BiologyISC Class 11 · Excretory Products and their Elimination

Excretory Products, Nephron Structure and the Steps of Urine Formation

Updated 17 October 2026 · Fact-checked

Animals excrete nitrogenous wastes as ammonia (ammonotelic), urea (ureotelic) or uric acid (uricotelic). The nephron, with its Malpighian body, PCT, loop of Henle, DCT and collecting duct, forms urine in three steps: glomerular filtration, selective reabsorption and tubular secretion; the counter current mechanism concentrates the urine.

Understand Nephron Structure and Urine Formation

Modes of excretion. Ammonotelic (ammonia: most toxic, needs a lot of water): bony fishes, aquatic amphibians, aquatic insects. Ureotelic (urea, made in the liver by the ornithine cycle; less toxic): mammals, terrestrial amphibians, marine fishes. Uricotelic (uric acid, excreted as a pellet or paste with the minimum loss of water): birds, reptiles, land snails, insects.

Human excretory system. A pair of kidneys, ureters, the urinary bladder and the urethra. Each kidney is a bean-shaped organ, 10 to 12 cm long, lying between the last thoracic and third lumbar vertebrae. The outer cortex and inner medulla (with medullary pyramids projecting into calyces) are traversed by renal columns; the concave side has the hilum leading to the renal pelvis.

Nephron. The functional unit; each kidney has about a million. It has a glomerulus (tuft of capillaries) enclosed by the Bowman's capsule (together the Malpighian body, in the cortex), a proximal convoluted tubule (PCT), the loop of Henle (descending and ascending limbs), a distal convoluted tubule (DCT) and the collecting duct. Cortical nephrons have a short loop; juxtamedullary nephrons have a long loop extending deep into the medulla, accompanied by the vasa recta.

Urine formation. (1) *Ultrafiltration* in the glomerulus through three layers (the endothelium, the basement membrane and the podocyte filtration slits); the glomerular filtration rate (GFR) is about 125 mL per minute (180 L per day). (2) *Selective reabsorption*: almost all glucose, amino acids and 70 to 80% of electrolytes and water in the PCT (by active and passive transport); the descending limb of the loop is permeable to water but not to salts, the ascending limb is impermeable to water but allows the transport of electrolytes; the DCT reabsorbs Na⁺ and water conditionally and HCO₃⁻; the collecting duct reabsorbs more water. (3) *Tubular secretion* of H⁺, K⁺ and NH₃ in the PCT and DCT maintains the ionic and acid-base balance.

Concentration of urine. The counter current mechanism of the loop of Henle and vasa recta keeps the osmolarity of the medullary interstitium rising from 300 mOsm L⁻¹ in the cortex to about 1200 mOsm L⁻¹ in the inner medulla (NaCl and urea), enabling urine up to four times as concentrated as the filtrate.

Nephron Structure and Urine Formation: key facts

Nitrogenous waste: Ammonia fishes
Urea mammals
Uric acid birds and reptiles; Ammonotelic, ureotelic, uricotelic.
Nephron: Glomerulus + Bowman's capsule
PCT
Loop of Henle
DCT
Collecting duct; About a million per kidney.
Filtration: $GF R ab o u t 125 m L mi n^{- 1} = 180 L p er d a y$; Urine formed: 1 to 1.5 L per day.
Osmolarity gradient: Cortex 300 → inner medulla 1200 mOsm L⁻¹; Counter current mechanism.

Ammonia	fishes
Urea	mammals
Uric acid	birds and reptiles

Nephron Structure and Urine Formation 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.

How much filtrate is reabsorbed?

Suppose a person has a GFR of 120 mL per minute and produces 1.8 L of urine in a day.

Find the volume of filtrate, the volume reabsorbed and the percentage reabsorbed.

Try it first, then show the solutionSolution

Filtrate per day = 120 mL × 60 × 24 = 172 800 mL = 172.8 L.
Reabsorbed = 172.8 − 1.8 = 171.0 L.
Percentage = 171.0/172.8 × 100 = 99.0%.

Answer: 172.8 L filtered; 171.0 L (99.0%) reabsorbed.

Memory hook: Almost all of the filtrate returns to the blood.

Tricks to remember Nephron Structure and Urine Formation

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

A-U-U
Ammonia (fish), Urea (mammals), Uric acid (birds), in decreasing need for water.
Use it when: Excretion types.
PCT does the most
The PCT reabsorbs nearly all nutrients and 70 to 80% of the water and salts.
Use it when: Reabsorption questions.

Nephron Structure and Urine Formation: solved examples

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

Example 1: Uricotelic animals · 1 mark · MCQBoard level

Uric acid is the main nitrogenous waste in:
(a) fishes
(b) mammals
(c) birds and reptiles
(d) tadpoles

Try it first, then show the solutionSolution

Uricotelism conserves water; it is found in birds, reptiles and insects.

Answer: (c) birds and reptiles

Example 2: Types of excretory products · 3 marks · Short answerBoard level

Compare ammonotelism, ureotelism and uricotelism with respect to the waste, toxicity, water needed and one example.

Try it first, then show the solutionSolution

Ammonotelic: ammonia, most toxic, needs much water; bony fishes. (1 mark)
Ureotelic: urea, less toxic, moderate water; mammals. (1 mark)
Uricotelic: uric acid, least toxic, least water; birds. (1 mark)

Answer: Ammonia (fishes), urea (mammals), uric acid (birds).

Example 3: Urine formation · 4 marks · Short answerHarder board level

Describe the three steps of urine formation with the site of each.

Try it first, then show the solutionSolution

Glomerular filtration (ultrafiltration) in the Malpighian body through the three-layered filtration membrane. (1.3 marks)
Selective reabsorption: in the PCT mostly, and the loop of Henle, DCT and collecting duct. (1.3 marks)
Tubular secretion of H⁺, K⁺ and ammonia in the PCT and DCT. (1.4 marks)

Answer: Filtration, reabsorption, secretion.

Example 4: GFR calculations · 4 marks · Short answerHarder board level

The kidneys of an adult together filter 125 mL of plasma per minute. Calculate the filtrate per day, and if the urine volume is 1.2 L per day find the percentage reabsorbed. The kidneys receive 1100 mL of blood per minute; what fraction of a cardiac output of 5500 mL per minute is this?

Try it first, then show the solutionSolution

125 × 60 × 24 = 180 000 mL = 180 L per day. (1 mark)
Reabsorbed = 180 − 1.2 = 178.8 L, that is 178.8/180 × 100 = 99.3%. (1.5 marks)
Fraction = 1100/5500 = 1/5. (1.5 marks)

Answer: 180 L; 99.3%; one-fifth.

Example 5: Counter current mechanism · 5 marks · Long answerHOTS / competency

Explain how the counter current mechanism in the loop of Henle and the vasa recta helps to produce concentrated urine. State the role of the descending and ascending limbs and the osmolarity of the inner medulla and the cortex. By what factor can the urine be concentrated?

Try it first, then show the solutionSolution

The two limbs of the loop of Henle run in opposite directions and the vasa recta run alongside them: a counter current. (1 mark)
The descending limb is permeable to water but not to salts; the filtrate becomes more concentrated as water leaves. (1 mark)
The ascending limb is impermeable to water but actively or passively transports electrolytes out, diluting the filtrate. (1 mark)
The interstitial osmolarity increases from about 300 mOsm L⁻¹ in the cortex to about 1200 mOsm L⁻¹ in the inner medulla. (1 mark)
Water leaves the collecting duct into the hyperosmotic medulla, so the urine can be about four times as concentrated as the filtrate (1200/300). (1 mark)

Answer: Loop and vasa recta create a gradient of 300 to 1200 mOsm L⁻¹; urine up to 4 times concentrated.

Where students lose marks in Nephron Structure and Urine Formation

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

Mistake: Reversing the permeability of the loop limbs
Saying the descending limb is impermeable to water.
Do this: Descending: permeable to water; ascending: impermeable to water.
Usually costs: 2 marks.
Mistake: Calling urea the waste of birds
Wrong nitrogenous waste.
Do this: Birds excrete uric acid.
Usually costs: 1 mark.
Mistake: Confusing secretion with reabsorption
Mixing up the two steps: writing that substances move from the tubule back to the blood under secretion.
Do this: Secretion is the active transfer of H⁺, K⁺ and NH₃ from the blood into the tubule, whereas reabsorption moves substances from the tubule back to the blood.
Usually costs: 1 mark.

Exam tips

Draw a labelled nephron with the blood supply.
State the site, process and substance for each step of urine formation.
Write the GFR calculation with units.
Link the type of excretion to the habitat.

Practice questions on Nephron Structure and Urine Formation

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
The glomerulus together with the Bowman's capsule is called the:
(a) nephron
(b) Malpighian body
(c) renal pyramid
(d) loop of Henle
Hint
It lies in the cortex.
Answer
(b) Malpighian body
Q2 · 2 marks · Short answerBoard level
Name the layers of the filtration membrane in the glomerulus.
Hint
Three layers.
Answer
The endothelium of glomerular blood vessels, the basement membrane and the epithelium of the Bowman's capsule with podocytes (filtration slits).
Q3 · 3 marks · Short answerHarder board level
Where in the nephron are glucose and amino acids reabsorbed?
Hint
The first tubule.
Answer
Almost entirely in the proximal convoluted tubule by active transport.
Q4 · 3 marks · Short answerHarder board level
Why is uricotelism an advantage to birds and desert animals?
Hint
Water conservation.
Answer
Uric acid is almost insoluble and is excreted as a semi-solid paste with very little water loss, which suits animals that must conserve water, and it is non-toxic.
Q5 · 3 marks · Assertion-reasonHarder board level
Assertion (A): Over 99% of the glomerular filtrate is reabsorbed. Reason (R): The filtrate contains useful substances such as glucose, amino acids and water that the body needs. Choose: (a) both true and R explains A (b) both true, R does not explain A (c) A true, R false (d) A false, R true.
Hint
Compare filtrate and urine volumes.
Answer
(a) Both true and R explains A.
Q6 · 4 marks · Case-basedHOTS / competency
Case study: Suppose a student studies the kidney of a mammal in a cut section.
(i) Name the outer region and the inner region. (1 mark)
(ii) Name the structures projecting into the calyces. (1 mark)
(iii) Name the extension of the renal pelvis. (1 mark)
(iv) State where the Malpighian bodies are found. (1 mark)
Hint
Cortex and medulla.
Answer
(i) Cortex (outer) and medulla (inner). (ii) Medullary pyramids. (iii) Calyces (the funnel-shaped extensions of the renal pelvis). (iv) In the cortex.

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.GFR?Show answer
About 125 mL min⁻¹.
2.Site of filtration?Show answer
The glomerulus.
3.Urine concentration factor?Show answer
About four times.
4.Birds excrete?Show answer
Uric acid.

Nephron Structure and Urine Formation: frequently asked questions

What is the role of the vasa recta?

It runs parallel to the loop of Henle and maintains the medullary osmotic gradient by counter current exchange.

What are podocytes?

Cells of the Bowman's capsule with foot-like processes forming filtration slits.

Why is the urine yellow?

Because of the pigment urochrome, from the breakdown of haemoglobin.