TalentJR

Excretory Products and their Elimination – Class 11 Biology Chapter 16 Notes & NCERT PDF

Class 11BiologyChapter 16NCERT book: Biology - Class 11

Notes and a simple summary of Chapter 16 of the NCERT Class 11 Biology book, with the official chapter PDF, flashcards, an MCQ quiz and an AI tutor for your doubts.

🎧 Listen to the content

Chapter 16: Excretory Products and their Elimination

Introduction

Animals accumulate ammonia, urea, uric acid, carbon dioxide, water and ions such as Na⁺, K⁺, Cl⁻, phosphate and sulphate, either from metabolism or excess intake. These must be removed totally or partially. This chapter explains how animals — and especially humans — get rid of nitrogenous wastes and keep the ionic and acid–base balance of body fluids steady.

Three forms of nitrogenous waste

  • Ammonia — most toxic; needs a large amount of water for elimination. Excreting it is ammonotelism. Examples: many bony fishes, aquatic amphibians, aquatic insects. Being readily soluble, it leaves by diffusion across body surfaces or gills (as ammonium ions); kidneys play no significant role.
  • Urea — less toxic; made in the liver from ammonia and filtered out by the kidneys. Animals excreting it are ureotelic: mammals, many terrestrial amphibians and marine fishes. Some urea may be retained in the kidney matrix to maintain osmolarity.
  • Uric acid — least toxic; removed as pellet or paste with minimum water loss. Uricotelic animals: reptiles, birds, land snails and insects.
    • Terrestrial adaptation required less toxic wastes (urea, uric acid) to conserve water.

    Excretory structures across the animal kingdom

  • Protonephridia / flame cells — Platyhelminthes (e.g., Planaria), rotifers, some annelids and the cephalochordate Amphioxus; mainly osmoregulation.
  • Nephridia — earthworms and other annelids.
  • Malpighian tubules — most insects including cockroaches.
  • Antennal glands / green glands — crustaceans such as prawns.
  • Kidneys — vertebrates.

    • Key Concepts

    16.1 Human Excretory System

    A pair of kidneys, a pair of ureters, a urinary bladder and a urethra. Kidneys are reddish-brown, bean-shaped, between the last thoracic and third lumbar vertebra, near the dorsal inner wall of the abdominal cavity. Each adult kidney is 10–12 cm long, 5–7 cm wide, 2–3 cm thick, weighing about 120–170 g.

  • Hilum — notch on the inner concave surface where ureter, blood vessels and nerves enter.
  • Renal pelvis — funnel-shaped space inside the hilum with projections called calyces.
  • Outer tough capsule; inside, outer cortex and inner medulla. The medulla forms medullary pyramids; the cortex extends between them as Columns of Bertini.

    • Nephron — each kidney has nearly one million; the functional unit. Two parts:

  • Glomerulus — tuft of capillaries formed by the afferent arteriole (from the renal artery); blood leaves by the efferent arteriole.
  • Renal tubule — begins as the double-walled cup, Bowman's capsule. Glomerulus + Bowman's capsule = Malpighian body / renal corpuscle. Then the PCT, the hairpin Henle's loop (descending and ascending limbs), the DCT, which opens into a collecting duct; collecting ducts open into the renal pelvis through the pyramids.
  • Malpighian corpuscle, PCT and DCT lie in the cortex; the loop of Henle dips into the medulla.
  • Cortical nephrons — short loop (majority). Juxta medullary nephrons — long loop deep in the medulla.
  • The efferent arteriole forms peritubular capillaries; one vessel parallel to Henle's loop forms the U-shaped vasa recta (absent or reduced in cortical nephrons).

    • 16.2 Urine Formation — three processes

  • Glomerular filtration — 1100–1200 ml of blood filtered per minute (about 1/5th of the blood pumped by each ventricle per minute). Glomerular capillary pressure filters blood through three layers: endothelium, Bowman's capsule epithelium and the basement membrane between them. Podocytes leave filtration slits (slit pores). Everything except proteins passes — ultrafiltration.

    • - GFR ≈ 125 ml/minute = 180 litres/day.

    - Juxta glomerular apparatus (JGA) — modified DCT and afferent arteriole cells at their contact; a fall in GFR makes JG cells release renin to restore it.

  • Reabsorption — 180 L filtrate vs 1.5 L urine ⇒ nearly 99% is reabsorbed. Glucose, amino acids, Na⁺ are reabsorbed actively; nitrogenous wastes passively; water passively in the initial segments.
  • Tubular secretion — H⁺, K⁺ and ammonia are secreted into the filtrate, maintaining ionic and acid–base balance.

    • 16.3 Function of the Tubules

  • PCT — simple cuboidal brush-border epithelium; reabsorbs nearly all essential nutrients and 70–80% of electrolytes and water; secretes H⁺ and ammonia and absorbs HCO₃⁻.
  • Henle's loop — minimum reabsorption in the ascending limb, but maintains high medullary osmolarity. Descending limb: permeable to water, almost impermeable to electrolytes (filtrate concentrates). Ascending limb: impermeable to water, transports electrolytes (filtrate dilutes).
  • DCT — conditional reabsorption of Na⁺ and water; reabsorbs HCO₃⁻; secretes H⁺, K⁺ and NH₃.
  • Collecting duct — large water reabsorption for concentrated urine; lets small amounts of urea into the medullary interstitium; secretes H⁺ and K⁺.

    • 16.4 Counter Current Mechanism

    Filtrate in the two limbs of Henle's loop, and blood in the two limbs of vasa recta, flow in opposite directions — a counter current. This builds an osmolarity gradient from 300 mOsmol L⁻¹ in the cortex to about 1200 mOsmol L⁻¹ in the inner medulla, mainly due to NaCl and urea. NaCl from the ascending limb is exchanged with the descending limb of vasa recta and returned by its ascending portion; urea enters the thin ascending limb and is returned by the collecting tubule. Water then leaves the collecting tubule easily — urine can be about four times as concentrated as the initial filtrate.

    16.5 Regulation of Kidney Function

  • ADH (vasopressin) — excessive fluid loss activates osmoreceptors, which stimulate the hypothalamus to release ADH from the neurohypophysis; ADH increases water reabsorption from later parts of the tubule, preventing diuresis; it also constricts blood vessels, raising BP and GFR.
  • Renin–angiotensin mechanism — low glomerular blood flow/pressure/GFR → JG cells release renin → angiotensinogen → angiotensin I → angiotensin II (powerful vasoconstrictor; raises glomerular pressure and GFR; activates the adrenal cortex to release aldosterone, which causes Na⁺ and water reabsorption from distal tubule).
  • ANF (Atrial Natriuretic Factor) — more blood flow to the atria → ANF → vasodilation → lower BP; a check on renin–angiotensin.

    • 16.6 Micturition

    Stretch receptors in the filled bladder signal the CNS; motor messages contract bladder smooth muscle and relax the urethral sphincter. The release of urine is micturition; the neural mechanism is the micturition reflex. An adult excretes 1 to 1.5 litres/day of light-yellow, slightly acidic (pH 6.0) urine with 25–30 g urea/day. Glycosuria and ketonuria indicate diabetes mellitus.

    16.7 Other Organs in Excretion

  • Lungs — about 200 mL CO₂/minute and water.
  • Liver — bile containing bilirubin, biliverdin, cholesterol, degraded steroid hormones, vitamins and drugs.
  • Skin — sweat (NaCl, small amounts of urea, lactic acid) and sebum (sterols, hydrocarbons, waxes).
  • Small amounts of nitrogenous waste in saliva.

    • 16.8 Disorders

  • Uremia — urea accumulates in blood; treated by haemodialysis: blood + anticoagulant heparin passes through a coiled cellophane tube in dialysing fluid (same as plasma minus nitrogenous wastes); wastes diffuse out; anti-heparin is added before return via a vein.
  • Kidney transplantation — ultimate correction of acute renal failure; donor preferably a close relative.
  • Renal calculi — stones of crystallised salts (oxalates). Glomerulonephritis — inflammation of glomeruli.

    • Diagrams to Know (in words)

  • L.S. of kidney: capsule, cortex, medulla with pyramids, Columns of Bertini, calyces, pelvis, hilum, ureter.
  • Nephron: glomerulus in Bowman's capsule → PCT → descending and ascending limbs of Henle's loop → DCT → collecting duct; afferent/efferent arterioles, peritubular capillaries, vasa recta.
  • Counter current: loop of Henle beside vasa recta with arrows in opposite directions; osmolarity labels 300 → 1200 mOsmol L⁻¹.

    • Key-Facts Sheet

  • GFR ≈ 125 ml/min ≈ 180 L/day; urine ≈ 1.5 L/day; ≈ 99% reabsorbed
  • Blood filtered: 1100–1200 ml/min
  • PCT reabsorbs 70–80% of electrolytes and water
  • Medullary gradient 300 → 1200 mOsmol L⁻¹; urine ~4× concentrated
  • Urine pH ≈ 6.0; urea 25–30 g/day; lungs remove ~200 mL CO₂/min

    • Exam Focus

  • Define GFR and explain its autoregulation by JGA (renin).
  • Counter current mechanism — who carries NaCl and urea where.
  • True/False from the exercises: micturition is a reflex (true); ADH helps water elimination making urine hypotonic (false — ADH conserves water); protein-free fluid is filtered into Bowman's capsule (true); Henle's loop helps concentrate urine (true); glucose is actively reabsorbed in PCT (true).
  • Name: chordate with flame cells — Amphioxus; cortical portions between pyramids — Columns of Bertini; capillary loop parallel to Henle's loop — vasa recta.

    • Key Terms

  • Ammonotelism (अमोनियाउत्सर्जन)
  • Ureotelic (यूरिया-उत्सर्जी)
  • Uricotelic (यूरिक-अम्ल-उत्सर्जी)
  • Nephron (वृक्काणु)
  • Glomerulus (केशिकागुच्छ)
  • Bowman's capsule (बोमैन सम्पुट)
  • Ultrafiltration (परानिस्यंदन)
  • Reabsorption (पुनरवशोषण)
  • Counter current mechanism (प्रतिधारा क्रियाविधि)
  • Vasa recta (वासा रेक्टा)
  • Antidiuretic hormone (प्रतिमूत्रल हॉर्मोन)
  • Micturition (मूत्रण)
  • Osmoregulation (परासरण-नियमन)
  • Uremia (यूरिमिया)
  • Haemodialysis (रुधिर अपोहन)

    • Common Mistakes

  • Saying ADH makes urine dilute — it does the opposite.
  • Mixing the limbs: the descending limb is permeable to water; the ascending limb is impermeable to water.
  • Calling filtration selective — glomerular filtration is non-selective ultrafiltration; selectivity comes during reabsorption and secretion.
  • Forgetting that ANF lowers BP and counters the renin–angiotensin system.

    • 💡 Key Learning Points

    • ✓Ammonia, urea and uric acid differ in toxicity and water need, giving ammonotelic, ureotelic and uricotelic animals.
    • ✓The nephron (glomerulus + renal tubule) is the kidney's functional unit; about one million per kidney.
    • ✓Urine forms by glomerular filtration (GFR ≈ 125 ml/min), reabsorption (~99%) and tubular secretion.
    • ✓Henle's loop and vasa recta create a 300→1200 mOsmol L⁻¹ gradient by the counter current mechanism.
    • ✓ADH, the renin–angiotensin–aldosterone system and ANF regulate kidney function and blood pressure.

    👨‍🏫 Teaching Tips

    • →Trace a single drop of plasma from afferent arteriole to renal pelvis on a large nephron sketch.
    • →Use a two-column chart for each tubule segment: what is reabsorbed, what is secreted, water permeability.
    • →Demonstrate counter current with two opposite-flowing tubes of coloured water to show gradient build-up.
    • →Build the renin–angiotensin chain as a flowchart students complete from memory.
    • →Link to the ADH/diabetes insipidus content of Chapter 19 to connect excretion with hormones.
    • →Discuss haemodialysis step by step, highlighting why heparin and anti-heparin are added.
    • →Use the book's True/False and Name-the-following exercises as quick oral quizzes.

    📋 Assessment Questions

    1. Define GFR and explain how the JGA regulates it.
    2. Describe the counter current mechanism and its role in concentrating urine.
    3. Compare functions of PCT, Henle's loop, DCT and collecting duct.
    4. Explain micturition and the role of lungs, liver and skin in excretion.
    5. Why are terrestrial animals ureotelic or uricotelic rather than ammonotelic?

    Ask the AI tutor about Class 11 Biology Chapter 16

    Get question answers, important questions, a quick MCQ quiz or a simple explanation of “Excretory Products and their Elimination”. Free — sign in with Google.

    How the AI tutor works →

    👩‍🏫 Class 11 Biology classes near you — online or at home

    Prefer a real teacher? TalentJR teachers take live online classes anywhere in India and visit homes in selected cities. Tell us what you need and our team will call you with timings and fees.

    • Live online classesSmall groups or 1:1 with a teacher, from home, any city.
    • Teacher at homeA TalentJR teacher visits your home (selected cities).
    • At a centreClasses at a TalentJR partner centre near you, where available.

    Keep learning

    All chapters in Biology - Class 11

    1. 1. The Living World
    2. 2. Biological Classification
    3. 3. Plant Kingdom
    4. 4. Animal Kingdom
    5. 5. Morphology of Flowering Plants
    6. 6. Anatomy of Flowering Plants
    7. 7. Structural Organisation in Animals
    8. 8. Cell: The Unit of Life
    9. 9. Biomolecules
    10. 10. Cell Cycle and Cell Division
    11. 11. Photosynthesis in Higher Plants
    12. 12. Respiration in Plants
    13. 13. Plant Growth and Development
    14. 14. Breathing and Exchange of Gases
    15. 15. Body Fluids and Circulation
    16. 16. Excretory Products and their Elimination
    17. 17. Locomotion and Movement
    18. 18. Neural Control and Coordination
    19. 19. Chemical Coordination and Integration