BiologyICSE Class 10 · Excretory System

Formation of Urine: Ultrafiltration, Selective Reabsorption and Tubular Secretion (ICSE Class 10 Biology)

Updated 16 October 2026 · Fact-checked

Urine forms in three steps in the nephron. In ultrafiltration, high pressure in the glomerulus forces water and small solutes into Bowman's capsule, leaving cells and proteins behind. In selective reabsorption, glucose, amino acids, most water and many salts return to the blood. In tubular secretion, ions such as potassium and hydrogen are added to the filtrate.

Understand Formation of Urine: Ultrafiltration, Reabsorption, Secretion

The idea. The kidney does not pick wastes out of the blood one by one. It first filters out almost everything small, then takes back what the body needs, and finally adds a few extra unwanted substances. What remains is urine.

Step 1: Ultrafiltration (in the Malpighian capsule). Ultrafiltration is the filtration of the liquid part of blood under high pressure in the glomerulus, into Bowman's capsule.

Pressure is high because the afferent arteriole is wider than the efferent arteriole.
Water, glucose, amino acids, salts and urea pass through the thin walls.
Blood cells and plasma proteins are too large and stay in the blood.
The fluid in the capsule is the glomerular filtrate. Its composition is like plasma without the proteins.

Step 2: Selective reabsorption (mainly in the proximal convoluted tubule). As the filtrate flows along the tubule, useful substances move back into the surrounding peritubular capillaries:

all the glucose and amino acids (normally);
most of the water (about 99% of the filtrate is reabsorbed);
many salts, as the body needs them.

Urea is reabsorbed only a little, so it becomes concentrated. 'Selective' means only useful substances, in the amounts the body needs, are taken back.

Step 3: Tubular secretion (in the tubule, mainly the distal part). Tubule cells actively move some substances from the blood into the filtrate: potassium (K⁺) and hydrogen (H⁺) ions, creatinine and some drugs. This helps keep the blood's salt balance and acidity right.

Composition compared.

Substance	Plasma	Glomerular filtrate	Urine
Proteins	present	absent	absent
Glucose	present	present	absent normally
Urea	low	low	high (concentrated)
Water	much	much	less (most reabsorbed)
Salts	present	present	present, amount regulated

Role of ADH. Antidiuretic hormone (vasopressin) from the pituitary makes the distal tubule and collecting duct reabsorb more water. More ADH → less, concentrated urine (on a hot day or after sweating). Less ADH → more, dilute urine (after drinking much water). Deficiency of ADH causes frequent passing of large volumes of dilute urine.

Urine. About 1 to 1.5 litres a day; mostly water with urea, salts and creatinine; yellow due to the pigment urochrome.

Flow chart. Blood in glomerulus → (ultrafiltration, high pressure) → glomerular filtrate → PCT (reabsorption of glucose, amino acids, water, salts) → loop of Henle (water) → DCT (secretion of K⁺, H⁺; more reabsorption, ADH-controlled) → collecting duct → urine → pelvis.

Draw-and-label checklist: urine formation on a nephron. Malpighian capsule labelled 'ultrafiltration'; PCT labelled 'selective reabsorption: glucose, amino acids, water, salts'; arrows from tubule into capillaries for reabsorption and from capillaries into tubule for secretion; collecting duct labelled 'urine'.

Formation of Urine: Ultrafiltration, Reabsorption, Secretion: key facts

Three steps: Ultrafiltration glomerulus into Bowman's capsule, high pressure
Selective reabsorption tubule back to blood, glucose, amino acids, water, salts
Tubular secretion blood into tubule, K⁺, H⁺, creatinine; Reabsorption goes out of the tubule; secretion comes into it.
What is filtered and what is not: Filtered: water, glucose, amino acids, salts, urea | Not filtered: blood cells, plasma proteins; Size decides.
ADH: More ADH more water reabsorbed, less and more concentrated urine
Less ADH more dilute urine
No ADH very large volumes of dilute urine; Made in the hypothalamus, released by the posterior pituitary.

Ultrafiltration	glomerulus into Bowman's capsule, high pressure
Selective reabsorption	tubule back to blood, glucose, amino acids, water, salts
Tubular secretion	blood into tubule, K⁺, H⁺, creatinine

More ADH	more water reabsorbed, less and more concentrated urine
Less ADH	more dilute urine
No ADH	very large volumes of dilute urine

How to answer Formation of Urine: Ultrafiltration, Reabsorption, Secretion questions

ICSE asks you to define ultrafiltration, name the step or site for a substance, compare plasma, filtrate and urine, and judge assertion-reason items about ADH.

1Define ultrafiltration with three keywords: glomerulus, high pressure, into Bowman's capsule.
2Ask of each substance: is it filtered? is it reabsorbed? is it secreted?
3Proteins and cells: never filtered. Glucose: filtered, fully reabsorbed. Urea: filtered, concentrated.
4For ADH questions, link the hormone to water reabsorption in the tubule.
5Name the part of the nephron for each step.

Formation of Urine: Ultrafiltration, Reabsorption, Secretion in real life

The idea, met in an everyday situation. Answer the question in your own words, then compare with the model answer. The one-line hook is what to remember.

Dark urine after cricket practice

Suppose after three hours of cricket practice in May, you pass only a little dark yellow urine.

Explain using reabsorption and ADH.

Try it first, then show the solutionSolution

Much water was lost as sweat, so the blood became more concentrated.
The pituitary released more ADH.
ADH made the tubules reabsorb more water into the blood.
Less water remained in the filtrate, so urine was small in volume and concentrated (darker).

Answer: More ADH increased water reabsorption, giving a small volume of concentrated urine.

Memory hook: ADH = Add water back.

Sugar in urine

Suppose a pharmacy sells urine test strips that change colour when glucose is present. A healthy person's strip stays unchanged after a meal, even though his blood plasma certainly contains glucose.

Explain why his urine has no glucose, and say when the strip would change colour.

Try it first, then show the solutionSolution

Glucose is filtered into Bowman's capsule with the plasma.
Normally all of it is taken back in selective reabsorption in the proximal tubule.
If blood glucose is very high (as in diabetes mellitus), more is filtered than the tubule can reabsorb.
The extra glucose remains in the filtrate and appears in urine.

Answer: Selective reabsorption normally returns all glucose; very high blood glucose overloads it.

Memory hook: Glucose in urine = reabsorption overloaded.

Tricks to remember Formation of Urine: Ultrafiltration, Reabsorption, Secretion

Short memory aids. Say each one aloud once, then use it on a question the same day.

Filter, Fetch back, Flush in
Ultrafiltration filters; reabsorption fetches back; secretion flushes extra in.
Use it when: Order and meaning of the three steps.
Proteins are Passengers who stay
Proteins and blood cells never leave the glomerulus.
Use it when: Filtrate composition.
K-H out
Secretion sends K⁺ and H⁺ out with the urine.
Use it when: Naming secreted ions.

Formation of Urine: Ultrafiltration, Reabsorption, Secretion: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Absent from the filtrate · 1 mark · MCQBoard level

Which of these is normally absent from the glomerular filtrate?
(a) glucose (b) urea (c) plasma proteins (d) sodium salts

Try it first, then show the solutionSolution

Plasma proteins are too large to be filtered.

Answer: (c) plasma proteins.

Example 2: Defining ultrafiltration · 1 mark · Very short answerBoard level

A student writes: 'Ultrafiltration is the cleaning of blood.' Rewrite the definition so that it would earn full marks. (Ultrafiltration was asked in 2024.)

Try it first, then show the solutionSolution

Include the site (glomerulus into Bowman's capsule) and the force (high pressure).

Answer: Ultrafiltration is the filtration of the liquid part of blood under high pressure in the glomerulus into Bowman's capsule.

Example 3: Fluid sampled along the nephron · 3 marks · Short answerHarder board level

Fluid is sampled at three points of a nephron: (i) inside Bowman's capsule, (ii) at the end of the proximal convoluted tubule, (iii) in the collecting duct. (a) Name the step of urine formation that has just acted on the fluid at each point. (b) State one difference between the fluids at (i) and (iii).

Try it first, then show the solutionSolution

(a) (i) Ultrafiltration in the Malpighian capsule; (ii) selective reabsorption, mainly in the PCT (1 mark).
(iii) Tubular secretion in the distal convoluted tubule, with further water reabsorption under ADH (1 mark).
(b) Fluid (i) contains glucose and amino acids and is dilute; fluid (iii) normally has no glucose and much more concentrated urea (1 mark).

Answer: (a) (i) Ultrafiltration (ii) Selective reabsorption (iii) Tubular secretion. (b) Glucose present at (i), absent at (iii); urea far more concentrated at (iii).

Example 4: ADH assertion · 1 mark · Assertion-reasonHarder board level

Assertion (A): A person whose posterior pituitary releases too little ADH passes large volumes of dilute urine.
Reason (R): All the glucose in the glomerular filtrate is normally reabsorbed in the proximal convoluted tubule.
(a) (A) is true and (R) is false.
(b) (A) is false and (R) is true.
(c) Both (A) and (R) are true, and (R) is the correct explanation of (A).
(d) Both (A) and (R) are true, but (R) is not the correct explanation of (A).
(An ADH assertion-reason item was asked in 2025.)

Try it first, then show the solutionSolution

Both statements are true, but the large urine volume is due to less water reabsorption in the distal tubule and collecting duct without ADH, not to glucose reabsorption. R does not explain A.

Answer: (d).

Example 5: Urea concentrated · 3 marks · Case-basedHOTS / competency

Suppose plasma contains 0.03 g of urea per 100 mL and urine contains 1.8 g per 100 mL. (a) How many times more concentrated is urea in urine? (b) Explain how the kidney achieves this. (c) Why is urea not concentrated in the glomerular filtrate itself?

Try it first, then show the solutionSolution

(a) 1.8 ÷ 0.03 = 60 times (1 mark).
(b) Most of the water in the filtrate is reabsorbed while little urea is, so urea is left in a smaller volume (1 mark).
(c) Ultrafiltration passes water and small solutes together, so the filtrate has about the same urea concentration as plasma (1 mark).

Answer: (a) 60 times. (b) Water reabsorbed, urea left behind. (c) Filtration does not separate urea from water.

Example 6: Low blood pressure · 2 marks · Short answerHOTS / competency

Suppose a patient's blood pressure falls sharply after heavy bleeding, and his urine output drops almost to zero. Explain why, step by step.

Try it first, then show the solutionSolution

Ultrafiltration depends on high pressure in the glomerulus; low blood pressure lowers glomerular pressure (1 mark).
Little filtrate is formed, and the body also reabsorbs more water to save it, so very little urine is produced (1 mark).

Answer: Low pressure cuts ultrafiltration; little filtrate and more reabsorption mean almost no urine.

Where students lose marks in Formation of Urine: Ultrafiltration, Reabsorption, Secretion

The slips examiners see most often, and what to write instead.

Mistake: Ultrafiltration without site or pressure
CISCE examiners' comments (2024): candidates did not mention the glomerulus and filtration of blood under high pressure.
Do this: Glomerulus + high pressure + into Bowman's capsule.
Usually costs: 1 mark.
Mistake: Not linking ADH to the tubule
CISCE examiners' comments (2025): candidates could not relate the role of the hormone to kidney tubule function, so chose distractors in an assertion-reason item.
Do this: ADH → more water reabsorbed in the tubules → less urine.
Usually costs: 1 mark.
Mistake: Reabsorption and secretion directions swapped
Both happen in the tubule, so students mix them up.
Do this: Reabsorption: tubule to blood. Secretion: blood to tubule.
Usually costs: 1 mark.

Exam tips

Ultrafiltration: glomerulus, high pressure.
Proteins and cells are not filtered.
Glucose is filtered and fully reabsorbed.
ADH increases water reabsorption.

Practice questions on Formation of Urine: Ultrafiltration, Reabsorption, Secretion

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
Selective reabsorption mainly takes place in the:
(a) Bowman's capsule (b) proximal convoluted tubule (c) ureter (d) urinary bladder
Hint
First coiled part.
Answer
(b).
Q2 · 1 mark · Very short answerBoard level
Name the pigment that gives urine its yellow colour.
Hint
Uro-.
Answer
Urochrome.
Q3 · 1 mark · Very short answerHarder board level
Name two substances added to the filtrate by tubular secretion.
Hint
Two ions.
Answer
Potassium ions and hydrogen ions (also creatinine, some drugs).
Q4 · 2 marks · Short answerHarder board level
Why is the reabsorption in the tubule called 'selective'?
Hint
Which substances, how much?
Answer
Only useful substances (glucose, amino acids, water, needed salts) are taken back, in the amounts the body needs, while wastes like urea are mostly left in the filtrate.
Q5 · 2 marks · Short answerHarder board level
Give two differences between glomerular filtrate and urine.
Hint
Glucose and urea.
Answer
Filtrate contains glucose and amino acids; urine normally does not. Urea is far more concentrated in urine because most water has been reabsorbed (filtrate volume is also much larger).
Q6 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Plasma proteins are absent from the urine of a healthy person.
Reason (R): Plasma proteins are filtered in the glomerulus and then completely reabsorbed in the proximal convoluted tubule.
(a) (A) is true and (R) is false.
(b) (A) is false and (R) is true.
(c) Both (A) and (R) are true, and (R) is the correct explanation of (A).
(d) Both (A) and (R) are true, but (R) is not the correct explanation of (A).
Hint
Are proteins filtered at all?
Answer
(a). Proteins are too large to be filtered, so they never enter the filtrate; they are not reabsorbed.
Q7 · 2 marks · Short answerHarder board level
When the body needs to save water, name (a) the hormone involved, (b) the gland that releases it, and (c) the two parts of the nephron on which it acts.
Hint
Anti-diuresis means less urine.
Answer
(a) Antidiuretic hormone (ADH, vasopressin). (b) The posterior pituitary (it is made in the hypothalamus). (c) The distal convoluted tubule and the collecting duct.
Q8 · 3 marks · Case-basedHOTS / competency
Read and answer.
Suppose 180 L of filtrate forms in a day and 1.5 L of urine is passed.
(a) What volume was reabsorbed? (1)
(b) What percentage of the filtrate becomes urine? (1)
(c) Name the main substance in the reabsorbed volume. (1)
Hint
Subtract, then divide.
Answer
(a) 180 − 1.5 = 178.5 L. (b) 1.5 ÷ 180 × 100 ≈ 0.83%. (c) Water.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Where does ultrafiltration occur?Show answer
In the Malpighian capsule (glomerulus into Bowman's capsule).
2.Which substance is in the filtrate but not normally in urine?Show answer
Glucose (also amino acids).
3.What does ADH do?Show answer
Increases water reabsorption in the kidney tubules.
4.How much urine is passed in a day?Show answer
About 1 to 1.5 litres.
5.What is tubular secretion?Show answer
Active transfer of substances such as K⁺ and H⁺ from blood into the filtrate.

Formation of Urine: Ultrafiltration, Reabsorption, Secretion: frequently asked questions

Is filtration in the kidney active or passive?

Ultrafiltration is driven by blood pressure; reabsorption of glucose and secretion use energy.

Why is so much filtrate made if 99% is reabsorbed?

Filtering the whole plasma many times a day clears wastes quickly; the useful parts are then recovered.

Do I need the counter-current mechanism?

No. A brief idea of the three steps and their relation to plasma and urine is enough.

Is diabetes insipidus the same as diabetes mellitus?

No. Insipidus is due to lack of ADH (much dilute urine); mellitus is due to lack of insulin (sugar in urine).