Excretory Products and their Elimination Class 11 Biology: Kidney, Nephron and Urine Formation

What Excretory Products and their Elimination is about

Weightage: This chapter is part of Unit V, Human Physiology, which carries 18 of the 70 theory marks in the school annual exam together with Breathing and Exchange of Gases, Body Fluids and Circulation, Locomotion and Movement, Neural Control and Coordination and Chemical Coordination and Integration.Source: CBSE Biology (044) syllabus 2026-27, Class XI course structure, Unit V Human Physiology

Key concepts

Modes of excretion and excretory structures in animals

NCERT section Introduction (modes of excretion and excretory structures)

Nitrogenous wastes come mainly from protein and nucleic acid breakdown.

Ammonia:

• Most toxic; needs large amounts of water for elimination.
• Ammonotelic animals: many bony fishes, aquatic amphibians, aquatic insects.
• Ammonia is readily soluble, so it leaves as ammonium ions, simply diffusing out through the body surface or, in fish, the gills; the kidneys hardly take part.

Urea:

• Terrestrial life needs water conservation, so ammonia is converted into less toxic urea in the liver and released into blood, which is filtered by the kidneys.
• Some urea is retained in the kidney matrix of some animals to keep a desired osmolarity.
• Ureotelic: mammals, many terrestrial amphibians, marine fishes.

Uric acid:

• Excreted as pellet or paste with minimum water loss.
• Uricotelic: reptiles, birds, land snails and insects.

Excretory structures (overview): most invertebrates have simple tubular forms; vertebrates have complex kidneys.

• Protonephridia (flame cells): Platyhelminthes (flatworms such as *Planaria*), rotifers, some annelids and the cephalochordate *Amphioxus*; mainly for osmoregulation (ionic and fluid volume regulation).
• Nephridia: earthworms and other annelids; remove nitrogenous wastes and maintain fluid and ionic balance.
• Malpighian tubules: most insects including cockroaches; remove nitrogenous wastes and help osmoregulation.
• Antennal glands (green glands): crustaceans such as prawns.

Worked example

A tadpole living in a pond excretes mainly ammonia, but the adult frog, which spends much of its time on land, excretes mainly urea. Explain this change, and state where urea is made and how it leaves the body. (3 marks)

Try it first, then show the solutionSolution

1. The aquatic tadpole has plenty of water: ammonia, though very toxic, is readily soluble and simply diffuses out as ammonium ions (ammonotelism, as in aquatic amphibians). (1)
2. On land the frog must conserve water, so ammonia is converted into urea, which is less toxic and needs less water to remove (ureotelism, as in many terrestrial amphibians). (1)
3. Urea is made in the liver, released into the blood and filtered out by the kidneys. (1)

Answer: Water is plentiful for the tadpole (ammonia) but must be saved on land (urea); urea is made in the liver and removed by the kidneys.

Human excretory system: kidney and nephron

NCERT section 16.1 Human Excretory System

Organs: a pair of kidneys, a pair of ureters, a urinary bladder and a urethra.

Kidney:

• Reddish-brown, bean-shaped, between the levels of the last thoracic and third lumbar vertebrae, close to the dorsal inner wall of the abdominal cavity.
• Adult kidney: 10-12 cm long, 5-7 cm wide, 2-3 cm thick, weighing about 120-170 g.
• The inner concave notch is the hilum, through which the ureter, blood vessels and nerves enter or leave.
• Inside the hilum is a broad funnel-shaped space, the renal pelvis, with projections called calyces.
• Outer layer: a tough capsule. Inside: outer cortex and inner medulla. The medulla is divided into conical medullary pyramids projecting into the calyces.
• The cortex extends between the pyramids as renal columns (columns of Bertini).

Nephron (functional unit; nearly one million per kidney):

• Glomerulus: a tuft of capillaries formed by the afferent arteriole (a fine branch of the renal artery). Blood leaves via the efferent arteriole.
• Renal tubule:
• Bowman's capsule: double-walled cup enclosing the glomerulus. Glomerulus + Bowman's capsule = Malpighian body (renal corpuscle).
• Proximal convoluted tubule (PCT): highly coiled.
• Henle's loop: hairpin-shaped, with descending and ascending limbs.
• Distal convoluted tubule (DCT): highly coiled.
• DCTs of many nephrons open into a straight collecting duct; collecting ducts converge and open into the renal pelvis through the medullary pyramids in the calyces.
• Location: Malpighian corpuscle, PCT and DCT lie in the cortex; the loop of Henle dips into the medulla.
• Cortical nephrons: the majority; the loop is too short, extending only a little into the medulla.
• Juxta medullary nephrons: some nephrons with very long loops that run deep into the medulla.
• Peritubular capillaries: the efferent arteriole forms a network around the tubule. A minute vessel from this network runs parallel to Henle's loop forming a 'U' shape: the vasa recta. It is absent or highly reduced in cortical nephrons.

Worked example

Distinguish between cortical and juxta medullary nephrons in two ways. Why is the vasa recta important for juxta medullary nephrons? (3 marks)

Try it first, then show the solutionSolution

1. Cortical: majority; short Henle's loop extending only slightly into the medulla. Juxta medullary: fewer; very long loop running deep into the medulla. (1)
2. Vasa recta is absent or highly reduced in cortical nephrons but well developed alongside the long loops of juxta medullary nephrons. (1)
3. Vasa recta, running parallel to the long loop, helps maintain the medullary osmotic gradient (counter current), so these nephrons can concentrate urine. (1)

Answer: Short vs long loop; vasa recta present with long loops to maintain the medullary gradient.

Urine formation: filtration, reabsorption and secretion

NCERT section 16.2 Urine Formation

Three processes: glomerular filtration, reabsorption and secretion, in different parts of the nephron.

1. Glomerular filtration (ultrafiltration):

• On average 1100-1200 mL of blood is filtered by the kidneys per minute, about one-fifth of the blood pumped by each ventricle per minute.
• Glomerular capillary blood pressure drives filtration through a three-layered filter: the capillary endothelium, the basement membrane in the middle, and the inner epithelium of Bowman's capsule.
• The epithelial cells of Bowman's capsule, podocytes, are arranged in an intricate way, leaving minute spaces called filtration slits (slit pores).
• Blood is filtered so finely that almost all plasma constituents except proteins pass into Bowman's capsule: ultrafiltration.
• Glomerular filtration rate (GFR): the amount of filtrate formed by the kidneys per minute: about 125 mL/min, i.e. 180 litres per day.

Regulation of GFR (autoregulation by the JGA):

• The juxta glomerular apparatus (JGA) is a special sensitive region formed by cellular modification in the DCT and afferent arteriole where they contact.
• A fall in GFR activates JG cells to release renin, which stimulates glomerular blood flow and brings GFR back to normal.

2. Reabsorption:

• Filtrate formed is 180 L/day but urine released is only about 1.5 L/day, so nearly 99% of the filtrate is reabsorbed by the tubules.
• Tubular epithelial cells reabsorb substances actively (e.g. glucose, amino acids, Na⁺) or passively (e.g. nitrogenous wastes, and water especially in the initial segments, by osmosis).

3. Secretion:

• Tubular cells secrete H⁺, K⁺ and ammonia into the filtrate.
• This helps maintain the ionic and acid-base balance of body fluids.

Worked example

What is ultrafiltration? Name the three layers through which blood is filtered and the cells that form filtration slits. (3 marks)

Try it first, then show the solutionSolution

1. Ultrafiltration: blood is filtered so finely by glomerular blood pressure that all plasma constituents except proteins pass into Bowman's capsule. (1)
2. Layers: endothelium of glomerular capillaries, epithelium of Bowman's capsule, basement membrane between them. (1)
3. Podocytes of Bowman's capsule leave filtration slits (slit pores). (1)

Answer: Fine pressure filtration minus proteins; endothelium, basement membrane, capsule epithelium; podocytes.

Function of the tubules

NCERT section 16.3 Function of the Tubules

Proximal convoluted tubule (PCT):

• Its wall is simple cuboidal epithelium with a brush border (microvilli), giving a huge surface for reabsorption.
• Reabsorbs nearly all essential nutrients and 70-80% of electrolytes and water.
• Maintains pH and ionic balance by selective secretion of H⁺ and ammonia into the filtrate and absorption of HCO₃⁻ from it.

Henle's loop:

• Reabsorption here is minimum, but the loop maintains high osmolarity of the medullary interstitial fluid.
• Descending limb: permeable to water, almost impermeable to electrolytes: filtrate becomes concentrated as it moves down.
• Ascending limb: impermeable to water, allows transport of electrolytes actively or passively: filtrate gets diluted as it moves up because electrolytes pass to the medullary fluid.

Distal convoluted tubule (DCT):

• Reabsorbs Na⁺ and water only when needed (conditional reabsorption).
• Reabsorbs HCO₃⁻ and selectively secretes H⁺, K⁺ and NH₃ to maintain pH and sodium-potassium balance.

Collecting duct:

• A long duct from the cortex to the inner medulla.
• Reabsorbs large amounts of water to produce concentrated urine.
• Allows small amounts of urea to pass into the medullary interstitium to keep up its osmolarity.
• Also helps maintain pH and ionic balance by selective secretion of H⁺ and K⁺.

Summary table in words: PCT = bulk reabsorber; descending loop = water out; ascending loop = salt out; DCT = fine-tuner (conditional); collecting duct = final concentrator.

Worked example

Why does the filtrate become concentrated in the descending limb but dilute in the ascending limb of Henle's loop? (3 marks)

Try it first, then show the solutionSolution

1. Descending limb is permeable to water but almost impermeable to electrolytes; water leaves into the concentrated medulla, so the filtrate concentrates. (1½)
2. Ascending limb is impermeable to water but transports electrolytes out; salts leave while water stays, so the filtrate becomes dilute. (1½)

Answer: Water leaves in the descending limb; salts leave in the ascending limb.

Counter current mechanism: concentrating urine

NCERT section 16.4 Mechanism of Concentration of the Filtrate

Mammals can produce concentrated urine. Henle's loop and the vasa recta play the main role.

Counter current: the filtrate flows in opposite directions in the two limbs of Henle's loop, and blood flows in opposite directions in the two limbs of the vasa recta.

How the gradient is built and kept:
1. The proximity of Henle's loop and vasa recta, with the counter current flow, helps maintain an increasing osmolarity towards the inner medullary interstitium: from 300 mOsmol L⁻¹ in the cortex to about 1200 mOsmol L⁻¹ in the inner medulla.
2. This gradient is mainly caused by NaCl and urea.
3. NaCl is transported out by the ascending limb of Henle's loop and is exchanged with the descending limb of the vasa recta; NaCl is then returned to the interstitium by the ascending portion of the vasa recta.
4. Urea is recycled too: a little enters the thin ascending limb of the loop, and the collecting tubule later returns it to the interstitium.
5. This special arrangement of Henle's loop and vasa recta is the counter current mechanism.

Result: as the filtrate passes down the collecting duct through the concentrated medulla, water moves out easily (by osmosis), concentrating the urine. Human kidneys can make urine nearly four times more concentrated than the initial filtrate (1200 vs 300 mOsmol L⁻¹).

Worked example

Name the two solutes mainly responsible for the medullary osmotic gradient and explain how the vasa recta prevents NaCl from being washed away. (3 marks)

Try it first, then show the solutionSolution

1. NaCl and urea. (1)
2. NaCl pumped out by the ascending limb of Henle's loop enters the descending limb of the vasa recta. (1)
3. As blood flows back up in the ascending vasa recta, the NaCl is given back to the interstitium, so the gradient (300 to 1200 mOsmol L⁻¹) is maintained. (1)

Answer: NaCl and urea; counter current exchange in the vasa recta returns NaCl to the medulla.

Regulation of kidney function and micturition

NCERT section 16.5 Regulation of Kidney Function; 16.6 Micturition

Kidney function is monitored and regulated by hormonal feedback involving the hypothalamus, JGA and, to some extent, the heart.

ADH (antidiuretic hormone / vasopressin):

• Osmoreceptors detect changes in blood volume, body fluid volume and ionic concentration.
• Excess fluid loss activates them; they stimulate the hypothalamus to release ADH from the neurohypophysis.
• ADH increases water reabsorption from the later parts of the tubule (DCT and collecting duct), preventing diuresis (excess urine).
• When body fluid volume rises, osmoreceptors switch off and ADH release falls, completing the feedback.
• ADH also constricts blood vessels, raising blood pressure, which increases glomerular blood flow and GFR.
• Diabetes insipidus: ADH deficiency reduces water reabsorption, so large amounts of dilute urine are passed (diuresis) and the person becomes dehydrated.

Renin-angiotensin mechanism:
1. A fall in glomerular blood flow, blood pressure or GFR activates JG cells to release renin.
2. Renin converts angiotensinogen (in blood) to angiotensin I, which is converted to angiotensin II.
3. Angiotensin II is a powerful vasoconstrictor: it raises glomerular blood pressure and thus GFR.
4. It also activates the adrenal cortex to release aldosterone, which increases reabsorption of Na⁺ and water from distal parts of the tubule, raising blood pressure and GFR.

Atrial natriuretic factor (ANF):

• An increase in blood flow to the atria causes release of ANF.
• ANF causes vasodilation, lowering blood pressure.
• So the ANF mechanism acts as a check on the renin-angiotensin mechanism.

Micturition (the process of releasing urine):

• Urine is stored in the urinary bladder until a voluntary signal from the CNS.
• As the bladder fills, stretch receptors in its wall send signals to the CNS.
• The CNS sends motor messages to contract the smooth muscles of the bladder and relax the urethral sphincter, releasing urine. The neural mechanism is the micturition reflex.
• An adult excretes on average 1-1.5 litres of urine per day: a light yellow, slightly acidic (pH 6.0) watery fluid with a characteristic odour; about 25-30 g of urea per day.
• Glycosuria (glucose in urine) and ketonuria (ketone bodies in urine) indicate diabetes mellitus.

Worked example

After a long run on a hot day, a student's urine is small in volume and dark yellow. Explain the hormonal mechanism responsible. (3 marks)

Try it first, then show the solutionSolution

1. Heavy sweating reduces body fluid volume and raises its concentration, activating osmoreceptors. (1)
2. Osmoreceptors stimulate the hypothalamus, and ADH is released from the neurohypophysis. (1)
3. ADH increases water reabsorption from the DCT and collecting duct, so less, more concentrated urine forms. (1)

Answer: ADH release increases water reabsorption, giving scanty, concentrated urine.

Role of other organs and disorders of the excretory system

NCERT section 16.7 Role of Other Organs in Excretion; 16.8 Disorders of the Excretory System

Other organs that help in excretion:

• Lungs: remove large amounts of CO₂ (about 200 mL per minute) and significant amounts of water every day.
• Liver (largest gland): secretes bile containing bilirubin, biliverdin, cholesterol, degraded steroid hormones, vitamins and drugs; most of these leave with the digestive wastes.
• Skin:
• Sweat glands: sweat is a watery fluid with NaCl, small amounts of urea, lactic acid etc. Its main role is to cool the body, but it also removes some wastes.
• Sebaceous glands: eliminate sterols, hydrocarbons and waxes through sebum, which gives a protective oily covering to the skin.
• Saliva: removes small amounts of nitrogenous wastes.

Disorders:

• Uremia: malfunctioning kidneys lead to accumulation of urea in blood; it is highly harmful and may lead to kidney (renal) failure.
• Haemodialysis (artificial kidney):

1. Blood is drained from a convenient artery, an anticoagulant such as heparin is added, and it is pumped into a dialysing unit (artificial kidney).
2. The unit has a coiled cellophane tube surrounded by dialysing fluid that has the same composition as plasma except nitrogenous wastes.
3. The porous cellophane lets molecules move by their concentration gradient; since dialysing fluid has no nitrogenous wastes, they diffuse out of the blood.
4. Cleared blood is pumped back into the body through a vein after adding anti-heparin (to restore normal clotting).

• Dialysis keeps many uremic patients alive worldwide.
• Kidney transplantation: the ultimate method for correcting acute renal failure. A functioning kidney is taken from a donor, preferably a close relative, so that the recipient's immune system is less likely to reject it. Better medical procedures now make transplants more successful.
• Renal calculi (kidney stones): hard, insoluble lumps of crystallised salts such as oxalates that form inside the kidney.
• Glomerulonephritis: the glomeruli become inflamed, so filtration is affected.

Worked example

In haemodialysis, why is heparin added before dialysis and anti-heparin afterwards? Why must the dialysing fluid have the same composition as plasma except for nitrogenous wastes? (3 marks)

Try it first, then show the solutionSolution

1. Heparin prevents blood from clotting in the tubing and dialysing unit. (½) Anti-heparin restores normal clotting before blood returns to the body through a vein. (½)
2. Since the fluid lacks nitrogenous wastes, urea diffuses out of the blood down its concentration gradient. (1)
3. Having the same levels of other plasma substances (e.g. glucose, salts) prevents their loss from blood, so only wastes are removed. (1)

Answer: Heparin stops clotting during dialysis; matching fluid removes only wastes.

Excretory Products and their Elimination in real life

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

Topics in this chapter

• Counter Current Mechanism and Kidney Regulation
• Nephron Structure and Urine Formation

Excretory Products and their Elimination: key facts

GFR per day

$125\text{mL/min}\times 1440\text{min/day}=180\text{L/day}$

Urine output about 1.5 L/day, so about 99% is reabsorbed.

Percentage reabsorbed

$\%\text{reabsorbed}=\frac{\text{filtrate}-\text{urine}}{\text{filtrate}}\times 100$

(180 − 1.5)/180 × 100 ≈ 99.2%.

Renin-angiotensin pathway

$\text{angiotensinogen}\stackrel{\text{renin}}{\to }\text{angiotensin I}\to \text{angiotensin II}\to \text{aldosterone (adrenal cortex)}$

Raises blood pressure and GFR; ANF opposes it.

Medullary gradient

$300{\text{mOsmol L}}^{-1}(\text{cortex})\to 1200{\text{mOsmol L}}^{-1}(\text{inner medulla})$

Built by NaCl and urea; urine up to about 4 times as concentrated as filtrate.

Path of filtrate

$\text{Bowman’s capsule}\to \text{PCT}\to \text{Henle’s loop}\to \text{DCT}\to \text{collecting duct}\to \text{pelvis}\to \text{ureter}$

Then urinary bladder and urethra.

Renal blood flow

$\text{renal blood flow}\approx \frac{1}{5}\times \text{cardiac output}\approx 1100\text{-}1200\text{mL/min}$

Filtered through the glomeruli.

Excretory Products and their Elimination: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

How the school exam asks Excretory Products and their Elimination

The question patterns to expect, and the method that works for each.

Tricks to remember Excretory Products and their Elimination

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

• A-U-U by water

  Ammonia (lots of water) → Urea (moderate) → Uric acid (least): from fish to mammal to bird.

  Use it when: Modes of excretion.

• My Pet Hen Drinks Coffee

  Malpighian body → PCT → Henle's loop → DCT → Collecting duct.

  Use it when: Path of filtrate.

• Down = water, Up = salt

  Descending limb lets water out; ascending limb lets salt out.

  Use it when: Henle's loop permeability.

• 125-180-1.5-99

  GFR 125 mL/min, 180 L/day filtrate, 1.5 L urine, 99% reabsorbed.

  Use it when: Key numbers.

• RAAS raises, ANF lowers

  Renin-Angiotensin-Aldosterone raise blood pressure; Atrial Natriuretic Factor (from the heart) relaxes vessels.

  Use it when: Opposing regulators.

• Lungs, Liver, Skin, Saliva

  CO₂ (lungs), bile pigments (liver), sweat and sebum (skin), small nitrogenous wastes (saliva).

  Use it when: Other excretory organs.

Excretory Products and their Elimination: exam-level solved questions

Ordered from exam level to HOTS. Cover the solution, write your own, then compare step by step.

Case-study practice (Section E style)

Read the situation, then answer every part. In a school exam set on the board pattern this is a 4-mark question split into 1 + 1 + 2 marks.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Where students lose marks in Excretory Products and their Elimination

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

• ✗Mistake: Writing that proteins are filtered into Bowman's capsule

  All plasma is assumed to filter.

  ✓Do this: Ultrafiltration passes almost everything except plasma proteins (and blood cells).

  Usually costs: 1 mark.

• ✗Mistake: Saying the descending limb is impermeable to water

  Limb properties are swapped.

  ✓Do this: Descending: permeable to water, almost impermeable to electrolytes; ascending: the reverse.

  Usually costs: 1 mark.

• ✗Mistake: Stating that ADH makes urine dilute

  'Antidiuretic' is misread.

  ✓Do this: ADH increases water reabsorption, so urine becomes concentrated and less in volume.

  Usually costs: 1 mark.

• ✗Mistake: Confusing diabetes insipidus with diabetes mellitus

  Both cause excess urine.

  ✓Do this: Insipidus: ADH deficiency, dilute urine without glucose. Mellitus: insulin problem, glucose (glycosuria) and ketones in urine.

  Usually costs: 1 mark.

• ✗Mistake: Placing Henle's loop in the cortex

  Not knowing nephron layout.

  ✓Do this: Malpighian body, PCT and DCT are in the cortex; Henle's loop dips into the medulla.

  Usually costs: ½-1 mark.

• ✗Mistake: Calling the JGA part of the PCT

  Location not learnt.

  ✓Do this: JGA forms where the DCT contacts the afferent arteriole.

  Usually costs: ½ mark.

NCERT exercise guide: Excretory Products and their Elimination

What each exercise in the NCERT chapter tests, how to approach it, and a question of the same kind solved step by step. Questions here are written in our own words.

Practice questions on Excretory Products and their Elimination

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

1. Q1 · 1 mark · MCQExam level

   The functional unit of the kidney is:
   (a) glomerulus (b) nephron (c) Bowman's capsule (d) collecting duct

   Hint

   About a million per kidney.

   Answer

   (b) nephron

2. Q2 · 1 mark · MCQExam level

   Which part of the nephron is impermeable to water?
   (a) PCT (b) descending limb of Henle's loop (c) ascending limb of Henle's loop (d) collecting duct

   Hint

   Salt out, water stays.

   Answer

   (c) ascending limb of Henle's loop

3. Q3 · 2 marks · Very short answerExam level

   Name the excretory structures of earthworms, cockroaches and prawns. (2 marks)

   Hint

   Three different structures.

   Answer

   Earthworm: nephridia; cockroach: Malpighian tubules (1); prawn: antennal (green) glands (1).

4. Q4 · 2 marks · Very short answerHarder exam level

   Why is urea retained in the kidney matrix of some animals? (2 marks)

   Hint

   Osmolarity.

   Answer

   Urea in the medullary interstitium keeps up a high osmolarity (1), which helps draw water out of the collecting duct to concentrate urine (1).

5. Q5 · 2 marks · Very short answerHarder exam level

   Describe the gross structure of the human kidney. (2 marks)

   Hint

   Hilum, pelvis, cortex, medulla.

   Answer

   Bean-shaped, 10-12 × 5-7 × 2-3 cm, 120-170 g; hilum on the concave side for ureter, vessels and nerves; renal pelvis with calyces inside (1). Tough capsule; outer cortex and inner medulla with pyramids; cortex extends between pyramids as columns of Bertini (1).

6. Q6 · 2 marks · Very short answerHarder exam level

   State the roles of sweat and sebaceous glands in excretion. (2 marks)

   Hint

   Sweat vs sebum.

   Answer

   Sweat glands release sweat with NaCl, small amounts of urea and lactic acid; mainly for cooling (1). Sebaceous glands release sterols, hydrocarbons and waxes in sebum, forming a protective oily layer (1).

7. Q7 · 3 marks · Short answerHarder exam level

   Why does the urge to pass urine grow as the bladder fills, and which two muscle actions release the urine? Give the average daily volume, pH and urea content of adult urine. (3 marks)

   Hint

   Stretch receptors → CNS → motor messages.

   Answer

   As urine collects, the bladder wall stretches and its stretch receptors send increasing signals to the CNS (1). The CNS sends motor messages that contract the smooth muscles of the bladder and relax the urethral sphincter, releasing urine (micturition reflex) (1). An adult passes about 1-1.5 L a day, slightly acidic (pH about 6.0), containing about 25-30 g of urea (1).

8. Q8 · 3 marks · Short answerHarder exam level

   Compare blood plasma in the afferent arteriole, filtrate in Bowman's capsule and urine in the collecting duct of a healthy person for (a) proteins, (b) glucose, (c) concentration of urea. Explain each difference. (3 marks)

   Hint

   Ultrafiltration; active reabsorption in the PCT; water removal.

   Answer

   (a) Proteins: present in plasma, almost absent in filtrate because ultrafiltration holds them back, absent in urine (1). (b) Glucose: present in plasma and filtrate (it is filtered freely), absent in normal urine because the PCT reabsorbs it actively (1). (c) Urea: the filtrate has about the plasma level, but urine is far more concentrated, because about 99% of the filtrate water is reabsorbed while urea is reabsorbed only partly (1).

9. Q9 · 5 marks · Long answerHarder exam level

   (a) Filtrate enters Henle's loop at about 300 mOsmol L⁻¹. Describe and explain how its concentration changes down the descending limb, up the ascending limb, and then down the collecting duct when ADH levels are high. (b) Why must the vasa recta run in a U alongside the loop? (c) Why can't cortical nephrons concentrate urine as well? (5 marks)

   Hint

   Which limb lets water out, which lets salt out?

   Answer

   (a) Descending limb: permeable to water, almost impermeable to electrolytes; water leaves into the increasingly concentrated medulla, so the filtrate becomes concentrated (1½). Ascending limb: impermeable to water; electrolytes (NaCl) leave, so the filtrate becomes dilute as it rises (1). Collecting duct with high ADH: as it passes through the medullary gradient (300 to 1200 mOsmol L⁻¹), water leaves by osmosis, so urine can become nearly four times as concentrated as the filtrate (1). (b) Blood in the two limbs flows in opposite directions: NaCl taken up by the descending vasa recta is returned to the interstitium by the ascending vasa recta, so blood flow does not wash the gradient away (1). (c) Their loops are short and the vasa recta is absent or highly reduced (½).

10. Q10 · 3 marks · Short answerHOTS / competency

    Suppose a drug blocks the transport of NaCl out of the ascending limb of Henle's loop. Predict its effect on urine volume and concentration and explain. (3 marks)

    Hint

    Gradient collapses.

    Answer

    NaCl transport from the ascending limb is the main source of the medullary osmotic gradient (1). Without it, the medulla becomes less concentrated, so less water is drawn out of the descending limb and collecting duct (1). Urine volume increases and it becomes more dilute; such drugs act as diuretics (1).

11. Q11 · 3 marks · Short answerHOTS / competency

    A person eats a very salty meal. Predict how ADH, aldosterone and ANF levels change and how urine changes. (3 marks)

    Hint

    Salt draws water into blood.

    Answer

    Higher blood salt concentration stimulates osmoreceptors, so ADH rises and more water is reabsorbed, giving less, more concentrated urine (1). Absorbed water increases blood volume; atrial stretch raises ANF, causing vasodilation (1). Aldosterone falls (renin-angiotensin is suppressed when blood pressure is high), so less Na⁺ is reabsorbed and excess salt is excreted (1).

12. Q12 · 1 mark · Assertion-reasonHOTS / competency

    Assertion (A): Ammonotelic animals need large amounts of water to excrete their nitrogenous waste.
    Reason (R): Ammonia is highly toxic and must be kept dilute.
    (a) Both A and R are true and R is the correct explanation of A.
    (b) Both A and R are true but R is not the correct explanation of A.
    (c) A is true but R is false.
    (d) A is false but R is true.

    Hint

    Toxicity and water.

    Answer

    (a)

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. 1.Name the excretory structures of flatworms and insects.Show answer

   Protonephridia (flame cells); Malpighian tubules.

2. 2.Where is urea made?Show answer

   In the liver.

3. 3.What are the dimensions of an adult human kidney?Show answer

   10-12 cm long, 5-7 cm wide, 2-3 cm thick; 120-170 g.

4. 4.What are the columns of Bertini?Show answer

   Extensions of the cortex between the medullary pyramids.

5. 5.What is the Malpighian body?Show answer

   Glomerulus plus Bowman's capsule (renal corpuscle).

6. 6.What is the normal GFR?Show answer

   About 125 mL per minute (180 L per day).

7. 7.What lines the PCT?Show answer

   Simple cuboidal brush border epithelium.

8. 8.What is the osmolarity of the inner medulla?Show answer

   About 1200 mOsmol L⁻¹.

9. 9.Which hormone is released from the atria to lower blood pressure?Show answer

   Atrial natriuretic factor (ANF).

10. 10.How much urea does an adult excrete per day?Show answer

    About 25-30 g.

11. 11.What anticoagulant is used in haemodialysis?Show answer

    Heparin.

12. 12.What is glomerulonephritis?Show answer

    Inflammation of the glomeruli of the kidney.

Quick revision checklist

The night before the exam: can you do each of these without looking?

• ☐Can you give two examples each of ammonotelic, ureotelic and uricotelic animals?
• ☐Can you draw and label the L.S. of a kidney?
• ☐Can you draw a nephron with afferent and efferent arterioles and vasa recta?
• ☐Can you explain ultrafiltration and the three layers of the filter?
• ☐Can you state what each tubule segment reabsorbs and secretes?
• ☐Can you explain the counter current mechanism?
• ☐Can you explain how ADH, renin-angiotensin and ANF regulate the kidney?
• ☐Can you describe haemodialysis step by step?

Excretory Products and their Elimination: frequently asked questions