Numerical problems from Kidney Function show up in NEET and CBSE boards every year. They look scary but are mostly plug-and-chug once the definitions are clear. We will work through five short problems that cover the high-yield ideas in nephron physiology — filtration, reabsorption, secretion, counter-current multiplier and urine concentration.
Quick facts you will need
- GFR ≈ 125 mL/min ≈ 180 L/day.
- ~99% of filtrate is reabsorbed; PCT handles ~65% of water and Na⁺.
- Loop of Henle sets up the medullary osmotic gradient.
- ADH acts on the collecting duct to insert aquaporins.
- Aldosterone raises Na⁺ reabsorption in DCT and collecting duct.
Problem 1 — baseline computation
A student is asked to compute the baseline value from the first fact above. Use the standard NCERT quantities and apply the definition directly.
Whenever a question asks for a rate or capacity, start from the textbook definition. In kidney function, the simplest equation is the one in fact 1.
Substitute the NCERT-quoted numbers. The calculation is a one-line arithmetic step that you should be able to do mentally in the exam.
The answer matches the textbook baseline in fact 1. Memorise it — examiners reuse the same numbers year after year.
Problem 2 — reverse calculation
A harder variation gives you the output and asks for one of the inputs.
Solve algebraically for the unknown before substituting numbers. This avoids sign and unit errors.
In biology numericals, mL vs L and min vs s cause half the mistakes. Convert everything to one system before the final multiplication.
Answer follows directly once the units are consistent. An unphysical result (say a GFR of 10 L/min) usually means you dropped a factor of 60.
Problem 3 — percentage change
Percentage-based question — for example, “if X drops by 20%, find the new Y”.
Write instead of subtracting the drop. Cleaner algebra, fewer mistakes.
Substitute into the main equation and simplify. The ratio usually comes out as a clean fraction.
The new value is 80% of the original whenever the relationship is linear. Non-linear dependencies change the factor, so always check.
Problem 4 — unit conversion
Values are given per second but the answer is asked per day.
s. Keep this number at your fingertips.
The final answer is the per-second rate times 86400. Three significant figures are enough for the exam.
Problem 5 — mixed concepts
A word problem that combines two facts from the list above. NEET uses this style to separate toppers from average students.
The final answer matches the NCERT baseline within about 10%. If it is wildly off, recheck units first.
For NEET, memorise the five baseline numbers above. Roughly one mark per paper comes from plugging them into a short arithmetic step.
Always write the formula first, then substitute numbers. Skipping the algebra step is where half the errors creep in.