure to high temperature combined with high humidity.
    • Note that humidity increases the effect of heat on the body because the body is less able to lose heat by evaporation (because the vapor pressure is high so water doesn't want to leave the body surface).
  • Heatstroke is defined as the "breakdown of heat loss capabilities".
  • Heat exhaustion is defined as "hypotension due to excessive loss of fluid via sweating"; this is an example of overactive heat loss.

• Hypothermia:
  • Hypothermia is defined as "unregulated temperature decrease".
  • In hypothermia, heat production and conservation capabilities are exceeded by environmental cold.

Temperature elevation in fever versus exercise

• Note that though both fever and exercise see an elevation of body temperature, in exercise the set point has not changed so there is a constant signaling saying "hey! it's too hot in here".

What else is the point on slide 16?

Environmental temperature acclimatization

• The body is able to acclimatize to heat by assuming a lower basal rectal temperature, a lower heart rate, and a larger sweat response.
• The body is able to acclimatize to cold by increasing the basal metabolic rate, increasing tissue insulation, and producing a larger cold-induced vasodilation response.
  • Recall that BMR (basal metabolic rate) is higher for both genders early in life when the body is growing.
  • Recall that BMR is higher for men.

Measuring BMR

• BMR can be measured by direct or indirect calorimetry.
• Direct calorimetry:
  • Direct calorimetry should take place under standard conditions: pt has fasted for 12 hours, early morning test (to measure lowest daily metabolic rate), room should be 25C, pt should be given 30-60 minutes of rest before the test.
  • The pt is then put in a small, well-insulated room.
  • Water is moved through a radiator in the room and measured before and after exposure to the room.
  • Knowing that 1 kcal heats 1 gram of H20 1 C, one can calculate how many kcal of heat the pt is generating.
  • Direct calorimetry results can be compared between separate individuals.
• Indirect calorimetry
  • Indirect calorimetry uses oxygen consumption to derive the metabolic rate.
  • Recall from biochemistry that carbs, fat, and protein all generate a certain amount of energy per oxygen molecule (because they all end up generating energy for the ETC which burns oxygen).
    • Carbs generate 5.0 kcal / L of O2.
    • Fats generate 4.7 kcal / L of O2.
    • Protein generates 4.6 kcal / L of O2.
    • On average, "food" generates 4.825 kcal / L of O2.
  • Indirect calorimetry requires a pt to breath a closed volume of oxygen over several minutes.
  • Measurements of the oxygen concentration are taken over time and thus rate of oxygen consumption can be generated.
  • With a rate of oxygen consumption (volume) and the knowledge that the pt's input generates 4.825 kcal / L of O2 we can calculate the metabolic rate.
  • MR = (rate of oxygen consumption * 4.825 kcal / L) / meter²
  • MR = *L of O2 / Hr * 4.825 kcal / L) / m²
  • MR = 40 kcal / m² / hr

• stopped here on 04/08/11.