Chapter 16: Excretory Products and their Elimination

Ammonia: Most toxic form of nitrogenous waste; requires large amounts of water for elimination; excreted by diffusion across body surfaces or gills.
Ammonotelism: Process of excreting ammonia; seen in bony fishes, aquatic amphibians, and aquatic insects.
Urea: Less toxic nitrogenous waste; produced in liver from ammonia; main excretory product in mammals, terrestrial amphibians, and marine fishes.
Ureotelism: Process of excreting urea; conserves water compared to ammonia.
Uric acid: Least toxic nitrogenous waste; excreted as pellet or paste with minimum water loss; seen in reptiles, birds, land snails, and insects.
Uricotelism: Process of excreting uric acid; highly adaptive for terrestrial life.

Protonephridia (Flame cells): Simple tubular excretory structures in Platyhelminthes (e.g., Planaria), rotifers, some annelids, and Amphioxus; mainly for osmoregulation.
Nephridia: Tubular excretory structures in earthworms and other annelids; remove nitrogenous wastes and maintain fluid/ionic balance.
Malpighian tubules: Excretory structures in most insects (e.g., cockroaches); remove nitrogenous wastes and aid osmoregulation.
Antennal glands (Green glands): Excretory organs in crustaceans (e.g., prawns).

Kidneys: Reddish-brown, bean-shaped organs (10–12 cm long); located between last thoracic and third lumbar vertebrae; produce urine.
Hilum: Notch on inner concave surface of kidney; entry point for ureter, blood vessels, and nerves.
Renal pelvis: Funnel-shaped space inside kidney; receives urine from calyces.
Calyces: Cup-like projections of renal pelvis that collect urine from medullary pyramids.
Cortex: Outer zone of kidney; contains Malpighian corpuscles and convoluted tubules.
Medulla: Inner zone; divided into medullary pyramids (conical masses).
Columns of Bertini (Renal columns): Cortical tissue extending between medullary pyramids.
Ureters: Tubes carrying urine from kidneys to urinary bladder.
Urinary bladder: Stores urine temporarily.
Urethra: Tube that expels urine from bladder to outside.

Nephrons: ~1 million per kidney; functional units of kidney; consist of glomerulus + renal tubule.
Cortical nephrons: Majority; short Henle’s loop; confined mostly to cortex.
Juxtamedullary nephrons: Fewer; very long Henle’s loop extending deep into medulla; crucial for concentrated urine.
Glomerulus: Tuft of capillaries formed by afferent arteriole; site of blood filtration.
Bowman’s capsule: Double-walled cup enclosing glomerulus; first part of renal tubule.
Malpighian body / Renal corpuscle: Glomerulus + Bowman’s capsule.
Proximal convoluted tubule (PCT): Highly coiled; lined by brush-border epithelium; major site of reabsorption.
Henle’s loop: Hairpin-shaped; has descending limb (water permeable) and ascending limb (electrolyte transport).
Distal convoluted tubule (DCT): Coiled part after ascending limb; conditional reabsorption and secretion.
Collecting duct: Receives DCTs of many nephrons; runs through medulla; final concentration of urine.
Podocytes: Epithelial cells of Bowman’s capsule with filtration slits/slit pores.
Peritubular capillaries: Capillary network around renal tubule formed by efferent arteriole.
Vasa recta: U-shaped capillary loops running parallel to Henle’s loop (present in juxtamedullary nephrons).

Glomerular filtration: First step; blood filtered under pressure through glomerular capillaries, Bowman’s capsule epithelium, and basement membrane.
Ultra filtration: Extremely fine filtration; almost all plasma constituents except proteins enter Bowman’s capsule.
Glomerular filtration rate (GFR): Volume of filtrate formed per minute (~125 ml/min or 180 L/day in healthy adult).
Juxta glomerular apparatus (JGA): Structure formed by DCT and afferent arteriole; regulates GFR via renin release.
Reabsorption: Process by which useful substances (glucose, amino acids, Na⁺, water, etc.) are reabsorbed from filtrate back into blood (99% of filtrate reabsorbed).
Tubular secretion: Active secretion of H⁺, K⁺, ammonia, etc., into filtrate by tubular cells; helps maintain pH and ionic balance.

Proximal convoluted tubule (PCT): Reabsorbs nearly all nutrients, 70–80% electrolytes and water; secretes H⁺ and ammonia; maintains pH/ionic balance.
Henle’s loop – Descending limb: Permeable to water, impermeable to electrolytes; concentrates filtrate.
Henle’s loop – Ascending limb: Impermeable to water; actively/passively transports electrolytes; dilutes filtrate.
Distal convoluted tubule (DCT): Conditional reabsorption of Na⁺ and water; secretion of H⁺, K⁺, NH₃; reabsorption of HCO₃⁻.
Collecting duct: Reabsorbs large amounts of water (under ADH); allows small urea passage into interstitium; final urine concentration.

Counter current mechanism: Flow of filtrate in Henle’s loop and blood in vasa recta in opposite directions; maintains high osmolarity gradient (300 to 1200 mOsmol L⁻¹) in medullary interstitium.
Medullary interstitial fluid osmolarity: Increases from cortex (300 mOsmol L⁻¹) to inner medulla (1200 mOsmol L⁻¹) due to NaCl and urea.

Osmoreceptors: Detect changes in blood volume, fluid volume, and ionic concentration.
Antidiuretic hormone (ADH) / Vasopressin: Released from neurohypophysis; increases water reabsorption in distal tubules and collecting duct; prevents diuresis.
Renin-Angiotensin mechanism: Triggered by low GFR; renin → angiotensin II → vasoconstriction + aldosterone release → Na⁺ and water reabsorption + increased blood pressure/GFR.
Aldosterone: Hormone from adrenal cortex; promotes Na⁺ and water reabsorption in distal tubule.
Atrial natriuretic factor (ANF): Released from heart atria; causes vasodilation and decreases blood pressure; opposes renin-angiotensin system.

Micturition: Voluntary release of urine; reflex involving stretch receptors in bladder, CNS, and contraction of bladder muscles + relaxation of urethral sphincter.
Micturition reflex: Neural mechanism controlling urination.
Urine: Light yellow, slightly acidic (pH ~6.0), 1–1.5 L/day; contains ~25–30 g urea/day.

Lungs: Remove CO₂ and water vapour.
Liver: Secretes bile containing bilirubin, biliverdin, cholesterol, degraded hormones, etc.
Skin: Sweat glands remove NaCl, urea, lactic acid; sebaceous glands remove sterols, hydrocarbons, waxes.
Saliva: Minor elimination of nitrogenous wastes.

Uremia: Accumulation of urea in blood due to kidney malfunction; can lead to kidney failure.
Hemodialysis: Process using artificial kidney (dialysing unit) to remove urea from blood in uremic patients.
Renal calculi: Kidney stones (crystallised salts like oxalates).
Glomerulonephritis: Inflammation of glomeruli.

Brush border epithelium: Microvilli-lined cells in PCT increasing surface area for reabsorption.
Filtration slits / Slit pores: Spaces between podocytes allowing ultra-filtration.
Osmoregulation: Maintenance of osmotic balance of body fluids (mentioned in exercises).
Renal artery / Renal vein: Blood vessels supplying/draining kidney.