NCERT Biology Class 11 - Chapter 16: Excretory Products and their Elimination - Notes

Learning Objectives

Understand the modes of excretion in different organisms
Learn the structure and functions of the human excretory system
Study the process of urine formation and its regulation
Understand the counter-current mechanism and role of hormones
Learn about excretory disorders and dialysis

Key Concepts

Modes of Excretion

Ammonotelism: Excretion of ammonia (highly toxic, requires large water volume). Found in aquatic animals — bony fishes, aquatic amphibians. Ureotelism: Excretion of urea (less toxic). Found in mammals, terrestrial amphibians, cartilaginous fishes. Urea synthesized in liver by ornithine cycle (urea cycle). Uricotelism: Excretion of uric acid (least toxic, insoluble, needs least water). Found in reptiles, birds, insects, land snails.

Human Excretory System

Two kidneys (bean-shaped, retroperitoneal), two ureters, one urinary bladder, one urethra. Each kidney has outer cortex and inner medulla (medullary pyramids). Hilum — indented region where renal artery, renal vein, and ureter connect. Functional unit: Nephron (~1 million per kidney).

Nephron structure: Bowman's capsule (encloses glomerulus) → Proximal Convoluted Tubule (PCT) → Loop of Henle (descending and ascending limbs) → Distal Convoluted Tubule (DCT) → Collecting duct. Types: Cortical nephrons (85%, short loop of Henle in cortex) and Juxtamedullary nephrons (15%, long loop extending deep into medulla — important for urine concentration).

Urine Formation

Glomerular Filtration: Blood is filtered in glomerulus under high pressure. Filtration rate: GFR = 125 mL/min = 180 L/day. Filtrate contains water, glucose, amino acids, urea, salts. Blood cells, proteins NOT filtered. Net filtration pressure = Glomerular blood pressure - (Bowman's capsule pressure + Colloid osmotic pressure).

Tubular Reabsorption: PCT reabsorbs ~65% of filtrate. Substances reabsorbed: glucose, amino acids, Na⁺, water (obligatory reabsorption). PCT also secretes H⁺, NH₃, K⁺ into filtrate. Loop of Henle: descending limb is permeable to water (reabsorbs water), ascending limb is impermeable to water but transports NaCl out (creating medullary osmotic gradient).

Tubular Secretion: Active transport of substances from blood into tubular fluid. Occurs in PCT, DCT, and collecting duct. Secretes H⁺, K⁺, urea, drugs.

Counter-Current Mechanism

Maintained by Loop of Henle and vasa recta (peritubular capillaries running parallel to loop). Creates and maintains osmotic gradient in medulla (300 mOsmol/L in cortex to 1200 mOsmol/L in inner medulla). This enables concentration of urine. NaCl transported out of ascending limb creates high osmolarity in medulla. Water reabsorbed from descending limb and collecting duct. Urea recycling also contributes to medullary gradient.

Hormonal Regulation

ADH (Antidiuretic Hormone / Vasopressin): From posterior pituitary. Increases water reabsorption from DCT and collecting duct. Deficiency causes diabetes insipidus (large volume of dilute urine). Aldosterone: From adrenal cortex. Increases Na⁺ and water reabsorption from DCT. ANF (Atrial Natriuretic Factor): From heart atria. Reduces Na⁺ reabsorption → increases Na⁺ excretion → lowers blood volume and pressure. Renin-Angiotensin-Aldosterone System (RAAS): JGA cells release renin when blood pressure drops → Angiotensinogen → Angiotensin I → Angiotensin II → stimulates aldosterone secretion → Na⁺ and water retention → BP increases.

Disorders

Uremia: Accumulation of urea in blood. Treated by haemodialysis (artificial kidney — cellulose membrane separates blood from dialysing fluid). Kidney stones: Calcium oxalate/phosphate crystals in kidney. Glomerulonephritis: Inflammation of glomeruli. Renal failure: May require kidney transplant.

Summary

Excretion removes nitrogenous wastes. Animals are classified as ammonotelic, ureotelic, or uricotelic based on excretory products. The human kidney with its nephrons performs filtration, reabsorption, and secretion to form urine. Counter-current mechanism concentrates urine. ADH, aldosterone, ANF, and RAAS regulate urine volume and composition.

Important Terms

Nephron: Functional unit of kidney; ~1 million per kidney
GFR: Glomerular Filtration Rate — 125 mL/min or 180 L/day
Counter-current mechanism: System maintaining medullary osmotic gradient
ADH: Antidiuretic hormone; increases water reabsorption
RAAS: Renin-Angiotensin-Aldosterone System for BP regulation
Haemodialysis: Artificial kidney process for removing urea from blood
JGA: Juxtaglomerular Apparatus; releases renin for BP regulation
Diabetes insipidus: Condition due to ADH deficiency; excessive dilute urine

Quick Revision

Ammonotelic: aquatic animals; Ureotelic: mammals; Uricotelic: birds, reptiles, insects
GFR = 125 mL/min = 180 L/day; only ~1.5 L urine formed
PCT: maximum reabsorption (~65% filtrate)
Counter-current: Loop of Henle + vasa recta → medullary gradient (300-1200 mOsmol/L)
ADH: water reabsorption; Aldosterone: Na⁺ reabsorption; ANF: Na⁺ excretion
Descending loop: permeable to water; Ascending loop: impermeable to water, transports NaCl out
Haemodialysis = artificial kidney for uremia
Urea cycle (ornithine cycle) occurs in liver