NCERT Summary: Excretory Products And Their Elimination

Excretory Mechanisms in Animals

Animals accumulate wastes such as ammonia, urea, uric acid, carbon dioxide, water and ions (Na⁺, K⁺, Cl^-, phosphate, sulfate) that must be eliminated.

Forms of Nitrogenous Wastes

• Ammonia - most toxic; requires large amounts of water for elimination.
• Urea - less toxic; conserves water.
• Uric acid - least toxic; eliminated with minimal water loss.

Types of Excretion

• Ammonotelism - excretion of ammonia; ammonia diffuses across body or gill surfaces; kidneys play little role.
• Ureotelism - ammonia converted to urea in the liver, released into blood and excreted by kidneys; some animals retain urea in the kidney matrix to control osmolarity.
• Uricotelism - excretion of uric acid as a paste or pellet, minimizing water loss.

Excretory Structures in Animals

• Protonephridia / Flame cells - tubular structures mainly for osmoregulation and ionic/fluid balance.
• Nephridia - tubular excretory organs that remove nitrogenous wastes and maintain fluid and ionic balance.
• Malpighian tubules - tubular system aiding removal of nitrogenous wastes and osmoregulation.
• Antennal (green) glands - excretory organs that perform filtration and excretion functions.

Human Excretory System

• Components: a pair of kidneys, pair of ureters, urinary bladder and urethra.
• Kidneys: reddish-brown, bean-shaped; outer tough capsule; two zones - cortex (outer) and medulla (inner).
• Medulla forms medullary pyramids projecting into calyces; cortex forms renal columns (Columns of Bertini).
• Each kidney contains ~one million nephrons, the basic functional units.

Nephron Structure

• Main parts: glomerulus (capillary tuft) and renal tubule.
• Glomerulus is supplied by an afferent arteriole and drained by an efferent arteriole.
• Renal tubule begins with Bowman's capsule; together with glomerulus forms the renal corpuscle (Malpighian body).
• Tubular segments: proximal convoluted tubule (PCT), loop of Henle (dips into medulla), distal convoluted tubule (DCT), and collecting duct.
• Podocytes of Bowman's capsule form filtration slits enabling ultrafiltration.
• Two nephron types: cortical (short loop) and juxtamedullary (long loop reaching deep medulla).

Urine Formation

• Three processes: glomerular filtration, reabsorption, and secretion.
• Glomerular filtration: kidneys receive ~1100-1200 ml blood per minute; filtration across endothelium, basement membrane and Bowman's epithelium yields ultrafiltrate (plasma components except proteins).
• Glomerular Filtration Rate (GFR): ~125 ml/min (~180 litres/day); regulated in part by the juxtaglomerular apparatus (JGA); JG cells release renin when GFR falls.
• Reabsorption: ~180 litres filtrate/day reduced to ~1.5 litres urine/day - about 99% reabsorbed by tubular epithelial cells via active and passive transport; essentials (glucose, amino acids, Na⁺) reabsorbed actively; water reabsorbed passively in initial segments.
• Secretion: tubular cells secrete H+, K+ and ammonia into filtrate to maintain ionic and acid-base balance.

Functions of Renal Tubule Segments

• PCT: lined by brush-border cuboidal epithelium; reabsorbs most nutrients, ~70-80% electrolytes and water; secretes H⁺ and NH₃; reabsorbs HCO₃^⁻ to maintain pH.
• Loop of Henle: descending limb permeable to water (concentrates filtrate); ascending limb impermeable to water and transports electrolytes, establishing medullary osmolarity.
• DCT: conditional reabsorption of Na⁺ and water; reabsorbs HCO₃^⁻ and secretes H⁺, K⁺ and NH_₃ to maintain pH and Na^⁺/K^⁺ balance.
• Collecting duct: reabsorbs large amounts of water to produce concentrated urine; permits urea recycling to medulla; selectively secretes H⁺ and K⁺.

Mechanism of Concentration of the Filtrate (Counter Current Mechanism)

• Counter current arrangement of Henle's loop and the vasa recta maintains an increasing osmolarity from cortex (~300 mOsm/L) to inner medulla (~1200 mOsm/L).
• Gradient established mainly by NaCl transport (ascending limb and vasa recta exchange) and urea recycling.
• Counter current mechanism enables efficient water reabsorption from collecting ducts; human kidneys can produce urine up to about four times more concentrated than initial filtrate.

Regulation of Kidney Function

• Osmoreceptors & ADH: osmoreceptors detect fluid loss; hypothalamus triggers release of antidiuretic hormone (ADH) from neurohypophysis; ADH increases water reabsorption in renal tubules and can constrict blood vessels to raise blood pressure and GFR; ADH release is suppressed when body fluid volume increases.
• Juxtaglomerular Apparatus (JGA): decreased glomerular flow or GFR activates JG cells to release renin, initiating the renin-angiotensin pathway that produces angiotensin II (vasoconstrictor) and stimulates aldosterone release.
• Aldosterone: promotes Na⁺ and water reabsorption from distal tubules, raising blood pressure and GFR (renin-angiotensin mechanism).
• Atrial Natriuretic Factor (ANF): released in response to increased atrial blood flow; causes vasodilation, lowers blood pressure and counters the renin-angiotensin mechanism.

Micturition

• Urine is stored in the bladder; bladder stretch receptors signal the CNS as it fills.
• CNS triggers contraction of bladder smooth muscle and relaxation of urethral sphincter - the micturition reflex.
• Typical adult urine output ~1-1.5 litres/day; urine is light yellow, watery, slightly acidic (pH ≈ 6.0); about 25-30 g urea excreted daily.
• Urine analysis helps diagnose metabolic and renal disorders.

Role of Other Organs in Excretion

• Lungs - remove large amounts of CO₂ (~200 ml/min) and water vapour.
• Liver - secretes bile containing bilirubin, biliverdin, cholesterol, degraded hormones, vitamins and drugs that are eliminated with digestive wastes.
• Skin - sweat glands excrete NaCl, small amounts of urea and lactic acid; sebaceous glands secrete sebum (sterols, hydrocarbons, waxes).
• Saliva - can remove small amounts of nitrogenous wastes.

Disorders of the Excretory System

• Uremia - accumulation of urea in blood due to kidney failure.
• Renal calculi - formation of insoluble crystalline stones in the kidney.
• Glomerulonephritis - inflammation of the glomeruli.

Clinical Interventions

• Hemodialysis - blood is diverted to an artificial kidney where anticoagulant (e.g., heparin) prevents clotting; a porous cellophane membrane separates blood from dialysing fluid that lacks nitrogenous wastes, allowing wastes to diffuse out; cleared blood is returned with anti-heparin.
• Kidney transplantation - transplantation of a healthy kidney is the most effective treatment for acute renal failure; related donors reduce risk of immune rejection; modern techniques have improved success rates.