Lecture 13 - Temperature Regulation Flashcards
Heat is a ___ resulting from the ___ of the various metabolic pathways
metabolic byproduct
inefficiency
Factors that Determine Heat Production
- BMR (Basic Metabolic Rate)
- Muscle activity (i.e., shivering)
- Thyroxin
- Norepinephrine and epinephrine
- Increased cellular chemical activity
- Extra metabolism for digestion, absorption, and food storage
Factors that Determine Rate of Heat Loss
- How rapidly heat can be conducted from body core to skin
- How rapidly heat can be transferred from skin to surroundings:
- Skin and subQ especially act as heat insulators.
- Continuous venous plexus in subQ is supplied by inflow of blood from capillaries from dermis.
- Rate of blood flow into the plexus can be as great as 30% of total cardiac output.
A ___ amount of heat is transferred by the respiratory system.
small
How rapidly heat can be transferred from skin to surroundings
- About an 8x increase in conductance between fully vasoconstricted state to fully vasodilated state.
- Heat conduction to skin is controlled by: •Degree of vasoconstriction of arterioles and the arteriovenous anastomoses that supply blood to the venous plexus of the skin.
- Vasoconstriction is controlled almost entirely by sympathetic system in responses to core temperature and environmental temperature
Mechanisms of Heat Loss from Skin Surface
Radiation
Conduction
Convection
Radiation
- Loss in the form of infrared heat rays
- Radiated by all objects not at absolute zero
- If temperature of body is greater than ambient temperature, more heat is radiated from the body than to the body.
- Refers to thermal energy transferred to objects in the external environment.
- Amount transferred depends on temperature difference and ability of object to absorb energy
Conduction
Kinetic energy of the molecules of the skin is transferred to the air if the air is colder than the skin
•Transfer of energy from one body to another when they are in close contact.
Convection
Removal of heat from the body by convection air currents
•Heat is transferred between two objects by air or water (fluid).
Slide 8
body cooling via wind and circulation
Heat Loss by percentage
radiation: 60%
conduction to objects: 3%
conduction to air: 15%
evaporation: 22%
Note that heated air must be moved away from the skin for continued heat loss to occur through conduction to air
Evaporation
Heat is dissipated by the use of thermal energy to cause a change from fluid to gas
Stimulation of Sweating
- Stimulation of anterior hypothalamus-pre-optic area in the brain electrically or by excess heat
- Cholinergic sympathetic nerve fibers (muscarinic)
- Circulating epinephrine and norepinephrine
What effect does aldosterone have on sweat composition?
Increased resorption of sodium: sodium loss in urine is decreased under aldosterone stimulation. Increased resorption of water, with consequent expansion of extracellular fluid volume. This is an osmotic effect directly related to increased resorption of sodium.
Strong stimulation of sweat glands
- Large amounts of precursor secretion are formed.
- Ducts reabsorb only about half the sodium chloride
- Concentrations of sodium and chloride ions are about 50 to 60 mEq/L
- Little water is reabsorbed
Unacclimatizedperson normally produces about __ liter sweat per hour (or less).
one liter or less
Person exposed to hot weather for 1 to 6 weeks may produce__ liters of sweat per hour, increasing heat removal by factor of __.
2 to 3
10
Due to changes in internal sweat gland cells
(Climatization)
body core temp. is stable (98⁰F − 100⁰F between environmental temperature range of ___?
55F - 130F
Regulation of core body temperature is essential because…
most of the metabolic processes necessary for life are strongly temperature dependent.
What are the principal areas of the brain that affect body temperature?
•Anterior hypothalamic nuclei •Medial pre-optic area
Anterior preoptic hypothalamus
Anterior pre-optic area:
Contains warm-sensitive, cold-sensitive, and temperature insensitive neurons: •Warm sensitive neurons: •Increase firing rate 2-10x in response to a 10°C increase in body temperature. •Cold-sensitive neurons: •Increase firing rate when temperature falls
The hypothalamus is one of the few brain areas where neurons are found that are themselves directly sensitive …
to physical or chemical variables (i.e., temperature, plasma osmolarity, plasma glucose, and hormone levels).
Warm-sensitive neurons comprise __% of neuronal pool while cold-sensitive neurons comprise _%.
30, 5
The preoptic anterior hypothalamus is referred to as the ___, while the posterior hypothalamus is referred to as ___.
heat dissipation center
heat generation/conservation center
Warm sensitive neurons have membrane receptors that are sensitive to changes in brain and blood temp above __?
Cold sensitive neurons…
37*
do not have temp sensitive receptors
Disinhibition due to decrease in discharge of warm-sensitive neurons results in ____ which are driven by tonic inputs from temperature insensitive neurons.
increase in discharge of cold-sensitive neurons
T/F Temp sensitive neurons are most numerous in the preoptic anterior hypothalamus.
FALSE
Temp set-point is essentially a function of …
the activity of the warm-sensitive neurons of the pre-optic anterior hypothalamus
Mechanisms to Reduce Body Heat
- Pre-optic anterior hypothalamus: •Heating this area immediately causes: •Dilation of skin blood vessels over the entire body •Profuse sweating over the entire body •Inhibition of excess heat production
- Vasodilation of skin blood vessels: •Caused by inhibition of sympathetic centers in posterior hypothalamus that cause vasoconstriction.
- Sweating
- Decrease in heat production: •Due to inhibition of shivering and thermogenesis
Mechanisms to Increase Body Heat
- Skin vasoconstriction
- Piloerection
- Increase in thermogenesis:
- Shivering
- Metabolic pathways
- Thyroxin secretion
Shivering
Dorsomedial portion of the posterior hypothalamus: •Primary motor area for shivering
Normally inhibited by signals from heat center in anterior hypothalamic preoptic area
Excited by cold signals from skin and spinal cord •Shivering is the most potent mechanism for increasing heat production.
Shivering is orchestrated by the posterior hypothalamus. •When activated, transmits signals into lateral columns of spinal cord to anterior motor neurons.
Chemical Thermogenesis
- Increase in rate of cellular metabolism
* Due to sympathetic stimulation (or norepinephrine in blood)
How is chemical thermogenesis related to epinephrine/norepinephrine?
Uncouples oxidative phosphorylation
How is chemical thermogenesis related to brown fat?
Describe brown fat distribution in humans
Degree of thermogenesis is directly related to amount of brown fat
Interscapularspace in infants
What effect does increased thyroxine output have on cellular metabolism?
Activates uncoupling protein: •A mitochondrial inner membrane protein that can dissipate the proton gradient before it can be used to provide the energy gradient for oxidative phosphorylation → energy is used to generate heat.
What is the critical body core temperature?
How does this core temperature relate to heat loss and heat production?
37.1 °C (98.8 °F)
Heat loss is greater at temperatures above this temperature and heat production is greater at temperatures below this temperature.
What is the “set-point” of the temperature control mechanism?
Level at which sweating begins or shivering begins in order to return to critical core body temperature
What is the feedback gain (and how is it calculated) of the temperature control system and how does it compare to that of other biological control systems?
= (change in environmental temperature/change in body core temperature) –1.0 = (28/1) –1 = 27
What are the physiological mechanisms that alter the critical set point?
Primarily skin temperature changes
Set point ____ as skin temperature decreases.
amount of sweating ___ as skin temperature falls
increases
decreases
Fever
Body temperature above the usual range of normal
Most bacteria proliferate poorly at temperatures above
39*
What are pyrogensand how do they relate to the set point of the hypothalamic thermostat?
Give examples of pyrogens.
- Pyrogens increase set point temperature by increasing production of intereukin-1, TNF, IL-6, and INF in phagocytic cells.
- IL-1 causes anterior pituitary to produce prostaglandins.
How do the following affect fever
•IL-1, prostaglandins
•Aspirin
- IL-1, prostaglandins> Increase set point temperature
- Aspirin> Decreases set-point temperature by inhibiting cyclooxygenase which results in decrease in production of prostaglandins
Febrile Condition
characteristics?????
Under what conditions is heat stroke likely to occur?
•Occurs when body temperature increases to point of tissue damage. •Normal response (sweating) is impaired and core temperature continues to increase
Heat exhaustion
•Caused by excessive sweating •Blood volume and arterial blood pressure decreases, resulting in fainting
Malignant hyperthermia:
•Caused in susceptible individuals by inhalation anesthetics •Characterized by massive increase in oxygen consumption and heat producJon by skeletal muscle → rapid rise in body temperature
Hypothermia:
•Ambient temperature is so low that heat-generating mechanisms cannot maintain core temperature near set point.
