ZOO 3210 Lecture Notes - Lecture 6: Vo2 Max, Allometry, Wader
2 May 2018
School
Department
Course
Professor
Define symmorphosis and give an example in
animals
1.
Present a research protocol of how you would test
the hypothesis of symmorphosis in an animal
2.
Learning outcomes:
Increased capillary density
!
Increase Hb-O2 affinity
!
Efficient mitochondria
!
Must keep up with demand of O2 by
mitochondria
○
Need systems to take away CO2 produced
○
Organ systems are arranged in series
○
E.g. well-trained athlete
!
Symmorphosis: close match between the various structural
and functional parameters involved in a physiological
process
"animals are not built in wasteful ways, animals are
economically designed"
!
"…all organ systems that serve a single function are
interactively adapted to have roughly equal limits
because it would make no sense for any one system
to have evolved capabilities that could never be used
because of more restrictive limits in other systems"
!
"…formation of structural elements is regulated to
satisfy but not exceed the requirements of the
functional system"
!
Symmorphosis: from the Greek 'balanced formation'
High O2 diffusing capacity of lungs
○
High heart pumping ability
○
High rates of O2 delivery to muscle fibers
(pectoral muscle) due to dense capillary
networks
○
High intracellular mitochondrial density
○
High [enzyme] involved in energy metabolism
in muscle
○
High supply of metabolic fuels from the gut
○
*steps 1-5 all run in series (see slide)
Hummingbirds: highest mass-specific metabolic rate
!
Sequence from 'fuel' to 'force'
Animals have limited resources and lots of
competition
!
They will have an advantage if they use their fuel
(energy) and physiological system wisely
!
Lineages with more successful progeny than other
competing lineages will prevail
!
Less complex systems save energy and even small
energetic advantages can have a big effect over time
!
Economy of 'design' or 'engineering'
Horses select to run over a limited range of
preferred speeds for each gate
!
Will change gate depending on energetic costs
at a certain speed
○
When forced to run, the preferred speed corresponds
to speed with the minimal costs (O2 consumption)
!
Conclusion: horses are economic, they select the
appropriate speed for each activity that minimizes
the energy costs (i.e. O2 uptake)
!
Energy Minimization in Nature: Horses
In there an economy of design among physiological
systems?
!
Ex. Can cardiac output be 10x from rest to
exercise, but mitochondrial oxidative
phosphorylation only be 2x higher?
○
Or does one part of the system have a much higher
capacity?
!
So, animals save energy when they can at the whole
animal level, but do they also save energy by linking
physiological systems designed to run at matched
performance?
*see slide
!
At VO2 max, the capacity on the demand side
needs to be matched by the capacity on the
supply side
○
Only at VO2 max is it useful to examine the
capacity of the multiple parts to determine if they
are matched to or surpass the demand of the whole
system
!
Increase in blood flow to skeletal muscles by
20x
○
Decreased blood flow to kidney and liver
○
No change in blood flow to brain (obligatory
demand)
○
Increase in whole body blood flow by 4x
○
Lungs always receive 100% of blood flow
○
*see distribution of blood flow among circulatory
subsystems at rest and during max exercise
!
*optimization is done within certain limits (e.g. flow
to brain)
Are the capacities of any part of the circulatory
system wasteful?
!
Capacity for performance (maximal O2
consumption)
!
Functional capacity of muscle
mitochondria to generate ATP
(mitochondrial volume)
!
Capacity of blood circulation to deliver
O2 to muscle (capillary volume)
!
Capacity of lungs to capture O2 from air
(O2 diffusion capacity)
!
Exercised range of mammals on customized
treadmills to quantify:
○
Double-logarithmic plots (allometric
comparisons) of maximal O2
consumption, as well as
structural/functional elements in
respiratory chain, against body mass
!
Note: athletic species have higher
oxygen diffusing capacity,
mitochondrial volume…etc.
!
Mitochondrial volume (almost
identical relationship to body
mass)
"
Capillary volume
"
--> evidence for symmorphosis
because they have similar
capacities (in series)
Features of respiratory system balanced
with maximal O2 consumption:
!
Oxygen diffusing capacity of
lungs is > whole animal oxygen
consumption (and other
components)
"
Feature of the respiratory system that
have an over capacity:
!
Results:
○
East Africa: mice, mongooses, wildebeest, lions,
eland, steers, horses (large body mass range)
!
Flexibility to chose habitats with
different O2 levels
!
Lungs can't be remodeled like other
organs (alveolar surface area in
mammals doesn't increase in adults)
!
Due to…
○
*in contrast, reptiles increase lung mass with
feeding
○
Are lungs in mammals constructed in a
somewhat wasteful way so that their capacity
is on the safe side?
○
Further studies have confirmed that lung capacity is
2-fold greater than what is needed for maximal O2
uptake (note all evidence presented so far have been
on mammals)
!
Most tests of symmorphosis hypothesis use the respiratory
system
04/03/18
Define safety factors of biological systems and the
considerations that affect their values
1.
Present argument for and against the concept of
symmorphosis
2.
Learning Outcomes:
With O2 diffusing capacity that are twice as large as
needed for maximal O2 consumption, the design of
lungs may be wasteful but it is also safe
!
SF = maximum O2 diffusing capacity / O2
diffusing capacity needed for max O2 uptake
○
Safety factor (SF) = capacity / maximum natural
load
!
When SF=1, there is an exact match between
maximum demand and capacity of system
(=symmorphosis)
!
Design failure can have disastrous
consequences
○
Deterioration of system is expected with use
○
Variability of overload on system is expected
○
In general, SF >1 when:
!
Cost of initial construction, maintenance,
operation, and rebuilding
○
In general, SF decreases with:
!
Cables of fast passenger elevators have SF =
12
○
Able to withstand load 12x more than stated
○
Relevance of safety factors to engineering and the
construction of elevator cables:
!
Engineering SF are fixed while biological SF can
change (= adaptive regulation)
!
Safety Factors:
Highest safety factor: human teeth
!
Lowest safety factor: shell of squid
!
Any organ that is duplicated have lower safety
factors
!
Variability of overload is also a factor
!
Shaft of feathers > Wing bones
○
Due to pressure/wind forces feathers are more
likely to break than bones
○
Ex. Wings of birds
!
Animals that run at high speeds have higher
SF of bones
○
All bones have same SF in organisms (even though
may be more likely to fracture) b because they are
made of the same structure and have same
density --> due to costs
!
*SF are balance of costs and benefits
!
Tail of lizards (geckos)
○
SF <1 --> spiders: attachment of legs to body (more
advantageous to lose leg than to be eaten)
!
Relevance of Safety Factors in Biology
CO2 excretion and acid-base balance
○
Evaporative water loss (thermoregulation)
○
Besides O2 uptake lungs are involved in:
!
Animal structure and function are really a
'continua of imperfection'
○
Since selection pressures change over time,
evolution by natural selection does not
necessarily lead to optimal design
○
Although there is good evidence for symmorphosis,
clearly the principles of elegant economic design
need qualification. Critics point out that…
!
Multiple Design Criteria:
Question: are the structural components of the
respiratory system in the locust quantitatively
adjusted to satisfy, but not exceed, maximum O2
uptake?
!
Increased tracheole lumen volume,
inner cuticle SA and diffusing
capacity --> same fold difference
!
Increased mitochondrial volume
!
Flight muscles required more O2 than hoping
muscles
○
*see table
!
Symmorphosis is upheld in the design of the
tracheal system but not in relation to the
amount of mitochondria
○
--> safety factors
○
Can determine limitations and gain
understanding of where safety factors are
within system
○
Conclusion: no more tracheole structure exists than
is needed at max performance
!
Testing for evidence of symmorphosis in the insect
respiratory system
VO2 = femoral blood flow x (arterial -venous
O2 difference)
○
Using knee extensor (KE) exercise --> VO2 max
assessed by direct Fick method:
!
Measure maximal mitochondrial respiration
rate
○
VO2 max assessed in vitro from muscle biopsy:
!
*see slide
!
Maximum mitochondrial O2 consumption in
muscles is proportional to O2 consumption
during KE exercise in untrained
○
O2 consumption during KE is fairly
constant in trained, despite variations in
mitochondrial O2 consumption
(independent)
!
Maxiumum mitchondrial O2 consumption in
trained individuals is higher than in untrained
○
--> suggests that endurance trained have over
mitochondrial respiration rate capacity
○
Evidence of a relationship between maximal
mitochondrial O2 consumption and maximal O2
consumption during KE exercise in untrained, but
not in trained subjects
!
Untrained --> O2 supply > O2
consumption
!
Have increased O2 supply and
increased mitochondrial
consumption
"
O2 consumption in muscles is not
increased as much
"
Trained --> mitochondrial O2
consumption > O2 supply > muscle O2
consumption
!
QO2 max = femoral blood flow x arterial O2
content
○
Among untrained individuals, VO2 max
is limited by the capacity of the
mitochondria to consume O2, despite an
excess in O2 supply
!
In trained individuals, VO2 max is
limited by supply of O2 to the
mitochondria, despite and excess in
mitochondrial respiratory capacity
!
The observation that the limitation to
VO2 max shift with endurance training
in humans conflicts with concept of
symmorphosis
!
Conclusions:
○
Maximal O2 supply is assessed as:
!
Testing for evidence of symmorphosis in the capacity for
O2 supply and consumption in the skeletal muscles of
untrained and endurance-trained humans
Mice at 23C had decreased small
intestine mass and food intake
!
Lactating mice had increased food
intake and small intestine mass, but
those at 5C had even higher
measurements
!
Mass of small intestine increases with food
intake:
○
However, the slope of the relationship is
<1 (the increase intestine mass does not
match the increase in food intake)
!
Ability to digest all of the food
consumed will be reduced (with
high intake)
"
Consequences
!
The intestine of mice undergoes hypertrophy
to match the increase in energy requirements
due to cold ambient temperatures, lactation,
and expanded litter size
○
However, given the relationship
food intake and small intestine
mass, this means that the
absorption capacity of the small
intestine does not keep up with
increases in energy requirements
"
As evidence of symmorphosis, nutrient
transport capacity and brush border
enzymatic activity per gram of small
intestine tissue stays the same with
intestine hypertrophy
!
Conclusion: the safety factor observed
under relaxed conditions (e.g. in non-
lactating mice at 23C) levels off under
conditions of increasing food intake
(e.g. in lactating mice at 5C)
!
Safety factor decreases (3 --> <1) as glucose
intake increases (--> increased cost)
○
Question: what is the relationship between food
intake and small intestine mass in mice experiencing
different levels of energy expenditure?
!
Liver and intestine mass is correlated with
gizzard mass
○
As evidence of symmorphosis, there is an
allometric relationship between the organ
sizes among shorebird species
○
In the shorebird species that specialize on
eating molluscs (bivalves), the need to crush
shells may explain the larger gizzards relative
to liver size
○
Question: what is the relationships between small
intestine mass or liver mass, and gizzard mass in a
range of shorebird species
!
Gizzard and intestine are essential for
breakign down bivalves and acquiring
nutrients
!
Relationship is only seen in Iceland
because they are trying to incrase mass,
rather than just maintaining their body
mass in winter
!
Relationship between intestine and gizzard
mass is much steeper during the spring
refuelling in Iceland than in the winter
○
Liver has much greater capacity to
process nutrients in winter than needed
in circumstances (at capacity in spring)
!
Liver mass only correlated with gizzard mass
at the spring stopover site
○
Depends when components of system
are being pushed to their limits
!
Re-fuel --> symmorphosis
!
Conclusion: there is evidence of
symmorphosis with a qualification
○
Question: what is the relationships between intestine
mass or liver mass and gizzard mass in the red knot
during the northward migration in Iceland or during
the winter in Western Europe?
!
Testing for symmorphosis in the digestive system
Symmorphosis: the principle that evolved body
designs avoid excess capacity (e.g. in cascades of
serial physiological processes), now widely accepted
as a useful design principle
!
Null hypotheis: same capacity relationship?
○
Symmorphosis may be better seen as a useful null
hypothesis of organismal performance rather than a
hypothesis with very precise and rigid criteria for
rejection
!
A consideration of safety factors in organismal
performance provides some biological insight into
the limitations of economy-based designs
!
Conclusions:
Physiological Systems
#$%&'()*+, -)&.$, /0+,/123
2425,6-
Define symmorphosis and give an example in
animals
1.
Present a research protocol of how you would test
the hypothesis of symmorphosis in an animal
2.
Learning outcomes:
Increased capillary density
!
Increase Hb-O2 affinity
!
Efficient mitochondria
!
Must keep up with demand of O2 by
mitochondria
○
Need systems to take away CO2 produced
○
Organ systems are arranged in series
○
E.g. well-trained athlete
!
Symmorphosis: close match between the various structural
and functional parameters involved in a physiological
process
"animals are not built in wasteful ways, animals are
economically designed"
!
"…all organ systems that serve a single function are
interactively adapted to have roughly equal limits
because it would make no sense for any one system
to have evolved capabilities that could never be used
because of more restrictive limits in other systems"
!
"…formation of structural elements is regulated to
satisfy but not exceed the requirements of the
functional system"
!
Symmorphosis: from the Greek 'balanced formation'
High O2 diffusing capacity of lungs
○
High heart pumping ability
○
High rates of O2 delivery to muscle fibers
(pectoral muscle) due to dense capillary
networks
○
High intracellular mitochondrial density
○
High [enzyme] involved in energy metabolism
in muscle
○
High supply of metabolic fuels from the gut
○
*steps 1-5 all run in series (see slide)
Hummingbirds: highest mass-specific metabolic rate
!
Sequence from 'fuel' to 'force'
Animals have limited resources and lots of
competition
!
They will have an advantage if they use their fuel
(energy) and physiological system wisely
!
Lineages with more successful progeny than other
competing lineages will prevail
!
Less complex systems save energy and even small
energetic advantages can have a big effect over time
!
Economy of 'design' or 'engineering'
Horses select to run over a limited range of
preferred speeds for each gate
!
Will change gate depending on energetic costs
at a certain speed
○
When forced to run, the preferred speed corresponds
to speed with the minimal costs (O2 consumption)
!
Conclusion: horses are economic, they select the
appropriate speed for each activity that minimizes
the energy costs (i.e. O2 uptake)
!
Energy Minimization in Nature: Horses
In there an economy of design among physiological
systems?
!
Ex. Can cardiac output be 10x from rest to
exercise, but mitochondrial oxidative
phosphorylation only be 2x higher?
○
Or does one part of the system have a much higher
capacity?
!
So, animals save energy when they can at the whole
animal level, but do they also save energy by linking
physiological systems designed to run at matched
performance?
*see slide
!
At VO2 max, the capacity on the demand side
needs to be matched by the capacity on the
supply side
○
Only at VO2 max is it useful to examine the
capacity of the multiple parts to determine if they
are matched to or surpass the demand of the whole
system
!
Increase in blood flow to skeletal muscles by
20x
○
Decreased blood flow to kidney and liver
○
No change in blood flow to brain (obligatory
demand)
○
Increase in whole body blood flow by 4x
○
Lungs always receive 100% of blood flow
○
*see distribution of blood flow among circulatory
subsystems at rest and during max exercise
!
*optimization is done within certain limits (e.g. flow
to brain)
Are the capacities of any part of the circulatory
system wasteful?
!
Capacity for performance (maximal O2
consumption)
!
Functional capacity of muscle
mitochondria to generate ATP
(mitochondrial volume)
!
Capacity of blood circulation to deliver
O2 to muscle (capillary volume)
!
Capacity of lungs to capture O2 from air
(O2 diffusion capacity)
!
Exercised range of mammals on customized
treadmills to quantify:
○
Double-logarithmic plots (allometric
comparisons) of maximal O2
consumption, as well as
structural/functional elements in
respiratory chain, against body mass
!
Note: athletic species have higher
oxygen diffusing capacity,
mitochondrial volume…etc.
!
Mitochondrial volume (almost
identical relationship to body
mass)
"
Capillary volume
"
--> evidence for symmorphosis
because they have similar
capacities (in series)
Features of respiratory system balanced
with maximal O2 consumption:
!
Oxygen diffusing capacity of
lungs is > whole animal oxygen
consumption (and other
components)
"
Feature of the respiratory system that
have an over capacity:
!
Results:
○
East Africa: mice, mongooses, wildebeest, lions,
eland, steers, horses (large body mass range)
!
Flexibility to chose habitats with
different O2 levels
!
Lungs can't be remodeled like other
organs (alveolar surface area in
mammals doesn't increase in adults)
!
Due to…
○
*in contrast, reptiles increase lung mass with
feeding
○
Are lungs in mammals constructed in a
somewhat wasteful way so that their capacity
is on the safe side?
○
Further studies have confirmed that lung capacity is
2-fold greater than what is needed for maximal O2
uptake (note all evidence presented so far have been
on mammals)
!
Most tests of symmorphosis hypothesis use the respiratory
system
04/03/18
Define safety factors of biological systems and the
considerations that affect their values
1.
Present argument for and against the concept of
symmorphosis
2.
Learning Outcomes:
With O2 diffusing capacity that are twice as large as
needed for maximal O2 consumption, the design of
lungs may be wasteful but it is also safe
!
SF = maximum O2 diffusing capacity / O2
diffusing capacity needed for max O2 uptake
○
Safety factor (SF) = capacity / maximum natural
load
!
When SF=1, there is an exact match between
maximum demand and capacity of system
(=symmorphosis)
!
Design failure can have disastrous
consequences
○
Deterioration of system is expected with use
○
Variability of overload on system is expected
○
In general, SF >1 when:
!
Cost of initial construction, maintenance,
operation, and rebuilding
○
In general, SF decreases with:
!
Cables of fast passenger elevators have SF =
12
○
Able to withstand load 12x more than stated
○
Relevance of safety factors to engineering and the
construction of elevator cables:
!
Engineering SF are fixed while biological SF can
change (= adaptive regulation)
!
Safety Factors:
Highest safety factor: human teeth
!
Lowest safety factor: shell of squid
!
Any organ that is duplicated have lower safety
factors
!
Variability of overload is also a factor
!
Shaft of feathers > Wing bones
○
Due to pressure/wind forces feathers are more
likely to break than bones
○
Ex. Wings of birds
!
Animals that run at high speeds have higher
SF of bones
○
All bones have same SF in organisms (even though
may be more likely to fracture) b because they are
made of the same structure and have same
density --> due to costs
!
*SF are balance of costs and benefits
!
Tail of lizards (geckos)
○
SF <1 --> spiders: attachment of legs to body (more
advantageous to lose leg than to be eaten)
!
Relevance of Safety Factors in Biology
CO2 excretion and acid-base balance
○
Evaporative water loss (thermoregulation)
○
Besides O2 uptake lungs are involved in:
!
Animal structure and function are really a
'continua of imperfection'
○
Since selection pressures change over time,
evolution by natural selection does not
necessarily lead to optimal design
○
Although there is good evidence for symmorphosis,
clearly the principles of elegant economic design
need qualification. Critics point out that…
!
Multiple Design Criteria:
Question: are the structural components of the
respiratory system in the locust quantitatively
adjusted to satisfy, but not exceed, maximum O2
uptake?
!
Increased tracheole lumen volume,
inner cuticle SA and diffusing
capacity --> same fold difference
!
Increased mitochondrial volume
!
Flight muscles required more O2 than hoping
muscles
○
*see table
!
Symmorphosis is upheld in the design of the
tracheal system but not in relation to the
amount of mitochondria
○
--> safety factors
○
Can determine limitations and gain
understanding of where safety factors are
within system
○
Conclusion: no more tracheole structure exists than
is needed at max performance
!
Testing for evidence of symmorphosis in the insect
respiratory system
VO2 = femoral blood flow x (arterial -venous
O2 difference)
○
Using knee extensor (KE) exercise --> VO2 max
assessed by direct Fick method:
!
Measure maximal mitochondrial respiration
rate
○
VO2 max assessed in vitro from muscle biopsy:
!
*see slide
!
Maximum mitochondrial O2 consumption in
muscles is proportional to O2 consumption
during KE exercise in untrained
○
O2 consumption during KE is fairly
constant in trained, despite variations in
mitochondrial O2 consumption
(independent)
!
Maxiumum mitchondrial O2 consumption in
trained individuals is higher than in untrained
○
--> suggests that endurance trained have over
mitochondrial respiration rate capacity
○
Evidence of a relationship between maximal
mitochondrial O2 consumption and maximal O2
consumption during KE exercise in untrained, but
not in trained subjects
!
Untrained --> O2 supply > O2
consumption
!
Have increased O2 supply and
increased mitochondrial
consumption
"
O2 consumption in muscles is not
increased as much
"
Trained --> mitochondrial O2
consumption > O2 supply > muscle O2
consumption
!
QO2 max = femoral blood flow x arterial O2
content
○
Among untrained individuals, VO2 max
is limited by the capacity of the
mitochondria to consume O2, despite an
excess in O2 supply
!
In trained individuals, VO2 max is
limited by supply of O2 to the
mitochondria, despite and excess in
mitochondrial respiratory capacity
!
The observation that the limitation to
VO2 max shift with endurance training
in humans conflicts with concept of
symmorphosis
!
Conclusions:
○
Maximal O2 supply is assessed as:
!
Testing for evidence of symmorphosis in the capacity for
O2 supply and consumption in the skeletal muscles of
untrained and endurance-trained humans
Mice at 23C had decreased small
intestine mass and food intake
!
Lactating mice had increased food
intake and small intestine mass, but
those at 5C had even higher
measurements
!
Mass of small intestine increases with food
intake:
○
However, the slope of the relationship is
<1 (the increase intestine mass does not
match the increase in food intake)
!
Ability to digest all of the food
consumed will be reduced (with
high intake)
"
Consequences
!
The intestine of mice undergoes hypertrophy
to match the increase in energy requirements
due to cold ambient temperatures, lactation,
and expanded litter size
○
However, given the relationship
food intake and small intestine
mass, this means that the
absorption capacity of the small
intestine does not keep up with
increases in energy requirements
"
As evidence of symmorphosis, nutrient
transport capacity and brush border
enzymatic activity per gram of small
intestine tissue stays the same with
intestine hypertrophy
!
Conclusion: the safety factor observed
under relaxed conditions (e.g. in non-
lactating mice at 23C) levels off under
conditions of increasing food intake
(e.g. in lactating mice at 5C)
!
Safety factor decreases (3 --> <1) as glucose
intake increases (--> increased cost)
○
Question: what is the relationship between food
intake and small intestine mass in mice experiencing
different levels of energy expenditure?
!
Liver and intestine mass is correlated with
gizzard mass
○
As evidence of symmorphosis, there is an
allometric relationship between the organ
sizes among shorebird species
○
In the shorebird species that specialize on
eating molluscs (bivalves), the need to crush
shells may explain the larger gizzards relative
to liver size
○
Question: what is the relationships between small
intestine mass or liver mass, and gizzard mass in a
range of shorebird species
!
Gizzard and intestine are essential for
breakign down bivalves and acquiring
nutrients
!
Relationship is only seen in Iceland
because they are trying to incrase mass,
rather than just maintaining their body
mass in winter
!
Relationship between intestine and gizzard
mass is much steeper during the spring
refuelling in Iceland than in the winter
○
Liver has much greater capacity to
process nutrients in winter than needed
in circumstances (at capacity in spring)
!
Liver mass only correlated with gizzard mass
at the spring stopover site
○
Depends when components of system
are being pushed to their limits
!
Re-fuel --> symmorphosis
!
Conclusion: there is evidence of
symmorphosis with a qualification
○
Question: what is the relationships between intestine
mass or liver mass and gizzard mass in the red knot
during the northward migration in Iceland or during
the winter in Western Europe?
!
Testing for symmorphosis in the digestive system
Symmorphosis: the principle that evolved body
designs avoid excess capacity (e.g. in cascades of
serial physiological processes), now widely accepted
as a useful design principle
!
Null hypotheis: same capacity relationship?
○
Symmorphosis may be better seen as a useful null
hypothesis of organismal performance rather than a
hypothesis with very precise and rigid criteria for
rejection
!
A consideration of safety factors in organismal
performance provides some biological insight into
the limitations of economy-based designs
!
Conclusions:
Physiological Systems
#$%&'()*+, -)&.$, /0+,/123 2425,6-
Define symmorphosis and give an example in
animals
1.
Present a research protocol of how you would test
the hypothesis of symmorphosis in an animal
2.
Learning outcomes:
Increased capillary density
!
Increase Hb-O2 affinity
!
Efficient mitochondria
!
Must keep up with demand of O2 by
mitochondria
○
Need systems to take away CO2 produced
○
Organ systems are arranged in series
○
E.g. well-trained athlete
!
Symmorphosis: close match between the various structural
and functional parameters involved in a physiological
process
"animals are not built in wasteful ways, animals are
economically designed"
!
"…all organ systems that serve a single function are
interactively adapted to have roughly equal limits
because it would make no sense for any one system
to have evolved capabilities that could never be used
because of more restrictive limits in other systems"
!
"…formation of structural elements is regulated to
satisfy but not exceed the requirements of the
functional system"
!
Symmorphosis: from the Greek 'balanced formation'
High O2 diffusing capacity of lungs
○
High heart pumping ability
○
High rates of O2 delivery to muscle fibers
(pectoral muscle) due to dense capillary
networks
○
High intracellular mitochondrial density
○
High [enzyme] involved in energy metabolism
in muscle
○
High supply of metabolic fuels from the gut
○
*steps 1-5 all run in series (see slide)
Hummingbirds: highest mass-specific metabolic rate
!
Sequence from 'fuel' to 'force'
Animals have limited resources and lots of
competition
!
They will have an advantage if they use their fuel
(energy) and physiological system wisely
!
Lineages with more successful progeny than other
competing lineages will prevail
!
Less complex systems save energy and even small
energetic advantages can have a big effect over time
!
Economy of 'design' or 'engineering'
Horses select to run over a limited range of
preferred speeds for each gate
!
Will change gate depending on energetic costs
at a certain speed
○
When forced to run, the preferred speed corresponds
to speed with the minimal costs (O2 consumption)
!
Conclusion: horses are economic, they select the
appropriate speed for each activity that minimizes
the energy costs (i.e. O2 uptake)
!
Energy Minimization in Nature: Horses
In there an economy of design among physiological
systems?
!
Ex. Can cardiac output be 10x from rest to
exercise, but mitochondrial oxidative
phosphorylation only be 2x higher?
○
Or does one part of the system have a much higher
capacity?
!
So, animals save energy when they can at the whole
animal level, but do they also save energy by linking
physiological systems designed to run at matched
performance?
*see slide
!
At VO2 max, the capacity on the demand side
needs to be matched by the capacity on the
supply side
○
Only at VO2 max is it useful to examine the
capacity of the multiple parts to determine if they
are matched to or surpass the demand of the whole
system
!
Increase in blood flow to skeletal muscles by
20x
○
Decreased blood flow to kidney and liver
○
No change in blood flow to brain (obligatory
demand)
○
Increase in whole body blood flow by 4x
○
Lungs always receive 100% of blood flow
○
*see distribution of blood flow among circulatory
subsystems at rest and during max exercise
!
*optimization is done within certain limits (e.g. flow
to brain)
Are the capacities of any part of the circulatory
system wasteful?
!
Capacity for performance (maximal O2
consumption)
!
Functional capacity of muscle
mitochondria to generate ATP
(mitochondrial volume)
!
Capacity of blood circulation to deliver
O2 to muscle (capillary volume)
!
Capacity of lungs to capture O2 from air
(O2 diffusion capacity)
!
Exercised range of mammals on customized
treadmills to quantify:
○
Double-logarithmic plots (allometric
comparisons) of maximal O2
consumption, as well as
structural/functional elements in
respiratory chain, against body mass
!
Note: athletic species have higher
oxygen diffusing capacity,
mitochondrial volume…etc.
!
Mitochondrial volume (almost
identical relationship to body
mass)
"
Capillary volume
"
--> evidence for symmorphosis
because they have similar
capacities (in series)
Features of respiratory system balanced
with maximal O2 consumption:
!
Oxygen diffusing capacity of
lungs is > whole animal oxygen
consumption (and other
components)
"
Feature of the respiratory system that
have an over capacity:
!
Results:
○
East Africa: mice, mongooses, wildebeest, lions,
eland, steers, horses (large body mass range)
!
Flexibility to chose habitats with
different O2 levels
!
Lungs can't be remodeled like other
organs (alveolar surface area in
mammals doesn't increase in adults)
!
Due to…
○
*in contrast, reptiles increase lung mass with
feeding
○
Are lungs in mammals constructed in a
somewhat wasteful way so that their capacity
is on the safe side?
○
Further studies have confirmed that lung capacity is
2-fold greater than what is needed for maximal O2
uptake (note all evidence presented so far have been
on mammals)
!
Most tests of symmorphosis hypothesis use the respiratory
system
04/03/18
Define safety factors of biological systems and the
considerations that affect their values
1.
Present argument for and against the concept of
symmorphosis
2.
Learning Outcomes:
With O2 diffusing capacity that are twice as large as
needed for maximal O2 consumption, the design of
lungs may be wasteful but it is also safe
!
SF = maximum O2 diffusing capacity / O2
diffusing capacity needed for max O2 uptake
○
Safety factor (SF) = capacity / maximum natural
load
!
When SF=1, there is an exact match between
maximum demand and capacity of system
(=symmorphosis)
!
Design failure can have disastrous
consequences
○
Deterioration of system is expected with use
○
Variability of overload on system is expected
○
In general, SF >1 when:
!
Cost of initial construction, maintenance,
operation, and rebuilding
○
In general, SF decreases with:
!
Cables of fast passenger elevators have SF =
12
○
Able to withstand load 12x more than stated
○
Relevance of safety factors to engineering and the
construction of elevator cables:
!
Engineering SF are fixed while biological SF can
change (= adaptive regulation)
!
Safety Factors:
Highest safety factor: human teeth
!
Lowest safety factor: shell of squid
!
Any organ that is duplicated have lower safety
factors
!
Variability of overload is also a factor
!
Shaft of feathers > Wing bones
○
Due to pressure/wind forces feathers are more
likely to break than bones
○
Ex. Wings of birds
!
Animals that run at high speeds have higher
SF of bones
○
All bones have same SF in organisms (even though
may be more likely to fracture) b because they are
made of the same structure and have same
density --> due to costs
!
*SF are balance of costs and benefits
!
Tail of lizards (geckos)
○
SF <1 --> spiders: attachment of legs to body (more
advantageous to lose leg than to be eaten)
!
Relevance of Safety Factors in Biology
CO2 excretion and acid-base balance
○
Evaporative water loss (thermoregulation)
○
Besides O2 uptake lungs are involved in:
!
Animal structure and function are really a
'continua of imperfection'
○
Since selection pressures change over time,
evolution by natural selection does not
necessarily lead to optimal design
○
Although there is good evidence for symmorphosis,
clearly the principles of elegant economic design
need qualification. Critics point out that…
!
Multiple Design Criteria:
Question: are the structural components of the
respiratory system in the locust quantitatively
adjusted to satisfy, but not exceed, maximum O2
uptake?
!
Increased tracheole lumen volume,
inner cuticle SA and diffusing
capacity --> same fold difference
!
Increased mitochondrial volume
!
Flight muscles required more O2 than hoping
muscles
○
*see table
!
Symmorphosis is upheld in the design of the
tracheal system but not in relation to the
amount of mitochondria
○
--> safety factors
○
Can determine limitations and gain
understanding of where safety factors are
within system
○
Conclusion: no more tracheole structure exists than
is needed at max performance
!
Testing for evidence of symmorphosis in the insect
respiratory system
VO2 = femoral blood flow x (arterial -venous
O2 difference)
○
Using knee extensor (KE) exercise --> VO2 max
assessed by direct Fick method:
!
Measure maximal mitochondrial respiration
rate
○
VO2 max assessed in vitro from muscle biopsy:
!
*see slide
!
Maximum mitochondrial O2 consumption in
muscles is proportional to O2 consumption
during KE exercise in untrained
○
O2 consumption during KE is fairly
constant in trained, despite variations in
mitochondrial O2 consumption
(independent)
!
Maxiumum mitchondrial O2 consumption in
trained individuals is higher than in untrained
○
--> suggests that endurance trained have over
mitochondrial respiration rate capacity
○
Evidence of a relationship between maximal
mitochondrial O2 consumption and maximal O2
consumption during KE exercise in untrained, but
not in trained subjects
!
Untrained --> O2 supply > O2
consumption
!
Have increased O2 supply and
increased mitochondrial
consumption
"
O2 consumption in muscles is not
increased as much
"
Trained --> mitochondrial O2
consumption > O2 supply > muscle O2
consumption
!
QO2 max = femoral blood flow x arterial O2
content
○
Among untrained individuals, VO2 max
is limited by the capacity of the
mitochondria to consume O2, despite an
excess in O2 supply
!
In trained individuals, VO2 max is
limited by supply of O2 to the
mitochondria, despite and excess in
mitochondrial respiratory capacity
!
The observation that the limitation to
VO2 max shift with endurance training
in humans conflicts with concept of
symmorphosis
!
Conclusions:
○
Maximal O2 supply is assessed as:
!
Testing for evidence of symmorphosis in the capacity for
O2 supply and consumption in the skeletal muscles of
untrained and endurance-trained humans
Mice at 23C had decreased small
intestine mass and food intake
!
Lactating mice had increased food
intake and small intestine mass, but
those at 5C had even higher
measurements
!
Mass of small intestine increases with food
intake:
○
However, the slope of the relationship is
<1 (the increase intestine mass does not
match the increase in food intake)
!
Ability to digest all of the food
consumed will be reduced (with
high intake)
"
Consequences
!
The intestine of mice undergoes hypertrophy
to match the increase in energy requirements
due to cold ambient temperatures, lactation,
and expanded litter size
○
However, given the relationship
food intake and small intestine
mass, this means that the
absorption capacity of the small
intestine does not keep up with
increases in energy requirements
"
As evidence of symmorphosis, nutrient
transport capacity and brush border
enzymatic activity per gram of small
intestine tissue stays the same with
intestine hypertrophy
!
Conclusion: the safety factor observed
under relaxed conditions (e.g. in non-
lactating mice at 23C) levels off under
conditions of increasing food intake
(e.g. in lactating mice at 5C)
!
Safety factor decreases (3 --> <1) as glucose
intake increases (--> increased cost)
○
Question: what is the relationship between food
intake and small intestine mass in mice experiencing
different levels of energy expenditure?
!
Liver and intestine mass is correlated with
gizzard mass
○
As evidence of symmorphosis, there is an
allometric relationship between the organ
sizes among shorebird species
○
In the shorebird species that specialize on
eating molluscs (bivalves), the need to crush
shells may explain the larger gizzards relative
to liver size
○
Question: what is the relationships between small
intestine mass or liver mass, and gizzard mass in a
range of shorebird species
!
Gizzard and intestine are essential for
breakign down bivalves and acquiring
nutrients
!
Relationship is only seen in Iceland
because they are trying to incrase mass,
rather than just maintaining their body
mass in winter
!
Relationship between intestine and gizzard
mass is much steeper during the spring
refuelling in Iceland than in the winter
○
Liver has much greater capacity to
process nutrients in winter than needed
in circumstances (at capacity in spring)
!
Liver mass only correlated with gizzard mass
at the spring stopover site
○
Depends when components of system
are being pushed to their limits
!
Re-fuel --> symmorphosis
!
Conclusion: there is evidence of
symmorphosis with a qualification
○
Question: what is the relationships between intestine
mass or liver mass and gizzard mass in the red knot
during the northward migration in Iceland or during
the winter in Western Europe?
!
Testing for symmorphosis in the digestive system
Symmorphosis: the principle that evolved body
designs avoid excess capacity (e.g. in cascades of
serial physiological processes), now widely accepted
as a useful design principle
!
Null hypotheis: same capacity relationship?
○
Symmorphosis may be better seen as a useful null
hypothesis of organismal performance rather than a
hypothesis with very precise and rigid criteria for
rejection
!
A consideration of safety factors in organismal
performance provides some biological insight into
the limitations of economy-based designs
!
Conclusions:
Physiological Systems
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