NCERT Solutions for Class 11 Biology Chapter 16: Excretory Products and their Elimination (NCERT 2026–27)
These Class 11 Biology Chapter 16 solutions cover Excretory Products and their Elimination with every NCERT exercise question reproduced verbatim and answered in exam-ready prose. The chapter explains how animals remove nitrogenous wastes such as ammonia, urea and uric acid, the structure of the human excretory system, the working of the nephron, urine formation and concentration, and the hormonal regulation of kidney function — all updated for session 2026–27.
Class: 11Subject: BiologyChapter: 16Name: Excretory Products and their EliminationUnit: Human PhysiologySession: 2026–27
Chapter 16, Excretory Products and their Elimination, deals with the removal of harmful metabolic wastes — chiefly nitrogenous wastes (ammonia, urea, uric acid), CO2, water and excess ions — from the animal body. Animals are classified as ammonotelic, ureotelic or uricotelic based on the main nitrogenous waste they excrete, which depends largely on water availability in their habitat. In humans, the excretory system consists of a pair of kidneys, a pair of ureters, a urinary bladder and a urethra. The functional unit of the kidney is the nephron, where urine is formed by three processes — glomerular filtration, reabsorption and secretion. The counter current mechanism of Henle’s loop and vasa recta concentrates the urine, while hormones such as ADH, the renin–angiotensin–aldosterone system and ANF regulate kidney function. The chapter also describes micturition, the role of other organs in excretion, and disorders such as uremia, renal calculi and glomerulonephritis along with hemodialysis and kidney transplantation.
Key Concepts & Definitions
Excretion: the process of removal of harmful metabolic nitrogenous and other wastes from the body to maintain homeostasis.
Ammonotelism: excretion of nitrogenous waste mainly as ammonia (most toxic, needs much water) — e.g. many bony fishes, aquatic amphibians, aquatic insects.
Ureotelism: excretion mainly as urea (moderately toxic) — e.g. mammals, terrestrial amphibians, marine fishes; urea is made in the liver.
Uricotelism: excretion mainly as uric acid (least toxic, conserves water; passed as pellet/paste) — e.g. reptiles, birds, land snails, insects.
Nephron: the structural and functional unit of the kidney; about one million per kidney; has a glomerulus and a renal tubule (Bowman’s capsule → PCT → Henle’s loop → DCT → collecting duct).
Glomerular Filtration Rate (GFR): the amount of filtrate formed by the kidneys per minute — about 125 mL/minute (180 litres/day) in a healthy person.
Counter current mechanism: the arrangement and opposite flow in the two limbs of Henle’s loop and the two limbs of vasa recta that builds an osmolarity gradient (300–1200 mOsmolL–1) in the medullary interstitium to concentrate urine.
Micturition: the process of release of urine from the urinary bladder, controlled by the micturition reflex initiated by stretching of the bladder.
NCERT Exercise Solutions
All questions below are reproduced verbatim from the NCERT textbook (Reprint 2026–27). Answers are original and exam-ready.
1. Define Glomerular Filtration Rate (GFR).
ANSWERGlomerular Filtration Rate (GFR) is the total volume of glomerular filtrate formed by both the kidneys per minute. In a healthy individual it is approximately 125 mL/minute, which amounts to about 180 litres of filtrate per day. It is an important measure of how efficiently the kidneys are filtering the blood.
2. Explain the autoregulatory mechanism of GFR.
ANSWERThe kidneys have a built-in mechanism to keep the GFR fairly constant, carried out mainly by the juxtaglomerular apparatus (JGA). The JGA is a special sensitive region formed by cellular modifications at the point of contact between the distal convoluted tubule and the afferent arteriole.When the GFR falls, the JG cells are stimulated to release the enzyme renin. Renin increases the glomerular blood flow and blood pressure (through the renin–angiotensin pathway), which raises the GFR back towards normal. In this way the JGA automatically regulates and maintains the GFR.
3. Indicate whether the following statements are true or false:
(a) Micturition is carried out by a reflex.(b) ADH helps in water elimination, making the urine hypotonic.(c) Protein-free fluid is filtered from blood plasma into the Bowman’s capsule.(d) Henle’s loop plays an important role in concentrating the urine.(e) Glucose is actively reabsorbed in the proximal convoluted tubule.
ANSWER(a) True. Release of urine occurs through the micturition reflex, triggered by stretching of the urinary bladder.(b) False. ADH (antidiuretic hormone) promotes water reabsorption from the latter parts of the tubule, preventing diuresis and making the urine more concentrated (hypertonic), not hypotonic.(c) True. During glomerular filtration almost all plasma constituents except proteins pass into Bowman’s capsule, so the filtrate is essentially protein-free.(d) True. Henle’s loop, together with the vasa recta, maintains the medullary osmotic gradient and plays a key role in concentrating the urine.(e) True. Glucose is reabsorbed actively in the proximal convoluted tubule (PCT).
4. Give a brief account of the counter current mechanism.
ANSWERThe counter current mechanism enables mammals to produce concentrated urine. The two limbs of Henle’s loop carry filtrate in opposite directions, and the two limbs of the vasa recta carry blood in opposite directions — this opposite (counter current) flow, along with the close proximity of the loop and the vasa recta, builds and maintains an increasing osmolarity towards the inner medulla, from about 300 mOsmolL–1 in the cortex to about 1200 mOsmolL–1 in the inner medulla.This gradient is caused mainly by NaCl and urea. NaCl is transported by the ascending limb of Henle’s loop and exchanged with the descending limb of the vasa recta, and is returned to the interstitium by the ascending portion of the vasa recta. Small amounts of urea enter the thin ascending limb and are carried back to the interstitium by the collecting duct.The high interstitial osmolarity draws water out of the collecting duct, concentrating the filtrate. As a result, human kidneys can produce urine that is nearly four times more concentrated than the initial filtrate — an excellent mechanism for conserving water.
5. Describe the role of liver, lungs and skin in excretion.
ANSWERLiver: the largest gland, it secretes bile containing substances such as bilirubin, biliverdin, cholesterol, degraded steroid hormones, vitamins and drugs; most of these pass out along with the digestive (faecal) wastes. The liver also converts toxic ammonia into urea.Lungs: they remove large amounts of CO2 (about 200 mL/minute) along with significant quantities of water vapour every day.Skin: the sweat glands secrete sweat — a watery fluid containing NaCl, small amounts of urea and lactic acid — which, besides cooling the body, removes some wastes. The sebaceous glands eliminate sterols, hydrocarbons and waxes through sebum, which also forms a protective oily layer on the skin.
6. Explain micturition.
ANSWERUrine formed by the nephrons is carried to the urinary bladder, where it is stored until a voluntary signal is given by the central nervous system (CNS). As the bladder fills and stretches, the stretch receptors on its wall send signals to the CNS.In response, the CNS sends motor messages that cause contraction of the smooth muscles of the bladder and the simultaneous relaxation of the urethral sphincter, releasing urine. This process of releasing urine is called micturition, and the neural mechanism that causes it is the micturition reflex. An adult human excretes, on an average, 1 to 1.5 litres of urine per day.
7. Match the items of column I with those of column II:
ANSWEROsmoregulation is the active regulation of the water content and the concentration of salts (ions) in the body fluids, so as to maintain a constant internal osmotic concentration. It keeps the body’s fluid and ionic balance steady regardless of changes in the external environment, and is an important part of homeostasis carried out by excretory structures such as the kidneys.
9. Terrestrial animals are generally either ureotelic or uricotelic, not ammonotelic, why?
ANSWERAmmonia is the most toxic nitrogenous waste and requires a large amount of water for its elimination. Terrestrial animals have limited access to water and must conserve it. Therefore they cannot afford to excrete ammonia.Instead, they convert ammonia into less toxic urea (ureotelic) or the least toxic uric acid (uricotelic), both of which can be removed with much less water. This is a terrestrial adaptation for water conservation, which is why land animals are generally ureotelic or uricotelic rather than ammonotelic.
10. What is the significance of juxta glomerular apparatus (JGA) in kidney function?
ANSWERThe JGA is a specialised sensitive region formed at the contact between the afferent arteriole and the distal convoluted tubule. It plays a key role in the regulation of GFR and blood pressure.When the glomerular blood flow, blood pressure or GFR falls, the JG cells release renin, which converts angiotensinogen into angiotensin I and then to angiotensin II. Angiotensin II is a powerful vasoconstrictor that raises the glomerular blood pressure and GFR, and also stimulates the adrenal cortex to release aldosterone (promoting Na+ and water reabsorption). Thus the JGA helps restore normal GFR and maintain body fluid balance.
11. Name the following:
(a) A chordate animal having flame cells as excretory structures(b) Cortical portions projecting between the medullary pyramids in the human kidney(c) A loop of capillary running parallel to the Henle’s loop.
ANSWER(a)Amphioxus (a cephalochordate), which has protonephridia or flame cells as excretory structures.(b) Columns of Bertini (renal columns).(c) Vasa recta.
12. Fill in the gaps:
(a) Ascending limb of Henle’s loop is _______ to water whereas the descending limb is _______ to it.(b) Reabsorption of water from distal parts of the tubules is facilitated by hormone _______.(c) Dialysis fluid contain all the constituents as in plasma except _______.(d) A healthy adult human excretes (on an average) _______ gm of urea/day.
ANSWER(a) Ascending limb is impermeable to water, whereas the descending limb is permeable to it.(b)ADH (antidiuretic hormone / vasopressin).(c)Nitrogenous wastes (e.g. urea).(d) About 25–30 gm of urea per day.
Extra Practice Questions
Short Answer Type Questions
Q1. Name the three major nitrogenous wastes excreted by animals in order of decreasing toxicity.
ANSWERAmmonia (most toxic) > urea (moderately toxic) > uric acid (least toxic). The less toxic forms require less water for elimination.
Q2. What is ultrafiltration in the context of urine formation?
ANSWERUltrafiltration is the filtration of blood at the glomerulus through three layers — the endothelium of the glomerular capillaries, the basement membrane and the epithelium (podocytes) of Bowman’s capsule. The filtration slits are so fine that almost all plasma constituents except proteins pass into Bowman’s capsule, hence it is called ultrafiltration.
Q3. Differentiate between cortical and juxtamedullary nephrons.
ANSWERIn cortical nephrons the loop of Henle is short and extends only slightly into the medulla, and the vasa recta is absent or highly reduced; they form the majority of nephrons. In juxtamedullary nephrons the loop of Henle is long, runs deep into the medulla, and has a well-developed vasa recta; they are important in concentrating urine.
Q4. What is uremia and how is it managed?
ANSWERUremia is the accumulation of urea in the blood due to malfunctioning of the kidneys; it is highly harmful and may lead to kidney failure. It is managed by hemodialysis using an artificial kidney, and in severe cases by kidney transplantation from a compatible donor.
Q5. State the functions of the proximal convoluted tubule (PCT).
ANSWERThe PCT, lined by a brush-bordered cuboidal epithelium, reabsorbs nearly all essential nutrients and 70–80% of electrolytes and water. It also maintains the pH and ionic balance of body fluids by secreting H+ and ammonia into the filtrate and absorbing HCO3– from it.
Long Answer Type Questions
Q1. Describe the structure of the human kidney.
ANSWEREach kidney is a reddish-brown, bean-shaped organ measuring about 10–12 cm in length, 5–7 cm in width and 2–3 cm in thickness, weighing 120–170 g, situated between the last thoracic and third lumbar vertebrae against the dorsal abdominal wall. The inner concave surface has a notch called the hilum, through which the ureter, blood vessels and nerves pass. Internal to the hilum is the funnel-shaped renal pelvis with projections called calyces. The kidney is covered by a tough capsule and has two zones — an outer cortex and an inner medulla. The medulla is divided into conical medullary pyramids projecting into the calyces, and the cortex extends between them as the Columns of Bertini. Each kidney has nearly one million nephrons, the functional units, each consisting of a glomerulus and a renal tubule.
Q2. Explain urine formation in the nephron.
ANSWERUrine formation involves three processes. (1) Glomerular filtration: about 1100–1200 mL of blood is filtered per minute through the three filtration layers at the glomerulus, forming about 125 mL of protein-free filtrate per minute (GFR). (2) Reabsorption: nearly 99% of the filtrate is reabsorbed by the tubules — glucose, amino acids and Na+ are reabsorbed actively (mainly in the PCT), water and nitrogenous wastes largely passively. The descending limb of Henle’s loop reabsorbs water, the ascending limb reabsorbs electrolytes, and the DCT and collecting duct carry out conditional reabsorption of water and Na+. (3) Tubular secretion: the tubular cells secrete H+, K+ and ammonia into the filtrate to maintain the ionic and acid–base balance. The final concentrated urine, about 1–1.5 litres/day, then passes to the bladder.
Q3. Describe the hormonal regulation of kidney function.
ANSWERKidney function is regulated by feedback mechanisms involving the hypothalamus, the JGA and the heart. ADH: when body fluid is lost, osmoreceptors stimulate the hypothalamus to release ADH (vasopressin) from the neurohypophysis, which promotes water reabsorption and prevents diuresis; rising fluid volume switches off ADH release. Renin–angiotensin–aldosterone: a fall in glomerular blood flow/pressure/GFR makes the JG cells release renin, which forms angiotensin II — a powerful vasoconstrictor that raises blood pressure and GFR and stimulates the adrenal cortex to release aldosterone, which increases Na+ and water reabsorption from distal tubules. ANF: increased blood flow to the atria releases Atrial Natriuretic Factor, which causes vasodilation, lowering blood pressure and acting as a check on the renin–angiotensin mechanism.
For each Assertion–Reason question, choose: (A) Both true and the Reason correctly explains the Assertion; (B) Both true but the Reason is not the correct explanation; (C) Assertion true, Reason false; (D) Assertion false, Reason true.
A-R 1. Assertion: Terrestrial animals are usually ureotelic or uricotelic.
Reason: Urea and uric acid require less water for elimination than ammonia, helping conserve water.
A-R 2. Assertion: ADH increases the volume of urine.
Reason: ADH facilitates reabsorption of water from the distal parts of the tubule.
A-R 3. Assertion: The glomerular filtrate is normally free of proteins.
Reason: During ultrafiltration the filtration slits allow almost all plasma constituents except proteins to pass into Bowman’s capsule.
A-R 4. Assertion: The counter current mechanism helps in concentrating urine.
Reason: The opposite flow in the limbs of Henle’s loop and vasa recta maintains an osmolarity gradient in the medulla.
A-R 5. Assertion: The JGA releases renin when GFR rises above normal.
Reason: Renin lowers the glomerular blood pressure to bring the GFR down.
Answer key: 1-(A), 2-(D), 3-(A), 4-(A), 5-(D).
Common Mistakes to Avoid
Watch out for these
Confusing the roles of ADH (promotes water reabsorption, concentrates urine) with diuresis — ADH does NOT cause water elimination.
Mixing up the limbs of Henle’s loop — the descending limb is permeable to water, the ascending limb is impermeable to water but transports electrolytes.
Writing that proteins are filtered into Bowman’s capsule — the filtrate is essentially protein-free.
Reversing the toxicity order — remember ammonia > urea > uric acid in toxicity, and uric acid needs the least water.
Stating GFR as 180 mL/min — GFR is about 125 mL/min (180 litres/day).
Saying urea is formed in the kidney — urea is produced in the liver and excreted by the kidneys.
How to score full marks in this chapter
Learn the exact figures — GFR (125 mL/min, 180 L/day), urine output (1–1.5 L/day), urea excreted (25–30 g/day) and the osmolarity gradient (300→1200 mOsmolL–1). Be able to draw and label the nephron and trace the path of the filtrate (Bowman’s capsule → PCT → Henle’s loop → DCT → collecting duct). For regulation questions, present ADH, the renin–angiotensin–aldosterone system and ANF clearly with their effects. Use precise terms such as ultrafiltration, counter current mechanism, ammonotelic/ureotelic/uricotelic, and always link structure to function.
Frequently Asked Questions
What is Class 11 Biology Chapter 16 about?
Chapter 16, Excretory Products and their Elimination, explains how animals remove nitrogenous wastes (ammonia, urea, uric acid) and other wastes, the structure of the human excretory system and nephron, urine formation and concentration through the counter current mechanism, hormonal regulation of kidney function, micturition, and disorders of the excretory system.
What is the difference between ammonotelic, ureotelic and uricotelic animals?
Ammonotelic animals (e.g. bony fishes) excrete ammonia, which is most toxic and needs the most water; ureotelic animals (e.g. mammals) excrete urea, which is moderately toxic; uricotelic animals (e.g. birds, reptiles, insects) excrete uric acid, which is least toxic and conserves the most water.
How is the GFR regulated in the kidney?
GFR is regulated mainly by the juxtaglomerular apparatus (JGA). A fall in GFR makes the JG cells release renin, which (through angiotensin II) raises glomerular blood pressure and restores normal GFR; ANF from the heart acts as a check by lowering blood pressure.
Are these Class 11 Biology Chapter 16 solutions free?
Yes. All solutions are free and follow the official NCERT Biology textbook for session 2026–27, with every exercise question answered in exam-ready format.
