Chapter overview: excretion and nitrogenous wastes
Metabolism and excess intake add ammonia, urea, uric acid, carbon dioxide, water and ions to the body. Excretion removes these materials and helps maintain water, ionic and acid-base balance.
Water trade-off Ammonia is most toxic and needs much water; uric acid is least toxic and is removed with the least water loss. Urea lies between them.
| Type | Main waste | Examples / route |
|---|---|---|
| Ammonotelic | Ammonia | Many bony fishes, aquatic amphibians and aquatic insects; diffuses across body or gill surfaces as ammonium ions. |
| Ureotelic | Urea | Mammals, many terrestrial amphibians and marine fishes; ammonia is converted to urea in liver, then filtered by kidneys. |
| Uricotelic | Uric acid | Reptiles, birds, land snails and insects; expelled as pellets or paste with minimal water loss. |
Excretory structures across animals
| Structure | Occurrence | Principal role |
|---|---|---|
| Protonephridia / flame cells | Platyhelminthes, rotifers, some annelids and Amphioxus | Mainly ionic and fluid-volume regulation (osmoregulation). |
| Nephridia | Earthworms and other annelids | Remove nitrogenous wastes; maintain fluid and ionic balance. |
| Malpighian tubules | Insects including cockroach | Nitrogenous-waste removal and osmoregulation. |
| Antennal / green glands | Crustaceans such as prawns | Excretion. |
| Kidneys | Vertebrates | Complex tubular organs for excretion and homeostasis. |
16.1 Human excretory system and kidney
The human urinary system has a pair of kidneys, a pair of ureters, urinary bladder and urethra. Kidneys are reddish-brown, bean-shaped organs near the dorsal inner abdominal wall, between the last thoracic and third lumbar vertebrae.
| Feature | NCERT facts |
|---|---|
| Kidney size | About 10-12 cm long, 5-7 cm wide and 2-3 cm thick; average mass 120-170 g. |
| Hilum and pelvis | Hilum is the inner concave notch for ureter, vessels and nerves. The renal pelvis is the inner funnel-shaped space; it has calyces. |
| Zones | Tough outer capsule; outer cortex and inner medulla. Medulla forms pyramids projecting into calyces; cortex between pyramids forms Columns of Bertini. |
| Nephron | About one million per kidney; functional unit made of glomerulus and renal tubule. |
Nephron: parts, blood supply and types
| Part | Location / significance |
|---|---|
| Glomerulus + Bowman’s capsule | Glomerulus is a capillary tuft supplied by afferent arteriole and drained by efferent arteriole. Together they form the Malpighian body / renal corpuscle. |
| Renal tubule | Bowman’s capsule -> PCT -> descending and ascending limbs of Henle’s loop -> DCT -> collecting duct -> renal pelvis through pyramids and calyces. |
| Cortical nephron | Short loop of Henle extending only slightly into medulla. |
| Juxtamedullary nephron | Long loop extending deep into medulla. |
| Peritubular capillaries and vasa recta | Efferent arteriole forms peritubular capillaries. Vasa recta runs parallel to Henle’s loop; absent or highly reduced in cortical nephrons. |
16.2 Urine formation: filtration, reabsorption and secretion
Urine formation occurs through glomerular filtration, selective reabsorption and tubular secretion.
| Process | Key facts |
|---|---|
| Glomerular filtration | Blood pressure filters through glomerular endothelium, Bowman’s epithelium and basement membrane. Podocyte slit pores admit almost all plasma constituents except proteins: ultrafiltration. |
| GFR | Glomerular filtration rate is about 125 mL min-1, or 180 L day-1. Kidneys receive 1100-1200 mL blood min-1 (about one-fifth of cardiac output). |
| Reabsorption | Nearly 99% of filtrate is reabsorbed. Glucose, amino acids and Na+ are actively reabsorbed; nitrogenous wastes and initial water reabsorption are passive. |
| Secretion | Tubular cells add H+, K+ and ammonia to filtrate, supporting ionic and acid-base balance. |
| JGA and GFR | Juxtaglomerular apparatus is at DCT-afferent arteriole contact. A fall in GFR stimulates JG cells to release renin, restoring glomerular flow and GFR. |
16.3 Functions of nephron tubules
| Segment | Permeability and function |
|---|---|
| PCT | Simple cuboidal brush-border epithelium. Reabsorbs nearly all essential nutrients and 70-80% of electrolytes and water; secretes H+ and ammonia, and reabsorbs HCO3-. |
| Descending limb of Henle’s loop | Permeable to water but almost impermeable to electrolytes; filtrate becomes concentrated while moving down. |
| Ascending limb | Impermeable to water; electrolytes leave actively or passively, so filtrate becomes dilute. Reabsorption is minimum here. |
| DCT | Conditional Na+ and water reabsorption; reabsorbs HCO3-; secretes H+, K+ and NH3. |
| Collecting duct | Reabsorbs large amounts of water, passes a little urea into medullary interstitium, and secretes H+ and K+. |
16.4 Concentration of filtrate: counter-current mechanism
The opposite flows in the two limbs of Henle’s loop and in vasa recta form counter currents. Their close arrangement maintains a corticomedullary osmotic gradient from about 300 mOsmol L-1 in cortex to 1200 mOsmol L-1 in inner medulla.
How the gradient persists The ascending loop transports NaCl; it is exchanged with descending vasa recta and returned to interstitium by ascending vasa recta. Urea cycles from collecting duct to the thin ascending limb and back to interstitium. Water can then leave the collecting duct easily, producing urine nearly four times more concentrated than initial filtrate.
16.5 Regulation of kidney function
| Control | Stimulus and effect |
|---|---|
| ADH / vasopressin | Fluid loss activates osmoreceptors, stimulating hypothalamus and neurohypophysis. ADH increases water reabsorption in latter tubules and prevents diuresis; it also constricts vessels, raising blood pressure. |
| Renin-angiotensin-aldosterone mechanism | Low glomerular flow/pressure/GFR -> renin -> angiotensin I -> angiotensin II. Angiotensin II constricts vessels and stimulates aldosterone; aldosterone increases distal Na+ and water reabsorption. |
| ANF | Increased atrial blood flow releases atrial natriuretic factor. It vasodilates and lowers blood pressure, checking renin-angiotensin action. |
16.6-16.8 Micturition, accessory excretion and disorders
Bladder stretch receptors signal CNS as urine accumulates. CNS triggers bladder smooth-muscle contraction and urethral-sphincter relaxation: the micturition reflex. An adult excretes 1-1.5 L urine daily; it is light yellow, slightly acidic (pH 6.0), and contains about 25-30 g urea daily.
| Topic | Key point |
|---|---|
| Clinical urine clues | Glycosuria (glucose) and ketonuria (ketone bodies) may indicate diabetes mellitus. |
| Other organs | Lungs eliminate ~200 mL CO2 min-1 and water. Liver releases bilirubin, biliverdin, cholesterol, degraded hormones, vitamins and drugs in bile. Skin removes NaCl, water, urea and lactic acid in sweat; sebum removes sterols, hydrocarbons and waxes. |
| Uremia and dialysis | Urea accumulation from kidney malfunction. In haemodialysis, heparinised arterial blood flows through cellophane tubing surrounded by dialysing fluid lacking nitrogenous wastes; cleared blood returns through a vein after anti-heparin. |
| Kidney failure | Transplantation is the ultimate correction for acute renal failure; a closely related donor reduces rejection risk. |
| Renal calculi / glomerulonephritis | Renal calculi are crystallised salt stones in kidney. Glomerulonephritis is inflammation of glomeruli. |
NCERT revision prompts
- Compare ammonotelism, ureotelism and uricotelism.
- Trace filtrate through the nephron and state where cortical and juxtamedullary nephrons differ.
- Define GFR and explain its autoregulation by JGA.
- Account for the three steps of urine formation.
- Compare the permeability and functions of PCT, Henle’s loop, DCT and collecting duct.
- Explain the counter-current mechanism and the 300-1200 mOsmol L-1 gradient.
- Outline ADH, renin-angiotensin-aldosterone and ANF control.
- Explain micturition and name non-renal excretory organs.
- Describe haemodialysis and distinguish renal calculi from glomerulonephritis.