Biology: Class 11 — Chapter 16: Excretory Products and Their Elimination

• Ammonotelism: The process of excreting ammonia. Ammonia is highly toxic and requires large amounts of water for elimination. Examples include many bony fishes, aquatic amphibians, and aquatic insects.
• Ureotelism: Excretion of urea. Ammonia is converted to urea in the liver and excreted by kidneys to conserve water. Examples include mammals, terrestrial amphibians, and marine fishes.
• Uricotelism: Excretion of uric acid in the form of pellets or paste with minimal water loss. Uric acid is the least toxic. Examples include reptiles, birds, land snails, and insects.

• Protonephridia (Flame cells): Primarily for osmoregulation in Platyhelminthes (e.g., Planaria), rotifers, some annelids, and cephalochordates (e.g., Amphioxus).
• Nephridia: Tubular excretory structures in earthworms and other annelids for waste removal and ionic balance.
• Malpighian Tubules: Excretory structures in insects (e.g., cockroaches) for nitrogenous waste removal and osmoregulation.
• Antennal Glands (Green Glands): Excretory structures in crustaceans like prawns.

• Location & Size: Reddish-brown, bean-shaped, situated between the last thoracic and third lumbar vertebra. Measures 10-12 cm in length, 5-7 cm in width, and weighs 120-170 g.
• Hilum: A notch on the inner concave surface through which ureters, blood vessels, and nerves enter.
• Renal Pelvis: A broad funnel-shaped space inner to the hilum with projections called calyces.
• Internal Zones: An outer cortex and an inner medulla. The medulla is divided into conical masses called medullary pyramids projecting into calyces.
• Columns of Bertini: Extensions of the cortex that project between the medullary pyramids.

Nephrons are the functional units of the kidney, with nearly one million per kidney. Each nephron consists of two parts:

• Glomerulus: A tuft of capillaries formed by the afferent arteriole. Blood is carried away by the efferent arteriole.
• Renal Tubule: Begins with a double-walled cup-like Bowman's capsule enclosing the glomerulus. Together they form the Malpighian body (renal corpuscle).
• Tubular Segments: The tubule continues into the Proximal Convoluted Tubule (PCT), a hairpin-shaped Henle's loop (with descending and ascending limbs), and the Distal Convoluted Tubule (DCT).
• Collecting Duct: A straight tube where DCTs of multiple nephrons open, converging into the renal pelvis.
• Nephron Types:
  • Cortical Nephrons: Loop of Henle is short and extends only slightly into the medulla.
  • Juxtamedullary Nephrons: Loop of Henle is very long and runs deep into the medulla.
• Vasa Recta: A U-shaped minute vessel running parallel to Henle's loop, absent or highly reduced in cortical nephrons.

• Process: Filtration of blood through three layers: endothelium of glomerular blood vessels, epithelium of Bowman's capsule (containing specialized cells called podocytes), and a basement membrane. This is a process of ultrafiltration.
• Glomerular Filtration Rate (GFR): The amount of filtrate formed by the kidneys per minute (approx. 125 ml/min or 180 liters per day in a healthy individual).
• Juxtaglomerular Apparatus (JGA): A sensitive cellular region regulating GFR. A fall in GFR triggers JGA to release renin to restore normal blood flow and GFR.

• Reabsorption: Nearly 99% of the filtrate is reabsorbed by renal tubules. Active transport reabsorbs glucose, amino acids, and $Na^+$, while nitrogenous wastes and water are reabsorbed passively.
• Secretion: Tubular cells secrete substances like $H^+$, $K^+$, and ammonia into the filtrate to maintain ionic and acid-base balance of body fluids.

• PCT: Lined by simple cuboidal brush border epithelium to increase surface area. Reabsorbs 70-80% of electrolytes and water. Maintains pH by secreting $H^+$ and $NH_3$ and absorbing $HCO_3^-$.
• Henle's Loop: Plays a major role in maintaining high osmolarity of medullary interstitial fluid. The descending limb is permeable to water but impermeable to electrolytes (concentrating the filtrate). The ascending limb is impermeable to water but allows active/passive transport of electrolytes (diluting the filtrate).
• DCT: Conditional reabsorption of $Na^+$ and water occurs here. It also reabsorbs $HCO_3^-$ and selectively secretes $H^+$, $K^+$, and $NH_3$.
• Collecting Duct: Reabsorbs large amounts of water to produce concentrated urine. Allows passage of small amounts of urea into the medullary interstitium to maintain osmolarity.

• Components: The flow of filtrate in the two limbs of Henle's loop and the flow of blood in the two limbs of vasa recta form a counter-current mechanism.
• Osmolarity Gradient: Proximity and counter-current flow maintain an increasing gradient from 300 mOsmol/L in the cortex to 1200 mOsmol/L in the inner medulla.
• Role of Solutes: This gradient is maintained by $NaCl$ (transported by the ascending limb of Henle's loop and exchanged with vasa recta) and urea (recycled from the collecting duct to the ascending limb of Henle's loop).
• Outcome: The gradient facilitates easy passage of water from the collecting tubule, concentrating the urine up to four times compared to the initial filtrate.

• Hypothalamus (ADH): Osmoreceptors detect changes in blood volume and ionic concentration. Excessive fluid loss triggers the release of Antidiuretic Hormone (ADH) or vasopressin, facilitating water reabsorption in distal tubules. ADH also constricts blood vessels to increase blood pressure and GFR.
• JGA (Renin-Angiotensin System): A fall in GFR activates JG cells to release renin, which converts angiotensinogen to angiotensin I and then to angiotensin II. Angiotensin II is a powerful vasoconstrictor that increases GFR and stimulates the adrenal cortex to release aldosterone, promoting $Na^+$ and water reabsorption.
• Heart (ANF): An increase in blood flow to the atria triggers the release of Atrial Natriuretic Factor (ANF), causing vasodilation to decrease blood pressure and act as a check on the renin-angiotensin system.

The process of releasing urine from the urinary bladder.

• Mechanism: Stretch receptors on the bladder wall send signals to the CNS when the bladder fills. The CNS sends motor messages to contract smooth muscles of the bladder and relax the urethral sphincter. This neural pathway is the micturition reflex.
• Urine Characteristics: An adult human excretes 1-1.5 liters of urine daily. It is light yellow, slightly acidic (pH 6.0), and contains 25-30 g of urea.

• Lungs: Eliminate large amounts of $CO_2$ (approx. 200 mL/min) and water.
• Liver: Secretes bile containing bilirubin, biliverdin, cholesterol, steroid hormones, vitamins, and drugs for elimination.
• Skin: Sweat glands eliminate water, $NaCl$, small amounts of urea, and lactic acid (primarily for cooling). Sebaceous glands eliminate sterols, hydrocarbons, and waxes through sebum.

• Uremia: Accumulation of urea in the blood due to kidney malfunction, which can lead to kidney failure.
• Hemodialysis: Artificial removal of urea from blood. Blood is drained from an artery, mixed with heparin, passed through a dialysing unit with a cellophane tube, cleared of wastes, and returned via a vein with anti-heparin.
• Kidney Transplantation: The ultimate treatment for acute renal failure, utilizing a functioning kidney from a compatible donor.
• Renal Calculi: Stones or insoluble masses of crystallized salts (e.g., oxalates) formed within the kidney.
• Glomerulonephritis: Inflammation of the glomeruli of the kidney.