Excretion and Osmoregulation | Zoology Optional Notes for UPSC PDF Download

Introduction

• Various metabolic activities in the body produce by-products that need elimination.
• Homeostasis, maintaining a constant internal environment, involves excretion and osmoregulation.

Homeostasis:

• Regulation of a constant body fluid or internal environment.
• Maintains constant temperature, water amount, and glucose concentration.

Excretion

• Removal of unwanted metabolic waste products.
• Waste products include CO2, H2O, bile pigments, nitrogenous wastes, excess inorganic salts, hormones, and vitamins.
• Nitrogenous wastes (ammonia, urea, uric acid) result from the breakdown of proteins.
• Skin, lungs, and liver act as accessory excretory organs, with the kidney playing a major role in vertebrates.

Modes of Excretion

1. Ammonotelism:

   • Excretion of ammonia as the main product.
   • Ammonia is toxic, highly soluble, and requires large amounts of water for elimination.
   • Found in aquatic invertebrates, bony fishes, tadpole larva of frogs, and salamanders.

2. Ureotelism:

   • Excretion of nitrogenous waste in the form of urea.
   • Urea is less toxic than ammonia, soluble in water, and stored as urine.
   • Requires less water for elimination.
   • Found in terrestrial animals like frogs, turtles, toads, mammals, and marine fishes.

3. Uricotelism:

   • Excretion of nitrogenous waste in the form of uric acid.
   • Uric acid is the least toxic, can be retained longer, and eliminated in solid pellets.
   • Found in land snails, terrestrial insects, reptiles, and birds.

Gout: Excess uric acid deposition in joints, causing painful arthritis.

Human Excretory System

1. Kidneys:
   • Dark red, bean-shaped structures located on either side of the backbone, protected by the last two ribs.
   • Approximately 10-12 cm in length, 5-7 cm in width, 4 cm thick, and weigh around 120-170g.
   • Right kidney is slightly lower than the left.
   • Inner concave structure with a hilum at the center.
   • Composed of an outer cortex and inner medulla.
   • Functions include fluid balance regulation, electrolyte concentration regulation, acid-base balance maintenance, removal of various substances, secretion of rennin and erythropoietin.

1. Blood Supply to Kidney:
   • Renal Vein: Drains the kidney, connecting it to the inferior vena cava.
   • Renal Artery: Arises from the abdominal aorta, supplies blood to the kidney. Renal arteries carry a large portion of the total blood flow to the kidneys.
2. Structure of Kidney:
   • Renal Cortex: Outer dark part containing Bowman’s capsule and nephrons.
   • Renal Medulla: Composed of conical pyramids, with renal columns of Bertini extending between pyramids.
   • Renal Pelvis: Funnel-shaped space near the hilum, connected to the ureter.

1. Ureters:

   • Thin and muscular tubes arising from the hilum of the kidney and opening into the urinary bladder.

2. Urinary Bladder:

   • Single, large, thin, muscular elastic bag in the abdominal cavity.
   • Pear-shaped structure lined by transitional epithelium.
   • Triangular area called trigone internally, containing openings of ureters and urethra.

3. Urethra:

   • Short canal, longer in males (20 cm) and shorter in females (4 cm).
   • Serves as a passage for both urine and semen in males.

4. Nephrons:

   • Microscopic functional units of the kidney.
   • Thin-walled, coiled duct lined by a single layer of epithelial cells.
   • Comprises the Malpighian body (Bowman’s capsule and glomerulus) and renal tubule (PCT, Henle’s loop, DCT, and collecting duct).

• Cortical Nephrons: About 85% located in the cortex, without vasa rectae.
• Juxtamedullary Nephrons: About 15% located in the medulla, highly supplied with vasa rectae.

Mechanism of Urine Formation

1. Ultrafiltration (Glomerular Filtration):

   • Takes place in the glomerulus.
   • Dissolved substances filtered into Bowman’s capsule due to blood pressure.
   • Afferent arteriole wider than efferent, creating hydrostatic pressure for ultrafiltration.
   • Effective filtration pressure (EFP) determined by glomerular hydrostatic pressure, osmotic pressure of blood, and hydrostatic pressure of glomerular capsule.
   • Net filtration pressure = Capillary hydrostatic pressure – (osmotic pressure + filtrate hydrostatic pressure).
   • About 180 liters of fluid filtered, but only about 1.5 liters of urine produced daily.

1. Selective Reabsorption:
   • About 99% of filtrate reabsorbed.
   • Passive transport (osmosis) and active transport (using ATP molecules) involved.
   • Proximal convoluted tubule (PCT) responsible for reabsorption.
   • High threshold substances (glucose, amino acids) completely reabsorbed.
   • Low threshold substances (uric acid, urea) partially reabsorbed.
   • Water reabsorbed by osmosis in PCT, DCT, and descending limb of the loop of Henle (obligatory water reabsorption).
2. Tubular Secretion:
   • Takes place in distal convoluted and collecting tubules.
   • Unwanted substances (uric acid, hippuric acid, creatine, ammonia, K+, H+) secreted into tubular fluid.
   • Exchange occurs between blood in peritubular capillaries and tubular fluid.
   • Na, Cl, and Ca moved from urine to blood to regulate ion concentration.
   • Water reabsorbed or secreted in DCT, controlled by antidiuretic hormone (ADH).

• Depends on water intake, diet, temperature, mental state, and physiological state.
• About 1.2 to 1.5 liters of urine produced daily.
• Transparent, pale yellow, usually acidic.
• Specific gravity varies (isotonic, hypotonic, hypertonic).
• Contains urea, ammonia, uric acid, creatinine, hippuric acid, sodium chloride, and small amounts of inorganic salts, vitamins, oxalic acid, phenolic substances.

Role of Kidney in Osmoregulation

• Osmoregulation regulates blood concentration and osmotic pressure.
• ADH regulates water absorption from filtrate based on body's water needs.
• Aldosterone maintains sodium ion concentration.
• Calcitonin and parathormone regulate calcium ion concentration.

Kidney Failure:

• Acute kidney injury (reversible) and chronic kidney disease (often irreversible).
• Detected by decreased urine production, waste products in blood, hematuria, proteinuria.
• Associated with fluid imbalance, acid-base issues, electrolyte imbalances, anemia, cardiovascular risks.

• Acute Kidney Injury (AKI): Rapid loss of renal function with various causes.
• Chronic Kidney Disease (CKD): Develops slowly, shows few symptoms initially.
• Used for renal failure treatment.
• Artificial replacement for lost kidney function.
• Diffusion and ultrafiltration used in waste and fluid removal.
• Imperfect compared to healthy kidney functions.

• Solid piece formed in kidneys from minerals in urine.
• Causes pain, nausea, vomiting, blood in urine.
• Types include calcium-containing, uric acid, struvite stones.

• Deceased-donor or living-donor transplantation.
• Living-donor classified as genetically related or unrelated.
• Supports end-stage renal disease patients.

• Hormonal feedback involves hypothalamus, JGA, and heart.
• Osmoreceptors activated by changes in blood volume, fluid volume, ionic concentration.
• Hormones like ADH, ANF, renin-angiotensin mechanism regulate blood pressure, water absorption, and electrolyte balance.

Accessory Excretory Organs

1. Skin:

   • Sweat and sebaceous glands contribute to excretion.
   • Sweat contains sodium-chloride, lactic acid, urea, amino acids, and glucose.
   • Sebum excretes lipids, waxes, and fatty acids.
2. Lungs:
   • Eliminate carbon dioxide and excess water vapor during expiration.
   • Help maintain body's internal equilibrium.
3. Liver:
   • Involved in metabolic processes.
   • Participates in the endocrine system, producing erythropoietin, calcitriol, and renin.

• Presence of excessive urea in blood.
• Associated with kidney failure.

• Inflammation of both kidneys.
• Characterized by hematuria, proteinuria, hypertension, edema, oliguria.