Composition of Fluids in Kidney Question

ShaShaBear

Okay, so I might be in the wrong place, but it's worth a shot!

I'm revising Human Biology for my college exams in two weeks and I've just seen a question in a past exam paper that I don't understand. I've been given a table which tells me the concentration of different substances in the blood entering the glomerulus, and the same concentration of these substances in the urine in the collecting duct. I have to use these figures to determine the filtrate in the renal corpuscle. I thought it might be simple addition or subtraction, but, for example, it says there is 0.1 Glucose in the blood, but 0 in the urine. So how can I work out filtrate from that?

Totally at a loss here guys, any help would be appreciated.

Tree

The glomerulus filters based on molecular size, so small molecules like glucose would pass into the filtrate. Many small molecules are actively reabsorbed in the tubules though.

Essentially the filtrate just after the glomerulus is the same as the blood but lacking in molecules over 70kDa (afair). So having a fair idea of what gets filtered might be handy. And also what's likely to be actively transported in and out of the filtrate in the tubule.

So I reckon it wont be simple addition or subtraction, but identifying what would have normally passed through the glomerulus.

ShaShaBear

So what I really need to know is where these things normally end up, in the filtrate or reabsorbed? And based on molecule size I can work out what should be in the filtrate, if at all?

Tree

yep, pretty much. Most of it is size based. There's a lot of swapping of small molecules in the tubules like bicarb and hydrogen ions.

ShaShaBear

Thanks, that makes it ten times easier. The lecturer tends to make our notes as she remembers the information, so knowing what exactly I'm looking for makes it easier to pick out answers.
Cheers!!