Topic 9 - Detoxification

Link to PDF - From 2021 Ask Weber

Miscellaneous topics

Bowman's capsule

'Recently, protein has appeared in John's urine. Deduce which part of the nephron must be damaged and justify your answer (2 marks).

  1. What is filtered out in the Bowman's capsule vs PCT

The Bowman's capsule is effectively a 'strainer' that prevents BIG (like big proteins and cells) from getting through - if this is damaged, large proteins can pass into the ultrafiltrate (Which then gets individual components resorbed into the body and what youre left with is urine).

The PCT is correctly the site of resorption. However, the PCT resorbs using channels (e.g. SGLT, Na+/Protein cotransporters). These channels cannot resorb big proteins, POSSIBLY very small ones but more so amino acids.

As a result, if it was the PCT damaged, you would get amino acids in urine, and maybe some small proteins (like ?albumin) but this wouldn't really cause a large protein load in the urine (At least not one that is easily detectable).

However if the Bowman's capsule is destroyed, it's like cutting big holes in a strainer - proteins (And even some cells) might end up getting through. There is no mechanism in the nephron which can take up large proteins, and you will end up with a massive protein load in the ultrafiltrate. Even if the PCT could resorb them, you will saturate these channels (just like how you can saturate the SGLT2 channels with high blood sugars) and they'll still end up in the urine.

That being said, from this bit of the question ALONE, I would say there are many caveats:

  • Depends on the protein load - Tam Horsfall protein is NORMALLY found in urine
  • If you had multiple myeloma, you can get Bence Jones protein in urine without actually having damage to the nephron
  • If you have Acute Tubular Necrosis, you could argue that damage of PCT could result in proteinuria (As the PCT cells die and release their insides/proteins into the ultrafiltrate)

However those above 3 reasons are pretty terrible.

An example of why John might have protein in his urine is:

  1. Glomerulonephrosis (effectively damage to the Bowman's capsule, resulting in protein getting through)
  2. Diabetic glomerulonephritis (more common, damage to Bowman's capsule resulting in protein getting through, in diabetics with chronic hyperglycaemia and glucose exposure to the kidneys)