Excretory Products and Their Elimination Class 11 Notes

This page is the complete revision version of Excretory Products and Their Elimination, Class 11 Biology Chapter 16.

Every concept the chapter teaches appears here — the three nitrogenous wastes, invertebrate excretory organs, the human excretory system and nephron, urine formation and its three processes, the counter current mechanism, hormonal regulation, micturition, the supporting role of lungs, liver and skin, and the disorders of the excretory system — condensed for a single revision sitting. These notes follow the 2026-27 session.

Every fact on this page traces to the NCERT Class 11 Biology textbook. If you want the full text and the original diagrams, download the official NCERT chapter PDF. For the rest of the unit, browse the Class 11 Biology notes index.

Reference: NCERT Class 11 Biology textbook, chapter Excretory Products and Their Elimination.

Chapter Map: How the Excretory Story Builds

The chapter builds layer by layer, and each idea is the prerequisite for the next. This is the order in which NCERT develops the topic on pages 205 to 216.

1. Why wastes must be removed — metabolism and excess ingestion produce ammonia, urea, uric acid, carbon dioxide, water and ions that have to be eliminated totally or partially.
2. The three nitrogenous wastes — ammonia, urea, uric acid — and which animal groups excrete which.
3. Simple excretory organs of invertebrates — flame cells, nephridia, Malpighian tubules, green glands.
4. The human excretory system and the nephron — organs, kidney zones, the functional unit.
5. The three processes of urine formation — glomerular filtration, reabsorption, secretion.
6. Segment-by-segment tubule function — PCT, Henle’s loop, DCT, collecting duct.
7. The counter current mechanism — how Henle’s loop and vasa recta concentrate urine.
8. Hormonal regulation — ADH, the renin-angiotensin mechanism, ANF.
9. Micturition and the supporting organs — lungs, liver, skin.
10. Disorders — uremia, hemodialysis, renal calculi, glomerulonephritis.

If you need the rest of Class 11, the Class 11 notes hub collects every chapter, and the CBSE notes library holds all classes and subjects.

Excretion and Nitrogenous Wastes: Ammonia, Urea and Uric Acid

Metabolic activities and excess ingestion load the body with unwanted substances — ammonia, urea, uric acid, carbon dioxide, water and ions such as \( Na^+ \), \( K^+ \), \( Cl^- \), phosphate and sulphate. These must be removed totally or partially (NCERT, p. 205).

The three nitrogenous wastes differ sharply in toxicity, and that difference decides how much water an animal must spend to get rid of them: ammonia is the most toxic and needs a large amount of water for elimination, urea is less toxic and its formation in the liver is a water-conserving adaptation of terrestrial life, and uric acid is the least toxic and can be removed as a paste or pellet with minimum water loss (NCERT, p. 205).

Mnemonic (toxicity order): run the wastes as A-U-U from most to least toxic — Ammonia, Urea, Uric acid. The water cost follows the same line: Ammonia Always Needs (a lot of) Water, Urea Uses Less, Uric acid Uses the Least.

Invertebrates manage excretion with simple tubular structures (NCERT, p. 206):

Human Excretory System: Organs and Their Jobs

In humans the excretory system is a pair of kidneys, a pair of ureters, a urinary bladder and a urethra (NCERT, p. 206). The kidneys are reddish-brown, bean-shaped organs lying between the levels of the last thoracic and third lumbar vertebra, close to the dorsal inner wall of the abdominal cavity.

Each adult kidney measures 10-12 cm in length, 5-7 cm in width and 2-3 cm in thickness, with an average weight of 120-170 g.

On the inner concave surface is the hilum — the notch through which the ureter, blood vessels and nerves enter. Inner to it lies the broad funnel-shaped renal pelvis, whose projections are the calyces. Figure 16.1 shows the whole system at a glance.

Trace the pathway in the figure once: urine forms in the kidneys → the ureters carry it to the urinary bladder, where it is stored → the urethra carries it out of the body. If you can reproduce this chain in order, you can label the system correctly.

Inside the Kidney: Cortex, Medulla and the Nephron

Inside the tough outer capsule, the kidney has two zones — the outer cortex and the inner medulla (NCERT, p. 206). The medulla is divided into conical masses called medullary pyramids, which project into the calyces. Cortex extends between the pyramids as renal columns called Columns of Bertini (NCERT, p. 207). Figure 16.2 shows all these zones in a longitudinal section.

Each kidney holds about one million nephrons, the functional units of the organ (NCERT, p. 207). Each nephron has a glomerulus and a renal tubule. The glomerulus is a tuft of capillaries formed by the afferent arteriole, a fine branch of the renal artery; blood leaves it through the efferent arteriole.

The renal tubule begins as a double-walled cup, Bowman’s capsule, which encloses the glomerulus — together they form the Malpighian body (renal corpuscle). The tubule then runs as the coiled proximal convoluted tubule (PCT), the hairpin-shaped Henle’s loop (descending and ascending limbs), and the coiled distal convoluted tubule (DCT).

The DCTs of many nephrons open into a straight collecting duct; the collecting ducts converge and open into the renal pelvis through the medullary pyramids in the calyces.

Figure 16.3 shows the complete nephron with its blood vessels; Figure 16.4 zooms into the renal corpuscle, where the capillary tuft sits inside Bowman’s capsule.

Where each part sits: the Malpighian corpuscle, PCT and DCT lie in the cortex, while the loop of Henle dips into the medulla. In most nephrons — cortical nephrons — the loop is short; in juxta medullary nephrons, the loop is long and runs deep into the medulla.

Blood supply: the efferent arteriole forms a fine capillary network, the peritubular capillaries, around the renal tubule. A minute vessel of this network runs parallel to Henle’s loop as a U-shaped vasa recta, which is absent or highly reduced in cortical nephrons (NCERT, p. 208).

Mnemonic (nephron order): Good Boys Pass Down All Creatures’ Dirt — Glomerulus → Bowman’s capsule → PCT → Descending limb → Ascending limb → DCT → Collecting duct. (Down = descending limb, All = ascending limb.)

Urine Formation: Filtration, Reabsorption and Secretion

Urine formation runs on three processes in the nephron: glomerular filtration, reabsorption and secretion (NCERT, p. 208).

Glomerular filtration. On average 1100-1200 ml of blood is filtered by the kidneys per minute — roughly one-fifth of the blood pumped out by each ventricle of the heart in a minute. Glomerular capillary blood pressure pushes the blood through three layers: the endothelium of the glomerular blood vessels, the epithelium of Bowman’s capsule, and the basement membrane between them.

The capsule’s epithelial cells, the podocytes, are arranged so intricately that they leave minute filtration slits (slit pores). Almost all plasma constituents except proteins pass into the lumen of Bowman’s capsule, which is why the process is called ultra filtration (NCERT, p. 208).

GFR and its regulation. The filtrate formed per minute is the glomerular filtration rate (GFR) — about 125 ml in a healthy individual, or 180 litres per day (NCERT, p. 209).

The juxta glomerular apparatus (JGA), a sensitive region formed by cellular modifications in the DCT and the afferent arteriole where they contact each other, does the monitoring: a fall in GFR activates its JG cells to release renin, which stimulates glomerular blood flow and restores GFR to normal (NCERT, p. 209).

Reabsorption. Compare 180 litres of filtrate per day with about 1.5 litres of urine: nearly 99 per cent of the filtrate must be reabsorbed by the renal tubules. Glucose, amino acids and \( Na^+ \) are reabsorbed actively; nitrogenous wastes move passively; water is reabsorbed passively in the initial segments (NCERT, p. 209).

Tubular secretion. Tubule cells secrete \( H^+ \), \( K^+ \) and ammonia into the filtrate, maintaining the ionic and acid-base balance of body fluids.

Segment by Segment: What Each Tubule Does

Each segment of the tubule has a specific load of work (NCERT, pp. 209-210):

Figure 16.5 compresses the same information into one diagram: the arrows indicate the direction of movement of materials at each part of the nephron. Materials moving out of the tubule toward the blood are being reabsorbed; materials moving into the tubule are being secreted. Reading the arrows segment by segment reconstructs the whole story.

Counter Current Mechanism: How the Kidney Concentrates Urine

Counter current simply means flow in opposite directions. The filtrate flows down the descending limb of Henle’s loop and up the ascending limb; blood flows in a counter current pattern in the two limbs of the vasa recta.

Because the loop and the vasa recta run close together and their flows oppose each other, substances can be exchanged at every level of the medulla. The result is an increasing osmolarity from 300 mOsmol per litre in the cortex to about 1200 mOsmol per litre in the inner medulla (NCERT, p. 211).

Two solutes build this gradient (NCERT, p. 211):

• NaCl — transported out by the ascending limb of Henle’s loop, exchanged into the descending limb of the vasa recta, and returned to the interstitium by the ascending portion of the vasa recta.
• Urea — small amounts enter the thin segment of the ascending limb of Henle’s loop and are transported back to the interstitium by the collecting tubule.

This coordinated transport, made possible by the special arrangement of Henle’s loop and vasa recta, is the counter current mechanism. Its payoff: the gradient pulls water out of the collecting tubule, concentrating the filtrate so strongly that human kidneys can produce urine nearly four times more concentrated than the initial filtrate (NCERT, p. 212).

Figure 16.6 shows the two systems side by side. Follow the filtrate down the descending limb, around the hairpin bend and up the ascending limb; then follow the blood down and up the U-shaped vasa recta. The two counter-flowing streams exchange NaCl and urea at every level while the collecting duct runs down through the gradient they maintain.

Analogy (double flow): picture two escalators moving in opposite directions, side by side. Passengers pass each other at every step, so contact is continuous along the whole length.

The loop of Henle and vasa recta work the same way: two streams flowing opposite ways beside each other, trading NaCl and urea at every level instead of letting the gradient mix away.

Hormonal Regulation: ADH, Renin-Angiotensin and ANF

Kidney function is monitored by hormonal feedback involving the hypothalamus, the JGA and, to some extent, the heart (NCERT, p. 212). Three mechanisms matter.

ADH (vasopressin). Osmoreceptors in the body respond to changes in blood volume, body fluid volume and ionic concentration. Excessive fluid loss activates them; the hypothalamus then releases ADH from the neurohypophysis. ADH facilitates water reabsorption from the latter parts of the tubule, preventing diuresis.

When body fluid volume rises, the osmoreceptors switch off and ADH release is suppressed, closing the feedback loop. ADH also constricts blood vessels — blood pressure rises, increasing glomerular blood flow and GFR.

Renin-angiotensin mechanism. A fall in glomerular blood flow, glomerular blood pressure or GFR activates the JG cells to release renin. Renin converts angiotensinogen to angiotensin I and then to angiotensin II, a powerful vasoconstrictor that raises glomerular blood pressure and so restores GFR.

Angiotensin II also activates the adrenal cortex to release aldosterone, which causes \( Na^+ \) and water reabsorption from the distal parts of the tubule — again raising blood pressure and GFR.

ANF. Increased blood flow to the atria of the heart releases atrial natriuretic factor (ANF), which causes vasodilation, lowers blood pressure, and acts as a check on the renin-angiotensin mechanism.

Real-life link — winter versus summer urine output. The ADH loop explains why you pass urine more often in winter than in summer. In summer, heavy sweating removes water through the skin; body fluid volume falls, osmoreceptors fire, ADH release rises, the kidneys reabsorb more water, and urine output stays low and concentrated.

In winter, sweating almost stops, body fluid volume stays high, osmoreceptors switch off, ADH release falls, and the kidneys let more water through — urine output rises. Both situations are the same feedback loop defending a steady body fluid volume and ionic concentration.

Micturition and the Other Excretory Organs

Micturition is the release of urine, and it works as a reflex (NCERT, p. 213): urine is stored in the urinary bladder until a voluntary signal comes from the central nervous system. As the bladder fills, it stretches; stretch receptors in its wall fire and send signals to the CNS.

The CNS replies with motor messages that contract the smooth muscle of the bladder and simultaneously relax the urethral sphincter — urine is released. This neural pathway is the micturition reflex.

Normal urine (NCERT, p. 213): an adult excretes 1-1.5 litres per day; the urine is light yellow, slightly acidic (pH about 6.0) and has a characteristic odour; about 25-30 g of urea leaves per day. Glucose in urine (glycosuria) and ketone bodies (ketonuria) indicate diabetes mellitus, which is why urine analysis is a routine clinical diagnostic tool.

Disorders of the Excretory System: Uremia to Renal Calculi

The clinical conditions in this chapter are definition-heavy. Learn each pair (NCERT, pp. 213-214):

Hemodialysis is the treatment for uremia. Blood is drained from a convenient artery and an anticoagulant such as heparin is added, then pumped into an artificial kidney — a dialysing unit containing a coiled cellophane tube surrounded by dialysing fluid with the same composition as plasma except for nitrogenous wastes.

The porous cellophane membrane lets molecules move along their concentration gradient, so the nitrogenous wastes leave the blood. The cleared blood is pumped back into a vein after adding anti-heparin.

Kidney transplantation is the ultimate correction of acute renal failure (kidney failure): a functioning kidney from a donor, preferably a close relative, is implanted to minimise immune rejection by the host (NCERT, p. 214).

Key Terms and Definitions at a Glance

The chapter’s vocabulary in one table (terms from NCERT pp. 205-214):

Key Numbers and Values to Remember

This chapter carries no equations, so the numbers themselves are the formulas. These are the values a complete answer depends on:

Worked Examples: Applying the Ideas

Worked example 1: From GFR to daily urine volume (calculation)

Suppose a person’s GFR is 120 ml per minute. How much filtrate forms per hour and per day, and how much urine is released if nearly 99 per cent is reabsorbed?

Worked example 2: One urea molecule — renal artery to the exterior (trace)

Follow a single urea molecule through the system, naming the structure and the process at each stop (NCERT, pp. 208-213):

1. Renal artery: urea arrives in blood plasma and enters the afferent arteriole.
2. Afferent arteriole → glomerulus: plasma reaches the tuft of capillaries.
3. Glomerular filtration (ultra filtration): plasma minus proteins passes through the endothelium of the glomerular vessels, the basement membrane and the filtration slits of the podocytes into the lumen of Bowman’s capsule.
4. Bowman’s capsule → PCT: filtrate enters the proximal segment, where glucose and amino acids are actively reabsorbed; urea, a nitrogenous waste, largely stays in the filtrate.
5. Descending limb of Henle’s loop: water leaves (the limb is permeable to water), so the urea-containing filtrate concentrates.
6. Ascending limb: electrolytes leave while water cannot, so the filtrate dilutes; urea continues upward.
7. DCT → collecting duct: \( Na^+ \) and water are conditionally reabsorbed; the collecting duct allows small amounts of urea into the medullary interstitium to keep up osmolarity, so a little urea leaves here while the rest continues.
8. Collecting duct → renal pelvis: many collecting ducts converge and open into the renal pelvis through the medullary pyramids in the calyces.
9. Ureter → urinary bladder → urethra: urea (now part of urine) is carried to the bladder, stored, then released during micturition through the urethra.

Conclusion to state: the molecule is filtered at the glomerulus, carried along the tubule (concentrated in the descending limb, diluted in the ascending limb), surrounded by nearly complete water reabsorption, and finally leaves the body as part of urine through the urethra.

Common Mistakes and How to Avoid Them

Seven errors students make repeatedly in this chapter — and the correction for each:

Exam Notes: Writing Complete Answers to the Textbook Exercises

The NCERT exercises for this chapter (pages 215-216) ask six kinds of answers. Match your opening move to the type:

Definitions — Q1 (GFR) and Q8 (osmoregulation)

Open with the term, then give the defining number or function in one or two lines. A complete answer to Q1: GFR is the volume of filtrate formed by the kidneys per minute; in a healthy individual it is approximately 125 ml per minute, about 180 litres per day.

For Q8, define osmoregulation as the regulation of ionic and fluid volume balance in body fluids.

Explanation questions — Q2, Q4, Q5, Q6, Q9, Q10

Pattern: name the key term → give the events in order → close with the functional outcome. Worked skeletons:

• Q2 (autoregulatory mechanism of GFR): a fall in GFR activates the JG cells → renin is released → glomerular blood flow is stimulated → GFR returns to normal.
• Q6 (micturition): filling stretches the bladder → stretch receptors signal the CNS → motor messages contract the bladder’s smooth muscle and relax the urethral sphincter → urine is released (micturition reflex).
• Q9 (terrestrial animals): ammonia excretion demands a large amount of water, which land habitats cannot supply; urea and uric acid are less toxic and conserve water, with uric acid leaving as a paste or pellet.

True or false — Q3

Verdict first, then the corrected statement for anything false. Of the five statements, (a) micturition is a reflex, (c) protein-free fluid is filtered into Bowman’s capsule, (d) Henle’s loop plays an important role in concentrating urine and (e) glucose is actively reabsorbed in the PCT are true.

Statement (b) is false: ADH does not help water elimination — it facilitates water reabsorption from the latter parts of the tubule, preventing diuresis and making urine more concentrated, not hypotonic.

Matching, naming and fill-in-the-gaps — Q7, Q11, Q12

These test exact vocabulary, so answer them as complete sets.

Revision Recap: The Chapter on One Page

The entire chapter, in one breath — no new material here, only what the sections above already taught:

1. Wastes and animals: ammonia (most toxic, needs lots of water; ammonoteles: bony fishes, aquatic amphibians, aquatic insects) → urea (liver; ureoteles: mammals, terrestrial amphibians, marine fishes) → uric acid (least toxic, paste/pellet; uricoteles: reptiles, birds, land snails, insects). Invertebrate organs: flame cells, nephridia, Malpighian tubules, green glands.
2. Human system: kidneys → ureters → bladder → urethra. Kidney zones: cortex + medulla (pyramids, Columns of Bertini); hilum → renal pelvis → calyces.
3. Nephron (about one million per kidney): glomerulus + Bowman’s capsule = Malpighian body; PCT → Henle’s loop (descending, ascending) → DCT → collecting duct → renal pelvis. Cortex holds the corpuscle, PCT and DCT; the loop dips into the medulla. Cortical versus juxta medullary nephrons; U-shaped vasa recta.
4. Urine formation: glomerular filtration (1100-1200 ml blood/min; GFR 125 ml/min; ultra filtration through three layers and podocyte slits) → reabsorption (nearly 99 per cent; active for glucose, amino acids, \( Na^+ \); passive for nitrogenous wastes and water) → secretion (\( H^+ \), \( K^+ \), ammonia).
5. Segments: PCT (nutrients + 70-80 per cent of electrolytes and water; secretes \( H^+ \) and ammonia; absorbs \( HCO_3^- \)); descending limb (water out — concentrates); ascending limb (salts out — dilutes); DCT (conditional \( Na^+ \)/water; secretes \( H^+ \), \( K^+ \), \( NH_3 \)); collecting duct (large water reabsorption; small urea leak; secretes \( H^+ \), \( K^+ \)).
6. Counter current: opposite flows in the limbs of Henle’s loop and vasa recta → gradient of 300 to 1200 mOsmol/L built by NaCl and urea → water pulled from the collecting duct → urine about four times more concentrated than the filtrate.
7. Hormones: ADH (water reabsorption, anti-diuresis, vasoconstriction); renin → angiotensin II (vasoconstrictor) → aldosterone (\( Na^+ \) and water reabsorption); ANF (vasodilation, checks the renin-angiotensin mechanism).
8. Micturition: bladder stretch → stretch receptors → CNS → motor message → bladder contraction + sphincter relaxation (micturition reflex).
9. Other organs: lungs (\( CO_2 \), about 200 ml/min, plus water); liver (bile with pigments, steroids, drugs → digestive wastes); skin (sweat: \( NaCl \), urea, lactic acid; sebum: sterols, hydrocarbons, waxes); saliva (traces).
10. Disorders: uremia → hemodialysis or kidney transplantation; renal calculi; glomerulonephritis.

Frequently Asked Questions

Six doubts that come up while revising this chapter, answered directly.

Why do terrestrial animals excrete urea or uric acid instead of ammonia?

Because ammonia is the most toxic nitrogenous waste and needs a large amount of water for its elimination. On land, water is scarce, so terrestrial animals convert ammonia into less toxic wastes: urea (formed in the liver) needs much less water, and uric acid leaves as a paste or pellet with minimum water loss.

What is the difference between cortical nephrons and juxta medullary nephrons?

Cortical nephrons — the majority — have a short loop of Henle that extends only a little into the medulla. Juxta medullary nephrons have a very long loop that runs deep into the medulla, and the vasa recta is present around them (it is absent or highly reduced in cortical nephrons).

The long loop lets juxta medullary nephrons contribute more strongly to the medullary osmotic gradient that concentrates urine.

Why is glomerular filtration called ultra filtration?

Because the three-layer filter is extremely fine: blood passes through the endothelium of the glomerular vessels, the basement membrane, and the epithelium of Bowman’s capsule with its podocyte filtration slits. Almost all plasma constituents except proteins pass into Bowman’s capsule, so the filtration goes beyond ordinary filtration — hence the name ultra filtration.

How are ADH and aldosterone different in their action on the kidney?

ADH (released from the neurohypophysis) acts mainly on water: it facilitates water reabsorption from the latter parts of the tubule, preventing diuresis and concentrating urine. Aldosterone (released by the adrenal cortex under angiotensin II) acts mainly on sodium: it causes \( Na^+ \) and water reabsorption from the distal parts of the tubule, raising blood pressure and GFR.

In short, ADH regulates water balance; aldosterone regulates \( Na^+ \) (with water following), and its release follows a fall in GFR.

How does the kidney produce urine about four times more concentrated than the filtrate?

Through the counter current mechanism. Filtrate and blood flow in opposite directions in the limbs of Henle’s loop and the vasa recta, and this builds an osmotic gradient from 300 mOsmol/L in the cortex to about 1200 mOsmol/L in the inner medulla, maintained by NaCl and urea.

As the filtrate moves down the collecting duct through this steep gradient, water is pulled out of it, so the final urine can reach about four times the concentration of the initial filtrate.

Which organs besides the kidneys help in excretion?

A trio: lungs remove large amounts of \( CO_2 \) (about 200 ml per minute) and significant water; the liver secretes bile containing bilirubin, biliverdin, cholesterol, degraded steroid hormones, vitamins and drugs, which pass out with digestive wastes; and the skin removes \( NaCl \), small amounts of urea and lactic acid through sweat, plus sterols, hydrocarbons and waxes through sebum.

Small amounts of nitrogenous wastes also leave through saliva.

Reference: NCERT Class 11 Biology textbook, chapter Excretory Products and Their Elimination.

Official source: download the NCERT textbook free from ncert.nic.in.