Is it true that human blood has approximatly the same salt content as sea water? Have there ever been any scientific studies on this issue? Any info will help. Thanks
Well, salt water is about 3.5% salt (not all just sodium chloride). (I guess I should really say “solute” instead of salt)
According to: The American Association of Blood Banks
Actually, 3.5% and 10% doesn’t seem like such a close match to me. Depends on how much weight you want to put into the “approximately.”
Part of an idea knocking around in my head is that body fluids would be similar in solute content to sea water because if life had spent much of its evolutionary history in the sea, our internal fluids should be almost isotonic to prevent large amounts of water loss or gain through osmosis. Thinking about it a little more, I think this is the sort of generalization that some professors tell undergraduates just to make a point, regardless of the truth or falsehood in the statement. That may apply to marine and amphibious organisms, but its application to humans is suspect.
Actually most vertebrates have very similar osmotic concentations in their blood, even fish (sharks are an exception). You’re almost right about blood being isotonic with seawater, but remember that when the first chordates evolved the oceans were much less saline than they are after 600 million years or more of gunk has washed into them.
Blood has a Na concentration of about 140 mMol/L and a Cl concentration of about 100 mMol/L. I suspect this is hypotonic with respect to sea water. This is well studied in the physiology literature; salty solutions should cause the red blood cells to crenate.
Normal saline, which is roughly isotonic to blood, is 0.9% NaCl. If seawater is 3.5% NaCl…
Huh. Good point. I didn’t think about that. 
But wouldn’t that mean that internal fluids should have less solute than the current oceans rather than more?
Could you point me to your source for the similarity in blood solute concentrations across vertebrates? Thanks, it’s been a long time since Physiology for me.
BTW, is the 10% figure from the blood banks accurate?
The blood does have a lower solute concentration than the ocean, around 180 millimoles Cl/l as opposed to 550 for seawater in terms of ionic concentration.
The only reference I have to hand are is from my old undergrad biology text, Raven and Johnson ‘Biology’, which quotes approximate figures for freshwater fish of around 130 millimoles Cl/l, 135 for amphibians, 140 for terrestrial vertebrates and up to 210 for some marine fish. There area lot better texts available I’m sure. Anything appropriate by Knut Schmidt-Neilsen should be good.
Is 10% accurate. No idea. There’s a way to work out salt concentration (I assume w/w) from solute concentration but I don’t have a periodic table and I’m too lazy anyway. Suffice it to say blood is much less saline than sea water by a factor of about 3.
OK, I finally dug up my physiology textbook. From Schmidt-Nielsen, Knut. 1990. Animal Physiology. A partial representation of a table found on p. 313:
*value insignificant
All the taxa represented above are currently marine (or amphibious), I didn’t find the values for any terrestrial vertebrates. However, I think it’s safe to say that in the light of the above, there is considerable variation in salt concentration of vertebrate blood plasma, and that to say that vertebrate body fluids are approximately isotonic with sea water is a little bit of an overgeneralization.
I’m curious about what the values for humans are, if anyone happens to know…
Which goes to demonstrate that two authorities can totally and wholeheartedly disagree. I’m surprised actually since both Schmidt-Neilsen and Johnson are very respected authors. I got fed the line about most animals having similar osmotic concentrations back in first year and having seen it quoted several times (as in Raven and Johnson) I’ve always taken it as gospel. Still I have more respect for Shmidt-Neilsen than Johnson when it comes to animal physiology so I’m willing to stand corrected.
thank you for enlightening me.
Cheers.
I had always been taught as well that saline of 0.9% NaCl was isotonic to the human body… so it would stand to reasion that 3.5%…
Damn, Dr_Paprika bet me to it. We do seem to agree alot
For the other values…
Na= 135-145 mmol/L
K = 3.5-5.0 mmol/L
Urea is maybe 6-10 mmol/L
Blood glucose around 4-10 mmol/L
Blood osmolality is 280-300 mmol/L
Blood osmolality is often quickly approximated by
2*[Na] + [urea] + [glucose]
I was pretty surprised too. I remembered my physiology professor mentioning that the similarity between body fluids and blood was often overstated, but when you posted I thought I had made a mistake or he had, so I had to look it up (eventually 
).
best regards,
Oh, and thanks for the details on humans, Dr_Paprika!