Lecture 14 Temperature Regulation Flashcards
Factors that determine Heat production
Heat is a metabolic byproduct resulting from the inefficiency of the various metabolic pathways
- BMR
- muscle activity
- Thyroxin
- Norepinephrine and epinephrine
- Increased cellular chemical activity
- Extra metabolism for digestion ,absorption and food storage
Malignant hyperthermia
Syndrome
- Heat production is far greater than heat dissipation
- Probably due to genetic abnormalities in the ryanodine receptors in skeletal muscle which leads to excess releases of sarcoplasmic calcium ion,leading to prolonged excitation-contraction coupling.
- triggered by anesthetics
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
Small amount of heat is transferred by respiratory system
How rapidly heat can be transferred from skin to surroundings
- About 8x increase in conductance between fully vasoconstricted state to fully vasodilated state
- Heat conduction to skin is controlled by:
- -Degree of vasoconstrcition 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 response to core temperature and environmental temperature
Mechanisms of Heat Loss from the skin surface
S=(M-Wk) +/- (C+K+R)-E
- Low velocity wind has a cooling effect proportional to the square root of 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
- 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
S=(M-Wk) +/- (C+K+R)-E
S=Heat storage
M-Metabolism
Wk=External work
C=convection(removal of heat from the body by convection currents
K=Conduction(kinetic energy of the molecules of the skin is transferred to the air if the air is colder than the skin
R=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
Heat Transfer
- Radiation:
- -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:Transfer of energy from one body to another when they are in close contact
- Convection:Heat is transferred between two objects by air or water(fluid)
- Evaporation:Heat is dissipated by the use of thermal energy to cause a change from fluid to gas
What stimulates sweat ?
- Stimulation of anterior hypothalamus-pre-optic-area in teh brain electrically or by excess heat
- cholinergic sympahetic nerve fibers(muscarinic)
- circulating epinephrine and norepinephrine
Precursor secretion for sweat
- Composition similiar to that of plasma w/o proteins:
- -Na+:142 mEq/L ; Cl-: 104mEq/L
- Compare concentrations of constituents when flow of precursor through the duct is low (slight stimulation of glands) versus rapid(strong stimulation)
- Aldosterone- lowers the amount of salt in the sweat when secreted and helps with acclimation
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
What are the principal areas of the brain that affect body temperature?
- Anterior hypothalamic pre-optic area
- pre-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 10C increase in body temperature
- Cold sensitive neurons:increase firing rate when temperature falls
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
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
Mechanisms to increase body heat
- skin vasoconstriciton
- piloerection
- increase in thermogenesis:
- -shivering
- -metabolic pathways
- -thyroxin secretion
