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Body Fluid Salinity

Post of the Month: May 2005


Subject:    Re: Blood Plasma Salinity for r norman
Date:       28 May 2005

On Sat, 28 May 2005 16:52:15 +0200, "Uncle Davey" wrote:

>R Norman wrote:
>I have just checked every post in this thread you (UD) wrote and I
>can't find any mention whatsoever of blood plasma salinity. As a
>comparative physiologists, I happen to know something about that
>subject. So if you have some point about plasma salinity, please post
>it and you will get a response. I would greatly prefer you starting a
>new thread titled "blood plasma salinity" or something like that.
>Don't just change the subject line in a response to this -- news
>readers still catalog it under the original subject and there is so
>much garbage here (talk origins) that I don't even try to read every
>My point is that the salinity of plasma of humans and most other animals -
>practically every vertebrate - is closer to freshwater than salt water.
>This means that in saltwater cells need additional resources to those needed
>in freshwater in order to osmoregulate.
>My point is, that if evolution did take place, then the sea must have been a
>lot less salty than it is now at the time the salinity of plasma became
>fixed in the higher animal.
>Some people initially agreed with this statement, others disagreed, but
>consensus did not get reached before the topic moved on.
>That's about it. What is your take on it?

I am leaving the cross posting to and free.christians because you, Uncle Davey, included them. I don't want anyone to even get a hint that you know what you are talking about or that your proposition can't be answered. However I can't imagine why anyone in either of those groups would have the slightest interest in the subject.

Probably the best written and the earliest popular exposition of the evolution of the body fluids in the vertebrate animals is "From Fish to Philosopher" (John Wilkins, take note -- you have lots of finny competition!) by Homer Smith. I believe it was originally published in 1953 by Little, Brown. My copy is a special paper edition put out by Ciba Pharmaceuticals in 1959. The subtitle is "The Story of Our Internal Environment" and it describes exactly the problem you raise as well as the evolution of the vertebrate and the mammalian kidney. Dr. Smith was a noted renal physiologist who spent some 30+ years studying that particular subject. For more current discussions, see "Animal Physiology: Adaptation and Environment" by K. Schmidt-Nielsen, 5th Edition, 1997, Cambridge U Press although I must confess that I only have the 4th edition, 1990. The subject has not been modified drastically in the last 15 years. Another excellent source is "Eckert Animal Physiology: Mechanisms and Adaptations", by Randall, Burggren, and French, 5th Edition, 2002, Freeman. That one I do have and have taught from for many years going back several editions. These two are probably the most widely used textbooks in Animal Physiology. Many good introductory biology textbooks, the encyclopedic ones specifically written for biology majors, also treat the subject.

In order to explain the situation to you, I am afraid I have to go into quite a bit of background to prepare you for the argument. Fortunately for me, I am a college professor and lecturing at you comes quite naturally. Unfortunately for you, if you want to understand what I am saying, you will have to read all this. Normally it would take at least a full hour lecture with explanations and examples and so on. You would also get my atrocious jokes. I will try to be as brief as I can, but still try to be at least a bit understandable and to leave out my attempts at humor.

The most important basic fact of physiology to understand is that osmotic regulation is a really major problem for animals. Animal cells have no cell wall and cannot withstand an osmotic difference between the intracellular and extracellular fluid. The intracellular fluid must necessarily be rather concentrated to maintain its biochemical and biophysical activity. Hence the extracellular fluid must be exactly equally concentrated. The specific salts inside the cell are quite different from those outside, but the osmotic pressure is the same so that the cells are in osmotic balance. The importance of the "regulation of the internal environment", which refers to a major degree to just this osmotic regulation, was proposed by C. Bernard in the mid 19th Century and is considered one of the major landmarks in the beginning of the subject of physiology.

The interface between the inside of the cell and the outside across the cell membrane is only part of the problem. The other part is the interface between the extracellular fluids outside the cell, but inside the organism, (that is, the body fluids) and the external fluids; fresh or salt water (terrestrial animals are a special case and are treated separately). There is no special reason why the body fluids (including the blood plasma) should be the same osmotic concentration as the external environment except for the very big fact that it is an enormous metabolic expense to regulate the body fluids if they are different. Even if you make the external surface relatively impermeable, the gills are necessarily directly exposed to the external environment, they have very large surface area and are extremely permeable to oxygen and carbon dioxide. One of the examples of "incredibly poor design" in animals is our inability to produce a membrane permeable to gases but not to salts and to water. As a result, any difference between the body fluids and the environment means a very large osmotic exchange of water and a diffusion exchange of salts that must be compensated somehow.

There are basically three ways of living in water. If your body fluids match the environment, you have no problem -- life is easy. This is the way virtually all marine invertebrates live. They do have body fluids virtually the same as sea water. They may have somewhat different Ca++ and Mg++ concentrations than sea water, but that the NaCl concentration is pretty close as is the osmotic pressure. Hence they have an easy life. You might expect all marine creatures to live this way, but no -- see below.

The second way is to have body fluids significantly more concentrated than the environment. That is the situation for all fresh-water animals. Fresh water is so dilute there is no way to allow the body fluids to match it because that would require the intracellular fluids also to match it which appears to be incompatible with life. So to keep the cells happy, the body fluids must be salty, hence must be more concentrated than the environment. Therefore these animals are constantly taking in water by osmosis and losing salt. As a result, these animals never drink and seek a source of salt in their food. They also have evolved excretory organs (kidneys, in the case of the vertebrates) specialized in producing a very large volume of urine with special mechanisms to reabsorb as much salt as possible from the urine. They also have special mechanisms in their gills and their gut to actively transport salts from the very dilute water they live in to the more concentrated body fluids. These pumps use a substantial fraction of their metabolic energy, a severe evolutionary load. However they have no choice and, besides, all their competitors in fresh water have exactly same problem. Fresh water fish and amphibians live this way, as do fresh water worms, annelids, crustacea, and insects. Note in particular two things. First, these vertebrates have kidneys specialized to produce large volumes of very dilute urine and INwardly directly active transport systems (towards the body fluids) in kidney and other organs. (Invertebrates are similar, but their excretory organs are not kidneys and work a bit differently). Second, these vertebrates have body fluids significantly less concentrated than sea water, in fact roughly 1/3 the concentration of sea water. That fact means the problem is not as severe as it might be if their body fluids were, indeed, as concentrated as sea water.

The third way to live is to have body fluids significantly more dilute than the environment. This is the situation for virtually all salt-water fish, as well as reptiles, birds, and mammals that live oceanic lives and never encounter fresh-water. Because of this imbalance, they constantly lose water to the environment by osmosis and take on salts by diffusion. To compensate, they must constantly drink (only salt water fish actually "drink like a fish", fresh-water fish never drink!) and must excrete a very tiny volume of extremely concentrated urine. Also they must have OUTward directed active transport systems to eliminate the salt. Not only is it a surprise to find out that these animals do exist -- their lives would be much simpler if they just had body fluids the same as sea water. But it turns out that their kidneys are totally unable to produce concentrated urine or to transport salts in the OUTward direction at all! They have extremely reduced kidneys that produce tiny volumes of urine, but that urine is the same concentration as their body fluids. They do have specialized outwardly directly salt transport systems in their gills and gut, So the existence of these animals seems to be a problem.

Then there is the problem of terrestrial animals. For simplicity, I will only consider the terrestrial vertebrates -- amphibians, reptiles, birds, and mammals (OK, I know there is no such thing as "reptile", but no matter, the argument still holds). All these animals have body fluids roughly the same osmotic concentration as fresh-water fish, roughly 1/3 the concentration of sea water. In other words, this completely explains the original question -- why humans have body fluids significantly less salty than sea water even though life first evolved in sea water. The answer is most definitely NOT that oceans were 1/3 as salty back then. It most definitely IS that the earliest vertebrates did evolve in salt water and then moved into fresh water. As an adaptation to avoid the osmotic effect of living the "type-two" way described above, they dropped the salinity of their body fluids about as low as they could consistent with keeping their cells happy. From then on, all vertebrate evolution (including the lungfish and lobe-fin fish, the amphibians, the reptiles, the birds, and the mammals) all retained the ancestral body fluid concentration roughly 1/3 sea water. (Again, a technical detail. I am grossly oversimplifying vertebrate and tetrapod evolution and using incorrect terms for modern animals to describe the ancient and extinct transition forms. But that is simply the gross oversimplification ordinarily used in introductory expositions. If you wish, replace "lungfish, lobe-fin fish, amphibian, reptile" with the proper cladistic term, throw in the dinosaurs (although we don't know what their body fluids were like) and do it up right. You still get the same result.)

Okay, the original question is now answered. However, the full story is so beautiful and illustrates so well just how evolution is totally married to physiology, that I must continue.

Terrestrial vertebrates, the tetrapods (amphibians, reptiles, birds, and mammals) with good access to fresh water don't have the same problems as fish because (except for larval amphibians) these animals don't breath with gills. If you make the skin impermeable, which all except the amphibians have done, you really don't have the water and salt exchange problem. However, all these animals still retain the same physiological pattern as fresh-water fish: a tendency to seek sources of salt in our food and kidneys specialized to produce large volumes of very dilute urine using salt transport mechanisms to reabsorb salt INTO the body from the urine. Since we don't constantly take in water osmotically through our gills, we do have a change in that we constantly seek to drink water. Except for drinking water, we have kidneys and physiology "designed" for living in fresh water.

Now what about tetrapods that live either in deserts or in sea water and do not have access to fresh water? They constantly lose water by evaporation and take in salt with their food. As a result, they share the same problem with salt-water fish: how to obtain enough water and how to get rid of excess salt. Unfortunately, these animals are stuck with fresh-water kidneys -- kidneys that produce dilute urine and salt transport systems that are INward directed. This is a REALLY BIG example of INCREDIBLY STUPID design in animals. Amphibians can't cope -- they don't live in marine environments. (Yes, Rana cancrivora lives in salty mangrove swamps. It is a special case, see below. There are always special cases!) Reptiles and almost all birds, but especially including the truly marine birds like penguins, albatrosses, gulls, auks and puffins, ducks, etc cannot produce concentrated urine at all. They have evolved specialized glands in the eyes, nose, and mouth that excrete salt. (Of course these are not totally new inventions, they are derived from other glands like lachrymal and salivary glands). Songbirds (Passeriformes) and mammals have taken a different tack. They have devised an extremely clever trick in kidney structure to allow salt transport pumps which really take salt back INTO the body from the urine but still manage to use them to produce urine much more concentrated that their body fluids and so excrete salt FROM the body. I can't go into that at all here -- it is a counter-current concentrating mechanism which, for mammals, involves the Loops of Henle in the kidney. Birds and mammals use a similar type of trick but in very different ways, indicating separate evolution in these two groups.

The story is made even more tantalizing by some other special exceptions in the vertebrates. The hagfish has body fluids the same as sea water, just like invertebrates. The lamprey has body fluids more like fresh-water fish. The explanation is that the earliest craniates evolved in sea water but virtually all the rest of vertebrate evolution past the hagfish stage occurred in fresh. This was proposed very early in the 20th century from the physiological data. I believe this was even before other paleontological evidence demonstrated that it was correct. (Although I am sure the vertebrate paleontologists in this group -- if they got to read this far -- will promptly correct me). And, more interesting, it demonstrated that hagfish and lampreys were not all that closely related even though they were lumped together as "cyclostomes" until relatively recently. In other words, the body fluids of hagfish and lampreys very beautifully illustrate vertebrate evolution. Then most of the rest of the vertebrates, having evolved in fresh water, developed body fluids much more dilute than sea water. That is, human blood plasma salinity is another beautiful illustration of evolution, exactly contrary to Uncle Davey's claim that it cannot be explained! But wait, it gets even better! The sharks are virtually all salt-water fish, but they have body fluids whose concentration is even slightly MORE concentrated than sea water! How could that be if vertebrates after lampreys evolved in fresh water? It turns out that the SALT concentration of their body fluids is quite consistent with that of fresh water fish, much more DILUTE than salt water. But they make up the difference by all lowing urea to accumulate in their body fluids to make up the osmotic difference. As a result they have the same WATER problem as fresh-water animals (body fluids more concentrated than their environment) so their kidneys work OK. However they do have a SALT problem just the opposite. Their kidneys can't do the job -- remember, they inherited a fresh-water kidney. So they evolved special rectal glands to excrete salt. Even more interesting, the Coelecanth, that "fossil relic" of a lobe-fin fish discovered in deep oceanic water, does the same trick with urea. So does that weird frog, Rana cancrivora, the only amphibian that can tolerate concentrated salt. These illustrate convergent evolution -- a similar mechanism evolving quite independently in different groups to solve the same problem. Of course, there is no "irreducible complexity" involved -- vertebrate kidneys are very good at producing urea and vertebrate kidneys are ordinarily very good at excreting it. It doesn't take that much change to retain the urea and urea is not all that toxic or dangerous so having the cells develop a tolerance to it is not all that difficult. The salt-water fish have a better solution (see below) but they evolved separately from these others. Evolution doesn't always pick the best solution, just one that works well enough to keep you in the game. Another nice illustration of evolutionary principles.

So what about salt-water fish? Since they derived from fresh-water fish that then migrated back out to the ocean, they inherited kidneys and body fluids totally unsuited for the job. So many of them simply let their kidneys shrink into insignificance -- they are not that useful. They drink to take in water and have gill and gut OUTward transport systems to eliminate the salt. This seems much more effective than the urea trick. The only way you can easily understand this situation is to realize that they did in fact originate in fresh-water. Again, the evolutionary pattern exactly matches the physiology. Even more -- there are fresh water fish that have extremely reduced kidneys and have even weirder mechanisms to regulate body fluids. It turns out that these guys evolved from sea water fish that migrated back to fresh water! In other words, the pattern of evolution went from seawater (earliest craniates) to fresh (later craniates and teleosts) to seawater (saltwater teleosts) to fresh (this weirdos).

I have already covered the extremely strange situation of desert and salt water mammals. Even humans live in rather arid climates and often have little fresh water available. We can produce concentrated urine (though not nearly as concentrated as desert and marine mammals) but, remember, we too inherited kidneys totally unsuited for the task. Remember, the basic freshwater pattern is to produce lots of dilute urine but desert dwellers must produce a small amount of concentrated urine. So evolution to the rescue once again: build a Loop of Henle and use the INward directed salt pumps to simulate an OUTward directed transport system, hence get concentrated urine. Evolution wins, again!

Yes, I am getting just a little punchy, but this is a subject I truly love. Everything about biology, including human physiology, is a beautiful product of the evolutionary process. Read Homer Smith to get a more temperate and literate discussion of all this.

So, Uncle Davey, your claim that "the sea must have been a lot less salty than it is now" is completely wrong. You point that "in saltwater cells need additional resources to those needed in freshwater in order to osmoregulate" is entirely wrong for salt water invertebrates, for hagfish, and for any other animal that evolved in saltwater and remained their. It is true for salt water fish because they evolved from fresh water varieties. The pattern of blood plasma salinity in humans and in all vertebrate animals and, indeed, in all animals is a beautiful exposition of evolutionary principles, not a contradiction of evolutionary predictions.

Incidentally, if you ever do get stranded in the middle of the ocean on a life raft with no water, I suggest that you do NOT drink the sea water. Not only is it very salty, the high Mg concentration tends to produce diarrhea which makes your situation even worse. Instead, catch fish and drink their body fluid! It is significantly less salty than seawater. Do NOT catch marine invertebrates and drink their body fluids -- they are just as salty as the ocean.

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Faith and Truth

Post of the Month Runner-Up: May 2005


Subject:    Science, religion, truth, and lies: a comparison of standards.
Date:       4 May 2005

As we know, a lot of the heat in the evolution v. creationism debate comes from the fact that the two sides often talk past each other from radically different frames of reference. There are real differences in the moral weight that the cultures of science and religion attach to certain ideas. I think "truth" is one of those ideas. I also think -- and I think I can support this -- that science has far, far stricter moral standards in the 'truth' department than religion does.

To start with, creationist groups like AiG and ICR use a definition of "true" which I wouldn't call either scientific or religious, but political. (If you think that I'm implying a lower moral standard with that word, well spotted.)

By that standard, reality is what you can get away with. That's why you see things like quotes taken out of context repeated again and again as if they weren't total misrepresentations of what the original speaker actually said. That's why AiG's list of arguments creationists shouldn't use speaks in terms of "you could be caught out, and lose the argument, if you use these" rather than "it is morally wrong to use arguments that we know to be inaccurate." Read it for yourself, on their own site.

But deliberate dishonesty aside, there's still a difference in mindset between science and more genuine kinds of religion. Differing definitions of 'faith' and 'truth' are big parts of it.

In a religious context, 'faith' and 'truth' are almost synonyms. And faith is automatically good. If an idea is considered truth in your religion, and you don't have faith in it, that's a reflection on your failure as a faith-holder rather than the idea's failure to be true. If you don't have enough faith on a given subject, you should work harder at it.

In the sciences, that kind of faith is not a virtue; it's a personal failing. Imagine a bridge engineer being invited to "have more faith" that a design has enough steel in it to keep his bridge from collapsing. His faith has nothing to do with it; either the bridge stays up, or it falls down. Faith in the sense of 'letting yourself be persuaded without adequate evidence' is morally wrong in that context. If the bridge engineer does so, and people die in the collapse, he's murdered them.

Scientists, or the good ones, feel the same way about their theories that good engineers feel about their bridges. It's their job to make them right, not to convince themselves for their own emotional comfort that they're already right, pretty much, close enough.

If a scientist says "I have faith this theory is true," he doesn't or shouldn't mean it in the religious sense of "I commit myself to this no matter what the evidence may say, forever. Don't try to change my mind, here I stand."

Instead, he means or ought to mean "I've tested this theory, and I've seen the results of other people's tests, and I'm as sure as I can possibly get on the available evidence that this theory is as close to right as we can get. Unless something else really radical turns up. Keep me posted."

Which, incidentally, is one reason why scientists in their professional personas are very sparing with words like 'faith' and 'truth'. Just as the bridge engineer is supposed to know exact breaking strains rather than "probably close enough," scientists are expected to be able to state exactly how confident they are in a given proposition and why they feel that confidence. Faith and truth imply absolutes, which in a scientific context implies glossing over small details that might contradict those absolutes.

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