The most successful vertebrate The most successful vertebrate group evolved in aquatic group evolved in aquatic

environments with low and environments with low and variable concentrations of variable concentrations of

dissolved oxygendissolved oxygen

David RandallDavid RandallBiology and Chemistry, City University of Biology and Chemistry, City University of

Hong KongHong KongNovember 2009November 2009

The response of mammals and fish to The response of mammals and fish to hypoxiahypoxia

Variations in oxygen concentration Variations in oxygen concentration in time and spacein time and space

and

Permian crisisDevonian crisis

Mammals Reptiles and birds

Teleost fish(90% of all fish species)

Common ancestor

Lungfish more related to a cow than a salmon (Gardiner et al., 1979)

250 million years agoextinction of 90% of species

400 million years ago

Lobe-finned fishes

Ray-finned fishes

Mammals and hypoxiaMammals and hypoxia

Mammals rarely experience hypoxiaMammals rarely experience hypoxia BirthBirth Moles in holesMoles in holes DivingDiving AltitudeAltitude

Humans at altitude:Humans at altitude: Everest ( Everest ( 珠穆朗玛峰珠穆朗玛峰 ) 8848 ) 8848 metresmetres

Oxygen about 30% of that at sea level

Humans at Humans at altitudealtitude

Mount Everest (Mount Everest ( 珠穆朗玛峰珠穆朗玛峰 ): humans can just maintain ): humans can just maintain basal metabolic rate at this oxygen level (but birds fly basal metabolic rate at this oxygen level (but birds fly overhead).overhead).

Many humans died, but eventually Everest was climbed Many humans died, but eventually Everest was climbed without oxygen by Messner and Habeler in 1978, Messner without oxygen by Messner and Habeler in 1978, Messner again in 1980again in 1980

Messner is very fit, has a large lung capacity, has panache Messner is very fit, has a large lung capacity, has panache & climbed quickly on a fine day& climbed quickly on a fine day

No permanent human habitation above 5000 metres No permanent human habitation above 5000 metres Reduced human reproduction at altitude Reduced human reproduction at altitude (no Spanish child (no Spanish child

survived into childhood until fifty-three years after the city of Potosi was survived into childhood until fifty-three years after the city of Potosi was established in the 16established in the 16 thth century at 4000 meters in the Andes. The indigenous century at 4000 meters in the Andes. The indigenous population did not experience these problems. Spanish women descended population did not experience these problems. Spanish women descended to lower altitudes for pregnancy and birth, staying there until the child was to lower altitudes for pregnancy and birth, staying there until the child was at least one year old)at least one year old)

High energy, not so hairy High energy, not so hairy humans and other mammalshumans and other mammals

• Humans cannot survive for long periods at oxygen levels Humans cannot survive for long periods at oxygen levels lower than 60% of that at sea levellower than 60% of that at sea level

• Wallabies do not reproduce in hypoxic environmentsWallabies do not reproduce in hypoxic environments• Egg laying mammalian ancestors were around before the Egg laying mammalian ancestors were around before the

DinosaursDinosaurs• Mammals did not do well during the age of the dinosaurs Mammals did not do well during the age of the dinosaurs

when it was hot and hypoxicwhen it was hot and hypoxic• Radiation of mammals occurred after the demise of the Radiation of mammals occurred after the demise of the

dinosaurs, when oxygen levels increased and it was coolerdinosaurs, when oxygen levels increased and it was cooler• Evolution of placental mammals probably occurred only when Evolution of placental mammals probably occurred only when

oxygen levels were highoxygen levels were high

Low oxygen and high carbon dioxide levels at the end of the Triassic: humans could not have managed in these conditions

P. Ward (2006) Out of Thin Air.

QE861.6 E95 W37

Jurassic DinosaursJurassic Dinosaurs

• Conditions in the Jurassic favored dinosaurs Conditions in the Jurassic favored dinosaurs

• Oxygen levels were low between 12 and 14%Oxygen levels were low between 12 and 14%

• Today this would be the oxygen level at about 4500 Today this would be the oxygen level at about 4500 metres above sea level, Mont Blanc, highest mountain metres above sea level, Mont Blanc, highest mountain in European alps is 4810 metresin European alps is 4810 metres

• Some birds migrate at high altitude, bar-headed geese Some birds migrate at high altitude, bar-headed geese fly over Everestfly over Everest

• Birds and dinosaurs have a common reptilian ancestorBirds and dinosaurs have a common reptilian ancestor

Bipedal locomotion to Bipedal locomotion to aid reptile breathingaid reptile breathing

Lizards can’t breath and runLizards can’t breath and run Movement of front legs impairs breathingMovement of front legs impairs breathing Run on back legs, don’t use front legs: bipedal Run on back legs, don’t use front legs: bipedal

locomotionlocomotion Earliest dinosaurs (and birds) were bipedalEarliest dinosaurs (and birds) were bipedal Increase size and power of back legs, changing role of Increase size and power of back legs, changing role of

arms in locomotion (grasping or flying)arms in locomotion (grasping or flying) Evolution of dinosaurs and birds from bipedal reptilesEvolution of dinosaurs and birds from bipedal reptiles Large heavy dinosaurs subsequently returned to Large heavy dinosaurs subsequently returned to

walking on four legswalking on four legs

Eyed lizard (Eyed lizard (Lacerta lepida))

Dinosaurs and BirdsDinosaurs and Birds

Both birds and dinosaurs evolved from reptilian Both birds and dinosaurs evolved from reptilian ancestor during the Triassicancestor during the Triassic

Both birds and dinosaurs are bipedalBoth birds and dinosaurs are bipedal Bird lungs better than mammalian lungs in low oxygen Bird lungs better than mammalian lungs in low oxygen

situationssituations Dinosaurs survived in hypoxic conditions Dinosaurs survived in hypoxic conditions Perhaps bird type lung existed in the common ancestor Perhaps bird type lung existed in the common ancestor

of dinosaurs and birds, but not in other reptiles of dinosaurs and birds, but not in other reptiles (Connor and (Connor and

Claessens, Nature, 2005; Berner, VandenBrooks and Ward, Science, 2007)Claessens, Nature, 2005; Berner, VandenBrooks and Ward, Science, 2007)

Bird lungBird lung

Animal Physiology (2002) Randall Burggren and French

Bird lungBird lung

Parabronchi of a bird lung the equivalent of mammalian alveoli

Mammalian lung: tidal ventilation with large dead spaces: not as good as the bird lung when exposed to low oxygen

Dinosaur lungs?Dinosaur lungs?

Are dinosaur lungs like bird lungs? Are dinosaur lungs like bird lungs? (Connor and Claessens 2005)(Connor and Claessens 2005)

Don’t knowDon’t know Dinosaur bones have holes (for air sacs)Dinosaur bones have holes (for air sacs) Dinosaurs and birds closely relatedDinosaurs and birds closely related Dinosaurs (and birds) evolved in a low oxygen Dinosaurs (and birds) evolved in a low oxygen

environment during the Jurassic and during that time environment during the Jurassic and during that time dominated the terrestrial environmentdominated the terrestrial environment

Dinosaurs versus mammalsDinosaurs versus mammals

The hot hypoxic Jurassic favored the dinosaurs, possibly because The hot hypoxic Jurassic favored the dinosaurs, possibly because they had bird like lungsthey had bird like lungs

The cold, oxygen rich present favors mammals. Mammals can The cold, oxygen rich present favors mammals. Mammals can increase heat production to maintain body temperature in the coldincrease heat production to maintain body temperature in the cold

Reptiles do no increase heat production to maintain body Reptiles do no increase heat production to maintain body temperature temperature

Mammalian lung design to conserve heat but operates poorly in Mammalian lung design to conserve heat but operates poorly in low oxygen conditionslow oxygen conditions

Teleosts versus mammalsTeleosts versus mammals

Teleost fish also evolved in the TriassicTeleost fish also evolved in the Triassic Teleost fish are hypoxia tolerantTeleost fish are hypoxia tolerant Many fish can survive in water with oxygen levels as Many fish can survive in water with oxygen levels as

low as 25% saturation (cf Mammals in air at 60% low as 25% saturation (cf Mammals in air at 60% saturation)saturation)

Several species can survive in 10% oxygen saturationSeveral species can survive in 10% oxygen saturation Carp can survive the winter buried in mud at low Carp can survive the winter buried in mud at low

temperatures under anoxic conditions, produce temperatures under anoxic conditions, produce ethanolethanol

Fish gill: countercurrent exchange of blood and water, can extract more oxygen from environment

Large lung dead spaces limit gas transfer during hypoxia

Most of oxygen in atmosphere, only small amount dissolved Most of oxygen in atmosphere, only small amount dissolved in water. Water contains much less dissolved oxygen gas in water. Water contains much less dissolved oxygen gas (only 2 to 4%) than an equivalent volume of air(only 2 to 4%) than an equivalent volume of air

PhotosynthesisPhotosynthesis

OXYGENOXYGEN RespirationRespiration

Other reactionsOther reactions

AtmosphereLight

WATER

mixing

Vertical section @ longitude 136W

Minimum oxygen zone @ 600 metres: no light but many organisms

Aquatic hypoxia: a common Aquatic hypoxia: a common eventevent

• No oxygen production No oxygen production in the darkin the dark

• Diffusion slow (10Diffusion slow (10-5 -5

less than in air)less than in air)• Oxygen transfer by Oxygen transfer by

convectionconvection• Convection enhanced Convection enhanced

by wind, temperature by wind, temperature gradients, water flowgradients, water flow Natural short term

changes in oxygen (várzea lakes)

Anthropogenic effectsAnthropogenic effects

Sewage from humans and the animals they eat Sewage from humans and the animals they eat released into rivers and coastal watersreleased into rivers and coastal waters

Climate change:Climate change: Increased temperature = reduced oxygen solubilityIncreased temperature = reduced oxygen solubility decreased salinity = increased oxygen solubilitydecreased salinity = increased oxygen solubility altered ocean currentsaltered ocean currents

How do teleost fish tolerate How do teleost fish tolerate hypoxic environmentshypoxic environments

• Low metabolic rate/high energy stores (carp can last the winter Low metabolic rate/high energy stores (carp can last the winter without oxygen)without oxygen)

• Gills: countercurrent gas exchanger results in efficient extraction Gills: countercurrent gas exchanger results in efficient extraction of oxygen from the environmentof oxygen from the environment

• Responses to hypoxia aimed at maintaining arterial oxygen content Responses to hypoxia aimed at maintaining arterial oxygen content

• Increased Increased breathing, gill diffusing capacity and breathing, gill diffusing capacity and Hb-OHb-O22 affinity affinity enhances oxygen uptake across the gills into the bloodenhances oxygen uptake across the gills into the blood

• Fish move to colder water and this, plus the decrease in NTP/Hb Fish move to colder water and this, plus the decrease in NTP/Hb ratios, increases Hb-Oratios, increases Hb-O22 affinity affinity

• BUTBUT increased Hb-O increased Hb-O22 affinity should diminish oxygen delivery to affinity should diminish oxygen delivery to tissuestissues

20

40

60

80

100

120

140

0 20 40 60 80 100 120 140 160

time (min)

pO

2 (

mm

Hg

)

Mean RM

Mean pwO2

Mean paO2

Fall in muscle pO2 ameliorated during hypoxia

Fish subjected to hypoxia: muscle oxygen remains high

Muscle oxygen levelsMuscle oxygen levels

• Human, Rat, DogHuman, Rat, Dog

• PaO2 100 mmHgPaO2 100 mmHg

• PvO2 40 mmHgPvO2 40 mmHg

• Tissue OTissue O22 25-35 25-35 mmHgmmHg

• Muscle oxygen level Muscle oxygen level below mixed venous below mixed venous level level

• TroutTrout

• PaO2 ~ 100 mmHgPaO2 ~ 100 mmHg

• PvO2 40 mmHgPvO2 40 mmHg

• Tissue OTissue O22 61 mmHg 61 mmHg

• Muscle oxygen Muscle oxygen level midway level midway between arterial between arterial and mixed venous and mixed venous levellevel

Fish muscle oxygen levels high!Fish muscle oxygen levels high!How is this possible?How is this possible?

Fish hemoglobins Fish hemoglobins have a Root shift: have a Root shift: reduced pH decreases reduced pH decreases HbOHbO22 even at very even at very

high Ohigh O22 levels. levels.

Root shift not found Root shift not found in any other in any other vertebrate groupvertebrate group

Fish swimbladder filled with Fish swimbladder filled with oxygenoxygen

Oxygen moved from water via blood into bladder often against a gradient of many atmospheres oxygen

Swimbladder used for buoyancy

Is the Root shift being used to Is the Root shift being used to deliver oxygen to other deliver oxygen to other

tissues?tissues?

Countercurrent blood supply to teleost eye, Countercurrent blood supply to teleost eye, high retinal oxygen levels allow reduced high retinal oxygen levels allow reduced capillary density, decreasing the complexity of capillary density, decreasing the complexity of visual processingvisual processing

Muscle: but what is the source of protons to Muscle: but what is the source of protons to cause the Root off shift?cause the Root off shift?

Don’t know but…Don’t know but…

CO2 + H2O HCO3- + H+

CO2 + OH- HCO3-

CO2 hydration/dehydration reaction slow except when catalyzed by carbonic anhydrase (c.a.)

c.a.

HCO3

H+ + HbO2CO2

Gill epithelium

RBC

Water

There is no carbonic anhydrase available to plasma as it passes There is no carbonic anhydrase available to plasma as it passes through the gills, bicarbonate entry into RBC rate limiting step, through the gills, bicarbonate entry into RBC rate limiting step, plasma bicarbonate high in blood leaving gillsplasma bicarbonate high in blood leaving gills

c.a.

Blood

HCO3 -

HHb + O2CO2

RBC

Muscle endothelial carbonic anhydrase

Muscle

Blood plasma

As blood flows away from gills plasma As blood flows away from gills plasma bicarbonate is dehydrated to CO2bicarbonate is dehydrated to CO2Blood enters muscle capillaries about 1 to 3 Blood enters muscle capillaries about 1 to 3 seconds after leaving gillsseconds after leaving gillsCarbonic anhydrase available in muscle Carbonic anhydrase available in muscle capillary endothelia results in further capillary endothelia results in further increase in bicarbonate dehydration to CO2increase in bicarbonate dehydration to CO2CO2 acidifies RBC causing a Root off shift CO2 acidifies RBC causing a Root off shift driving oxygen into the muscledriving oxygen into the muscle

Muscle oxygen levels:Muscle oxygen levels:

• Non equilbrium state for bicarbonate in Non equilbrium state for bicarbonate in plasma entering muscle capillariesplasma entering muscle capillaries

• Carbonic anhydrase on muscle capillary Carbonic anhydrase on muscle capillary endothelium catalyses bicarbonate endothelium catalyses bicarbonate dehydration, resulting in RBC dehydration, resulting in RBC acidificationacidification

• Root off shift increases PaORoot off shift increases PaO2 2 and oxygen and oxygen diffuses into the muscle tissuediffuses into the muscle tissue

• Muscle endothial c.a activity? Muscle endothial c.a activity?

Oxygen delivery in teleosts during hypoxiaOxygen delivery in teleosts during hypoxia

Movement into tissues dependent on pH change Movement into tissues dependent on pH change rather than the oxygen gradientrather than the oxygen gradient

This allows for the selection of mechanisms to This allows for the selection of mechanisms to enhance uptake from the environment, which does enhance uptake from the environment, which does depend on oxygen diffusion gradients, maintaining depend on oxygen diffusion gradients, maintaining oxygen delivery and enabling the fish to tolerate oxygen delivery and enabling the fish to tolerate hypoxiahypoxia

• Radiation in the Triassic when oxygen levels are lowRadiation in the Triassic when oxygen levels are low• Subsequent radiation (when oxygen levels were high) Subsequent radiation (when oxygen levels were high)

related to swimming ability and feeding mechanismsrelated to swimming ability and feeding mechanisms

The teleosts are the most successful The teleosts are the most successful group of vertebratesgroup of vertebrates

• Teleost fish have low energy turnover and can survive hypoxic Teleost fish have low energy turnover and can survive hypoxic conditionsconditions

• Teleost fish have more species, numbers and biomass, than Teleost fish have more species, numbers and biomass, than mammalsmammals

• Mammals are hairy and have high energy turnover and are OK Mammals are hairy and have high energy turnover and are OK in the cold when there is plenty of food in the cold when there is plenty of food

• Mammals not so good when oxygen levels are low, fish Mammals not so good when oxygen levels are low, fish manage much better, and so did the dinosaursmanage much better, and so did the dinosaurs

““Higher and lower Higher and lower vertebrates”vertebrates”

This term reflects the narcissistic view of humansThis term reflects the narcissistic view of humans Mammals are not the most successful in terms of numbers of Mammals are not the most successful in terms of numbers of

species or biomassspecies or biomass Mammals are specialized “high energy use” machines that can Mammals are specialized “high energy use” machines that can

operate in the coldoperate in the cold Mammals are not hypoxia tolerant and need to eat all the timeMammals are not hypoxia tolerant and need to eat all the time The teleost demise is through increasing aquatic The teleost demise is through increasing aquatic hypoxiahypoxia and and

overfishingoverfishing to feed humans to feed humans LUNCH LUNCH

Thank you

pHa caO2(vol. %)

MO2(mmol Kg-1 h-1)

Normoxia 7.82 0.02 10.7 1.0 4.76 0.79

pwO2(100 mmHg)

7.81 0.03 9.9 0.8 5.11 0.90

pwO2(75 mmHg)

7.84 0.03 9.4 0.8 4.64 0.64

Recovery (1 hr) 7.80 0.02 10.6 0.5 4.48 0.97

None of the above variables were affected significantly by hypoxia. The conditions for O2 transfer to the tissues appear unchanged, except the blood:muscle pO2 gradient decreased during hypoxia.