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Reference values for sodium and potassium concentrations in blood tests are usually in the range of 135-145 mEq/L and 3.5-5.1 mEq/L, respectively.

My question is what could be the reason for this (relatively) large difference in the blood concentrations of these two ions which are together responsible for cells function and communication? What other roles does sodium play outside of cells that require it to be a lot more abundant in the blood than potassium?

Also, a follow-up question: is this proportion more-or-less maintained in other animal species?

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    $\begingroup$ This is much too broad. Some reading on your part about homeostasis is in order. If you still have a question then, and can explain why it is unclear from your reading, that would be a narrower question. The short answer is because it's necessary for life. When we die, equilibration occurs (more or less). $\endgroup$ – anongoodnurse Mar 22 '17 at 14:40
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    $\begingroup$ Although this is a fairly broad question, I think it could still be given an appropriately-scoped answer. One bit of information missing from the question that would make it more interesting is the imbalance between extra- and intra-cellular concentrations: not just that Na+ concentrations are greater than K+ in the blood but that the opposite is true in a typical cell. $\endgroup$ – Bryan Krause Mar 22 '17 at 17:14
  • $\begingroup$ @anongoodnurse, I have studied some basic physiology, including nervous system physiology, and I am aware of electrical activity in the cell, membrane potential, ion pumps etc. This question focuses on a very narrow (in my opinion) aspect of this very broad subject, which is merely the quantities. Actually, it came up when I was reviewing a patient's blood tests - I looked at the normal ranges and asked myself: "how come sodium blood concentration is 30-40 times greater than potassium?", or in other words, why did evolution settle on this proportion? Perhaps no one knows, but worth a try. $\endgroup$ – Don_S Mar 22 '17 at 17:44
  • $\begingroup$ @Don_S - Sorry, the question read like a beginner's. In that case, God only knows, but it is still too broad. (I was a Molecular Biologist before I became a physician.) $\endgroup$ – anongoodnurse Mar 22 '17 at 19:16
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    $\begingroup$ Well, thanks for sharing your thoughts. For me, this is the place to come before giving up... I hope the question won't be closed, at least 3 people liked it... $\endgroup$ – Don_S Mar 22 '17 at 19:23

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