Excretion — Telangana (SSC) Class 10 Science Solutions (Free)

Excretion: Removal of metabolic waste products from body (produced during cellular metabolism). Egestion: Removal of undigested food (from digestive tract); not metabolic waste; not excretion. Key difference: • Excretion: Metabolic waste (urea, CO₂); removed from cells/blood; requires energy • Egestion: Undigested food residue; removed from digestive tract; passive elimination Main excretory products in humans: 1. Carbon dioxide (CO₂): • From cellular respiration • Excretion site: Lungs (via breathing) • Transported by blood 2. Urea: • From amino acid deamination (protein breakdown) • Excretion site: Kidneys (via urine) • Transported by blood from liver 3. Water (H₂O): • From respiration + metabolism • Excretion sites: Kidneys (urine), lungs (breath), skin (sweat) 4. Mineral salts (ions): • Excess from diet • Excretion site: Kidneys (urine), skin (sweat) 5. Ammonia (NH₃): • Intermediate in amino acid deamination • Converted to urea in liver (less toxic)

Human kidney structure: • Bean-shaped organ (~10 cm long) • Outer layer: Renal cortex (contains filtering units) • Middle layer: Renal medulla (contains loops and collecting ducts) • Inner region: Renal pelvis (collects urine; leads to ureter) • One ureter per kidney → transports urine to bladder Microscopic structure: Renal capsule (Bowman's capsule): Cup-shaped structure surrounding glomerulus Glomerulus: Capillary network where filtration occurs; blood enters from afferent arteriole; leaves via efferent arteriole Proximal convoluted tubule (PCT): Reabsorbs useful substances (glucose, ions, amino acids) via active transport Loop of Henle: Creates osmotic gradient; reabsorbs water and ions Distal convoluted tubule (DCT): Fine-tunes reabsorption; responds to hormones Collecting duct: Final water reabsorption; concentration of urine Functional unit: Nephron (kidney tubule + associated capillaries) • Length: ~3 cm per nephron • Number: ~1 million nephrons per kidney • Each nephron filters ~1 liter blood daily

Stage 1 - Ultrafiltration (in Bowman's capsule): • Occurs at glomerulus (capillary network) • Blood pressure forces small molecules through capillary wall • Filtered substances: Water, glucose, amino acids, ions, urea (small solutes) • NOT filtered: Large proteins, blood cells, RBCs (too large) • Filtrate produced: ~180 liters per day • Primary filtrate contains useful + waste substances Stage 2 - Selective reabsorption (in PCT, loop of Henle, DCT): • Useful substances reabsorbed back into blood: 1. Glucose: Completely reabsorbed (PCT) - all reused 2. Amino acids: Completely reabsorbed (PCT) - all reused 3. Ions: Selectively reabsorbed - as needed 4. Water: Selectively reabsorbed - depends on hydration • Mechanism: Active transport (needs ATP) + diffusion • Waste (urea) NOT reabsorbed → remains in filtrate • Result: ~1-2 liters urine per day Stage 3 - Tubular secretion (in DCT and collecting duct): • Additional waste molecules secreted INTO tubular lumen • Examples: Excess ions, ammonia, drugs • Mechanism: Active secretion (ATP needed) • Fine-tunes urine composition • Final urine concentration regulated by ADH hormone

ADH (Antidiuretic hormone) - also called vasopressin: Production: • Produced by: Hypothalamus (brain) • Stored and released by: Posterior pituitary gland Role in osmoregulation: 1. Increases water reabsorption in kidneys: • Acts on collecting duct cells • Increases permeability to water • More water reabsorbed → more concentrated urine • Less water in urine → conserves body water 2. Stimulated by: • Increase in blood osmotic pressure (concentrated blood) • Dehydration (detected by osmoreceptors in hypothalamus) • Blood volume decrease 3. Effect on urine: • High ADH → high water reabsorption → small volume, concentrated urine • Low ADH → low water reabsorption → large volume, dilute urine When ADH is low: • Collecting duct becomes impermeable to water • Water NOT reabsorbed → remains in filtrate • Produces large volume (3-4 liters) of dilute, pale urine • Body loses excess water (occurs after drinking water) Disease - Diabetes insipidus: • Insufficient ADH production • Kidneys cannot reabsorb water • Produces excessive urine (up to 20 liters/day) • Patient experiences extreme thirst

Path of urine (from formation to excretion): 1. Formation in nephron: • Filtration in glomerulus • Reabsorption in PCT, loop of Henle, DCT • Secretion in DCT and collecting duct • Result: Final urine (1-2 liters/day) 2. Collection in renal pelvis: • Urine flows from collecting ducts → renal pelvis (funnel-shaped) 3. Transport via ureter: • Ureter (muscular tube) carries urine from kidney to bladder • Peristalsis (muscular contractions) moves urine downward • One ureter per kidney 4. Storage in bladder: • Bladder (elastic pouch) stores urine temporarily • Capacity: 400-600 mL • Walls stretch to accommodate increasing volume • Stretch receptors signal brain when ~200-300 mL accumulated 5. Excretion via urethra: • Micturition reflex: Bladder contracts, sphincters relax • Urethra (tube) carries urine from bladder to outside • In males: passes through prostate, penis (longer ~20 cm) • In females: shorter tube (~4 cm) Role of bladder: 1. Temporary storage → prevents continuous urine dribbling 2. Elastic walls → accommodate varying volumes 3. Allows voluntary control (in adults) via sphincters

Excretion in plants: Plants produce waste from metabolism but in much smaller quantities than animals. Waste removal is less prominent. Metabolic waste products in plants: 1. Oxygen (O₂): • Byproduct of photosynthesis • Released through stomata (leaves) and lenticels (stems) • Some used for respiration; excess excreted 2. Carbon dioxide (CO₂): • Product of respiration (especially roots) • Released through stomata, lenticels, root surfaces • Used in photosynthesis (some recycled) 3. Excess water (H₂O): • From respiration and photosynthesis • Transpired through stomata • Some stored in vacuoles 4. Excess mineral salts: • Absorbed in excess; stored in vacuoles • Some crystals form in cells (calcium oxalate) • Excreted in fallen leaves, dead plant parts 5. Tannins and alkaloids: • Secondary metabolites • Stored in vacuoles • Excreted when tissues die/fall Methods of excretion in plants: 1. Through stomata (leaves) → gases 2. Through lenticels (stems) → gases 3. Through root surfaces → mineral excretion 4. Abscission (leaf fall) → removal of stored waste 5. Exudation → mineral salts seep out from leaves Difference from animals: No specialized excretory system; waste removal gradual, passive

Excretory system comparison: Fish (aquatic): • Waste: Ammonia (highly toxic but diluted in water) • Excretory organs: Gills (main), kidney (minor) • Adaptation: Osmoregulation in freshwater (dilute urine); saltwater (concentrated urine) • Advantage: Ammonia easily diffuses into water; no need for detoxification Birds (terrestrial): • Waste: Uric acid (least soluble, least toxic) • Excretory organs: Kidney (no bladder) • Product: White paste-like urine (uric acid) + feces • Adaptation: No water loss; water conservation (desert animals) • Advantage: Uric acid requires minimal water → suited for dry environment Mammals (terrestrial): • Waste: Urea (moderately toxic, moderately soluble) • Excretory organs: Kidney, bladder, skin (sweat) • Product: Liquid urine + feces • Adaptation: Concentrated urine; osmoregulation via ADH • Advantage: Balanced water loss; urea easily filtered Osmoregulation: Maintenance of constant osmotic pressure (water-salt balance) in body fluids despite external environment changes. In freshwater fish: Absorbs water through skin; produces dilute urine; actively absorbs salts In saltwater fish: Loses water through skin; produces concentrated urine; drinks seawater, excretes salt through gills In mammals: ADH controls water reabsorption; kidneys adjust urine concentration