Excretory Products and their Elimination
Welcome to HSLC Guru! This page provides comprehensive notes, questions, and answers for Class 11 Biology Chapter 19 — Excretory Products and their Elimination, prepared strictly according to the ASSEB syllabus. The chapter explains how animals, especially humans, eliminate metabolic wastes such as urea, uric acid and ammonia through specialised organs to maintain a stable internal environment (homeostasis).
Summary
Metabolic activities in living organisms produce nitrogenous wastes such as ammonia, urea and uric acid. Animals are classified on the basis of the major nitrogenous waste they excrete. Ammonotelic animals (bony fishes, aquatic amphibians, aquatic insects) excrete ammonia, which is highly toxic but very soluble in water and requires plenty of water for elimination. Ureotelic animals (mammals, many terrestrial amphibians, marine fishes) excrete urea, a less toxic compound formed in the liver through the ornithine cycle. Uricotelic animals (reptiles, birds, land snails, insects) excrete uric acid as a semi-solid paste, conserving water — an adaptation for terrestrial and desert life.
The human excretory system consists of a pair of kidneys, a pair of ureters, a urinary bladder and a urethra. Each kidney is bean-shaped and lies on the posterior abdominal wall. Internally, a kidney shows an outer cortex, an inner medulla (containing renal pyramids), and a renal pelvis that opens into the ureter. The functional unit of the kidney is the nephron, of which there are about one million in each kidney. A nephron has a Malpighian (renal) corpuscle made of Bowman’s capsule and glomerulus, followed by the proximal convoluted tubule (PCT), the U-shaped Loop of Henle (descending and ascending limbs), the distal convoluted tubule (DCT), and finally the collecting duct. Cortical and juxtamedullary nephrons differ in the length of the Loop of Henle.
Urine formation involves three main processes — glomerular filtration, tubular reabsorption and tubular secretion. Blood under high pressure in the glomerulus is filtered into Bowman’s capsule (Glomerular Filtration Rate — about 125 mL/min in a healthy adult). Nearly 99% of the filtrate is reabsorbed in the PCT, Loop of Henle, DCT and collecting duct. The counter-current mechanism, operating in the Loop of Henle and vasa recta, maintains an osmotic gradient in the medulla (from 300 mOsmL⁻¹ in the cortex to about 1200 mOsmL⁻¹ in the inner medulla) and helps in producing concentrated (hypertonic) urine. Tubular secretion of H⁺, K⁺ and NH₃ helps maintain ionic and acid–base balance.
Kidney function is regulated hormonally. The juxtaglomerular apparatus (JGA) releases renin in response to a fall in blood pressure or sodium, activating the renin-angiotensin-aldosterone system. ADH (vasopressin) from the posterior pituitary increases water reabsorption in the DCT and collecting duct, while ANF (atrial natriuretic factor) from the heart causes vasodilation and reduces blood pressure. Micturition is the voluntary release of urine from the bladder. Other organs assist in excretion — lungs eliminate CO₂ and water vapour, the liver excretes bile pigments, cholesterol and drugs, and skin removes water, salts, urea and lactic acid through sweat. Common disorders include uraemia, renal failure, renal calculi (kidney stones) and glomerulonephritis. Patients with renal failure are treated by haemodialysis or peritoneal dialysis, and ultimately by kidney transplantation.
Very Short Answer Questions (1 Mark)
Q1. Define excretion.
Answer: Excretion is the biological process by which the metabolic nitrogenous waste products are eliminated from the body to maintain homeostasis.
Q2. What is meant by the term “ureotelic”?
Answer: Animals that excrete urea as their main nitrogenous waste are called ureotelic. Examples include mammals, marine fishes and adult amphibians.
Q3. Name the functional unit of the human kidney.
Answer: The nephron is the structural and functional unit of the kidney.
Q4. What is GFR?
Answer: Glomerular Filtration Rate (GFR) is the amount of filtrate formed by the kidneys per minute. In a healthy adult, it is about 125 mL/min, i.e., approximately 180 litres per day.
Q5. Name the hormone that increases water reabsorption in the kidneys.
Answer: Antidiuretic hormone (ADH) or vasopressin, secreted by the posterior pituitary, increases water reabsorption in the DCT and collecting duct.
Q6. What is micturition?
Answer: The process of voiding urine from the urinary bladder through the urethra is called micturition.
Q7. Where is the juxtaglomerular apparatus located?
Answer: The JGA is located at the point where the afferent arteriole comes in contact with the distal convoluted tubule (DCT) of the same nephron.
Q8. Name the two types of nephrons in the kidney.
Answer: Cortical nephrons (with short Loop of Henle) and juxtamedullary nephrons (with long Loop of Henle extending deep into the medulla).
Q9. What is uraemia?
Answer: Uraemia is the abnormal accumulation of urea and other nitrogenous waste in blood due to impaired kidney function.
Q10. What is the role of ANF?
Answer: Atrial Natriuretic Factor (ANF) released by the heart atria causes vasodilation and reduces blood pressure, thereby antagonising the renin-angiotensin system.
Short Answer Questions (2–3 Marks)
Q1. Differentiate between ammonotelism, ureotelism and uricotelism.
Answer: Ammonotelism is the excretion of ammonia, which is highly toxic and water-soluble; seen in bony fishes and aquatic invertebrates. Ureotelism is the excretion of urea, a less toxic compound formed in the liver; seen in mammals and marine fishes. Uricotelism is the excretion of uric acid as a semi-solid paste with minimum water loss; seen in reptiles, birds and insects.
Q2. Describe the structure of a nephron.
Answer: A nephron consists of two parts — the renal corpuscle and the renal tubule. The renal corpuscle includes a Bowman’s capsule (a double-walled cup) enclosing a network of capillaries called the glomerulus. The renal tubule comprises the proximal convoluted tubule (PCT), the U-shaped Loop of Henle (with descending and ascending limbs) and the distal convoluted tubule (DCT). Several DCTs open into a common collecting duct.
Q3. What is glomerular filtration? Mention the GFR in humans.
Answer: Glomerular filtration is the process by which blood entering the glomerulus under high hydrostatic pressure is filtered through three layers — endothelium of capillaries, basement membrane and podocytes of Bowman’s capsule. The filtrate, which is plasma minus proteins and blood cells, collects in Bowman’s space. The Glomerular Filtration Rate is about 125 mL/minute, equivalent to 180 L of filtrate per day.
Q4. Explain the role of the Loop of Henle in concentrating urine.
Answer: The Loop of Henle along with the vasa recta operates a counter-current multiplier mechanism. The descending limb is permeable to water but not to salts, so water leaves and the filtrate becomes concentrated. The ascending limb is impermeable to water but actively transports Na⁺ and Cl⁻ out, making the filtrate dilute. This builds an osmotic gradient (300 to 1200 mOsmL⁻¹) in the medulla, which helps the collecting duct produce concentrated urine.
Q5. What is dialysis? State its types.
Answer: Dialysis is the artificial removal of nitrogenous waste from the blood of patients with renal failure. The two types are haemodialysis, in which blood is passed through a dialyser containing a semi-permeable membrane, and peritoneal dialysis, in which the patient’s peritoneum acts as the natural dialysis membrane.
Q6. Mention the role of skin, lungs and liver in excretion.
Answer: The lungs eliminate CO₂ and water vapour during exhalation. The liver excretes bile pigments (bilirubin and biliverdin), cholesterol, drugs and degraded steroid hormones via bile. The skin (sweat glands) removes water, salts (NaCl), small amounts of urea, lactic acid and other metabolic wastes through sweat, also helping in thermoregulation.
Long Answer Questions (5–7 Marks)
Q1. Describe the structure of the human excretory system in detail.
Answer: The human excretory system consists of one pair of kidneys, one pair of ureters, a urinary bladder and a urethra. Each kidney is reddish-brown, bean-shaped and lies on the dorsal abdominal wall, one on each side of the vertebral column between T12 and L3 vertebrae. The right kidney is slightly lower than the left due to the position of the liver. The inner concave surface of each kidney has a notch called the hilum, through which renal artery, renal vein, nerves and ureter enter or leave. Internally, a kidney shows an outer cortex and an inner medulla. The medulla is divided into 8–18 conical renal pyramids whose tips (papillae) project into cup-shaped calyces that unite to form the renal pelvis. The pelvis continues as the ureter, which carries urine to the urinary bladder, a muscular sac that stores urine until it is voided through the urethra. Each kidney contains about a million nephrons — its functional units. Blood is filtered by the kidneys to remove urea, creatinine, uric acid and excess salts, while regulating water balance and pH of body fluids.
Q2. Describe the process of urine formation in detail.
Answer: Urine formation involves three processes — glomerular filtration, tubular reabsorption and tubular secretion. (i) Glomerular filtration: Blood enters the glomerulus through the afferent arteriole, where the high hydrostatic pressure forces plasma (without proteins and blood cells) into Bowman’s capsule. The filtrate, called glomerular filtrate, is formed at a rate of about 125 mL/min. (ii) Tubular reabsorption: About 99% of the filtrate is reabsorbed by the renal tubules. The PCT reabsorbs nearly all glucose, amino acids and 70–80% of electrolytes and water. The descending limb of Loop of Henle reabsorbs water; the ascending limb actively transports Na⁺ and Cl⁻. The DCT carries out conditional reabsorption of Na⁺ and water. (iii) Tubular secretion: Tubule cells secrete H⁺, K⁺, NH₃ and certain drugs into the filtrate to maintain acid–base balance. The collecting duct concentrates urine under the influence of ADH. Final urine, only about 1.5 L per day, is passed into the renal pelvis and then to the urinary bladder.
Q3. Explain the counter-current mechanism in the kidney.
Answer: The counter-current mechanism operates in the Loop of Henle and the vasa recta of juxtamedullary nephrons. The two limbs of the Loop of Henle and the two limbs of the vasa recta act as counter-current multipliers and counter-current exchangers respectively. As filtrate moves down the descending limb, water leaves passively because the limb is permeable to water but impermeable to salts; the filtrate becomes hypertonic. In the ascending limb, Na⁺ and Cl⁻ are actively transported out while water cannot leave; the filtrate becomes hypotonic. This creates an osmolarity gradient in the medulla rising from 300 mOsmL⁻¹ in the cortex to about 1200 mOsmL⁻¹ in the inner medulla. The vasa recta runs parallel and exchanges salt and water passively, maintaining the gradient. Urea recycling from the collecting duct further enhances medullary hypertonicity. As the collecting duct passes through this gradient, water moves out (under ADH influence), forming concentrated urine and conserving body water.
Q4. Discuss the hormonal regulation of kidney function.
Answer: Kidney function is regulated mainly by three mechanisms. (i) Renin-Angiotensin-Aldosterone system: The juxtaglomerular apparatus releases renin when blood pressure or Na⁺ concentration falls. Renin converts angiotensinogen into angiotensin I, which is further converted to angiotensin II — a powerful vasoconstrictor that raises BP and stimulates the adrenal cortex to release aldosterone. Aldosterone increases reabsorption of Na⁺ and water from the DCT, restoring blood volume. (ii) Antidiuretic hormone (ADH): Released from the posterior pituitary in response to dehydration or rise in plasma osmolarity, ADH increases water reabsorption from DCT and collecting duct, producing concentrated urine. (iii) Atrial Natriuretic Factor (ANF): Released by atria of the heart when blood volume increases, ANF causes vasodilation and inhibits renin release, lowering BP. Together, these mechanisms maintain water and electrolyte balance and homeostasis of body fluids.
Q5. Describe the common disorders of the excretory system and the methods of treatment.
Answer: Major disorders of the excretory system include: (i) Uraemia — accumulation of urea in blood due to kidney malfunction; toxic and may be fatal. (ii) Renal failure — partial or complete loss of kidney function; leads to retention of waste, fluid and electrolyte imbalance. (iii) Renal calculi (kidney stones) — insoluble crystallised masses of oxalates and other salts formed in the kidney; cause severe pain. (iv) Glomerulonephritis — inflammation of glomeruli, often following streptococcal infection; causes haematuria and proteinuria. Treatment includes haemodialysis, in which the patient’s blood is passed through an artificial kidney machine fitted with a semi-permeable membrane, separating wastes by diffusion against a dialysing fluid. Peritoneal dialysis uses the peritoneum as the membrane. In end-stage renal disease, kidney transplantation is the ultimate cure; the donor kidney is best obtained from a close, genetically related relative to minimise the chance of rejection. Lifelong immunosuppressive drugs are required after transplant.
Multiple Choice Questions (MCQs)
Q1. The functional unit of the kidney is —
(a) Neuron (b) Nephron (c) Hepatocyte (d) Alveolus
Answer: (b) Nephron
Q2. Bony fishes are —
(a) Ureotelic (b) Uricotelic (c) Ammonotelic (d) None
Answer: (c) Ammonotelic
Q3. Site of urea synthesis in humans is —
(a) Kidney (b) Liver (c) Spleen (d) Lungs
Answer: (b) Liver
Q4. ADH is secreted by —
(a) Anterior pituitary (b) Posterior pituitary (c) Adrenal cortex (d) Thyroid
Answer: (b) Posterior pituitary
Q5. The maximum reabsorption in the nephron occurs in —
(a) PCT (b) DCT (c) Loop of Henle (d) Collecting duct
Answer: (a) PCT
Q6. Glomerular filtration rate (GFR) in a healthy adult is —
(a) 125 mL/min (b) 250 mL/min (c) 25 mL/min (d) 1250 mL/min
Answer: (a) 125 mL/min
Q7. Renin is released from —
(a) Glomerulus (b) JGA (c) Adrenal medulla (d) Hypothalamus
Answer: (b) JGA
Q8. Counter-current mechanism is associated with —
(a) PCT (b) Bowman’s capsule (c) Loop of Henle (d) DCT
Answer: (c) Loop of Henle
Q9. Kidney stones are mainly composed of —
(a) Calcium carbonate (b) Calcium oxalate (c) Cholesterol (d) Urea
Answer: (b) Calcium oxalate
Q10. ANF causes —
(a) Vasoconstriction (b) Vasodilation (c) Increase in BP (d) Water retention
Answer: (b) Vasodilation
Fill in the Blanks
1. Birds and reptiles excrete nitrogenous waste mainly as ______.
Answer: uric acid
2. The U-shaped portion of nephron is called ______.
Answer: Loop of Henle
3. The voluntary release of urine from the bladder is called ______.
Answer: micturition
4. ADH is also known as ______.
Answer: vasopressin
5. The outer region of the kidney is called the ______.
Answer: cortex
True / False
1. Mammals are ureotelic. Answer: True
2. Glucose is normally present in human urine. Answer: False
3. ANF lowers blood pressure. Answer: True
4. The right kidney is slightly higher than the left. Answer: False
5. Bowman’s capsule encloses the glomerulus. Answer: True
Glossary
| Term | Meaning |
|---|---|
| Excretion | Removal of metabolic nitrogenous waste from the body. |
| Ammonotelism | Excretion of ammonia as the principal nitrogenous waste. |
| Ureotelism | Excretion of urea as the chief nitrogenous waste. |
| Uricotelism | Excretion of uric acid as semi-solid waste. |
| Nephron | Structural and functional unit of the kidney. |
| Glomerulus | Tuft of capillaries enclosed by Bowman’s capsule. |
| Bowman’s capsule | Cup-shaped structure enclosing glomerulus. |
| PCT | Proximal convoluted tubule — main site of reabsorption. |
| DCT | Distal convoluted tubule — site of conditional reabsorption. |
| Loop of Henle | U-shaped portion of nephron involved in counter-current mechanism. |
| GFR | Glomerular Filtration Rate, about 125 mL/min in humans. |
| JGA | Juxtaglomerular apparatus — secretes renin. |
| ADH | Antidiuretic hormone (vasopressin) — promotes water reabsorption. |
| ANF | Atrial natriuretic factor — lowers BP via vasodilation. |
| Micturition | Voluntary release of urine from urinary bladder. |
| Uraemia | Accumulation of urea in blood. |
| Renal calculi | Insoluble crystallised stones in the kidney. |
| Glomerulonephritis | Inflammation of glomeruli of the kidney. |
| Dialysis | Artificial removal of nitrogenous wastes from blood. |
| Kidney transplantation | Surgical replacement of a diseased kidney with a healthy donor kidney. |