Topic 13.1 · Supplement
Kidney and Nephron
Filtration on its own would strip your blood of useful glucose and ions every single time it passed through a kidney. The reason it doesn’t is a second, much more selective step — and keeping those two steps cleanly apart is where most of the marks in this section actually live.
Cortex and medulla
A section through the kidney shows two internal regions, and at this level you only need to identify them: the cortex, the outer region, and the medulla, the inner region. On an unfamiliar diagram, find the outer edge of the organ first — the tissue nearest that edge is the cortex, and the tissue further inward is the medulla. Resources sometimes label additional regions such as the renal pelvis or pyramids, but the current syllabus limits the required identification specifically to these two.
What a nephron actually does
A nephron is the structure inside the kidney where substances are filtered out of the blood and useful substances are then reabsorbed back into it. The syllabus deliberately limits how far you need to take this: you need the outline of filtration and reabsorption, not a detailed description of the proximal or distal convoluted tubule, the loop of Henle, the collecting duct, or the pressure and transport mechanisms that drive each step. Terms like Bowman’s capsule, ultrafiltration and selective reabsorption turn up often in wider resources and can help your understanding, but an answer isn’t wrong for leaving them out when a question doesn’t specifically ask for them.
Step one — filtration at the glomerulus
The glomerulus is the part of the nephron where filtration happens, and it filters four substances out of the blood: water, glucose, urea and ions. This step is physical and size-based — it has no way of telling a useful molecule from a waste one, so glucose and ions that the body still needs are pushed out of the blood alongside urea and excess water, purely because they’re all small enough to pass through at that point. Blood cells and large plasma proteins are too big to be filtered and simply stay in the blood, which is a useful check that filtration really is working by size rather than by usefulness.
Step two — reabsorption back into the blood
If filtration were the whole story, the body would lose useful glucose and ions every time blood passed through a kidney. Reabsorption is the step that fixes this, and it works in the opposite direction to filtration — from the nephron back into the blood — and, crucially, it’s selective rather than physical: it discriminates by what the body actually needs. The required proportions are specific and worth learning exactly as stated: all of the glucose is reabsorbed, some of the ions are reabsorbed, and most of the water is reabsorbed. Those three words — all, some, most — are not interchangeable, and swapping them is one of the most common ways this point loses marks.
What’s left forms urine
Once filtration and reabsorption are both done, what remains in the nephron becomes urine: urea, excess water and excess ions. The whole route can be written as one chain — blood enters the kidney, water, glucose, urea and ions are filtered from it at the glomerulus, then all the glucose, some of the ions and most of the water are reabsorbed back into the blood, leaving urea, excess water and excess ions to form urine.
| Substance | Filtered from blood? | Reabsorbed? | In final urine? |
|---|---|---|---|
| Glucose | yes | all | normally no |
| Water | yes | most | excess remains |
| Ions | yes | some | excess remains |
| Urea | yes | not reabsorbed | yes |
Why glucose is the example examiners keep using
Glucose is useful to the body, yet it’s still filtered out of the blood at the glomerulus — that’s the clearest possible demonstration that filtration can’t distinguish useful substances from waste. Because all of the filtered glucose is then reabsorbed in the normal required model, it shouldn’t appear in final urine at all. If a question describes glucose turning up in someone’s urine, that’s a signal something has gone wrong with reabsorption — but the detailed clinical explanation for that is an optional extension, not required Topic 13 content on its own.
Where urea comes from before it ever reaches the kidney
Everything on this page assumes urea is already present in the blood by the time it reaches the glomerulus — the kidney only removes urea, it never makes it. For the actual formation story, where the liver converts excess amino acids into urea by deamination, see deamination and urea. Once urine has formed here, its route out of the body is covered on the urinary system page.