Excretory Products and their Elimination

1. Chapter at a glance

• Animals eliminate nitrogenous wastes as ammonia (ammonotelic), urea (ureotelic) or uric acid (uricotelic) depending on habitat and water availability.
• Excretory structures include protonephridia/flame cells (Platyhelminthes), nephridia (annelids), Malpighian tubules (insects), antennal glands (crustaceans) and kidneys (vertebrates).
• Human excretory system comprises a pair of kidneys, ureters, urinary bladder and urethra; each kidney contains ~1 million nephrons as functional units.
• Urine formation occurs by glomerular filtration, tubular reabsorption and tubular secretion.
• Counter-current mechanism between Henle’s loop and vasa recta maintains medullary interstitial osmolarity (300–1200 mOsmolL⁻¹) enabling concentration of urine.
• Kidney function is regulated by JGA (renin-angiotensin), ADH/vasopressin and ANF.
• Micturition is a CNS-mediated reflex involving bladder contraction and urethral sphincter relaxation.
• Other organs (lungs, liver, skin) also eliminate CO₂, bile pigments, sweat and sebum.

2. Definitions and laws

• Ammonotelism: The process of excreting ammonia.
• Ureotelic animals: Mammals, many terrestrial amphibians and marine fishes that mainly excrete urea.
• Uricotelic animals: Reptiles, birds, land snails and insects that excrete nitrogenous wastes as uric acid in the form of pellet or paste.
• Protonephridia (flame cells): Excretory structures in Platyhelminthes, rotifers, some annelids and Amphioxus, primarily concerned with osmoregulation.
• Nephridia: Tubular excretory structures of earthworms and other annelids that remove nitrogenous wastes and maintain fluid/ionic balance.
• Malpighian tubules: Excretory structures of most insects that help in removal of nitrogenous wastes and osmoregulation.
• Glomerular filtration rate (GFR): The amount of the filtrate formed by the kidneys per minute; in a healthy individual ≈125 ml/minute (180 litres per day).
• Juxta glomerular apparatus (JGA): A special sensitive region formed by cellular modifications in the distal convoluted tubule and the afferent arteriole; releases renin when GFR falls.
• Counter current mechanism: The transport of substances (NaCl and urea) facilitated by the special arrangement of Henle’s loop and vasa recta that maintains an increasing osmolarity (300 mOsmolL⁻¹ cortex to 1200 mOsmolL⁻¹ inner medulla) in the medullary interstitium.
• Micturition: The process of release of urine; neural mechanisms causing it are called the micturition reflex.
• Uremia: Accumulation of urea in blood due to malfunctioning of kidneys.
• Hemodialysis: Process of removing urea from blood using an artificial kidney (dialysing unit).

3. Important diagrams and activities

• Figure 16.1 Human Urinary system – shows kidneys, ureters, urinary bladder and urethra in position.
• Figure 16.2 Longitudinal section (diagrammatic) of Kidney – cortex, medulla, medullary pyramids, Columns of Bertini, renal pelvis and calyces.
• Figure 16.3 Nephron showing blood vessels, duct and tubule – complete nephron with glomerulus, PCT, Henle’s loop, DCT, collecting duct, peritubular capillaries and vasa recta.
• Figure 16.4 Malpighian body (renal corpuscle) – glomerulus inside Bowman’s capsule with afferent and efferent arterioles.
• Figure 16.5 Reabsorption and secretion of major substances at different parts of the nephron – arrows indicating movement of glucose, Na⁺, water, H⁺, K⁺, NH₃ and urea at PCT, loop, DCT and collecting duct.
• Figure 16.6 Diagrammatic representation of nephron and vasa recta showing counter current mechanisms – opposite flow in limbs of Henle’s loop and vasa recta maintaining medullary gradient.

4. Common misconceptions and exam pitfalls

• Kidneys play no significant role in ammonia removal (diffusion across body/gill surfaces).
• Not all nephrons have long loops of Henle; only juxtamedullary nephrons do; cortical nephrons have short loops and lack or have reduced vasa recta.
• Reabsorption of water in collecting duct is conditional (ADH-dependent), not automatic.
• Counter-current mechanism involves both Henle’s loop and vasa recta; urea also contributes to the gradient.
• GFR is 125 ml/min but daily urine output is only ~1.5 L because ~99 % of filtrate is reabsorbed.
• ADH prevents diuresis (water reabsorption); it does not cause water elimination.
• Urea is retained in medullary interstitium to maintain osmolarity; it is not only a waste.

5. Formula sheet