The short answer
How the nephron filters ~180 L of blood a day and fine-tunes water and ions through filtration, reabsorption and secretion.
Written and checked by GAMSAT tutors — not AI-generated.
Try the reasoning style
We treat forgetting as a failure — a lapse to be patched with reminders and records. Yet a mind that kept everything could not think; it would drown in the undifferentiated noise of every moment it had ever lived. To forget is not so much to lose information as to decide, mostly without our noticing, what was never worth keeping.
The author's argument relies most directly on which unstated assumption?
Pick an option to see how the tutor reasons to the answer — not just whether you were right.
Not quite — the answer is B.
Work backwards from the conclusion: a mind that ‘kept everything’ supposedly ‘could not think.’ That only follows if thinking means leaving most of experience out — so B is the premise the argument quietly rests on. A raises reliability, which the passage never weighs; C contradicts ‘mostly without our noticing’; D smuggles in a claim about intellect the passage never makes. The question rewards finding the hidden premise, not recalling a fact.
Your kidneys filter around 180 litres of blood every day and return almost all of it — you only excrete ~1.5 L as urine. The unit doing the work is the nephron, and it manages three jobs that are worth knowing cold.
Three jobs of the nephron
Filtration (at the glomerulus): pressure forces water and small solutes out of the blood into the tubule — but keeps cells and proteins in. Reabsorption (along the tubules): the useful stuff (glucose, most water, ions) is taken back into the blood. Secretion: extra waste and ions are added from the blood into the tubule for removal.
The journey of the filtrate
Glomerulus → Bowman's capsule
High pressure filters plasma. Glucose, ions, water and urea pass; blood cells and proteins stay behind.
Proximal convoluted tubule (PCT)
The workhorse of reabsorption — most glucose, ions and water are taken back here.
Loop of Henle
Sets up a salt gradient in the medulla that lets the kidney concentrate urine and conserve water.
Distal tubule + collecting duct
Fine-tuning under hormonal control — this is where ADH and aldosterone act.
The two hormones to know
ADH (antidiuretic hormone) makes the collecting duct more permeable to water → more water reabsorbed → concentrated urine. Aldosterone drives Na⁺ reabsorption (water follows). When you're dehydrated, ADH rises and you produce less, darker urine — a very common GAMSAT scenario.
Worked example
A hiker becomes dehydrated. Predict what happens to their ADH level, the permeability of the collecting duct, and their urine.
Check yourself
Where in the nephron does filtration of the blood actually occur?
The paper does not ask you to name the parts. It prints a set of clearance measurements and asks you to work out what the tubule did to each substance. Below is that shape.
Passage· 3 questions on it
The following is adapted from a renal physiology study in a healthy adult volunteer.
| Substance | Plasma concentration (mg/L) | Filtered load (mg/min) | Excreted in urine (mg/min) |
|---|---|---|---|
| Inulin | 10 | 1.25 | 1.25 |
| Glucose | 900 | 112.5 | 0.0 |
| Urea | 250 | 31.25 | 15.6 |
| Para-aminohippurate | 20 | 2.50 | 12.5 |
Q1 of 3 on this passage
What must the nephron be doing to para-aminohippurate?
Q2 of 3 on this passage
Using the inulin measurements, what volume of plasma is being filtered each minute?
Q3 of 3 on this passage
The second volunteer has twice the plasma glucose of the first and the same filtration rate, yet excretes 18 mg of glucose a minute where the first excreted none. The best explanation is that
Key takeaways
- Nephron jobs: filtration (glomerulus), reabsorption (tubules), secretion.
- Filtration keeps cells and proteins in the blood; small solutes pass.
- The PCT reabsorbs most glucose, ions and water.
- ADH ⇒ more water reabsorbed at the collecting duct ⇒ concentrated urine.
- Aldosterone ⇒ more Na⁺ reabsorption (water follows the salt).
Practise this with real GAMSAT-style questions
A timed diagnostic, real practice questions with worked explanations, and your first essay marked on our rubric built from ACER's two published criteria.