Re: why a low [K+] & a high [Na+] are commonly observed together?

That is a great observation - and I'm really glad you're asking about why 
the concentration of Na+ is higher than K+ in physiological saline 
solutions rather than just accepting it as a fact.

As it turns out, the answer to your question is relatively straight 
forward.  In the cytoplasm of essentially all living cells, the K+ ion 
concentration is much higher than the Na+ ion concentration ( [Na+] is 
about 10mM and [K+] is about 145mM).  The solution outside the cell, on the 
other hand, is reversed with Na+ having a concentration of about 150mM and 
K+ having a concentration of about 4mM or so.  Because of this, there are 
gradients of both K+ and Na+ ions across the cell membrane.  Also, there 
are gradients for other ions such as Cl-, Ca++, and Mg++ - but for 
simplicity, I won't discuss these ions here.

Now these gradients are EXTREMELY important.  They are in fact the basis 
for the electrical aspects of cellular physiology.  Without these ionic 
gradients, all electrical activity (in brain cells and in muscle cells, for 
example) would completely stop.  In a sense, the ionic gradients of Na+ and 
K+ are equivalent to little "batteries" that power the body's electrical 
circuits.

So, when you are injecting or perfusing a patient with a solution, one must 
used physiological saline which contains high Na+ and low K+.  This is 
especially true when a patient is undergoing kidney dialysis.  Usually, the 
kidney very tightly controls the amount of Na+ and K+ in the blood, but 
during dialysis, your physician is taking over this control - so he needs 
to be very careful.  Even slight changes in the concentrations of either 
Na+ or K+ (and also the relative amounts of the two) can have severe 
consequences.

So how did these Na+ and K+ gradients evolve?  As I mentioned before, these 
gradients are very important for the electrical aspects of cellular 
physiology.  But why couldn't you have a gradient of just Na+?  Why does 
high Na+ always go with low K+, as your question asks?  It's because the 
TOTAL concentration of positive ions must be the same of either side of the 
membrane.  If there is a difference, then there will be an osmotic 
gradient, which basically means that the cell will either shrink or expend 
until the total concentration of positive ions is the same.  Thus, if you 
want to have a gradient of Na+, you also must have an opposite gradient of 
K+.  But why is Na+ high on the outside of the cell rather than the other 
way around?  This is probably due to the fact that in nature, most 
solutions (like sea water, for example) are much higher in Na+ than in K+. 
 Since our ancient single-celled ancestors had a much easier time changing 
their internal environment rather than their external environment, it made 
sense to have low Na+ and high K+ on the inside of the cell.  It has 
remained that way ever since.

I hope this answers your questions.  I could also go on to tell you the 
fascinating story of how cells control the electrical voltage across their 
membranes using the Na+ and K+ gradients, but this has probably been 
discussed elsewhere on the Mad Scientist site.  You can also read much more 
about these things in any general biology textbook.