Temperature Regulation Flashcards
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
What are the three 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
What are the three mechanisms to increase body heat?
- skin vasoconstriction
- piloerection
- increase in thermigenesis
What are methods of thermogenesis?
- shivering
- metabolic pathways
- thyroxin secretion
Where is the primary motor area for shivering located? How does this area relate to the anterior hypothalamic preoptic area? Under what conditions is this area activated?
- dorsomedial portion of posterior hypothalamus
- normally inhibited by signals from heat center in anterior hypothalamic preoptic area
- excited by cold signals from skin and spinal cord
How does the dorsomedial of the posterior hypothalamus initiate shivering?
- when activated, transmits signals into lateral columns of spinal cord to anterior motor neurons -> alpha motor neurons and gamma neurons are activated
- nonrhythmical signals increase muscle tone of skeletal muscles throughout the body
- shivering begins when tone rises above a certain critical level
- may involve feedback oscillation of muscle spindle stretch reflex mechanism
What is chemical thermogenesis?
-increase in rate of cellular metabolism due to sympathetic stimulation or NE in blood
How is chemical thermogenesis related to epi/NE?
-uncoupled oxidative phosphorylation
How is chemical thermogenesis related to brown fat?
-degree of thermogenesis is directly related to amount of brown fat
Where is brown fat in humans?
Interscapular space in infants
What effect does increased thyroxine output have on cellular metabolism?
-activates uncoupling protein -> dissipates proton gradient before it can provide the energy gradient for oxphos
What is critical body core temperature?
37.1 C or 98.8 F
How does core temp relate to heat loss and heat production?
-heat loss is greater at temps above this temp and heat production is greater at temps below this temp
What is the “set point” of the temperature control mechanism?
-level at which sweating or shivering begins in order to maintain critical core body temperature
What is the definition of heat?
-a metabolic byproduct resulting from the inefficiency of the various metabolic pathways
What compares set point temp with recorded temp?
-anterior hypothalamus
What senses temperature in skin and relays it to the anterior hypothalamus?
-skin sensors
What happens with the core temp is less than the set point? What happens when core temp is higher than set point?
- posterior hypothalamus activates heat generating mechanisms
- anterior hypothalamus activates heat loss mechanisms
What is the feedback gain of the temperature control system and how does it compare to that of other biological control systems?
-change in environmental temp/change in body core temp - 1.0 = (28/1)-1 =27
What are the physiological mechanisms that alter the critical set point?
-primarily skin temp changes
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?
- pyrogens increase set point temp by increasing production of IL-1 in phagocytic cells
- IL-1 causes anterior pituitary to produce prostagladins
How does IL-1 and prostaglandins affect fever?
-increase set point temperature
How does aspirin affect fever?
-decreases set point temperature by inhibiting cyclooxygenase which results in decrease in production of prostaglandins
How does arachidonic acid affect fever?
-generates PGE2 which elevates set point temperature
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
What heat exhaustion?
- caused by excessive sweating
- blood volume and arterial blood pressure decreases, resulting in fainting
What is malignant hyperthermia?
- caused in susceptible individuals by inhalation anesthetics
- characterized by massive increase in oxygen consumption and heat production by skeletal muscle -> rapid rise in body temperature
What is hypothermia?
-ambient temperature is so low that heat generating mechanisms cannot maintain core temperature near set point
