L-5 Temp Regulation Flashcards
what is heat and what determines heat production?
heat is a metabolic byproduct resulting from the inefficiency of the various metabolic pathways.
- BMR
- muscle activity (shivering)
- thyroxin
- norepinephrine and epinephrine
- incr cellular chemical activity
- extra metabolism for digestion
describe malignant hyperthermia
its a syndrome
heat production is far greater than heat dissipation.
whats it due to/
probably due to genetic abnormalities in the ryanodine receptors in skeletal muscle which leads to excess release of sarcoplasmic calcium ion, leading to prolonged excitation-contraction coupling.
its triggered by anesthetics
what are factors that determine rate of heat loss
- how rapidly heat can be conducted from body core to skin
- how rapidly heat can be transferred frmo 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
*note that a small amount of heat is transferred by respiratory system
elaborate on how rapidly heat can be transferred from skin to surroundings:
- about an 8x incr 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 temp
what are the 3 mechanisms of heat loss from skin surface
radiation
conduction
convection
define 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.
define conduction
- kinetic energy of the molecules of the skin is transferred to the air if the air is colder than the skin
define convection
- removal of heat from the body by convection air currents
what is the heat storage equation
S = (M - Wk) +/- (C+K+R) - E
S= heat storage M= metabolism Wk= external work C= convection K= conduction R= radiation E= evaporative heat transfer
elaborate on mechanisms of heat loss from skin surface
- low velocity wind has a cooling effect proportional to the square 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 (scuba diving)
how many Calories of heat is lost per gram of water that evaporates from the body?
0.58 Calories
define insensible perspiration:
type of heat loss youre unaware of
- occurs at a rate of 600 to 700 ml/day
- causes a continual heat loss at a rate of 16 to 19 Calories/day
heat loss by percentages:
- radiation: 60%
- conduction to objects: 3%
- conduction to air: 15%
- evaporation: 22%
*note: heated air must be moved away from skin for continued heat loss to occur thru conduction to air
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
heat transfer: 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
describe the stimulating factors for 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 is the precursor to secretion?
- composition similar to that for plasma without proteins
what effect does aldosterone have on sweat composition?
decrease in sweat rate and incr in sweat sodium concentration
describe the 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
how much sweat per hour does an unacclimatized person normally produce?
1 liter or less
what happens when a person is exposed to hot weather for 1-6 weeks?
they may produce 2-3 liters/hour, increasing the heat removal by a factor of 10; due to changes in internal sweat gland cells
why is regulation of core temp essential?
because most of the metabolic processes necessary for life are strongly tempt-dependent
range: 55F-130F
what are the principal areas of the brain that affect body temperature?
- anterior hypothalamic nuclei
- medial pre-optic area
describe the anterior pre-optic area
contains warm-sensitive, cold-sensitive and temp insensitive neurons
describe warm sensitive neurons
- increase firing rate 2-10x in response to a 10C incr in body temp
describe cold sensitive neurons
- increase firing rate when temp falls (below 37 degree)
- comprise about 5% of neuronal pool in APO area byt are more prevalent in the posterior hypothalamic nucleus
what is the pre-optic anterior hypothalamus also referred to?
as the heat dissipation center
what about the posterior hypothalamus?
heat generation/conservation center
why is this?
because of the higher concentration of warm-sensitive neurons in the POA-hypothalamus that promote heat loss;
and
the higher concentration of cold-sensitive neurons in the posterior hypothalamus that promote heat generation
T/F: warm sensitive neurons have membrane receptors that are sensitive to changes in brain and blood temps above 37 degrees?
true
T/F: cold-sensitive neurons have intrinsic temp-sens receptors
False
fun fact between hypothalamus and body temp
disinhibition due to decrease in discharge of warm-sensitive neurons results in increase in discharge of cold-sensitive neurons which are driven by tonic inputs from temperature insensitive neurons
which neurons are the most numerous of the neurons in the POA hypothalamus?
temperature-insensitive (60%)
what is the temperature set point essentially a function of?
the activity of the warm-sensitive neurons of the pre-optic anterior hypothalamus
what are some mechanisms to reduce body heat?
- action by pro-optic anterior hypothalamus
- vasodilation of skin blood vessels
- sweating
- decrease in heat production (due to inhibition of shivering and thermogenesis)
describe action by POA 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
how is vasodilation caused?
caused by inhibition of sympathetic centers in posterior hypothalamus that cause vasoconstriction
what are the mechanisms that increase body heat?
- skin vasoconstriction
- piloerection
- incr in thermogenesis: (shivering; metabolic pathways; thyroxin secretion)
what is responsible for shivering?
- dorsomedial portion of the posterior hypothalamus
what inhibits or excites this area?
- normally inhibited by signals from heat center in anterior hypothalamic preoptic area
- excited by cold signals from skin and spinal cord
describe shivering
- the most potent mechanism for increasing heat production
- orchestrated by the posterior hypothalamus
- when activated, transmits signals into lateral columns of spinal cord to anterior motor neurons: alpha motor neurons and gamma motor neurons are activated
what increases muscle tone of skeletal muscles throughout body?
nonrhythmical signals
when does shivering begin?
when tone rises above a certain critical level
- may involve feedback oscillation of muscle spindle stretch reflex mechanisms
define chemical thermogenesis
- incr in rate of cellular metabolism
- due to sympathetic stimulation (or norepinephrine in blood)
how is chemical thermogenesis related to epinephrine/ norepinephrine?
- uncouples oxidative phosphorylatoin
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 incr 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
how is thyroxine output related to body temperature and the anterior hypothalamic-preoptic area?
textboook***
what is the critical body core temperature?
37.1 C (98.8F)
how does this core temp relate to heat loss and heat production?
heat loss is greater at temperatures above this temperature and heat production is greater at temps below this temp
what is the “set-point” of the temp control mechanism?
- level at which sweating begins or shivering begins in order to return to critical core body temp
describe what happens
- temperature sensors in skin and hypothalamus read core temp and relay to the anterior hypothalamus
- anterior hypothalamus compares recorded temp with set-point temp and:
- core temp < set point: posterior hypoth activates heat=generating mechanisms; vise verse with anterior hypoth and heat loss mech
what is the feedback gain of the temp control system and how does it compare to that of other biological control systems?
27
(change in environment 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 temperature 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 incr set point temp by incr’ing production of IL-1, TNF, IL-6, and INF in phagocytic cells
- IL-1 causes anterior pituitary to produce prostaglandins
how does IL-1 and prostaglandins effect fever?
- increases set point temp
how does aspirin effect fever?
decreases set-point temp by inhibiting cyclooxygenase, which results in decrease in production of prostaglandins
under what conditions is heat stroke likely to occur?
occurs when body temp increases to point of tissue damage
normal response (sweating) is impaired and core temp continues to incr
what are the symptoms of heat stroke? how are they related to circulatory shock?
TEXTBOOK
at what temperature point is the ability of the hypothalamus to regulate temp lost?
TEXTBOOK
what factors contribute to this loss of ability to regulate body temp?
TEXTBOOK
describe heat exhaustion
- caused by excessive sweating
- blood volume and arterial blood pressure decreases, resulting in fainting
describe hyperthermia
caused in susceptible individuals by inhaling anesthetics
- characterized by massive increase in oxygen consumption and heat production by skeletal muscle –> rapid rise in body temperature
describe hypothermia
ambient temperature is so low that heat-generating mechanisms cannot maintain core temperature near set point.
