Understanding sodium and the kidney

Last updated on 06 Jul 2026

I have memorised what on earth happens with sodium in the kidney countless times for exams over many years, but I can honestly say I have never truly understood it sufficiently that I could explain coherently it in one go.

Today I change that.

This is my quest to follow sodium's journey through the nephron and figure out exactly what happens and why, and how that makes my day as an anaesthetist more interesting.

Here goes.

What is sodium for?

I find physiology much easier to comprehend if I start from the premise of 'what is the body trying to achieve?' and work backwards.

In the case of sodium, we're trying to accomplish a few things:

Membrane electrical stability
Useful, coordinated action potentials
Maintaining a blood pressure compatible with life
Keeping the body's water where it's meant to be

And that's about it, quite a lot to ask of one humble ion.

Start with the sieve

The first thing that happens is the arterial blood pours into the glomerular colander and the vast majority of small things, (less than 70 000 daltons) fall out into the tubule.

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The exam term for 'falls out' is ultrafiltration.

This is how the kidney has evolved to work - throw everything out, and then claw back what it actually needs - and to be fair to mother nature, it works ridiculously well.

Some numbers

Assuming a glomerular filtration rate of 125 ml/min:

180 litres of fluid a day drains into the tubular system
Molecules over 70 000 daltons are not filtered
Molecules below 7000 daltons are freely filtered
Negatively charged molecules are repelled by the basement membrane and therefore have a reduced filtration fraction

Sodium ions are positively charged and very small indeed at just 23 daltons, so it pours merrily into the start of the proximal convoluted tubule.

Note that this process is largely due to physical or hydrostatic pressure, not electrochemical or concentration gradients.

This means you lose lots of sodium through the glomerular sieve, but you lose proportional amounts of water too, so the concentration of sodium remains essentially the same.

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Thus the concentration of sodium in the beginning of the proximal convoluted tubule is pretty much the same as the plasma concentration.

The proximal convoluted tubule

This is where the bulk of reabsorption of pretty much everything happens.