NCERT Solutions for Class 11 Biology Chapter 16 Excretory Products and Their Elimination
NCERT Solutions for Class 11 Biology Chapter 16 Excretory Products and Their Elimination covers revision notes and step-by-step solutions detailing modes of excretion, human excretory system, structure of nephron, urine formation, function of tubules, counter current mechanism, regulation of kidney function, micturition, and disorders of the excretory system.
Overview of Topics & Subtopics (Click to Jump)
1. Modes of Excretion & Human Excretory System Structure
Short Note:
• Modes of Nitrogenous Excretion:
- Ammonotelism: Excretion of toxic Ammonia requiring large amounts of water (e.g., aquatic amphibians, bony fishes, aquatic insects).
- Ureotelism: Excretion of Urea, which is less toxic and requires moderate water (e.g., mammals, terrestrial amphibians, marine fishes).
- Uricotelism: Excretion of Uric Acid as paste or pellet with minimal water loss (e.g., reptiles, birds, land snails, insects).
• Excretory Organs in Invertebrates: Protonephridia/Flame cells (Platyhelminthes), Nephridia (Annelids), Malpighian tubules (Insects), Antennal/Green glands (Crustaceans).
• Human Excretory System: Consists of a pair of bean-shaped Kidneys, a pair of Ureters, a Urinary Bladder, and a Urethra. Kidney tissue is divided into outer Cortex and inner Medulla.
2. Structure of Nephron & Mechanism of Urine Formation
Short Note:
• Structure of Nephron: Functional unit of kidney (~1 million per kidney).
- Malpighian Body: Glomerulus (tuft of capillaries) + Bowman's capsule (cup-like double-walled structure with podocytes).
- Renal Tubule: Proximal Convoluted Tubule (PCT) → Henle's Loop (descending and ascending limbs) → Distal Convoluted Tubule (DCT) → Collecting Duct.
- Types of Nephrons: Cortical nephrons (short Henle's loop; ~85%) and Juxtamedullary nephrons (long Henle's loop extending deep into medulla; ~15%).
• Urine Formation Steps:
1. Glomerular Filtration: Non-selective filtration under pressure (GFR = ~125 mL/min or 180 L/day). Filtration membrane consists of endothelium, basement membrane, and podocyte slit pores.
2. Reabsorption: Nearly 99% of filtrate reabsorbed. PCT reabsorbs 70–80% of electrolytes and water via brush border cuboidal epithelium.
3. Tubular Secretion: Active secretion of H⁺, K⁺, and NH₃ into filtrate to maintain ionic balance and pH.
3. Concentration of Filtrate (Counter-Current Mechanism) & Regulation
Short Note:
• Counter-Current Mechanism: Operates between the two limbs of Henle's Loop and Vasa Recta to maintain an increasing medullary interstitial osmolarity (300 mOsmol/L in cortex to 1200 mOsmol/L in inner medulla).
- Descending limb of Henle's loop is permeable to water, impermeable to electrolytes.
- Ascending limb of Henle's loop is impermeable to water, permeable to electrolytes (NaCl).
- NaCl transported by ascending limb of Henle's loop is exchanged with descending limb of Vasa Recta. Urea enters thin segment of ascending limb and is returned to interstitium by collecting duct.
• Regulation of Kidney Function:
- Hypothalamus (ADH / Vasopressin): High blood osmolarity triggers ADH release from neurohypophysis → increases water reabsorption in DCT and collecting duct → prevents diuresis.
- JGA (RAAS System): Fall in GFR/blood pressure stimulates Juxtaglomerular (JG) cells to release Renin → converts Angiotensinogen to Angiotensin I → Angiotensin II (vasoconstrictor) → stimulates adrenal cortex to release Aldosterone (increases Na⁺ and water reabsorption).
- ANF (Atrial Natriuretic Factor): High blood pressure causes heart atria to release ANF → causes vasodilation → inhibits RAAS mechanism.
4. Micturition, Other Excretory Organs & Disorders
Short Note:
• Micturition: Process of release of urine. Stretch receptors on bladder wall send signals to CNS → triggers motor impulses causing contraction of smooth muscle of bladder and relaxation of urethral sphincter.
• Other Organs in Excretion: Lungs (remove ~200 mL/min CO2), Liver (excretes bilirubin, biliverdin, cholesterol, degraded steroid hormones), Skin (sweat glands remove NaCl, urea, lactic acid; sebaceous glands eliminate sterols, hydrocarbons, waxes).
• Disorders:
- Uremia: Accumulation of urea in blood due to kidney failure; treated by Hemodialysis.
- Renal Calculi: Insoluble mass of crystallized salts (oxalates) formed within the kidney.- Glomerulonephritis: Inflammation of glomeruli of kidney.
Intext & Conceptual Questions
The counter-current mechanism involves the flow of filtrate in opposite directions through the two limbs of Henle's loop and flow of blood through the parallel limbs of Vasa Recta:
1. NaCl Transport: The ascending limb of Henle's loop actively pumps NaCl into the medullary interstitium, which is then carried away by the descending limb of Vasa Recta.
2. Osmotic Gradient: This creates a hyperosmotic gradient in the medullary interstitium, rising from 300 mOsmol/L in the cortex to 1200 mOsmol/L in the inner medulla.
3. Water Reabsorption: As fluid flows down the collecting duct through this hyperosmotic medullary region, water is reabsorbed passively via osmosis, producing concentrated urine (up to 4 times more concentrated than initial filtrate).
• Trigger: A decrease in glomerular blood flow/blood pressure or GFR activates the Juxtaglomerular (JG) cells to release the enzyme Renin into the bloodstream.
• Cascade: Renin converts plasma Angiotensinogen into Angiotensin I, which is converted to Angiotensin II.
• Actions:
1. Angiotensin II acts as a powerful vasoconstrictor, increasing glomerular blood pressure and GFR.
2. It stimulates the adrenal cortex to release Aldosterone, which causes reabsorption of Na⁺ and water from the DCT, increasing blood volume and restoring normal blood pressure.
Textbook Chapter End Exercise Questions
| Feature | Cortical Nephrons | Juxtamedullary Nephrons |
|---|---|---|
| Abundance | Form about 85% of total nephrons. | Form about 15% of total nephrons. |
| Henle's Loop Length | Loop of Henle is short and extends only slightly into medulla. | Loop of Henle is very long and runs deep into the inner medulla. |
| Vasa Recta | Vasa recta is absent or highly reduced. | Vasa recta is well-developed and prominent. |
| Main Function | Mainly involved in general excretion and volume control. | Crucial for concentrating urine during water scarcity. |
| Feature | Ammonotelism | Ureotelism | Uricotelism |
|---|---|---|---|
| Excretory Product | Ammonia (NH₃) | Urea | Uric Acid |
| Toxicity Level | Highly Toxic | Moderately Toxic | Least Toxic |
| Water Requirement | Requires large volume of water for excretion. | Requires moderate amount of water. | Requires minimum water (excreted as paste/pellet). |
| Examples | Bony fishes, aquatic amphibians, aquatic insects. | Mammals, terrestrial amphibians, marine fishes. | Reptiles, Birds, Land snails, Insects. |
Read More & Chapter Resources
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