Excretory Products and their Elimination is Chapter 16 of NCERT Class 11 Biology and part of Unit 5, Human Physiology, of the NEET UG syllabus. These notes cover modes of excretion, the human kidney and nephron, urine formation, tubule functions, the counter current mechanism, hormonal regulation, micturition, other excretory organs and disorders, with a free notes PDF.
Excretory Products and Their Elimination NEET Notes
Section-wise key points from NCERT Chapter 16, comparison tables, high-yield facts and common mistakes for NEET 2027 revision.
Excretory Products and Their Elimination at a Glance
| Item | Details |
|---|---|
| NEET UG syllabus unit | Unit 5: Human Physiology |
| NCERT textbook | Biology, Class 11, Chapter 16 (rationalised book) |
| Other chapters in the unit | Breathing and Exchange of Gases (Chapter 14), Body Fluids and Circulation (Chapter 15), Locomotion and Movement (Chapter 17), Neural Control and Coordination (Chapter 18), Chemical Coordination and Integration (Chapter 19) |
| Syllabus topics | Modes of excretion (ammonotelism, ureotelism, uricotelism); human excretory system, structure and function; urine formation; osmoregulation; regulation of kidney function (renin-angiotensin, atrial natriuretic factor, ADH and diabetes insipidus); role of other organs in excretion; disorders (uraemia, renal failure, renal calculi, nephritis); dialysis and artificial kidney |
| Official chapter | NCERT Chapter 16 |
| Notes PDF | View PDF |
The topic list follows the NEET UG 2026 syllabus released by the National Medical Commission (NMC) on 22 December 2025, which stays the reference until the 2027 syllabus is out. The full list is on the NEET syllabus page.
How do animals excrete nitrogenous wastes?
Ammonia, urea and uric acid are the major nitrogenous wastes, and which one an animal excretes depends mainly on its habitat, that is, on how much water is available.
| Mode | Main waste | Toxicity and water need | Examples |
|---|---|---|---|
| Ammonotelism | Ammonia | Most toxic; needs a large amount of water; excreted by diffusion across body surfaces or gill surfaces (in fish) as ammonium ions | Many bony fishes, aquatic amphibians, aquatic insects |
| Ureotelism | Urea (made from ammonia in the liver) | Less toxic; conserves water | Mammals, many terrestrial amphibians, marine fishes |
| Uricotelism | Uric acid as pellet or paste | Least toxic; removed with minimum loss of water | Reptiles, birds, land snails, insects |
- Ammonia is readily soluble and is generally excreted by diffusion; kidneys play no significant role in its removal.
- In ureotelic animals some urea may be retained in the kidney matrix to maintain a desired osmolarity.
Excretory Structures in Animals
| Structure | Animals | Role |
|---|---|---|
| Protonephridia (flame cells) | Platyhelminthes (flatworms, e.g. Planaria), rotifers, some annelids and the cephalochordate Amphioxus | Mainly ionic and fluid volume regulation (osmoregulation) |
| Nephridia | Earthworms and other annelids | Remove nitrogenous wastes; maintain fluid and ionic balance |
| Malpighian tubules | Most insects, including cockroaches | Remove nitrogenous wastes; osmoregulation |
| Antennal glands (green glands) | Crustaceans like prawns | Excretion |
| Kidneys | Vertebrates | Complex tubular excretory organs |
Human Excretory System and Kidney Structure
The human excretory system has a pair of kidneys, one pair of ureters, a urinary bladder and a urethra.
- Kidneys are reddish brown, bean-shaped organs between the levels of the last thoracic and third lumbar vertebra, close to the dorsal inner wall of the abdominal cavity.
- Each adult kidney is 10-12 cm long, 5-7 cm wide and 2-3 cm thick, with an average weight of 120-170 g.
- Hilum: a notch on the inner concave surface through which the ureter, blood vessels and nerves enter. Inner to it is the funnel-shaped renal pelvis with projections called calyces.
- The outer layer is a tough capsule. Inside are an outer cortex and an inner medulla; the medulla is divided into conical medullary pyramids projecting into the calyces.
- The cortex extends between the medullary pyramids as renal columns called Columns of Bertini.
Nephron: the Functional Unit
- Each kidney has nearly one million nephrons. Each nephron has two parts: the glomerulus and the 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 double-walled, cup-like Bowman’s capsule encloses the glomerulus; together they form the Malpighian body or renal corpuscle.
- Tubule sequence: Bowman’s capsule, proximal convoluted tubule (PCT), hairpin-shaped Henle’s loop (descending and ascending limbs), distal convoluted tubule (DCT), then the collecting duct. Collecting ducts open into the renal pelvis through the medullary pyramids in the calyces.
- The Malpighian corpuscle, PCT and DCT lie in the cortex; the loop of Henle dips into the medulla.
- The efferent arteriole forms the peritubular capillaries around the tubule. A minute vessel of this network runs parallel to Henle’s loop as the U-shaped vasa recta.
| Feature | Cortical nephron | Juxta medullary nephron |
|---|---|---|
| How common | Majority of nephrons | Some nephrons |
| Loop of Henle | Too short; extends only very little into the medulla | Very long; runs deep into the medulla |
| Vasa recta | Absent or highly reduced | Runs parallel to the long Henle’s loop |
How is urine formed?
Urine formation involves three main processes, glomerular filtration, reabsorption and secretion, which take place in different parts of the nephron.
| Process | Where | Key points |
|---|---|---|
| Glomerular filtration | Glomerulus into Bowman’s capsule | Driven by glomerular capillary blood pressure through 3 layers: endothelium of glomerular vessels, epithelium of Bowman’s capsule and a basement membrane between them. Podocytes leave filtration slits (slit pores). Almost all plasma constituents except proteins pass, so it is ultrafiltration. |
| Reabsorption | Renal tubules | Nearly 99% of the filtrate is reabsorbed. Glucose, amino acids and Na+ are reabsorbed actively, nitrogenous wastes passively; water is reabsorbed passively in the initial segments. |
| Secretion | Tubular cells | H+, K+ and ammonia are secreted into the filtrate, helping maintain ionic and acid-base balance of body fluids. |
- The kidneys filter 1100-1200 ml of blood per minute, roughly 1/5th of the blood pumped out by each ventricle of the heart in a minute.
- Glomerular filtration rate (GFR), the filtrate formed per minute, is about 125 ml/minute in a healthy person, i.e. 180 litres per day, against about 1.5 litres of urine released per day.
- Juxta glomerular apparatus (JGA): a sensitive region formed by cellular modifications in the DCT and the afferent arteriole where they touch. A fall in GFR activates JG cells to release renin, which brings GFR back to normal.
What does each part of the tubule do?
Each segment of the nephron differs in what it reabsorbs and secretes, and in its permeability to water and electrolytes.
| Segment | Feature | Function |
|---|---|---|
| PCT | Simple cuboidal brush border epithelium (more surface area) | Reabsorbs nearly all essential nutrients and 70-80% of electrolytes and water; secretes H+ and ammonia and absorbs HCO3- to keep pH and ionic balance |
| Descending limb of Henle’s loop | Permeable to water, almost impermeable to electrolytes | Concentrates the filtrate as it moves down |
| Ascending limb of Henle’s loop | Impermeable to water; transports electrolytes actively or passively | Minimum reabsorption; dilutes the filtrate; keeps medullary interstitial fluid at high osmolarity |
| DCT | Conditional reabsorption of Na+ and water | Reabsorbs HCO3-; secretes H+, K+ and NH3 to keep pH and sodium-potassium balance |
| Collecting duct | Long duct from cortex to inner medulla | Reabsorbs large amounts of water for concentrated urine; passes small amounts of urea into the medullary interstitium; secretes H+ and K+ |
Counter Current Mechanism: How Urine Is Concentrated
Mammals can produce concentrated urine because Henle’s loop and the vasa recta work as a counter current system.
- Filtrate flows in opposite directions in the two limbs of Henle’s loop, and blood flows in a counter current pattern in the two limbs of the vasa recta.
- Their proximity and counter current flow keep osmolarity rising towards the inner medulla, from 300 mOsmolL-1 in the cortex to about 1200 mOsmolL-1 in the inner medulla.
- This gradient is mainly caused by NaCl and urea.
- NaCl is transported by the ascending limb of Henle’s loop, exchanged with the descending limb of the vasa recta, and returned to the interstitium by the ascending portion of the vasa recta.
- Small amounts of urea enter the thin segment of the ascending limb of Henle’s loop and are transported back to the interstitium by the collecting tubule.
- The gradient lets water pass easily out of the collecting tubule, so human kidneys can make urine nearly four times as concentrated as the initial filtrate.
How is kidney function regulated?
Kidney function is monitored by hormonal feedback mechanisms involving the hypothalamus, the JGA and, to a certain extent, the heart.
| Mechanism | Trigger | Effect |
|---|---|---|
| ADH (vasopressin) | Excessive fluid loss activates osmoreceptors, which stimulate the hypothalamus; ADH is released from the neurohypophysis | Water reabsorption from the latter parts of the tubule, preventing diuresis. ADH also constricts blood vessels, raising blood pressure, glomerular blood flow and GFR. A rise in body fluid volume switches off the osmoreceptors. |
| Renin-angiotensin | Fall in glomerular blood flow, glomerular blood pressure or GFR activates JG cells to release renin | Renin converts angiotensinogen to angiotensin I and then angiotensin II, a powerful vasoconstrictor that raises glomerular blood pressure and GFR. Angiotensin II also makes the adrenal cortex release aldosterone, which causes reabsorption of Na+ and water from distal parts of the tubule. |
| Atrial Natriuretic Factor (ANF) | Increased blood flow to the atria of the heart | Vasodilation, which decreases blood pressure; acts as a check on the renin-angiotensin mechanism |
- Diabetes insipidus, named in the syllabus, is explained in NCERT Chapter 19: impaired synthesis or release of ADH reduces the kidney’s ability to conserve water, leading to water loss and dehydration.
Micturition and Urine
Micturition is the release of urine from the urinary bladder, controlled by a neural reflex.
- Urine is stored in the urinary bladder until the central nervous system (CNS) gives a voluntary signal.
- As the bladder fills, stretch receptors in its wall signal the CNS, which sends motor messages to contract the bladder’s smooth muscles and relax the urethral sphincter. The neural mechanism is the micturition reflex.
- An adult excretes on average 1 to 1.5 litres of urine per day: a light yellow, watery, slightly acidic fluid (pH 6.0) with a characteristic odour.
- On average 25-30 g of urea is excreted per day.
- Glucose (glycosuria) and ketone bodies (ketonuria) in urine indicate diabetes mellitus.
Role of Other Organs in Excretion
Besides the kidneys, the lungs, liver and skin also help eliminate excretory wastes.
| Organ | What it eliminates |
|---|---|
| Lungs | Large amounts of CO2 (about 200 mL/minute) and significant quantities of water every day |
| Liver (largest gland) | Bile containing bilirubin, biliverdin, cholesterol, degraded steroid hormones, vitamins and drugs; most leave with digestive wastes |
| Skin: sweat glands | Sweat, a watery fluid with NaCl, small amounts of urea and lactic acid; its main role is cooling the body surface |
| Skin: sebaceous glands | Sterols, hydrocarbons and waxes through sebum, which also forms a protective oily covering |
| Saliva | Small amounts of nitrogenous wastes |
Disorders of the Excretory System
Kidney malfunction lets urea build up in the blood, and NCERT covers its treatment by dialysis and transplantation.
| Disorder or treatment | Key points |
|---|---|
| Uremia | Accumulation of urea in blood due to kidney malfunction; highly harmful and may lead to kidney failure |
| Haemodialysis (artificial kidney) | Blood from a convenient artery, with an anticoagulant like heparin, is pumped into a dialysing unit: a coiled cellophane tube in dialysing fluid that matches plasma except for nitrogenous wastes. Wastes move out along the concentration gradient, and the cleared blood returns through a vein after adding anti-heparin. |
| Kidney transplantation | The ultimate method to correct acute renal (kidney) failure; a donor, preferably a close relative, minimises rejection by the host’s immune system |
| Renal calculi | Stone or insoluble mass of crystallised salts (oxalates, etc.) formed within the kidney |
| Glomerulonephritis | Inflammation of the glomeruli of the kidney |
Most-asked Facts for NEET: Excretory Products and Their Elimination
Revise these one-line facts from the NCERT chapter just before the exam.
- Ammonia is the most toxic and uric acid the least toxic nitrogenous waste.
- Ammonotelic: many bony fishes, aquatic amphibians, aquatic insects. Ureotelic: mammals, many terrestrial amphibians, marine fishes. Uricotelic: reptiles, birds, land snails, insects.
- Flame cells: Planaria and Amphioxus; nephridia: earthworm; Malpighian tubules: cockroach; green glands: prawn.
- Kidneys lie between the last thoracic and third lumbar vertebra; each weighs 120-170 g.
- Columns of Bertini are cortical extensions between the medullary pyramids.
- Malpighian corpuscle, PCT and DCT lie in the cortex; the loop of Henle dips into the medulla.
- Vasa recta is absent or highly reduced in cortical nephrons.
- Filtration happens through endothelium, Bowman’s capsule epithelium and basement membrane; podocytes form filtration slits.
- GFR is about 125 ml/minute or 180 litres per day; nearly 99% of the filtrate is reabsorbed.
- PCT reabsorbs 70-80% of electrolytes and water.
- Descending limb is permeable to water; ascending limb is impermeable to water.
- Medullary osmolarity rises from 300 to about 1200 mOsmolL-1, mainly due to NaCl and urea.
- Urine can be nearly four times as concentrated as the initial filtrate.
- ADH (vasopressin) is released from the neurohypophysis and prevents diuresis.
- Renin is released by JG cells; angiotensin II is a powerful vasoconstrictor and releases aldosterone from the adrenal cortex.
- ANF causes vasodilation and checks the renin-angiotensin mechanism.
- Urine pH is about 6.0; 25-30 g of urea is excreted per day; lungs remove about 200 mL CO2 per minute.
- Heparin is added before haemodialysis and anti-heparin before the blood is returned.
Common Mistakes to Avoid
Watch for these mix-ups when you answer questions on this chapter.
- Saying the ascending limb of Henle’s loop is permeable to water. It is the descending limb that is permeable to water; the ascending limb is impermeable to water.
- Putting the loop of Henle in the cortex. The Malpighian corpuscle, PCT and DCT are in the cortex; the loop dips into the medulla.
- Calling marine fishes ammonotelic. NCERT lists marine fishes as ureotelic; many bony fishes are ammonotelic.
- Thinking proteins are filtered into Bowman’s capsule. Almost all plasma constituents except proteins pass, which is why it is called ultrafiltration.
- Mixing up the hormones: ADH comes from the neurohypophysis, renin from JG cells, aldosterone from the adrenal cortex, and ANF from the atria of the heart.
- Saying ANF raises blood pressure. It causes vasodilation and lowers blood pressure, acting against the renin-angiotensin mechanism.
- Thinking nitrogenous wastes are reabsorbed actively. Glucose, amino acids and Na+ are reabsorbed actively; nitrogenous wastes are absorbed passively.
- Confusing diabetes insipidus with diabetes mellitus. Diabetes insipidus is linked to ADH; glucose and ketone bodies in urine indicate diabetes mellitus.
- Swapping heparin and anti-heparin in dialysis: heparin is added before blood enters the dialyser, anti-heparin before it goes back.
Excretory Products and Their Elimination Notes PDF and Related Pages
Download the AglaSem notes for Excretory Products and their Elimination (key notes and important questions with answers) free: View PDF
- All chapters: NEET Biology notes
- Full topic list: NEET syllabus
- Practice: NEET question papers
- Source text: NCERT Chapter 16 (NCERT, Class 11 Biology)
NEET Biology Notes: All Chapters
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All chapters with their notes PDFs are on the NEET Biology notes page.
Excretory Products and Their Elimination NEET Notes FAQs
Which NCERT chapter is Excretory Products and their Elimination?
It is Chapter 16 of the NCERT Class 11 Biology textbook (rationalised edition), in Unit 5, Human Physiology. In the NEET UG syllabus it is part of Unit 5, Human Physiology.
Is Excretory Products and their Elimination in the NEET 2027 syllabus?
It is in the latest NEET UG syllabus (2026, released by NMC on 22 December 2025) under Unit 5, Human Physiology. NMC is expected to release the 2027 syllabus later; check the syllabus page once it is out.
What is the counter current mechanism?
It is the transport of NaCl and urea made possible by the opposite flows in the two limbs of Henle’s loop and of the vasa recta. It keeps medullary osmolarity rising from 300 to about 1200 mOsmolL-1, which lets water leave the collecting tubule and concentrates urine.
What is the difference between ADH and the renin-angiotensin mechanism?
ADH is released from the neurohypophysis when osmoreceptors sense fluid loss and increases water reabsorption. The renin-angiotensin mechanism starts when JG cells sense a fall in GFR; angiotensin II constricts vessels and triggers aldosterone release, raising GFR and blood pressure.
Are these notes enough for NEET questions on excretion?
No. Read NCERT Chapter 16 line by line first, because NEET Biology questions closely follow the NCERT textbooks. Use these notes and the PDF for quick revision, then practise previous year questions.