S5 Biology | Kidney and excretion prep guide (for tutors)

S5 Biology | Kidney and excretion prep guide (for tutors) | nephron, filtration, ADH

2026-11-01

Tutor lesson-prep guides · Secondary Biology

Students think the kidney manufactures urine, so they cannot explain how 180 litres of filtrate a day becomes 1.8 litres of urine. This lesson separates filtration from reabsorption.

Objectives — Students can: (1) label the glomerulus, Bowman capsule, proximal tubule, loop of Henle, distal tubule and collecting duct; (2) explain filtration as size-based sieving and reabsorption as selective recovery, with two examples each; (3) use ADH to explain why urine becomes smaller and more concentrated when water intake falls.

01 Prerequisites (3-minute check)

If any of these fails, fix it before the lesson starts.

Skill	Quick check	Pass mark
Blood composition	plasma versus cells	protein stays in blood
Osmosis	water moves to higher solute	get direction right
Active transport	reabsorption needs ATP	against the gradient

02 Lesson flow (55 minutes)

Time	Stage	Tutor does	Student does
0-10 min	Nephron structure	cortex versus medulla	label six parts
10-24 min	Filtration	GFR 125 mL/min	180 L filtrate per day
24-38 min	Reabsorption	99% recovered	only 1.8 L out
38-48 min	ADH and collecting duct	low versus high water intake	3.6x urine volume
48-52 min	Integrated question	urea concentration, dialysis	80x concentrated
52-55 min	Exit check	three questions collected	independent work

03 Teaching script (read it out)

Scene 1: A nephron is a sieve followed by a salvage line

The glomerulus is a capillary knot; blood pressure pushes water, salts, glucose and urea into Bowman capsule while cells and large proteins stay behind. This step is filtration, passive and pressure-driven.
At a GFR of 125 mL/min the daily filtrate is 180 litres. Voiding all of it would dehydrate a person, so reabsorption must follow. The proximal tubule recovers glucose and most water and salts, the loop of Henle builds the medullary gradient, and the distal tubule and collecting duct fine-tune under hormone control.

Student task: on a blank nephron, mark filtration, reabsorption and secretion, with one substance for each.

Scene 2: Reabsorb 99 per cent and 1.8 litres remain

From 180 litres of filtrate, recovering 99 per cent leaves 1.8 litres. For glucose, 5 mmol/L across 180 L filters 900 mmol a day, all recovered in the proximal tubule, so normal urine contains none. Glucose in urine means blood glucose has passed the transport maximum or the tubule is damaged.
Urea goes from 5 mmol/L in plasma to 400 mmol/L in urine, a 80-fold concentration, because water leaves while urea stays in the tubule. Reabsorption is either active (glucose, amino acids, against the gradient) or passive (water by osmosis).

Student task: compute the 900 mmol glucose load and explain why a diabetic produces more urine.

Scene 3: ADH: less water in, more concentrated urine out

Rising plasma osmotic pressure is detected by the hypothalamus, the posterior pituitary releases ADH, the collecting duct becomes more permeable to water, and urine falls from 1.8 L to about 0.50 L while total urea output stays similar. After a large drink the opposite happens: ADH falls, little water is recovered, urine is copious and dilute.
Stress that ADH controls water recovery, not urea excretion; long questions award marks for that distinction. Dialysis replaces filtration and ultrafiltration but not reabsorption, which is why dialysis patients lose nutrients and swing between electrolyte levels.

Student task: draw two flow lines (low intake, high intake) marking ADH level, duct permeability, urine volume and concentration.

04 Question types and worked steps

Type	Example	Steps	Answer
Structure labelling	nephron diagram	split cortex and medulla first	six parts each
Calculation	GFR 125 mL/min	multiply by 1440 minutes, divide by 1000	180 L per day
Explanation	why no glucose in urine	filtered load against transport maximum	900 mmol all recovered
Hormonal control	urine after low water intake	ADH up, water recovery up	down to 0.50 L

Sanity check (teach this): Filtrate at 180 L must exceed final urine at 1.8 L, otherwise the reabsorption figures are reversed. A 80-fold concentration looks dramatic, but 5 and 400 mmol/L are standard textbook values; a student reporting 8-fold has usually mixed litres and millilitres.

05 Independent practice (3 questions)

Question	Answer	What to watch
GFR 100 mL/min, daily filtrate in litres?	144 L	did they multiply by 1440
150 L filtrate, 98.5 per cent recovered, urine volume?	2.25 L	convert the percentage first
Insufficient ADH, what happens to urine?	large volume, dilute	separate volume from concentration

06 Common mistakes and what to say

Mistake	Student writes	Say this
Kidney as a urea factory	writes that the kidney makes urea	“The liver makes urea; the kidney only sieves and recovers.”
Filtration and reabsorption swapped	says glucose is recovered in the glomerulus	“Recovery happens in the tubule; the glomerulus only sieves.”
Units not unified	treats 180000 mL as 180000 L	“Write everything in litres first, then divide by 1000.”
ADH described as urea control	writes that ADH raises urea output	“ADH moves water; urea simply follows the water.”

07 Homework (paper, section and question numbers)

Secondary biology chapter index (cells, enzymes, genetics, plant transport, ecology) — do excretion before revising respiration
Secondary prep guide hub — all subjects

08 Exit check (last 5 minutes)

Question	Answer	Decision
180 L filtrate, 99 per cent recovered: urine volume?	1.8 L	wrong: redo act two
Glucose in urine: most likely cause?	high blood glucose or tubule damage	wrong: redo transport maximum
Effect of rising ADH on the collecting duct?	higher water permeability, concentrated urine	two wrong: back to act three

09 Tutor FAQ

Which year is this for?

S5 to S6; it covers the HKDSE excretion and homeostasis topics.

How many structures must be recalled?

Six in flow order: glomerulus, Bowman capsule, proximal tubule, loop of Henle, distal tubule, collecting duct.

Is there practice attached?

The homework list links to the biology chapter index, pairing with respiration and homeostasis.

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A TutorZone lesson-prep guide. Written by the TutorZone team; questions and lesson flow are original and are not taken from any school or publisher material.