Homeostatic Mechanisms-Thermoregulation

Question 1

body temp

Answer 1

• endotherms
• homeotherms-maintain core body temp in narrow range
• skin temp rises and falls with outside temp
• core temps vary by time of day, stage of menstrual cycle, level of activity, age

Question 2

consequences of deviations

Answer 2

• below 82, temp regulation lost
• 95-82 temp regulation impaired
• 98-102- regulation efficient in febrile disease, health, work
• 106-114, regulation seriously impaired, heat stoke or brain lesions
• 38-40 (100-104) febrile disease and hard work

Question 3

hypothalmic regulation

Answer 3

• compromised below 94/34, lost below 85/29
• cellular heat production decreased 2x for every 10 degree F decrease
• cardiac standstill at low temps
• heat stroke/ organ failure at high temps
• neg feedback with high gain (25-30)
• we change 1 C for every 25-30 C change in environment

Question 4

more regulation

Answer 4

• nude person can maintain core temp within 97-100 F after exposure for a few hours to dry air between 55-130F
• wind and moisture affect shape of curve
• regulatory mech is skin detectors and hypothalamus, some receptors in the core

Question 5

major mechanisms of heat gain/loss

Answer 5

• radiation
• convection
• evaporation

Question 6

radiation

Answer 6

• transfers heat as electromagnetic waves between objects that aren’t in contact
• rate of transfer is proportional to temp difference between body and environment
• at rest indoors ~60% of body heat is lost by radiation

Question 7

conduction

Answer 7

• intermolecular thermal heat transfer between solid objects in direct contact
• hot sand on feet, ice pack looses heat
• normally minimal in person with shoes and clothing`

Question 8

convection

Answer 8

• loss or gain of heat by movement of air or water over the body
• heat rises-air carries heat away
• immersion in water exchanges most heat this way

Question 9

evaporation

Answer 9

• losing water in its gas phase
• skin and resp tract
• insensible loss is ~650 ml/ day
• air circulation improves rate of evaporation of sweat from skin, humidity makes it less effective
• normally dissipates nearly all heat from exercise

Question 10

mass balance

Answer 10

• most heat generated in organs by cellular metabolism-inefficient
• rate of heat loss is determined by how fast heat is carried from core to skin and then from skin to surroundings
• most heat transferred from core to skin by convection in the blood, then lost to air and surroundings
• SNS regulates blood blow to the skin and sweating

Question 11

passive or unregulated heat transfer

Answer 11

• in steady state, heat production in core must match by flow to skin, and then to environment
• regulation not directly involved in heat can affect it
• sweating and hypoglycemia, blood pressure drops and flow change, and changes in metabolism from thyroid problems

Question 12

rate of heat transfer from core to skin

Answer 12

• nearly 0 to 30% of cardiac output
• skin and subQ fat are insulators and maintain core temp
• BV beneath skin have continuous venous plexus, blood comes from cap
• hands/feet/ears plexus supplied from arteries via arteriovenous anastomosis
• high rate of blood flow to skin is efficient way to conduct heat from the core, low rates decrease conduction
• dry clothing- insulates body by trapping air next to skin and reduces conduction to environment
• also reduces convective air currents
• suit decreases heat loss to 1/2, arctic to as little as 1/6

Question 13

radiator system

Answer 13

• changes in environmental temp can cause 8 fold increase in conductance compared to vasoconstricted state
• constricted in cold and dilate in heat
• SNS inhibited when temp increases-less constriction, and activated when temp falls-constriction
• our set pt in right in middle so we can vasoconstrict and dilate easily to increase or decrease heat conductance

Question 14

evaporative heat loss

Answer 14

• hot- thermal sensors in skin increase blood flow to skin and sweat production
• if core temp increases, can sweat a lot
• if humid, sweating not effective for heat reduction
• acclimatization (1-6weeks)- change in sweat glands to increase sweating capability
• decreases loss of NaCl to conserve body salt- aldosterone secretion from adrenal cortex
• congenital anhidrosis
• sweat gland innervated by Ach secreting sympathetic nerve

Question 15

cold and warm receptors

Answer 15

• programmed to work best at a certain range
• cold from 15-40
• hot from 30-50
• more in skin and periphery
• change AP frequency- anticipate changes in core temp
• skin has anatomically distinct receptors, 10x more cold in some parts
• distinct deep body receptors- sensitive primarily to cold in core prevent hypothermia
• project to control center in hypothalamus
• travel up afferent fibers via spinal cord to hypothalmic regulatory center

Question 16

hypothalamus

Answer 16

• integrates thermal information from skin and central temp receptors, directs changes in efferent activity resulting in vasoconstriction to conserve heat or shivering to produce more heat, or vasodilation and sweating to lose heat
• both heat and cold sensitive neurons in hypothalamus, more heat—>
• skin=environment-anticipatory feedback to prevent change
• core temp changes (exercise)-negative feedback responses to minimize change in core temp

Question 17

increased temp

Answer 17

• vasodilation
• sweating
• decreased heat production

Question 18

decreased temp

Answer 18

• vasoconstriction
• piloerection
• thermogenesis
• shivering, thyroid hormone production
• epi/norepi in babies

Question 19

homeostatic control systems

Answer 19

• maintain temp when room temp decreases by decreasing heat loss and increasing heat generation
• dec room temp
• heat loss from body, temp goes own
• vasoconstrict, curling up–decrease heat loss
• shivering-heat production
• return to normal temp

Question 20

regulated variabl

Answer 20

-body temp

Question 21

stimulus

Answer 21

decreased temp

Question 22

sensors

Answer 22

neurons in periphery and CNS

Question 23

integrator

Answer 23

hypothalamic neurons that compare input to set point

Question 24

effectors

Answer 24

sympathetic nerves regulating blood vessels and sweat glands

-hypothalamic motor centers regulating shivering

Question 25

body heat during exercise

Answer 25

• hyperthermia due to initial imbalance between rates of heat production and dissipation
• rate of heat production goes over rate of dissipation
• rise in core temp
• hypothalamus sense it
• activates dissipation effectors
• sweating more important than convection of radiation
• rise of 1C provides error that starts sweating mech
• mean skin temp nearly constant, decreases slightly from evaporative cooling

Question 26

fever

Answer 26

• increase in temperature set point and core temp due to pathological process-cytokines, IL, TNF cross hypothalamus cap and increase set point
• set point raised, mechanisms raise body temp, enhances heat conservation and production
• pyrogens sensed by hypothalamic control neurons
• degenerating tissues, lesions, tumors
• aspirins and acetaminophen inhibits prostaglandins and reduce fever
• chills come because you’re trying to keep up with new set point- piloerection, epi, constriction oare other effects
• break a fever and set point is back to normal-then heat dissipation kicks in- sweating/ vasodilation

Question 27

heat exhaustion

Answer 27

• failure of CV homeostasis
• decrease in circulating blood from dilation
• sweating induced decrease in central venous pressure
• upright-blood pools in limbs-syncope
• collapse during rest or exercise
• preceded by weakness, confusion, ataxia, anxiety, vertigo, headache, nauseam vomiting
• dilated pupils and sweats profusely
• rest in cool environment and fluid/electrolyte replacement

Question 28

heat stroke

Answer 28

• elevated core temp
• severe neuro disturbances
• loss of consciousness and convulsions
• cell/tissue damage
• classical-environmental stress overwhelms an impaired thermoregulatory system
• exertional-primary factor is high metabolic heat production
• over 41 C neural death and organ failure
• dry air-convection promote evaporation-can withstand higher temps
• humid air- over 94 F can elevate core
• need rapid lowering- immersion in cold water, hydration, airway maintenance, avoidance of aspiration

Question 29

malignant hyperthermia

Answer 29

• massive increase in MR, O2 consumption and heat production in skeletal muscle
• gene mutations in ryanodine receptor-disrupts calcium homeostasis in skeletal muscles
• may be triggered by inhalation of anesthetics and some depolarizing muscle relaxants
• discontinue triggering agent, use ryanodine receptor agonists and cool body

Question 30

hypothermia

Answer 30

• core temp less than 35 (95)
• immersion in cold water
• heat production can’t increase enough to compensate for heat loss
• drowsiness, slurred speech, bradycardia, hypoventilation- decreases in MR
• coma, hypotension, cardiac arrhythmias

Question 31

frostbite

Answer 31

• freezing of surface areas
• earlobes and fingers and toes
• necrotic damage from ice crystals forming in skin and subQ areas
• gangrene follows thawing
• sudden cold induced vasodilation is final protective response- occurs near freezing temperatures when smooth muscle in the vascular becomes paralyzed by cold