6 - Thermoregulation Flashcards
What is the simple understanding of Body Heat?
Body Heat = Heat Produced + Heat Transferred
Remember, heat transfer and be positive or negative! Can be used for heating or cooling
How does skin vs internal temperature compare?
Peripheral changes rapidly across a wider range, internal requires recise regulation
What are Diurnal Variations?
Physiological Fluctuations to temperature, minimal in morning peak in late afternoon
What is the range (deg C) for normal body T vs hard exercise?
36-37 deg C, hard ex = 40 deg C
What four factors acount for heat loss?
- Radiation
- Conduction
- Convection
- Evaporation
What factors affect heat production?
Basal Metabolic Rate - Abdominal viscera, heart, brain generate maj. of body heat
Muscular Activity - 75% of heat during exercise
Hormones - Thyroxine has direct effect to increase local metabolism, Epinephrine (SNS) increase metabolism
Thermic Effect of Food - Food consumption increases heat production of body
What is the major mode of acute heat production?
What brain center controls this?
Shivering
Hypothalamus receives cold stimulation
What controls the rate of blood flow at the skin, and how does it play into heat dissipation?
Degree of vasoconstriction of arterioles and arteriovenous anastomoes by the sympathetic nervous system
High Skin Flow = High heat conduction away from body
Low Skin Flow = Low heat conduction from core
What is the majority of head change with the body mechanisms?
Radiation (60%) heat waves
Conduction to Objects vs Conduction to Air
Conduction to Air much more important
“bubble” of personal spare is warmed by your temperature
Convection
Transfer of body heat by movement of air
You “warm” your bubble, and then that air is replace by new air to “warm”
What are two means of Evaporation loss?
Insensible Water Loss (always occuring, breaking, skin evap, etc) - no regulation
Sweating - highly regulated
WHen skin temperature is greater than ambient temperature, how is heat lost?
When ambient temperature is greater than that of skin, how does the body gain heat? How can the body get rid of heat?
Radiation, Conduction
Radiation, Conduction
Evaporation
What innervates sweat glands?
What occurs at the duct?
What occurs at the gland?
ACh secreting Sympathetic Nerves
Duct - Absorption (Na/Cl)
Gland - Primary secretion center
Stimulation of what area causes sweating?
Anterior Hypothalamus-preoptic area
What is the definition for sweat?
What can happen to ions prior to evaporation?
Solution with osmolar concentration below plasma
They can be reabsorbed
What is the normal acclimatization process for sweating?
After 1-6 weeks: Increase volume of sweat glands; increased capability
After 4-6 weeks: Secretion of aldosterone is increased, leading to reduced NaCl in sweat
What are the five components of the thermoregulatory system?
- Thermal Sensors
- Afferent Pathways
- Integration System in CNS
- Efferent Pathways
- Target Organs to Control Transfer / Generation
Where are receptors for body temperature located?
What is the reflex response to cold at the level of the skin?
Skin - Cold (more) / Warm receptors
- Shivering
- Inhibition of Sweating
- Skin vasoconstriction
Deep body temperature - spinal cord, abdominal viscera, around great veins in the upper abdomen
What role does the hypothalamus play in thermal regulation?
Preoptic / Anterior Hypothalamic Nuclei = Thermostatic Body Temperature control center
Contains both heat and cold sensitive neurons
What location serves as the summation center prior to pushing our marching orders to heat production or dissipation?
How does it do this?
Posterior Hypothalamus
It compares sensory information to desired “set point”
What mechanisms does the body employ when body too hot?
Skin Vasodilatione
Sweating
Decrease in Heat Production
Behavior Change - Sprawling, Light Clothing, Seek Shade
What mechanisms does the body employ when too cold?
- Skin Vasoconstriction
- Piloerection
- Increas Heat Production
a. Shivering
b. Chemical thermogenesis (uncoupling oxidative phosphorylation - Behavior CHanges - Curling up, dressing up, seeking warmth, voluntary exercsie
When the body is cold, what are the results of acute vs long term increases in heat production?
Acute - Increase Catecholamine release
Chronic - Increase in thyroid function
What can rapidly burn glucose to produce heat when activated by cold?
Uncoupling of Oxidative Metabolism in Brown Fat
Activated by Cold
Clinical: How does fever cause the hypothalamic set point to rise?
Pyrogens
- Proteins, their breakdown products, or lipopolysacccaride toxins
Clinical: How does aspirin lower a fever?
What circulating substance eventually reaches the brain and begins a signaling cascade in fever?
Blocks synthesis of prostaglandin (PGE2), preventing its stimulation of the hypothalamus
Cytokines
Clincial: Antipyretics
Drugs that reduce fever
Aspirin / Cycloxygenase Inhibitors block prostaglandin synthesis
Clinical: Why do you get the chills with a fever?
Why do you begin to sweat eventually?
The set point is suddenly raised by the hypothalamus, and your normal temp is well below new set point. You will “feel cold” and may shiver, vasoconstrict, piloerection, and secrete epinephrine.
Once you take medicine, or the fever “breaks” your set point returns to normal and you are now well above that value. Your body will feel “hot” and you will vasodilate, sweat.
Clinical: Heatstroke
Body temp 105 - 108 degF
Dizziness, abdominal distress, vomiting
Delirium
Loss of consciousness
Circulatory shock from loss of fluid/electrolytes
Clinical: Heat Exhaustion
Occurs afer excessive loss of salt and water due to sweating
May also occur when venous return is compromised by heavy exertion and maximal cutaneous vasodilation.
Body temperature MAY remain in normal values
Clinical: Hypothermia
Body at 94?
Body below 85?
Body at 77 (or less)?
General slowing of metabolism and of cardiac and neuronal excitabiity
94 - Thermoregulation Impaired
85 - Thermoregulation Lost
< 77- Death to heart fibrillation
Clinical: Frostbite
Damage due to frozen body surface
Lobes of ears, hands, feet
What substances can increase as we acclimatize to cold?
Increased chemical thermogenesis by elevated catecholamine and throxine levels
