Biology 2672a: Comparative Animal

Physiology

EndothermyDr Brent Sinclairbsincla7@uwo.ca

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