What factors determine heat production?
- BMR
- muscle activity
- thyroxin
- NE and epi
- increased cellular chemical activity
- extra metabolism for digestion, absorption, and food storage
What is malignant hyperthermia?
- heat production is far greater than heat dissipation
- due to genetic abnormalities in the ryanodine receptors in skeletal muscle which leads to excess release of Ca, leading to prolonged excitation-contraction coupling
- triggered by anesthetics
What are the 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
- small amount of heat is transferred via the respiratory system
Explain how heat is transferred from skin to environment.
- skin and subQ 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
- 8x increase in conductance between fully vasoconstricted state to fully vasodilaters state
- vasoconstriction is controlled almost entirely by sympathetic system in responses to core temperature and environmental temperature
What is heat conduction of the skin controlled by?
-degree of vasoconstriction of arterioles and the arteriovenous anastomosis that supply blood to the venous plexus of the skin
What are the three methods heat is removed from the body?
- radiation
- conduction
- convection
How is radiation lost?
- loss in the form of infrared heat rays
- radiated by all objects not at absolute zero
- if temp of body is greater than ambient temp, more heat is radiated from the body than to the body
How is conduction lost from the body?
-kinetic energy of the molecules of the skin is transferred to the air if the air is colder than the skin
How does convection work?
-removal of heat from the body by convection air currents
Low velocity has a cooling effect proportional to ______________.
The square root of the wind velocity
What has a greater specific heat, water or air? What does this mean?
- water
- rate of heat loss in water is usually many times greater than the rate of heart loss in the air
- for each gram of water evaporated from body, 0.58 calories of heat is lost
What is insensible perspiration?
- occurs at a rate of 600-700 mL/day
- causes a continual heat loss at a rate of 16-19 calories/day
Percentage of heat lost and method of loss
- evaporation (22%)
- radiation (60%)
- conduction to air (15%)
- conduction to objects (3%)
What is radiation in terms of heat transfer?
- thermal energy transferred to objects in the external environment
- amount transferred depends on temp difference and ability of object to absorb energy
What is conduction in terms of heat transfer?
-transfer of energy from one body to another when they are in close contact
What is convection in terms of heat transfer?
-heat is transferred between two objects by air or water
What is evaporation?
Heat is dissipated by the use of thermal energy to cause a change from fluid to gas
What stimulates sweating?
- stimulation of anterior hypothalamus- preoptic area in the brain electrically or by excess heat
- cholinergic sympathetic nerve fibers (muscarinic)
- circulating epi and NE
What is the composition of precursor secretion?
-similar to plasma without proteins
Na (142mEq/L)
Cl (104mEq/L)
What does strong stimulation of sweat glands do?
- large amounts of precursor secretion are formed
- ducts reabsorb only about half the NaCl
- concentrations of Na and Cl are about 50-60 mEq/L
- little water is reabsorbed
What are some of the differences between unacclimatized and acclimatized individuals when exposed to hot weather?
-unacclimatized person normally produces about 1 L sweat per hour
-person exposed to hot weather for 1-6 weeks may produce 2-3 L sweat/hr, increasing heat removal 10x
+due to changes in internal sweat gland cell
What are the principal areas of the brain that affect body temperature?
- anterior hypothalamic pre-optic area
- pre-optic area
What does the anterior pre-optic area contain? What do these do?
- heat sensitive neurons: increase firing rate 2-10x in response to a 10 degree C increase in body temp
- cold sensitive neurons: increase in firing rate when temp falls
What does heating of the pre-optic area cause?
- dilation of skin blood vessels over the entire body
- profuse sweating over the entire body
- inhibition of excess heat production
