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What happens in the thermoneutral zone?
Ambient temperature
Meta
bolic R
ate
Thermoneutral Zone
Tb
Endothermy & HomeothermyEndotherms generate their own
heatHomeotherms regulate their
body temperature by physiological or behavioural means
EndothermyHeat productionHeat retentionRegulation
Producing Heat I: All biochemical processes produce heat
Fig. 6.4
Producing Heat II: Shivering ThermogenesisUncoordinated
muscular contractions
Performs no locomotor function
Uses ATP, therefore generates heat
Producing Heat III: Non-shivering ThermogenesisFinding other ways to do
biochemical ‘work’ and to produce heat
Acclimation to 6°C
Initially, rats shivered uncontrollably…
After acclimation, they maintained their body temperature without shivering
Brown adipose tissue Highly vascularized
tissue with lots of mitochondria
Abundant in newborn mammals, declines with age
Can increase with acclimation/ acclimatisation (rats)
Localisation of Brown Adipose Tissue
Fig. 9.29
Generating heat through futile cyclesCircular biochemical reactions
that use up ATP and produce heat without achieving anything else
Mechanisms not well-understood…
Futile cycles Ion cycling across membraneBreakdown and building of
triglycerides
Uncoupling protein (thermogenin, UCP1)
Box 7.1
Uncoupling protein (thermogenin, UCP1)
Box 7.1
Uncoupling protein (thermogenin, UCP1)When thermogenin is activated,
proton transport into the mitochondrion is uncoupled from ATP synthesis ATP is expended pumping protons
out again, generating heatThe numerous mitochondria in
BAT are essentially futile pumping engines
But birds don’t have BAT?!A UCP2/3 homolog
has been found in hummingbird muscle Another tissue with
lots of mitochondria Dual use of muscles
because of weight restriction?
Being an endotherm is expensive!!
Clarke, 2006 Funct. Ecol. 20: 405-412.
Counter-current heat exchange
Fig 9.34
Countercurrent heat exchange
HotCold
• Heat is lost to the outside world
• More heat is then necessary to increase temperature upon return to the body
• A NET LOSS OF HEAT ENERGY
No transfer of heat between outgoing and incoming vessels
Countercurrent heat exchange
HotCold
Allow heat transfer between outgoing and incoming vessels
• Heat transfers to the incoming vessel
• Less heat is lost to the outside world
• Less energy is required to re-heat the incoming blood
Counter-current heat exchange
Fig 9.33
Regional heterothermy
Exposed body parts are often cooler than core temperature Heat & energy
conservation Testes and brains
are often also kept a little cooler
Fig. 9.30
Why fish are ectothermsFishes can generate heat just
like anything else with metabolism, but they have problems keeping it
Surrounded by thermally-conductive water
Fish gills act as a heat sink
Fig. 22.10c
Fish Gills
Why Fish are EctothermsFish are ectotherms because the
high blood flow across their large gill surface means that they lose heat to the environment really quickly
Heat retention is a major issue
Tuna temperature gradients in 19°C Water – Regional Endothermy
29 °C
31 °C
27 °C
23 °C
19 °C
23 °C
Rete mirabileA recurring theme – especially in
(but not exclusive to) fish Swim bladder Heater organs Oxygen delivery to retina
Allows for VERY effective countercurrent exchange of heat (and other things)
Red muscle in Tuna
Tuna Salmon
Area of
Rete
Fig. 9.43
Red muscle temperature is elevated above water temperature
Fig. 9.44
Thermal Mass
Where does the heat come from?Heat comes from the normal
heat produced by contractile activity of the red muscles The only difference is that the heat
is retained
Why?Good question!To allow long migration through
water of different temperatures?To allow better performance as a
predator chasing prey into colder water?
Improvements in power output of muscles?
Brain heaters in billfish
Heater organ in Billfish
Brain
Carotid Artery
Carotid Rete
Heater OrganEye
7.2°C8.5°C
4.8°C1.3°C
Heater organ in BillfishDerived from an extra-ocular eye
muscleNo contractile ability left63% mitochondria (by volume)Another 30% is sarcoplasmic
reticulumHeat is produced by futile
cycling of Ca2+ pumping
Large sharks are also regional endotherms
Regional endothermy evolved in bony fishes on at least three separate occasions
Fig 9.45
Brain heaters evolved in bony fishes on at least two separate occasions
Fig. 9.45
Superior rectus muscle developed into a brain heater
Lateral rectus muscle developed into a brain heater
Reading for next week(s) Two Guest Lectures
Hibernation Migration (are examinable) Note: Lecture Quiz 2 material ends NEXT
Thursday Reading to be announced on OWL Reading for Tues Oct 7 & Thu Oct 9:
Circulation Pp 611-641