Excretory Products and their Elimination

2.1 Core concepts

• Animals accumulate ammonia, urea, uric acid, CO₂, water and ions (Na⁺, K⁺, Cl⁻, phosphate, sulphate) from metabolism or excess ingestion; these must be removed partially or totally (NCERT Ch. 16 intro, p. 205).
• Ammonia is the most toxic nitrogenous waste and needs large amounts of water to eliminate; uric acid is least toxic and can be excreted with minimal water loss (NCERT §intro, p. 205).
• Ammonotelism — excretion of ammonia by diffusion across body/gill surfaces as ammonium ions; seen in many bony fishes, aquatic amphibians and aquatic insects; kidneys play no significant role (NCERT §intro, p. 205).
• Ureotelism — ammonia is converted to urea in the liver and excreted by kidneys; seen in mammals, many terrestrial amphibians and marine fishes; some urea may be retained in kidney matrix for osmolarity (NCERT §intro, p. 205).
• Uricotelism — excretion of uric acid as pellet/paste; seen in reptiles, birds, land snails and insects, with minimum water loss (NCERT §intro, p. 205).
• Invertebrate/lower-chordate excretory structures: Protonephridia (flame cells) in Platyhelminthes (e.g., Planaria), rotifers, some annelids and Amphioxus — chiefly osmoregulation; Nephridia — tubular structures in earthworms and other annelids, remove nitrogenous wastes and balance fluid/ions; Malpighian tubules — most insects including cockroaches, do excretion and osmoregulation; Antennal/green glands — crustaceans like prawns (NCERT §intro, p. 206).
• Human excretory system consists of a pair of kidneys, a pair of ureters, a urinary bladder and a urethra (NCERT §16.1, p. 206).
• Kidneys are reddish brown, bean-shaped, situated between the last thoracic and third lumbar vertebrae on the dorsal abdominal wall; each kidney is 10–12 cm long, 5–7 cm wide, 2–3 cm thick, weighing 120–170 g (NCERT §16.1, p. 206).
• The inner concave surface bears a hilum through which ureter, blood vessels and nerves enter; inner to the hilum is the funnel-shaped renal pelvis with projections called calyces (NCERT §16.1, p. 206).
• The kidney has an outer cortex and an inner medulla; the medulla is divided into conical medullary pyramids that project into the calyces, while the cortex extends between pyramids as the Columns of Bertini (NCERT §16.1, pp. 206–207).
• Each kidney has nearly one million nephrons, the functional units; each nephron has a glomerulus and a renal tubule (NCERT §16.1, p. 207).
• The glomerulus is a tuft of capillaries from the afferent arteriole (branch of renal artery); blood leaves through the efferent arteriole (NCERT §16.1, p. 207).
• The renal tubule begins with a double-walled cup, Bowman's capsule, enclosing the glomerulus; glomerulus + Bowman's capsule = malpighian body / renal corpuscle; it continues into PCT → Henle's loop (descending + ascending limbs) → DCT → collecting duct → renal pelvis (NCERT §16.1, pp. 207–208).
• Malpighian corpuscle, PCT and DCT lie in the cortex; the loop of Henle dips into the medulla. Cortical nephrons have a short loop barely entering the medulla; juxtamedullary nephrons have a long loop running deep into the medulla (NCERT §16.1, p. 208).
• The efferent arteriole forms peritubular capillaries around the tubule; a U-shaped vasa recta runs parallel to the loop of Henle and is absent or highly reduced in cortical nephrons (NCERT §16.1, p. 208).
• Urine formation has three main processes — glomerular filtration, reabsorption and secretion (NCERT §16.2, p. 208).
• ~1100–1200 mL of blood is filtered per minute (~1/5 of cardiac output per ventricle); filtration occurs through endothelium of glomerular capillaries, basement membrane, and the podocyte epithelium of Bowman's capsule with filtration slits/slit pores; everything except plasma proteins passes through — hence ultrafiltration (NCERT §16.2, p. 208).
• Glomerular Filtration Rate (GFR) in a healthy human ≈ 125 mL/min, i.e., 180 L/day; the juxtaglomerular apparatus (JGA) — at the contact of DCT and afferent arteriole — regulates GFR by releasing renin when GFR falls (NCERT §16.2, p. 209).
• ~99% of the 180 L/day filtrate is reabsorbed; glucose, amino acids, Na⁺ are actively reabsorbed, nitrogenous wastes by passive transport, and water passively in initial segments (NCERT §16.2, p. 209).
• Tubular cells secrete H⁺, K⁺ and ammonia into the filtrate to maintain ionic and acid-base balance (NCERT §16.2, p. 209).
• PCT — simple cuboidal brush-border epithelium reabsorbs nearly all essential nutrients and 70–80% of electrolytes and water; secretes H⁺, NH₃ and absorbs HCO₃⁻ (NCERT §16.3, p. 209).
• Henle's loop — descending limb is permeable to water but almost impermeable to electrolytes (filtrate gets concentrated); ascending limb is impermeable to water but transports electrolytes (filtrate gets diluted); minimum reabsorption in ascending limb; maintains the high osmolarity of medullary interstitial fluid (NCERT §16.3, p. 209).
• DCT — conditional reabsorption of Na⁺ and water; reabsorbs HCO₃⁻; secretes H⁺, K⁺ and NH₃ (NCERT §16.3, p. 209).
• Collecting duct — extends from cortex to inner medulla; reabsorbs large amounts of water to give concentrated urine; allows small amount of urea into medullary interstitium to maintain osmolarity; secretes H⁺ and K⁺ (NCERT §16.3, p. 210).
• Counter-current mechanism — the two limbs of Henle's loop, and the two limbs of vasa recta, carry fluid in opposite directions; their proximity maintains an interstitial osmolar gradient from ~300 mOsmol L⁻¹ in the cortex to ~1200 mOsmol L⁻¹ in the inner medulla, due mainly to NaCl and urea; human kidneys can produce urine nearly four times as concentrated as the initial filtrate (NCERT §16.4, pp. 210–212).
• Regulation involves the hypothalamus, JGA and (partly) the heart. Osmoreceptors detect fall in blood/body fluid volume, triggering release of ADH (vasopressin) from the neurohypophysis; ADH facilitates water reabsorption from later tubular segments (preventing diuresis) and also raises blood pressure by vasoconstriction (NCERT §16.5, p. 212).
• Renin-Angiotensin mechanism — fall in glomerular blood flow/pressure/GFR triggers JG cells to release renin; renin converts angiotensinogen → angiotensin I → angiotensin II (powerful vasoconstrictor) raising glomerular BP and GFR; angiotensin II also stimulates the adrenal cortex to release aldosterone, which causes Na⁺ and water reabsorption at distal tubule (NCERT §16.5, p. 212).
• Atrial Natriuretic Factor (ANF) is released when atrial blood flow increases; it causes vasodilation, lowering BP, and thus acts as a check on the renin-angiotensin mechanism (NCERT §16.5, p. 212).
• Micturition — urine is stored in the urinary bladder; stretching activates stretch receptors that signal the CNS; CNS triggers bladder contraction and urethral sphincter relaxation, releasing urine. The neural process is the micturition reflex (NCERT §16.6, pp. 212–213).
• Adult humans excrete 1–1.5 L of urine/day; urine is light yellow, slightly acidic (pH ~6.0), with a characteristic odour; ~25–30 g of urea per day; presence of glucose (glycosuria) or ketone bodies (ketonuria) indicates diabetes mellitus (NCERT §16.6, p. 213).
• Other excretory organs — Lungs remove ~200 mL/min of CO₂ and significant water; Liver secretes bile-containing bilirubin, biliverdin, cholesterol, degraded steroid hormones, vitamins and drugs (mostly leave with digestive wastes); Sweat (NaCl, urea, lactic acid) is mainly for cooling but aids excretion; Sebaceous glands eliminate sterols, hydrocarbons, waxes via sebum; small amounts of nitrogenous wastes are eliminated through saliva (NCERT §16.7, p. 213).
• Disorders — Uraemia: accumulation of urea in blood; treated by haemodialysis through an artificial kidney (coiled cellophane tube in dialysing fluid of plasma-like composition without nitrogenous wastes; heparin added before, anti-heparin after). Kidney transplantation corrects acute renal failure — donor preferably a close relative to reduce rejection. Renal calculi — stones/insoluble crystallised salts (e.g., oxalates) in the kidney. Glomerulonephritis — inflammation of glomeruli (NCERT §16.8, pp. 213–214).

2.2 Definitions to memorise

2.3 Diagrams / processes to remember

• Figure 16.1 (p. 206) — Human urinary system: kidneys, renal artery/vein, ureters, urinary bladder, urethra, dorsal aorta, inferior vena cava and adrenal gland.
• Figure 16.2 (p. 207) — Longitudinal section of kidney: cortex, medulla, medullary pyramids, renal columns (Bertini), calyx, renal pelvis, hilum.
• Figure 16.3 (p. 207) — Nephron with blood supply: afferent and efferent arterioles, glomerulus, Bowman's capsule, PCT, descending and ascending limbs of loop of Henle, DCT, collecting duct, vasa recta.
• Figure 16.4 (p. 208) — Malpighian body / renal corpuscle (Bowman's capsule enclosing glomerulus).
• Figure 16.5 (p. 210) — Sites of reabsorption and secretion: HCO₃⁻, NaCl, water, K⁺, H⁺, NH₃, nutrients, urea — across PCT, loop of Henle, DCT and collecting duct.
• Figure 16.6 (p. 211) — Counter-current mechanism: numerical osmolarity values (300 → 1200 mOsmol L⁻¹), NaCl and urea movements between vasa recta, Henle's loop and collecting duct.

2.4 Common confusions / NTA trap points

• Toxicity vs water economy direction: Ammonia > urea > uric acid in toxicity, but ammonia needs the most water and uric acid the least — students often invert one of the two scales.
• Cortical vs juxtamedullary nephrons: Vasa recta is well-developed in juxtamedullary nephrons and absent/reduced in cortical nephrons — NTA flips this regularly.
• Loop of Henle permeability: Descending limb is permeable to water (impermeable to salts); ascending limb is the opposite — confusing the limbs is a classic distractor.
• GFR figures: 1100–1200 mL/min is the blood filtered, while 125 mL/min is the filtrate formed (GFR) — NTA tests both numbers in the same MCQ.
• ADH effect: ADH causes water reabsorption (producing concentrated/hypertonic urine), it does not promote water elimination or hypotonic urine — Exercise Q3(b) tests this trap directly.
• Renin vs renal calculi vs glomerulonephritis — three different "renal" words; renin is a JGA enzyme, calculi are stones, glomerulonephritis is glomerular inflammation.
• PCT vs DCT — PCT reabsorbs ~70–80% of electrolytes & water (obligate); DCT performs conditional water reabsorption under ADH and aldosterone control.
• Micturition reflex — initiated by stretch receptors in the bladder wall when filled with ~300 mL urine; controlled by the CNS via pelvic nerves (NCERT §16.5, p. 305).

2.5 Quick comparison table — excretion at a glance