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Why is body temperature homeostasis critical?
The majority of the key chemical reactions within the body occur at optimal rates when then take place at body temperature.
What are normal body temperatures and what factors affect them?
- Normal is 37.0 C or 98.6 F
- Sex - women are colder then men
- Colder in the morning than in the afternoon
- women - luteal phase/contraceptives are warmer than follicular phase and men
What is the relationship between total metabolism (energy consumption), efficiency, and portion lost as heat.
Of the total energy we consume, only about 20% of it is converted into useful work, making us 20% efficient. It also means that 80% of our energy is converted to heat.
What are the components of total metabolism?
- Metabolism - Heat Gain
- Evaporation - Heat Loss
- Convection - Gain/Lose Heat
- Conduction - Gain/Lose Heat
- Radiation - Gain/Lose Heat
How do age and gender affect resting metabolic rate?
- Rate decreases with age in both sexes
- Males tend to have a greater RMR than females
What is the difference between shivering and non-shivering thermogenesis?
- Shivering thermogenesis causes a maximum of 3-10 times RMR and is sustainable for ~ 2 hours
- Non-shivering thermogenesis ("futile cycling") utilizes uncoupling proteins which alllow for protons to leak from the intermembrane space to the mitochondrial matrix, which releases teh energy stored in the gradient as heat.
How do uncoupling proteins work in general?
- uncoupling proteins alllow for protons to leak from the
- intermembrane space to the mitochondrial matrix, which releases the energy stored in the gradient as heat.
Where is UCP1 found and what does it do?
- It is found only in mitochondria of brown adipose tissue (BAT)
- Allows for non-shivering thermogenesis in hibernating mammals and infants.
- They decrease the proton gradient generated in oxidative phosphorylation. They do this by increasing the permeability of the inner mitochondrial membrane, allowing protons that have been pumped into the intermembrane space to return to the mitochondrial matrix.
Where is UCP2 found and what is its link to reactive oxygen species and type II diabetes?
- UCP2 is found in BAT, WAT, the heart, and muscle
- It acts to remove Reactive oxygen species
- Polymorphism causing higher UCP2 expression in beta cells leads to reduced insulin production in response to glucose, twofold increase in risk of type II diabetes
What is the difference between convection and conduction?
- Convection is the heat transfer by the movement of fluid molecules - dependant on:
- 1. The temperature difference between the air close to the skin and the surrounding air
- 2. the surface area of skin exposed to air
- 3. the convective heat coefficient hc
- Conduction is the exchange of heat between surfaces in contact - dependant on:
- 1. The temperature difference between the skin and the surface
- 2. the surface area of skin touching the surface
- 3. the conductive heat transfer coefficient hk
what is the difference between long wavelength (terrestrial) and short-wavelength (solar) radiation?
- Amount of long wavelength radiation we gain or lose depends on the difference between the temperature of our skin and them temperature of our surroundings, the surface area exposed to our surroundings and the radiative coefficient.
- The amount of solar radiation we absorb depends on how strong the radiation is and how much skin surface area is exposed to it.
What is specific heat capacity? How is it affected by body fat composition? what is its effect on heat storage?
- The heat required to raise the temperature of a substance one degree Celsius
- Higher fat composition leads to lower specific heat capacity.
- Larger SHC leads to smaller temperature changes (less fat -> smaller temperature changes)
What layer of the skin are innervated by thermoreceptors and what types of fibers are those thermoreceptors?
- Thermooreceptors mainly ly within the dermis and epidermis
- A-Gamma fibers- myelinated, large, faster
- C fibers - unmyelinated, small, slower
How are thermoTRP channels involved in perception of temperature?
- TRP channels are large, diverse family of cation channels
- TPRV1 (Trancient receptor potential Vanilloid receptor 1) detects painfully hot temperatures (>43C)
- A series of TRp channels with divergent response characteristics may be responsible for our ability to measure a wide range of temperatures
What evidence suggests that keratinocytes might behave as specialized temperature receptor cells
- 1. can secrete bioactive substances - neurotropins, ATP, Beta-endorphin, interleukins
- 2. TRPV3 and TRPV4 more detectable in keratinocytes than in sensory neurons
- 3. TRPV1 present. Capsaicin can trigger interleukin release
What are the two ways that vasodilation/constriction in the periphery is controlled?
- 1. Norepinephrine is tonically released with more release causing peripheral vasoconstriciton adn less release causing vasodilation
What sweat glands are primarily used for thermoregulation?
Eccrine sweat glands - particularly dense in acral regions (hands and feet)
How is shivering controlled by the hypothalamus?
cold temperatures release the inhibition of the dorsomedial posterior hypothalamus by the POAH
What is hypohydrotic ectodermal dysplasia?
- x-linked recessive condition
- They have fewer and/or non-functioning sweat glands
How might age, exercise, drugs, and lesions affect thermoregulation?
- If you exercise more, you have increased heat tolerance andincreased sweat sensitivity
- As you age, you have reduced sweat sensitivity
- Drugs like anticholinergics and alcohol can strongly effect thermoregulation
- lesions to the POAH can lead to chronic hypo- and hyperthermia
What is the difference between fever and exercise in terms of our model of how the hypothalamus works to regulate temperature?
- Heat stroke from exercise overwhelms body thermoregulation and the Tcore increases above 41 C and hypothalamus ceases to function
- Fever causes the Tcore to increase as a result in the change in Temperature set point.
what is the mechanism by which sustained exercise can lead to ischemic injury to the gut?
Exercise diverts blood flow from the gut and internal organs to the muscles being used. sustained exercise can lead to an ischemic injury (lack of blood flow) to the gut when too much of the blood is being diverted and the intestines are not receiving the blood required to function properly
What mechanisms to humans have for adapting to heat and cold? How do they work in light of our model of hypothalamic control of temperature and biophysics of heat exchange?
- Heat Acclimatization -
- There temperature and heart rate do not change as much as a result of exercise whereas their sweat glands are more active
- Sweat gland activity reduces in response to their dehydration
- Release less sodium in their sweat
- Cold Acclimatization -
- Have a lower core temperature threshold for shivering and may have a lower shell conductance: reduced blood flow to the periphery
- Cold-induced vasodilation occurs earlier, with higher blood flow, and rhythmic patterns of vasodilation and vasoconstriction (lewis response)
- Have lower thresholds for peripheral vasodilation
Why is regulation of [Na+] released during sweating physiologically important?
- Amount of sodium in your sweat determines where the water in your sweat comes from:
- High sodium means water is lost primarily from the extracellular fluid
- Low sodium means water is lost primarily from the intracelluar fluid
- Low Na in sweat maintains plasma volume which leads to cardiovascular effects
Describe the stages of hypothermia. How do the effects of ambient temperature vary with falling core temperature?
- Mild - Tcore 32-35 C (sluggishness, shivering)
- Moderate - Tcore 28-32 C (confusion, speech disterbances
- Severe - Tcore <28 C Loss of consciousness
- You become less sensitive to changes in ambient temperature as a response to decreased core temperature
how does alcohol affect thermoregulation?
- Alcohol reduces Tset and the precision of thermoregulation by :
- depressing shivering
- increasing peripheral vasodilation
- sedative effects on CNS