Lecture 09: Temperature Regulation Flashcards
Factors that determine Heat Production:
- BMR
- Muscle activity (i.e., shivering)
- Thyroxin
- Norepinephrine and epinephrine
- Increased cellular chemical activity
- Extra metabolism for digestion, absorption, and food storage
How rapidly heat can be conducted from body core to skin
Factors that Determine Rate of Heat Loss
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.
*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.
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.
Conduction:
Kinetic energy of the molecules of the skin is transferred to the air if the air is colder than the skin.
Convection:
Removal of heat from the body by convection air currents.
Heat Loss from Skin Surface
Low velocity wind has a cooling effect proportional to the square root of the wind velocity.
Water has a specific heat several thousand times as great as that of air.
Note that the rate of heat loss in water is usually many times greater than the rate of heat loss in air.
For each gram of water that evaporates from the body surface
0.58 Calories of heat is lost.
Insensible perspiration:
- Occurs at a rate of 600 to 700 ml/day
* Causes a continual heat loss at a rate of 16 to 19 Calories/day
Stimulating factors for sweating:
- Stimulation of anterior hypothalamus-pre-optic area in the brain electrically or by excess heat
- Cholinergic nerve fibers
- Circulating epinephrine and norepinephrine
Precursor secretions of sweating:
- Composition similar to that of plasma w/o proteins: • Na+: 142 mEq/L; Cl-: 104 mEq/L
- Compare concentrations of constituents when flow of precursor through the duct is low (slight stimulation of glands) versus rapid (strong stimulation).
What effect does aldosterone have on sweat composition?
…
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
Unacclimatized person normally produces
about 1 liter sweat per hour (or less).
Person exposed to hot weather for 1 to 6 weeks
may produce 2 to 3 liters of sweat per hour, increasing heat removal by factor of 10.
• Due to changes in internal sweat gland cells
What are the principal areas of the brain that affect body temperature?
- Anterior hypothalamic pre-optic area
* Pro-optic area
Anterior pre-optic area:
- Contains both heat-sensitive and cold-sensitive neurons
- Heat 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
Effect of hypothalamic temperature on evaporative heat loss and on heat production (critical point)
Where the two graphs cross, of heat production and evaporative heat loss: Begin evaporative heat loss
Pre-optic area:
- 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
List mechanisms to reduce body heat
…• 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
List mechanisms to increase body temperature
- Skin vasoconstriction
- Piloerection
- Increase in thermogenesis: • Shivering
- Metabolic pathways
- Thyroxin secretion
Where is the primary motor area for shivering located?
Dorsomedial portion of posterior hypothalamus.
How does this area relate to the anterior hypothalamic preoptic area?
Normally inhibited by signals from heat center in anterior hypothalamic preoptic area.
Under what conditions is this area activated?
Excited by cold signals from skin and spinal cord
How does this area initiate shivering?
• •
When activated, transmits signals into lateral columns of spinal cord to anterior motor neurons.
• •
Shivering begins when tone rises above a certain critical level.
Nonrhythmical signals increase muscle tone of skeletal muscles throughout body.
May involve feedback oscillation of muscle spindle stretch reflex mechanisms.
Define: 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?
Degree of thermogenesis is directly related to amount of brown fat.
Describe brown fat distribution in humans.
Interscapular space in infants
What effect does increased thyroxine output have on cellular metabolism?
..
How is thyroxine output related to body temperature and the anterior hypothalamic- preoptic area?
..
What is the critical body core temperature?
37.1 °C (98.8 °F)
How does this core temperature relate to heat loss and heat production?
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
Refer to Figures 73-8 and 73-9
Define: “fever.”
Body temperature above the usual range of normal
What are pyrogens and how do they relate to the set point of the hypothalamic thermostat?
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Give examples of pyrogens.
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How do the following effect fever:
- IL-1,prostaglandins
- Aspirin
- Arachidonic acid
List and describe characteristics of the febrile condition.
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Under what conditions is heat stroke likely to occur?
.
What are the symptoms of heat stroke? How are they
related to circulatory shock?
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At what temperature point is the ability of the hypothalamus to regulate temperature lost?
.
What factors contribute to this loss of ability to regulate body temperature? (EXTREME COLD)
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