DSE Biology | Kidney and Osmoregulation: filtration, reabsorption and the ADH loop

2026-11-05

Students answer “the kidney removes water” and stop. The marks sit in the middle: what is reabsorbed, from where to where, and why the loop counts as negative feedback. This lesson pins those three questions onto one diagram.

Objectives — Students can: (1) separate glomerular filtration from tubular reabsorption and name the site of each; (2) use filtration rate and urine volume to calculate the percentage reabsorbed; (3) draw the ADH loop from plasma osmolarity to urine concentration and explain every arrow.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Diffusion and osmosis	Which way does water move?	From higher to lower water potential
Active transport	How is glucose reabsorbed?	Active transport, using energy
Negative feedback	Which mechanism controls body temperature?	Negative feedback pulling back to set point

02 Lesson flow (45 minutes)

Time	Stage	Tutor does	Student does
0-5 min	Two questions	Ask what the kidney removes and what it keeps	List three of each
5-17 min	Three-step filtration	Fill a table: site, driving force, kept versus passed	Fill it before checking
17-25 min	Reabsorption by numbers	Calculate percentage reabsorbed from GFR and urine volume	Press the calculator themselves
25-35 min	ADH loop	Draw from osmolarity to urine concentration, naming each arrow	Draw and label every arrow
35-41 min	One exam question	Full answer with named structures	Work alone and self-mark
41-45 min	Exit check	Collect and mark three questions	Work alone

03 Teaching script (read it out)

Scene 1: Two stages: filtration at the glomerulus, selection at the tubule

Students treat the kidney as one filter and answer “it filters out waste and extra water”, which earns nothing. Draw two stages at the start: glomerular filtration, passive and pressure-driven, holding back cells and large proteins; and tubular reabsorption, active and selective, returning glucose, amino acids, most water and some salts to the blood.
Each stage owns its own verbs. Wrong verbs mean wrong location. Ask students to write what is kept and what passes at each stage, not the organ name, because marks follow the verb and the site.

Students do: write three substances kept and three passed at each stage, with no umbrella words.

Scene 2: Numbers make reabsorption concrete: 180 L in, 1.5 L out

Students know most water is reabsorbed but cannot say how much, so calculation questions defeat them. Give two numbers: filtration rate 125 mL per minute over 1,440 minutes is 180 L per day; urine is 1.5 L, so 178.5 L is reabsorbed, about 99.2 percent.
That turns “almost all” into a stated ratio and explains why urine still forms on little water: filtration does not stop, the percentage reabsorbed rises. Link that to ADH and act two flows into act three.

Students do: recalculate for a urine volume of 20 L and name the condition that produces it.

Scene 3: ADH: one loop, four arrows

Marks are lost because students write “more ADH, more reabsorption” and drop the middle. Fix four arrows: plasma osmolarity rises, hypothalamus detects it, posterior pituitary releases ADH, collecting duct permeability rises, water reabsorption rises, urine becomes more concentrated, plasma osmolarity falls.
Every arrow gets “up” or “down”, and the last arrow needs a line drawn back to the start labelled negative feedback. Students who can draw it can write the six-mark answer by reading their own diagram.

Students do: draw the full loop and explain in one sentence why it is called negative feedback.

04 Question types and worked steps

Type	Example	Steps	Answer
Structure and function	What does each kidney region do?	Choose the verb, then the site	Filtration (passive) and reabsorption (active, selective)
Percentage calculation	Filtered 180 L, urine 1.5 L	Subtract, then divide, units matched	About 99.2 percent
Negative feedback description	How does the body respond to dehydration?	Write the four-arrow loop sentence by sentence	Osmolarity up, ADH up, permeability up, urine concentrated
Data question	Compare osmolarity of two urine samples	Read concentration, then infer ADH	More concentrated means more ADH and more water reabsorbed

Sanity check (teach this): Sanity check: filtration is passive and pressure-driven; reabsorption is active, selective and costs energy; ADH acts on the collecting duct, never on the glomerulus; percentage calculations need the same unit above and below the line; 180 L minus 1.5 L is the volume reabsorbed and the numbers must close before any conclusion is written.

05 Independent practice (3 questions)

Question	Answer	What to watch
Filtration and reabsorption	Three kept and three passed at each site	Check for umbrella words instead of substances
Reabsorption percentage	Filtered 180 L, urine 1.5 L	Check that units were matched
ADH loop	Draw four arrows with up or down	Check the return arrow is drawn

06 Common mistakes and what to say

Mistake	Student writes	Say this
Filtration and reabsorption merged	“The kidney filters out extra water”	“Filtration stays at the glomerulus. Which segment takes the water back?”
ADH answer stops at reabsorption	“More ADH, more water reabsorbed”	“From osmolarity to urine concentration, which two organs and which structure sit in between?”
Mixed units in a percentage	125 mL per minute divided by 1.5 L	“Match the units first: convert per minute into per day.”
Wrong site for ADH	“ADH makes the glomerulus filter more water”	“Which duct wall changes permeability? Point at it on your diagram.”

07 Homework (paper, section and question numbers)

Redo this chapter (Paper 1B Q7-Q9 style) — Redraw the ADH loop and label all four arrows
Secondary biology index — Other chapters and their question types
All prep guides — The same ten-section structure in other subjects

08 Exit check (last 5 minutes)

Question	Answer	Decision
Two stages	Filtration and reabsorption, each with its verb	Wrong verb means repeat act one
Percentage reabsorbed	About 99.2 percent with the calculation order	No subtract-then-divide means repeat act two
Four ADH arrows	Osmolarity, ADH, permeability, urine concentration	Missing middle steps means repeat act three

09 Tutor FAQ

Can this be taught before the nephron structure?

Teach three terms first: glomerulus, tubule, collecting duct; the rest hangs on the two verbs.

Is unit conversion worth a whole example?

Yes. Students lose the mark between mL per minute and L per day, not on the biology.

Should ADH and aldosterone be taught together?

Keep them apart. Stabilise water control with ADH before adding sodium control.

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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.