Mammalian Gills
Logan Kearsley
it capable of breathing under water. The first things that come to mind are
adding something like starfish gills, little thin-walled bumps scattered all
over the skin for gas exchange, or adding something more like fish or
mollusk gills.
There are three major considerations I can think of- protecting the gills
out of water, making sure that they can sift out enough oxygen to support
the animal, and integrating the gills with the rest of the body
structurally. Skin gills are the easiest as far as structure, and I was
thinking perhaps they could lie flat and continuous with the rest of the
skin when dry and just inflate under water, but they're necessarily limited
to less than the surface area of the skin, which is far less than the
surface area in a mammalian lung. So, probably they're out.
So, looking instead at fish/mollusk type gills, it obviously won't work to
try and put them back into the head and neck; there's just not enough room,
and even if there were, we've used up our former gill arches on other
things. Having just read the _Venus Prime_ series, I rather like the
solution used there- how about building gills into the chest, just behind
the ribs? An extra barrier would have to be added between the ribs and the
organ cavity to create a space for the gills for water to flow through.
Exactly how to locate the intakes presents a bit of trouble- perhaps at the
base of the neck, just above the clavicles? One could put gill slits between
every set of ribs, but to minimize the number of connections to the outside,
and thus the chance of infection, I'm thinking of putting just two slits
along the side of the body between the seventh and eighth ribs. Out of
water, flaps of skin would seal over the ribs, to keep the gill cavity
isolated. I expect you'd have to expand the rib cage a bit to provide extra
room to fit the gills in, but the chest ought to provide ample room for
plenty of gill surface area.
The only major problem I can think of with that is how to pump water through
the gill cavity. If the lungs are deflated and sealed off, could the
diaphragm and rib muscles expand and contract the gill cavity just like the
lungs, with the intake and outlet slits alternating opening and closing?
-l.
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Mike Van Pelt
Logan Kearsley <chrono...@verizon.net> wrote:
>I've been thinking about how one could take a mammal, like a human,
>and make it capable of breathing under water.
Another issue is that mammals are warm-blooded. The gills have
to pass the blood through a large surface area separated from
the water by a thin membrane to allow gas exchange. This also
allows heat exchange.
One fix is to do it the way tuna do -- Tuna maintain a relatively
constant body temperature considerably higher than the water they
swim through. The blood vessels leading to and from the gills
act as a counterflow heat exchanger to minimize heat loss.
--
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Robert O'Connor
<chrono...@verizon.net> wrote:
>I've been thinking about how one could take a mammal, like a human, and make
>it capable of breathing under water. The first things that come to mind are
>adding something like starfish gills, little thin-walled bumps scattered all
>over the skin for gas exchange, or adding something more like fish or
>mollusk gills.
The biggest problem is that you won't be able to extract enough oxygen
out of any plausible body of water to meet metabolic demand.
Oyxgen isn't all that soluble. The flow rates of water through the
gill apparatus will need to be large, or the gill itself very big.
Mike van Pelt's countercurrent exchanger will limit heat loss across
the gill, but this will minimally slow the onset of dangerous
hypothermia.
Dr. Robert O'Connor
Logan Kearsley
news:44befd6f$0$84241$d36...@news.calweb.com...
> In article <_MCvg.10874$Oj.1934@trnddc05>,
> Logan Kearsley <chrono...@verizon.net> wrote:
> >I've been thinking about how one could take a mammal, like a human,
> >and make it capable of breathing under water.
>
> Another issue is that mammals are warm-blooded. The gills have
> to pass the blood through a large surface area separated from
> the water by a thin membrane to allow gas exchange. This also
> allows heat exchange.
>
> One fix is to do it the way tuna do -- Tuna maintain a relatively
> constant body temperature considerably higher than the water they
> swim through. The blood vessels leading to and from the gills
> act as a counterflow heat exchanger to minimize heat loss.
Lots of animals use systems like that (called a rete mirabile). It's in
webbed-footed birds' legs, fish swim bladders, and dogs' and giraffes' necks
(not always for heat exchange, though).
I figure that problem is pretty much solved, although the details of
re-arranging blood vessels going to the rib cage and the gill cavity wall
might be tricky.
Logan Kearsley
news:md0ub21k8pevn6d5c...@4ax.com...
It seems to work for tuna. We could just restrict gill-modified mammals to
swimming in warm water, but then colder water holds more oxygen.
So, we need some way to minimize metabolic demand, then. How about just
allwoing the body to work at a lower temperature? Camels can handle body
temperature swings of 11 degrees Fahrenheit / 6 Celsius. Any other ideas?
When you say "flow rates... will need to be large", just how large are we
talking about?
John Schilling
<chrono...@verizon.net> wrote:
>"Robert O'Connor" <rob...@ozemail.com.au> wrote in message
>news:md0ub21k8pevn6d5c...@4ax.com...
>> On Thu, 20 Jul 2006 03:30:02 GMT, "Logan Kearsley"
>> <chrono...@verizon.net> wrote:
>>
>> >I've been thinking about how one could take a mammal, like a human, and
>make
>> >it capable of breathing under water. The first things that come to mind
>are
>> >adding something like starfish gills, little thin-walled bumps scattered
>all
>> >over the skin for gas exchange, or adding something more like fish or
>> >mollusk gills.
>>
>> The biggest problem is that you won't be able to extract enough oxygen
>> out of any plausible body of water to meet metabolic demand.
>>
>> Oyxgen isn't all that soluble. The flow rates of water through the
>> gill apparatus will need to be large, or the gill itself very big.
>>
>> Mike van Pelt's countercurrent exchanger will limit heat loss across
>> the gill, but this will minimally slow the onset of dangerous
>> hypothermia.
>
>It seems to work for tuna. We could just restrict gill-modified mammals to
>swimming in warm water, but then colder water holds more oxygen.
>So, we need some way to minimize metabolic demand, then. How about just
>allwoing the body to work at a lower temperature?
Specifically, the temperature of the surrounding water, or something
very close to it - even tuna are only good for running five or ten
degrees high. And, since it's not just temperature regulation but
all the other metabolic activity that you need oxygen for, you're
going to have to slow down to the point where a drunken ground sloth
would seem hyperkinetic by comparison.
Congratulations; you've just reinvented the fish. If mammals could
grow gills, whales and dolphins would have done it long ago - or been
displaced by fish that evolved otherwise mammalian characteristics.
Instead, the cetacens are at the top of the aquatic food chain by
virtue of *not* having gills, but instead realizing that the way to
rule the ocean deep is to hold your breath.
Or, to put it another way, one might consider that water is the
most commonly used fire-extinguishing chemical on Earth, before
recommending its use as an oxidizer.
--
*John Schilling * "Anything worth doing, *
*Member:AIAA,NRA,ACLU,SAS,LP * is worth doing for money" *
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Logan Kearsley
news:h1kvb2164btpfaknc...@4ax.com...
Temperature regulation is a biggy, though. The amount you'd have to slow
down could be reduced by increasing the size of or the flow rate through the
gills, which is why I propose covering the whole interior of the rib cage
with gills, and why I ask just how large do we mean when we say "flow
rates... will need to be large"; or by reducing non-essential energy
expenditure.
So, what are some other things besides temperature regulation that could be
turned down under water? I have no idea how energy-intensive kidneys are,
but they could certainly be shut down for several hours at least. And
digestive glands, I expect. Ideally, I'm looking for a solution that would
allow the creature to survive indefinitely under water, and obviously
shutting off the kidneys and digestive tract won't allow that, but they're
the only things to come immediately to mind; if reducing activity to the
level of a drunken ground sloth is the only way, I guess I'll just have to
cut back on my goals or accept it, but I'm rather sceptical that the
situation is really that extreme.
I'm not looking to create a mammalian fish. If I were, I'd've asked "how can
we modify a fish to give it mammalian characteristics" or something along
those lines. I'm just looking to figure out how to modify a mammal so it can
breathe underwater.
I've eliminated skin gills- fine. It's been determined that body temperature
will need to be lowered- great, no problem! Now, is there any other problem
with the chest-gills I proposed? If not, I'd really like to know what sorts
of things could be done to minimize energy usage and what reasonable maximum
flow rates are and such, but otherwise, mission accomplished.
> Congratulations; you've just reinvented the fish.
Most fish don't have the option of using lungs to take a breath of air for
extra oxygen, or of climbing out of the water whenever they want and
switching over to a higher energy mode.
> If mammals could
> grow gills, whales and dolphins would have done it long ago - or been
> displaced by fish that evolved otherwise mammalian characteristics.
> Instead, the cetacens are at the top of the aquatic food chain by
> virtue of *not* having gills, but instead realizing that the way to
> rule the ocean deep is to hold your breath.
Even if mammals could naturally grow gills, they probably still would've
gone for the 'hold your breath' route, since it's so much easier. But
holding your breath restricts your time under water and requires coming to
the surface periodically, even if the periods are relatively long- whales
have been known to drown. Something that could stay under water indefinitely
*if it had to* would have an advantage over both fish and cetaceans.
Robert O'Connor
<chrono...@verizon.net> wrote:
>When you say "flow rates... will need to be large", just how large are we
>talking about?
Solubility of oxygen in water at 20 degrees C is 0.0434g/kg water per
atmosphere pressure oxygen.
For air, this is about 7mL oxygen per litre at 20 degrees (assume 21%
O2). At zero degrees C, multiply by 1.6. Cooler water doesn't help
much with regard to oxygen content.
Resting metabolic requirement for man is 3mL oxygen per kg body mass
per minute. Due to the properties of haemoglobin, the required oxygen
flux is closer to 14mL oxygen per kg per minute.
So you need to pass 2 litres per kg per minute across the gill *at
rest*. Human factorial scope (ratio of maximum to resting metabolism)
can be very high - to 25-30 times - which makes the required flow very
high indeed.
I suppose you could reduce the required flow rate if there was some
way of boosting the oxygen content of the water that had passed
through the gill - 'rebreathing'. However, I am deeply concerned that
the work required to pump all that water would exceed 100% of resting
metabolic rate!
>So, what are some other things besides temperature regulation that could be
>turned down under water?
You don't want to do this; non-hibernating mammals don't tolerate
hypothermia well. Brain, heart, liver and kidney perfusion is largely
non-negotiable.
Take some cues from diving mammals. Adaptations include:
- greatly increased muscle and visceral myoglobin levels for increased
tissue oxygen storage;
- intense vasoconstrictor response in skin, gut, and non-exercising
muscles on diving.
I really think that you would have to abandon the idea of homeothermy
to give this a chance to work.
So the critters would display torpor until they warmed up, like other
pokilotherms.
>Now, is there any other problem with the chest-gills I proposed?
How does the creature reroute blood from the lung to the gill and vice
versa? The circulation of the lungfish is instructive. It isn't
divided into two circulations like those of birds and mammals
(systemic and pulmonary). Thinking about creating a lungfish-like
circulation in a mammal makes my head hurt.
I'd think that the transition from air to water breathing (or vice
versa) would be quite unpleasant until recovery from hypoxia occurs
(there'd be a period of many seconds, maybe a minute or more, of
significant hypoxaemia).
Factors that would increase pulmonary vascular resistance (hypoxia,
hypercarbia, pain, acidosis) would also have interesting effects as
the gill would be preferentially perfused. This could be awkward out
of the water.
Hmm... uplift some lungfish - it may be easier than radically
modifying a mammal!
Dr. Robert O'Connor
sigi...@yahoo.com
Robert O'Connor wrote:
> So you need to pass 2 litres per kg per minute across the gill *at
> rest*.
Okay. An average 70 kg human has a lung capacity of about 6 kg. Tidal
volume -- the amount of air breathed in and out during normal
respiration -- is much lower, less than a liter.
For argument's sake, let's say the modified lungs are no longer tidal
but flow-through; the aquahuman breathes through the mouth, then expels
the water through the gill slits along the ribs. We will handwave away
the issue of blood circulation. [handwave]
Our 70 kg human needs to "breathe" 140 liters/minute at rest. That's a
full "breath" of water every 0.9 seconds.
That sound possible, if uncomfortable. You'd be panting heavily, and
each breath would be a double lungful of water. The caloric burn just
to maintain breathing would be significant. Still, it maybe could just
barely work.
However, things get ugly fast if you try anything more active than
passive drifting with the current. A "walking" human is burning at
about 2 times rest rate. Two breaths per second... um. And a
"jogging" human, forget it.
So, a water-breathing mammal would indeed be pretty sluggish. Maybe it
could find a niche as a drifting filter feeder, but anything more
active is right out. Our poor aquahuman would spent most of his energy
simply breathing, and even very modest exertion would cause him to
instantly black out.
Hm. Okay, what about dropping body temperature?
> You don't want to do this; non-hibernating mammals don't tolerate
> hypothermia well. Brain, heart, liver and kidney perfusion is largely
> non-negotiable.
Anteaters and sloths have mildly variable core temperatures. Sloths do
fine down to 31 C, and anteaters down to 32. Naked-mole rats go down
to 30 C, but they're deeply weird. Never mind them.
Anteaters, at least, are reasonably active creatures; they don't hurry
much, but they're strong, roam over large areas, show bursts of intense
activity when ripping open nests to feed, and survive just fine despite
the presence of large predators. Yeah, they have really tiny brains,
but we're talking proof-of-concept here. It's clearly possible to have
a large, moderately active mammal with a core temperature 5 degrees
lower than yours or mine.
If our aquahuman can tolerate a core temperature of 32 C, things do get
better. Not great, but better. You cut oxygen requirements by about
half. So, we now have a creature that mostly drifts, but can swim
slowly, and occasionally exert itself for a minute or two.
In warm tropical waters, where heat loss is no big deal, it could maybe
work.
Pretty marginal, though.
Doug M.
Logan Kearsley
> On Thu, 20 Jul 2006 04:31:02 GMT, "Logan Kearsley"
> <chrono...@verizon.net> wrote:
>
> >When you say "flow rates... will need to be large", just how large are we
> >talking about?
>
> Solubility of oxygen in water at 20 degrees C is 0.0434g/kg water per
> atmosphere pressure oxygen.
>
> For air, this is about 7mL oxygen per litre at 20 degrees (assume 21%
> O2). At zero degrees C, multiply by 1.6. Cooler water doesn't help
> much with regard to oxygen content.
>
> Resting metabolic requirement for man is 3mL oxygen per kg body mass
> per minute. Due to the properties of haemoglobin, the required oxygen
> flux is closer to 14mL oxygen per kg per minute.
Couldn't the blood be modified to pick up a higher proportion of the oxygen,
like high-altitude animals?
What's the effect of increasing the gill surface area?
> So you need to pass 2 litres per kg per minute across the gill *at
> rest*. Human factorial scope (ratio of maximum to resting metabolism)
> can be very high - to 25-30 times - which makes the required flow very
> high indeed.
>
> I suppose you could reduce the required flow rate if there was some
> way of boosting the oxygen content of the water that had passed
> through the gill - 'rebreathing'. However, I am deeply concerned that
> the work required to pump all that water would exceed 100% of resting
> metabolic rate!
As am I. 2 litres per kilogram-minute doesn't seem too bad, but 60 is
definitely not workable for anything much more than a tube of muscle with
gills in it.
> >So, what are some other things besides temperature regulation that could
be
> >turned down under water?
>
> You don't want to do this; non-hibernating mammals don't tolerate
> hypothermia well. Brain, heart, liver and kidney perfusion is largely
> non-negotiable.
We could just restrict the gill-modification to hibernating animals then, or
make modification to withstand hypothermia a pre-requisite for the
gill-modification, no?
> Take some cues from diving mammals. Adaptations include:
> - greatly increased muscle and visceral myoglobin levels for increased
> tissue oxygen storage;
Not going to help much with reducing oxygen collection requirements after
the stores are used up, but if the resting oxygen collection rate is a
little higher than the requirements for resting metabolism, we could get a
creature that hangs around lazily most of the time with occasional bursts of
high activity, right?
> - intense vasoconstrictor response in skin, gut, and non-exercising
> muscles on diving.
>
> I really think that you would have to abandon the idea of homeothermy
> to give this a chance to work.
> So the critters would display torpor until they warmed up, like other
> pokilotherms.
>
> >Now, is there any other problem with the chest-gills I proposed?
>
> How does the creature reroute blood from the lung to the gill and vice
> versa? The circulation of the lungfish is instructive. It isn't
> divided into two circulations like those of birds and mammals
> (systemic and pulmonary). Thinking about creating a lungfish-like
> circulation in a mammal makes my head hurt.
Why not just split the pulmonary arteries to go to the lungs and gills, with
restrictor muscles on each branch?
> I'd think that the transition from air to water breathing (or vice
> versa) would be quite unpleasant until recovery from hypoxia occurs
> (there'd be a period of many seconds, maybe a minute or more, of
> significant hypoxaemia).
I can see that as a problem coming out of the water, but why going in? The
animal could just take in oxygen through the lungs before diving, and rely
on that like a cetacean during the transition to using gills.
The problem coming out of the water might be solved by just sitting around
lazily for a while to store oxygen in the blood and body tissues just before
the transition.
> Factors that would increase pulmonary vascular resistance (hypoxia,
> hypercarbia, pain, acidosis) would also have interesting effects as
> the gill would be preferentially perfused. This could be awkward out
> of the water.
With branching pulmonary arteries, couldn't the gills just be mostly shut
off all the time out of water?
> Hmm... uplift some lungfish - it may be easier than radically
> modifying a mammal!
Well, that's obvious- it's already happened in nature, while mammals growing
gills hasn't.
Logan Kearsley
news:1153484404.5...@m79g2000cwm.googlegroups.com...
>
> Robert O'Connor wrote:
>
> > So you need to pass 2 litres per kg per minute across the gill *at
> > rest*.
>
> Okay. An average 70 kg human has a lung capacity of about 6 kg. Tidal
> volume -- the amount of air breathed in and out during normal
> respiration -- is much lower, less than a liter.
>
> For argument's sake, let's say the modified lungs are no longer tidal
> but flow-through; the aquahuman breathes through the mouth, then expels
> the water through the gill slits along the ribs. We will handwave away
> the issue of blood circulation. [handwave]
Oo, not modified lungs. Lots of tiny little passageways that would be very
difficult to force water through. Completely new, separate gill organs.
Which I did propose to be flow-through, with water expelled through slits
between the ribs.
> Our 70 kg human needs to "breathe" 140 liters/minute at rest. That's a
> full "breath" of water every 0.9 seconds.
>
> That sound possible, if uncomfortable. You'd be panting heavily, and
> each breath would be a double lungful of water. The caloric burn just
> to maintain breathing would be significant. Still, it maybe could just
> barely work.
Don't bother breathing through the mouth- it's a relatively small opening
for passing multi-liter volumes of water through. I was suggesting long
slits at the base of the neck, just above the clavicles, but I'm not really
sure about that. Better suggestions for how to position the intakes would be
appreciated. The caloric burn from sucking water through opening much larger
than the trachea, and pushing it through a large tube filled with feathery
gills, ought to be much less than trying to fill the lungs with water
through the trachea.
> However, things get ugly fast if you try anything more active than
> passive drifting with the current. A "walking" human is burning at
> about 2 times rest rate. Two breaths per second... um. And a
> "jogging" human, forget it.
>
> So, a water-breathing mammal would indeed be pretty sluggish. Maybe it
> could find a niche as a drifting filter feeder, but anything more
Ambush hunter? Spend most of your time sitting still, storing up oxygen,
until something dumb and edible wanders by.
> active is right out. Our poor aquahuman would spent most of his energy
> simply breathing, and even very modest exertion would cause him to
> instantly black out.
>
> Hm. Okay, what about dropping body temperature?
>
> > You don't want to do this; non-hibernating mammals don't tolerate
> > hypothermia well. Brain, heart, liver and kidney perfusion is largely
> > non-negotiable.
>
> Anteaters and sloths have mildly variable core temperatures. Sloths do
> fine down to 31 C, and anteaters down to 32. Naked-mole rats go down
> to 30 C, but they're deeply weird. Never mind them.
>
> Anteaters, at least, are reasonably active creatures; they don't hurry
> much, but they're strong, roam over large areas, show bursts of intense
> activity when ripping open nests to feed, and survive just fine despite
> the presence of large predators. Yeah, they have really tiny brains,
> but we're talking proof-of-concept here. It's clearly possible to have
> a large, moderately active mammal with a core temperature 5 degrees
> lower than yours or mine.
>
> If our aquahuman can tolerate a core temperature of 32 C, things do get
> better. Not great, but better. You cut oxygen requirements by about
> half. So, we now have a creature that mostly drifts, but can swim
> slowly, and occasionally exert itself for a minute or two.
There are lizards that are active at body temperatures of only 10 C, I
think, and hibernating mammals can survive extremely low temperatures (some
squirrels and other small rodents can even survive sub-zero temperatures
during hibernation). It's probably not feasible to let core temperatures
drop all the way to freezing, like arctic fish, and that really shouldn't
ever be necessary (just restrict the animal to not going swimming in really
cold water), but if we use counter-current systems to isolate the
extremities, and let those just match the ambient temperature, and let the
core temperature drop to, say, 25 C, that ought to provide even more
advantage.
> In warm tropical waters, where heat loss is no big deal, it could maybe
work.
>
> Pretty marginal, though.
-l.
Robert O'Connor
>Our 70 kg human needs to "breathe" 140 liters/minute at rest. That's a
>full "breath" of water every 0.9 seconds.
... of a substance nearly 1000x more dense than air, at a flow rate
nearly 30x that of resting cardiac output. That is a huge amount of
work.
On Fri, 21 Jul 2006 20:10:17 GMT, "Logan Kearsley"
<chrono...@verizon.net> wrote:
>Couldn't the blood be modified to pick up a higher proportion of the oxygen,
>like high-altitude animals?
Left-shifting the oxygen-haemoglobin dissociation curve isn't going to
help you. It just changes the operating range of partial pressure of
oxygen.
>What's the effect of increasing the gill surface area?
A point of diminishing returns will be reached where heat loss will be
excessive. You need to radically change how haemoglobin works.
>Not going to help much with reducing oxygen collection requirements after
>the stores are used up, but if the resting oxygen collection rate is a
>little higher than the requirements for resting metabolism, we could get a
>creature that hangs around lazily most of the time with occasional bursts of
>high activity, right?
Sounds like someone with moderate-severe heart or lung failure, who
gets short of breath on minimal exertion. It really depends on how
much "a little higher than the requirements for resting metabolism"
is.
Abandoning homeothermy will make this easier, but requires lots of
biochemical tweaks so that metabolism can work across a broader range
of temperatures.
>Why not just split the pulmonary arteries to go to the lungs and gills, with
>restrictor muscles on each branch?
OK... local hypoxia in the lung or gill causes vasoconstriction
upstream so that blood flows to the more useful exchange organ at the
time. If this vasoconstrictor reflex is fast enough, the period of
relative hypoxaemia will be minimised.
The right side of the heart will be need to be redesigned to cope with
an increased workload.
>> Factors that would increase pulmonary vascular resistance (hypoxia,
>> hypercarbia, pain, acidosis) would also have interesting effects as
>> the gill would be preferentially perfused. This could be awkward out
>> of the water.
>
>With branching pulmonary arteries, couldn't the gills just be mostly shut
>off all the time out of water?
In air, they would effectively be most of the time, but there would be
common situations where you could force blood flow to the gill. This
would cause a rather nasty downward hypoxic spiral I think.
>I can see that as a problem coming out of the water, but why going in? The
>animal could just take in oxygen through the lungs before diving, and rely
>on that like a cetacean during the transition to using gills.
It depends on how quickly switching between lung and gills occurs.
This is a non-trivial event with a mammalian ciirculation. It's like
the newborn transition from uterine life to air, each time - lots of
big cardiovascular and respiratory changes.
>The problem coming out of the water might be solved by just sitting around
>lazily for a while to store oxygen in the blood and body tissues just before
>the transition.
The lung is the primary oxygen reservoir in man. Increasing the amount
of substances like myoglobin in the tissues is something diving
animals do. Increasing the oxygen carrying capacity of blood really
means increasing haermoglobin concentrations, which soon leads to a
disastrous increase in blood viscosity.
So you really are left with increasing tissue oxygen stores which will
give you a bit more of a buffer against (non-brain) hypoxia.
Dr. Robert O'Connor
Logan Kearsley
> On Fri, 21 Jul 2006 20:10:17 GMT, "Logan Kearsley"
> <chrono...@verizon.net> wrote:
>
> >Couldn't the blood be modified to pick up a higher proportion of the
oxygen,
> >like high-altitude animals?
>
> Left-shifting the oxygen-haemoglobin dissociation curve isn't going to
> help you. It just changes the operating range of partial pressure of
> oxygen.
Why would that not help? It seems like increasing affinity for oxygen at
lower partial pressures would let the gills/lungs suck out a higher
proportion of the oxygen from the fluid moving through
> >What's the effect of increasing the gill surface area?
>
> A point of diminishing returns will be reached where heat loss will be
> excessive. You need to radically change how haemoglobin works.
Mewonders how to determining that point.
Perhaps don't modify the blood quite like high-altitude animals. What about
setting up chemical signaling system such that the haemoglobin (or
replacement analogue) strongly binds oxygen in the respiratory organs, and
releases it normally in the body tissues? I'm fairly sure there are
varieties of haemocyanin that work that way.
> >Not going to help much with reducing oxygen collection requirements after
> >the stores are used up, but if the resting oxygen collection rate is a
> >little higher than the requirements for resting metabolism, we could get
a
> >creature that hangs around lazily most of the time with occasional bursts
of
> >high activity, right?
>
> Sounds like someone with moderate-severe heart or lung failure, who
> gets short of breath on minimal exertion. It really depends on how
> much "a little higher than the requirements for resting metabolism"
> is.
>
> Abandoning homeothermy will make this easier, but requires lots of
> biochemical tweaks so that metabolism can work across a broader range
> of temperatures.
>
> >Why not just split the pulmonary arteries to go to the lungs and gills,
with
> >restrictor muscles on each branch?
>
> OK... local hypoxia in the lung or gill causes vasoconstriction
> upstream so that blood flows to the more useful exchange organ at the
> time. If this vasoconstrictor reflex is fast enough, the period of
> relative hypoxaemia will be minimised.
For going under, I was thinking more along the lines of reaction to skin
temperature / feeling wet. Just add it to the list of mammalian diving
reflexes triggered by the face being submerged.
A combination might be best, though. You want to activate the gills as
quickly as possible, so make that a diving reflex, but you don't want to
shut off the lungs when there's still usable oxygen in them, so make that a
reaction to local hypoxia.
Coming out of the water would be a bit different. You want the gills to shut
off quickly and the lungs to turn on again. Hypoxia in the gills ought to
work out just fine to shut them off quickly, but I'm not sure about the
lungs.
> The right side of the heart will be need to be redesigned to cope with
> an increased workload.
The only time I can think of an increased workload coming about as a result
of that setup would be if the arteries to both the lungs and gills were
constricted for a short time during transitions. Would that really be enough
extra load to require redesigning the heart?
Perhaps take an idea from mollusks and hagfish and have simple auxiliary
hearts just to help pump blood through the gills.
> >> Factors that would increase pulmonary vascular resistance (hypoxia,
> >> hypercarbia, pain, acidosis) would also have interesting effects as
> >> the gill would be preferentially perfused. This could be awkward out
> >> of the water.
> >
> >With branching pulmonary arteries, couldn't the gills just be mostly shut
> >off all the time out of water?
>
> In air, they would effectively be most of the time, but there would be
> common situations where you could force blood flow to the gill. This
> would cause a rather nasty downward hypoxic spiral I think.
Keeping the gills slits closed ought to help- any escaping oxygen would just
be trapped in the gill cavity and eventually re-absorbed. Using chemical
triggering to hugely increase oxygen affinity in the respiratory organs
ought to help, too.
Otherwise, I guess that's just a trade-off the animal will have to deal
with.
> >I can see that as a problem coming out of the water, but why going in?
The
> >animal could just take in oxygen through the lungs before diving, and
rely
> >on that like a cetacean during the transition to using gills.
>
> It depends on how quickly switching between lung and gills occurs.
>
> This is a non-trivial event with a mammalian circulation. It's like
> the newborn transition from uterine life to air, each time - lots of
> big cardiovascular and respiratory changes.
Even if the pulmonary arties just branch before the respiratory organs, and
the pulmonary veins re-combine afterwards? It doesn't seems like just
switching the blood flow from one set of organs to other should have much
effect on the rest of the circulatory system, especially since they're
isolated in their own special loop. Shutting off homeothermy is a big deal,
but most everything else can be taken care of by the already-extant
mammalian diving reflex.
So, the newborn transition analogy seems apt- starting to breathe air again
looks like it would present a much larger problem than the transition to
breathing water. As long as the problems with making a mammalian body work
breathing water through gills after the transition are solved, anyway.
> >The problem coming out of the water might be solved by just sitting
around
> >lazily for a while to store oxygen in the blood and body tissues just
before
> >the transition.
>
> The lung is the primary oxygen reservoir in man. Increasing the amount
> of substances like myoglobin in the tissues is something diving
> animals do. Increasing the oxygen carrying capacity of blood really
> means increasing haermoglobin concentrations, which soon leads to a
> disastrous increase in blood viscosity.
Well, yeah, but it seems kinda pointless to store extra oxygen in the lungs
right before a water-to-air transition, since the lungs will shortly be
filling with air anyway.
> So you really are left with increasing tissue oxygen stores which will
> give you a bit more of a buffer against (non-brain) hypoxia.
So how do diving animals buffer against brain hypoxia? Air stored in the
lungs?
Robert O'Connor
<chrono...@verizon.net> wrote:
>Perhaps don't modify the blood quite like high-altitude animals. What about
>setting up chemical signaling system such that the haemoglobin (or
>replacement analogue) strongly binds oxygen in the respiratory organs, and
>releases it normally in the body tissues? I'm fairly sure there are
>varieties of haemocyanin that work that way.
Haemocyanin performs more like myoglobin in that the oxygen-carrier
dissociation curve is a rectangular hyperbola rather than sigmoid.
It is optimally configured for an aquatic, or relatively hypoxic,
lifestyle.
As far as increasing haemoglobin's affinity for oxygen is concerned,
many animals use 'trigger compounds' to alter the conformation of the
globin chains. In man, this compound is 2,3 diphosphoglycerate
(2,3-DPG).
All 2,3-DPG does is change the range of operating pressures over which
haemoglobin will load/unload.
>The only time I can think of an increased workload coming about as a result
>of that setup would be if the arteries to both the lungs and gills were
>constricted for a short time during transitions. Would that really be enough
>extra load to require redesigning the heart?
>Perhaps take an idea from mollusks and hagfish and have simple auxiliary
>hearts just to help pump blood through the gills.
It depends on how quickly switching occurs. If you are going to try to
extract as much oxygen out of the lung as possible after entering the
water, a progressive decrease in lung blood flow will be required.
This implies a steady increase in resistance in this limb of the
circuit until blood flow ceases.
>Keeping the gills slits closed ought to help- any escaping oxygen would just
>be trapped in the gill cavity and eventually re-absorbed. Using chemical
>triggering to hugely increase oxygen affinity in the respiratory organs
>ought to help, too.
Is the partial pressure of oxygen in the gill space going to get so
high that you will be able to extract useful amounts of oxygen?
Probably not.
>Otherwise, I guess that's just a trade-off the animal will have to deal
>with.
It depends on how vigorous the reflex is. If they pass out from
hypoxia after they stub their toes or some other minor pain stimulus,
it could be inconvenient.
>So how do diving animals buffer against brain hypoxia? Air stored in the
>lungs?
Their brains get more blood flow, so they effectively get a larger
share of the air store in the lung. Other tissues sustain a huge
reduction in blood flow. Working muscles have a slightly reduced
oxygen requirement thanks to all the extra myoglobin there.
Dr. Robert O'Connor
sigi...@yahoo.com
Robert O'Connor wrote:
> Haemocyanin performs more like myoglobin in that the oxygen-carrier
> dissociation curve is a rectangular hyperbola rather than sigmoid.
>
> It is optimally configured for an aquatic, or relatively hypoxic,
> lifestyle.
Could you clarify this, please?
I was under the naive impression that hemocyanin was simply "inferior"
to hemoglobin.
Early chordates probably didn't use hemoglobin for oxygen transport.
AFAIK that's unique to vertebrates. Hemoglobin is a ubiquitous
molecule that pops up everywhere from clams to mushrooms, but only
vertebrates seem to use it for hauling oxygen around for respiration.
(There are organisms that use it to /get rid of/ oxygen, but that's
something else.)
So some proto-fish "chose" hemoglobin relatively late in chordate
evolution. Yet it seems to have been a great success. Certainly
aquatic vertebrates are more common and diverse than aquatic molluscs.
(No disrespect to our friends the squid.)
Hemocyanin simply carries less oxygen per unit mass or volume. I
vaguely assumed that the ur-mollusc "chose" hemocyanin because it was
small, so oxygen transport efficiency didn't matter so much. Then it
got "locked in", and subsequent molluscs were stuck with a second-rate
oxygen transport molecule. Upon consideration, though, that's silly;
the ur-vertebrate was probably small too.
Anyway -- you're saying that hemocyanin is in some ways better? Please
say more, tell how.
Doug M.
dwight...@gmail.com
Robert O'Connor wrote:
> On Mon, 24 Jul 2006 01:48:27 GMT, "Logan Kearsley"
> <chrono...@verizon.net> wrote:
>
> >Perhaps don't modify the blood quite like high-altitude animals. What about
> >setting up chemical signaling system such that the haemoglobin (or
> >replacement analogue) strongly binds oxygen in the respiratory organs, and
> >releases it normally in the body tissues? I'm fairly sure there are
> >varieties of haemocyanin that work that way.
>
> Haemocyanin performs more like myoglobin in that the oxygen-carrier
> dissociation curve is a rectangular hyperbola rather than sigmoid.
>
> It is optimally configured for an aquatic, or relatively hypoxic,
> lifestyle.
Anyone seen those pictures of small, very active, mammals frisking
around in a cage lined with selectively semipermeable membranes? The
ones entirely submerged in water? This goes back at least to the early
70's, when it was an illustration in one of the Time-Life science
series ("Plastics", I think), and I can think of several more
instances.
So, if these mice can sustain regular metabolic activitiy underwater
purely through the use of a purely passive device, why can't humans?
Oh, and btw, whoever wrote something about mammals not evolving gills
in preference to breathholding as proof that it's not possible . . .
that's not how evolution works. Somebody ought to reallly purge the
"just-so stories" explanations from the science textbooks.
Robert O'Connor
>> It is optimally configured for an aquatic, or relatively hypoxic,
>> lifestyle.
>
>Could you clarify this, please?
>
The molecule saturates with oxygen at a relatively low partial
pressure of oxygen, and unloads almost completely at the low partial
pressure of oxygen found in the organism's tissues.
Dwight Thieme wrote:
>Anyone seen those pictures of small, very active, mammals frisking
>around in a cage lined with selectively semipermeable membranes?
I have some familiarity with that line of research.
Paganelli C., Bateman N., Rahn, H. Artificial gills for gas exchange
in water. In: Proceedings of the Third Symposium of Underwater
Physiology, edited by C.J. Lambertson. Baltimore: Williams and
Wilkins, 1967, ch.38
The area of the gill was *very* large compared to the size of the
mouse, and the water was being aerated like any other fish tank. There
were 'human sized' gills proposed about the same time.
One of the human sized gills was a passive device, but endurance was
limited by nitrogen accumulation and subsequent emptying of the
reservoir bag.
The other required a current of several miles per hour to work (a fish
viewing cage just upstream from the Niagara falls, IIRC). The area of
membrane required was on the order of 9 square meters per side of the
cage and a 6 mph current.
Both used the semipermeable membranes you spoke of, or at least close
relatives.
>So, if these mice can sustain regular metabolic activitiy underwater
>purely through the use of a purely passive device, why can't humans?
The thread so far has been about modifying humans to carry portable
gills, using magic/SFnal biotech. Please read the earlier posts for an
overview of the (significant) problems.
The primary one is the relative lack of oxygen dissolved in water
compared to human metabolic requirements.
Dr. Robert O'Connor
Robert O'Connor
<dwight...@gmail.com> wrote:
>Anyone seen those pictures of small, very active, mammals frisking
>around in a cage lined with selectively semipermeable membranes? The
>ones entirely submerged in water?
I just realised that some of these mice may not have been immersed in
water.
Perfluorocarbon liquids were being experimented with at about the same
time as artificial oxygen carriers. Google 'liquid ventilation' for
more info - these substances can contain much more oxygen than an
equivalent amount of water.
Dr. Robert O'Connor
dwight...@gmail.com
I have; what makes you think I hadn't? It's a rather short thread,
after all. I've just pointed out a relatively easy fix (large area),
one furthermore, that fits right in with handwavy magic tech. Or are
you objecting to the notion that large-area semi-permeable membranes
aren't 'gills'? Fair enough, but I don't think the original poster
really cares; all he wants is a semi-plausibe fix that lets humans
function underwater.
sigi...@yahoo.com
Robert O'Connor wrote:
> The molecule saturates with oxygen at a relatively low partial
> pressure of oxygen, and unloads almost completely at the low partial
> pressure of oxygen found in the organism's tissues.
I'm sorry; I guess I was unclear. How is this /adaptively/ better?
Fish use hemoglobin, and don't seem to suffer any disadvantage. Quite
the opposite. Fish are far more widespread, more diverse, and make up
more of marine biomass than the hemocyanin-using molluscs.
This seems odd, if hemocyanin is really better. And using hemoglobin
for respiratory transport of oxygen arose late in chordate evolution,
so it wasn't a "bad early choice".
What am I missing?
Doug M.
Robert O'Connor
<dwight...@gmail.com> wrote:
> I've just pointed out a relatively easy fix (large area),
>one furthermore, that fits right in with handwavy magic tech. Or are
>you objecting to the notion that large-area semi-permeable membranes
>aren't 'gills'? Fair enough, but I don't think the original poster
>really cares; all he wants is a semi-plausibe fix that lets humans
>function underwater.
Logan seemed to want biologically based solutions for the problem.
Perhaps I was reading too much into the initial post.
In any case, giving humans integral tissue-based gills looks really
hard to do, at least to me.
I suppose you could use the 'big membrane' technofix.
Whether you directly oxygenate the blood or have these as a
modification to your scuba gear is a matter of taste. It just feels
very clumsy to me, not just because large sheets of clever plastics
are required.
Doug M. wrote:
>How is this /adaptively/ better?
>
>Fish use hemoglobin, and don't seem to suffer any disadvantage. Quite
>the opposite. Fish are far more widespread, more diverse, and make up
>more of marine biomass than the hemocyanin-using molluscs.
Haemoglobin is indeed more useful for the active lifestyle of the
fish. Many fishes have several different subtypes in their blood in
order to exploit a wide range of oxygen tensions.
Haemocyanin justs works well at low oxygen tensions. Sessile molluscs
don't need much else.
You hadn't missed anything. I explained myself poorly.
Dr. Robert O'Connor
sigi...@yahoo.com
Robert O'Connor wrote:
> Haemoglobin is indeed more useful for the active lifestyle of the
> fish. Many fishes have several different subtypes in their blood in
> order to exploit a wide range of oxygen tensions.
Really! Did not know that.
> Haemocyanin justs works well at low oxygen tensions. Sessile molluscs
> don't need much else.
Interesting. I was thinking of cephalopods, which are sometimes very
active indeed. Hemocyanin is clearly a handicap for them, because it
means they tire faster than fish of equivalent size. But presumably
the ancestral mollusc was sessile or slow-moving, a "clam" or a
"snail"; and by the time cephalopods evolved, the "choice" of
hemocyanin was locked in.
Hm. Given the ubiquity of hemoglobin, I half wonder that the
cephalopods didn't independently evolve a respiratory version of it.
On the other hand, it's not as easy as all that... hemocyanin is
dissolved directly in the cephalopod bloodstream, so cephalopods have
acellular blood. Respiratory hemoglobin is a bit trickier to handle
(viscosity issues), so vertebrates have to stick the hemoglobin into
red blood cells. So it's not just a question of switching molecules,
but of independently evolving a whole oxygen delivery system.
> You hadn't missed anything. I explained myself poorly.
I understand now. Thank you!
Doug M.