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Excretory Products and their Elimination – Class 11 Biology notes, important questions and MCQs

NCERT Class 11 Biology, chapter 16. Notes, key terms, important questions with answers and MCQs.

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Chapter overview

Animals remove nitrogenous wastes as ammonia, urea or uric acid. In humans, kidneys form urine through filtration, reabsorption and secretion, regulated by hormones.

Excretion types and kidney

  • Ammonotelic (bony fish), ureotelic (mammals, amphibians), uricotelic (birds, reptiles, insects).
  • Nephron: glomerulus + Bowman's capsule (Malpighian body), PCT, loop of Henle, DCT, collecting duct.
  • Glomerular filtration rate (GFR) ≈ 125 mL/min (≈ 180 L/day); about 99% is reabsorbed.

Regulation and disorders

  • Counter-current mechanism (loop of Henle and vasa recta) concentrates urine.
  • ADH (vasopressin) increases water reabsorption; renin–angiotensin–aldosterone system raises BP and Na⁺ reabsorption; ANF lowers BP.
  • Disorders: uraemia (treated by haemodialysis), renal calculi, glomerulonephritis; kidney transplant for failure.

Key terms

Nephron
structural and functional unit of the kidney
Ultrafiltration
filtration of blood in the glomerulus under pressure
Juxtaglomerular apparatus
specialised region where the DCT touches the afferent arteriole; releases renin when GFR falls

Important questions with answers

What is the role of the liver and lungs in excretion?2 marks

The liver converts ammonia into urea (urea cycle) and excretes bile pigments and some drugs through bile. The lungs remove CO₂ (about 200 mL/min) and some water vapour.

How does the renin–angiotensin mechanism control blood pressure and GFR?3 marks

A fall in GFR or BP makes the JG cells release renin, which converts angiotensinogen to angiotensin I and then angiotensin II. Angiotensin II constricts blood vessels (raising BP and GFR) and stimulates the adrenal cortex to release aldosterone, which increases Na⁺ and water reabsorption in the DCT, raising blood volume and pressure.

Explain urine formation (filtration, reabsorption, secretion) and the counter-current mechanism that concentrates urine.5 marks

Glomerular filtration: blood pressure forces plasma (without proteins and cells) through the glomerular capillaries into Bowman's capsule, about 180 L per day. Reabsorption: about 99% of the filtrate is reabsorbed — glucose, amino acids, Na⁺ and much water in the PCT; water in the descending limb; NaCl in the ascending limb; more water in the DCT and collecting duct under ADH. Secretion: H⁺, K⁺ and ammonia are secreted into the filtrate to keep the ionic and acid–base balance. Counter-current: filtrate flows in opposite directions in the two limbs of Henle's loop and blood flows in opposite directions in the vasa recta. NaCl pumped out of the ascending limb and urea from the collecting duct build an increasing osmotic gradient in the medulla (from about 300 to 1200 mOsm/L). As filtrate passes down the collecting duct through this gradient, water leaves by osmosis, so urine becomes up to about four times more concentrated than the filtrate.

Quick revision

  • GFR ≈ 125 mL/min.
  • PCT reabsorbs most of the filtrate.
  • ADH conserves water.

Exam tips

  • Lack of ADH causes diabetes insipidus (large volumes of dilute urine).
  • Draw a nephron and label what is reabsorbed where: PCT (glucose, amino acids, 70–80% water and Na⁺), descending limb (water), ascending limb (NaCl), DCT and collecting duct (water under ADH).

Practice MCQs

  1. Functional unit of the kidney is

    1. neuron
    2. villus
    3. alveolus
    4. nephron
    Show answer

    Answer: (d) nephron. Nephron.

  2. Humans are

    1. ammonotelic
    2. ureotelic
    3. uricotelic
    4. guanotelic
    Show answer

    Answer: (b) ureotelic. Urea.

  3. Birds excrete mainly

    1. ammonia
    2. urea
    3. creatine
    4. uric acid
    Show answer

    Answer: (d) uric acid. Uricotelic.

  4. Hormone that increases water reabsorption in kidneys is

    1. insulin
    2. glucagon
    3. ADH
    4. thyroxine
    Show answer

    Answer: (c) ADH. Vasopressin.

  5. Glucose is mainly reabsorbed in the

    1. glomerulus
    2. collecting duct
    3. loop of Henle
    4. PCT
    Show answer

    Answer: (d) PCT. Proximal convoluted tubule.

  6. Normal GFR in a healthy adult is about

    1. 12.5 mL/min
    2. 1250 mL/min
    3. 125 mL/min
    4. 1.25 L/min
    Show answer

    Answer: (c) 125 mL/min. ~180 L/day.

  7. Counter-current mechanism involves

    1. PCT and DCT
    2. loop of Henle and vasa recta
    3. glomerulus only
    4. ureter and bladder
    Show answer

    Answer: (b) loop of Henle and vasa recta. Concentrates urine.

  8. The descending limb of Henle's loop is

    1. impermeable to water
    2. permeable to water but almost impermeable to salts
    3. permeable to both
    4. impermeable to both
    Show answer

    Answer: (b) permeable to water but almost impermeable to salts. Filtrate gets concentrated going down.

All chapters of Class 11 Biology

Written by Kundali IQ from standard, well-established facts to help with revision. This is not an official NCERT publication; always read your textbook too.