Temperature Control

Question 1

Describe the Core/Shell model of thermoregulation

Answer 1

body has core and shell

core = heat generating - internal organs, CNS and skeletal muscle

temp in core is quite constant and subject to precise regulation - contains sites of internal heat production and set point control

Shell = insulating and exchanging = skin/subcutaneous fat = exchanges heat according to ambient temp and momentary insulating capacity of shell. Temp in shell is generally cooler and varies substantially

Question 2

how does heat move?

Answer 2

it moves down the thermal concentration gradient from warmer to cooler k

Question 3

what are the four mechanisms by which we lose heat?

Answer 3

Radiation

Conduction

Convection

Evaporation

Question 4

what is radiation?

Answer 4

Emission of heat from surface in form of electromagnetic waves (“heat” waves)

Body emits (heat loss) and absorbs (heat gain) radiant energy

Net transfer depends on ambient temperature

Transfers from warmer to cooler

Sources of gain: sun, radiator

Sources of loss: furniture, building walls

Humans tend to lose ~50% heat energy through radiation

Question 5

what is conduction?

Answer 5

Transfer of heat between objects of differing temperature that are in direct contact

Warmer object transfers to cooler

Faster-moving “warmer” molecules agitates cooler molecule and warms it up, in turn losing some thermal energy

Rate according to

Temperature difference

Thermal conductivity (air is poor, water is good)- which is why air feels warmer than water at the same temp

Question 6

what is convection?

Answer 6

Transfer of heat by air (or H2O) currents

Warm air is less dense than cold air

Warm air rises away [from skin], replaced by cooler air

Process carries heat away from body

Can be enhanced by forced air movements

Wind, fan, riding a bike

“wind chill factor”

Question 7

What is evaporation?

Answer 7

Heat loss from an object through evaporation of water from its surface

Heat needed to transform water from liquid to gaseous state is absorbed from the skin

cools body

Sites: respiratory airways, lining of mouth, skin

“insensible” water loss – not subject to control

Question 8

why do your vessels constrict when you’re cold?

Answer 8

because your body is trying to conserve heat

Question 9

what is a vasomotor response?

Question 10

what is the ‘thermoneutral zone’ ?

Answer 10

Range within which core To maintained by vasomotor responses alone

Question 11

why do you sweat more when it’s humid?

Answer 11

because the air is already saturated with water, so it cannot evaporate the sweat

Question 12

what neurotransmitter is particularly involved with sweating?

Answer 12

sweating = sympathetic response = acetylcholine

Question 13

what is the primary involuntary mechanism of heat production?

Answer 13

shivering = skeletal muscle activity generates heat - body harnesses principle to generate heat by shivering - all of the cheical energy liberated by shivering is converted to heat b/c no ‘work’ is to be accomplished by the muscle otherwise

Question 14

what region of your brain recieves information from the thermoreceptors in your body?

Answer 14

the Hypothalamus

Question 15

where are peripheral and core thermoreceptors located?

Answer 15

peripheral = monitor skin temp

core = located in hypothalamus, CNS and internal abdominal organs

Question 16

where in particular in the Hypothalamus are the temp. detectors located?

Answer 16

Anterior hypothalamus

Question 17

what does the hypothalamic neurons firing depend on?

Answer 17

on local temerature

• warm sensitive neurons and cold sensitive neurons - also respond to signals form the peripheries

Question 18

how do the anterior and posterior hypothalamus effect body temperature?

Answer 18

anterior = mediates decrease in body temp - activated by warmth and triggers heat loss reflexes

posterior = mediates increases in body temperature - activated by cold and triggers heat production and conservation

Question 19

describe the body’s coordinated response to cold

Answer 19

Impulses from peripheral/core thermoreceptors stimulate posterior hypothalamus

Shivering activated

1o involuntary means of increasing heat production

Peripheral vasoconstriction

Hair raising

Behavioural adaptations

Question 20

what is the body’s coordinated response to heat?

Answer 20

Impulses from peripheral/core thermoreceptors stimulate anterior hypothalamus

Skin vasodilation

Sweating

Behavioural adaptations

Fans, cold drink, light clothing

Question 21

when does the body’s core temperature normally vary?

Answer 21

Diurnal “body clock”

Core varies by ~1oC during normal day

Lowest in morning (6-7am), highest ~5-7pm

During exercise

Tremendous heat production due to (friction) caused by contraction of muscles (up by 4oF)

Menstrual cycle

Core raised ~0.9oF during last half of cycle (cause unknown)

Question 22

what happens in hypothermia?

Answer 22

Fall in core body To below normal accepted range

Generalized cooling exceeds ability of heat-producing and heat-causing mechanisms

Metabolic processes slow down

Higher cerebral functions affected quickly

Loss of judgment, apathy, disorientation

Progresses to depression of respiratory centres:

Reduced breathing

Reduced cardiac output

Question 23

is fever a failure of the thermoregulatory system?

Answer 23

no - it is a ‘reset’ of the ‘normal’ temperature set point

Question 24

what triggers a fever?

Answer 24

Endogenous pyrogen (examples interleukin 1beta and TNFalpha- released by macrophages in response to virus/bacteria

Question 25

What is the proposed function of fever?

Answer 25

may stimulate immune system and reduce bacterial replicaiton -

*aspirin and anti-pyretics- block pathway used by pyrogens to alter set-point thus reducing fever

Question 26

what are the main physiologic mechanisms regulating temperature?

Answer 26

metabolic activity

vasomotor responses

sweating

shivering

brown fat metabolism

hair raising

behavioural