DSE Biology | The kidney and osmoregulation: filtration, reabsorption, ADH and negative feedback

2026-11-04

Tutor lesson-prep guides · Secondary Biology

Students can recite that ADH makes urine more concentrated, but cannot trace the line from plasma osmotic pressure to the collecting duct. This lesson turns negative feedback into one arrow pointing backwards.

Objectives — By the end students can: 1) place filtration, reabsorption and secretion in the nephron; 2) calculate daily filtrate volume and the percentage reabsorbed; 3) explain ADH negative feedback from osmotic pressure to urine concentration; 4) compare urine volume and concentration after dehydration and after drinking a lot of water.

01 Prerequisites (3-minute check)

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

Skill	Quick check	Pass mark
Osmosis	Which way water moves	From higher to lower water potential
Active transport	How glucose is reabsorbed	Active transport
Negative feedback	How body temperature is held steady	A deviation triggers the opposite change

02 Lesson flow (60 minutes)

Time	Stage	Tutor does	Student does
0-8 min	Three processes	Filtration, reabsorption, secretion	Name the site of each
8-20 min	Filtration	Blood pressure pushes water and small solutes into the capsule	Cells and protein stay
20-34 min	Reabsorption arithmetic	180 litres of filtrate, 1.5 litres of urine	Get 99.2 percent
34-46 min	ADH feedback	Hypothalamus, pituitary, collecting duct	Draw the return arrow
46-54 min	Two situations	Dehydration against heavy drinking	Fill a comparison table
54-60 min	Exit check	Three questions collected and marked	Independent work

03 Teaching script (read it out)

Scene 1: Three processes: fix the positions first

Filtration happens in the glomerulus and Bowman capsule: high blood pressure forces water, glucose, urea and ions into the capsule while cells and large proteins stay in the blood. Reabsorption happens in the proximal tubule, where glucose and amino acids are recovered completely by active transport and water follows by osmosis. Secretion happens in the distal tubule, where excess hydrogen ions and drugs are actively moved into the tubule. Draw one tube on the board, write one verb at each site, and afterwards ask only which segment a process belongs to.
At 125 cubic centimetres a minute the daily filtrate is 125 times 60 times 24 divided by 1 000, which is 180 litres. With about 1.5 litres of urine the reabsorption rate is (1 minus 1.5 over 180) times 100, which is 99.2 percent. Plasma glucose is about 1 gram per litre, so 180 grams are filtered daily; none appears in normal urine, which means 100 percent is reabsorbed. Ask what appears in urine once blood glucose exceeds the transport maximum: glucose, which is the basis of diabetes screening.

ADH feedback: dehydration raises plasma osmotic pressure, the hypothalamus detects it, the posterior pituitary releases ADH, the distal tubule and collecting duct become more permeable to water, water is reabsorbed, urine becomes small in volume and concentrated, and osmotic pressure falls back. Drinking a lot of water reverses all of it, with ADH falling and daily urine rising from about 0.5 litre to 15 litres, a 30-fold difference. Have students draw the loop as arrows and label each one rising or falling.

04 Question types and worked steps

Type	Example	Steps	Answer
Calculation	Daily filtrate and reabsorption	Rate times time, then divide by urine volume	180 litres and 99.2 percent
Explanation	How ADH changes urine volume	From osmotic pressure to duct permeability	Dehydration gives small concentrated urine
Comparison	Urine in two situations	Compare volume, concentration and ADH	15 against 0.5 litres
Prediction	What never appears in filtrate	Check molecule size and blood cells	Protein and red cells

Sanity check (teach this): A reabsorption percentage must lie between 0 and 100, and filtrate volume must far exceed urine volume; otherwise the units are wrong, since cubic centimetres to litres divides by 1 000. Rising ADH always lowers urine volume and raises concentration, never both upwards. Normal urine contains no glucose.

05 Independent practice (3 questions)

Question	Answer	What to watch
Daily filtrate at 125 cm3 per minute	180 litres	Multiply by 60 and 24, divide by 1 000
Reabsorption percentage when urine is 1.5 litres	About 99.2 percent	One minus urine over filtrate
Glucose filtered daily at 1 gram per litre	180 grams	Filtrate volume times concentration
Effect of more ADH on urine volume	It falls	Higher water permeability in the collecting duct

06 Common mistakes and what to say

Mistake	Student writes	Say this
Treating filtration as active transport	Saying filtration needs energy	Filtration is pressure-driven and passive; glucose reabsorption needs energy.
Putting ADH in the wrong place	Saying ADH acts on the glomerulus	ADH changes water permeability in the distal tubule and collecting duct, not filtration.
Moving volume and concentration together	Answering small and dilute urine	Less urine means more concentration, because water was reclaimed.
Skipping unit conversion	Answering 180 cubic centimetres of filtrate a day	125 is per minute; multiply by 1 440 and divide by 1 000.

07 Homework (paper, section and question numbers)

Secondary biology chapter index — Redo the three calculation questions
Tutor preparation guide hub — Pair with the excretion and breathing chapter

08 Exit check (last 5 minutes)

Question	Answer	Decision
Daily filtrate at 125 cm3 per minute	180 litres	All three right: move to the nervous system
Effect of rising ADH on urine	Less urine, more concentrated	One wrong: redraw the feedback loop
Does normal urine contain glucose	No	Two wrong: restart act one

09 Tutor FAQ

Why reabsorb more than 99 percent?

Filtrate volume far exceeds the water the body needs to keep, so reclaiming it prevents rapid dehydration.

Where is ADH made and released?

Made in the hypothalamus and released from the posterior pituitary.

Why is urea secreted?

Some urea is reabsorbed passively, and the excess is actively secreted so it can be excreted.

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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.