Human Excretory System

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### Introduction to Excretion - **Excretion:** Process of eliminating metabolic waste products from the body. - **Metabolic wastes:** Ammonia, urea, uric acid, carbon dioxide, water, ions (Na+, K+, Cl-, phosphate, sulphate). - **Ammonotelic:** Animals excreting ammonia (e.g., bony fishes, aquatic amphibians, aquatic insects). Ammonia is highly toxic and requires large amounts of water for its elimination. - **Ureotelic:** Animals excreting urea (e.g., mammals, terrestrial amphibians, marine fishes). Urea is less toxic than ammonia and requires less water. - **Uricotelic:** Animals excreting uric acid (e.g., reptiles, birds, land snails, insects). Uric acid is least toxic and requires minimal water for excretion, often as a pellet or paste. ### Human Excretory System - Consists of a pair of kidneys, a pair of ureters, a urinary bladder, and a urethra. #### Kidneys - **Location:** Between the last thoracic and third lumbar vertebra, close to the dorsal inner wall of the abdominal cavity. - **Shape:** Reddish-brown, bean-shaped. - **Dimensions:** 10-12 cm long, 5-7 cm wide, 2-3 cm thick, average weight 120-170 gm. - **Hilum:** A notch on the inner concave surface where ureter, blood vessels, and nerves enter. - **Renal pelvis:** A broad funnel-shaped space inner to the hilum, with projections called calyces. - **Capsule:** Tough fibrous covering. - **Cortex:** Outer layer. - **Medulla:** Inner layer, divided into a few (8-12) conical masses called medullary pyramids (renal pyramids) projecting into the calyces. - **Renal columns of Bertini:** Parts of cortex extending between medullary pyramids. ### Structure of Nephron (Functional Unit) - Each kidney has ~1 million complex tubular structures called nephrons. - **Two parts:** 1. **Glomerulus:** A tuft of capillaries formed by the afferent arteriole (a fine branch of renal artery). Blood from glomerulus is carried away by the efferent arteriole. 2. **Renal Tubule:** - **Bowman's capsule:** Double-walled cup-like structure enclosing the glomerulus. - **Malpighian body/Renal corpuscle:** Glomerulus + Bowman's capsule. - **Proximal Convoluted Tubule (PCT):** Highly coiled, next to Bowman's capsule. - **Henle's Loop:** Hairpin-shaped, has a descending and an ascending limb. - **Distal Convoluted Tubule (DCT):** Highly coiled, next to Henle's loop. - **Collecting Duct:** Straight tube into which many DCTs open. Many collecting ducts converge and open into the renal pelvis through medullary pyramids. #### Types of Nephrons - **Cortical nephrons:** Majority (~85%), loop of Henle is short and extends only a little into the medulla. - **Juxtamedullary nephrons:** Loop of Henle is very long and runs deep into the medulla. #### Blood Supply to Nephron - **Peritubular capillaries:** A fine network of capillaries formed by the efferent arteriole, surrounding the renal tubule. - **Vasa recta:** A minute U-shaped vessel, a part of the peritubular capillary network, running parallel to Henle's loop in juxtamedullary nephrons. It is absent or highly reduced in cortical nephrons. ### Urine Formation - Involves three main processes: 1. **Glomerular Filtration (Ultrafiltration):** - Blood is filtered in the glomerulus. - **Glomerular capillary blood pressure:** Causes filtration of blood through 3 layers: - Endothelium of glomerular blood vessels. - Epithelium of Bowman's capsule. - Basement membrane between the two layers. - **Podocytes:** Epithelial cells of Bowman's capsule, arranged in an intricate manner leaving some minute spaces called filtration slits or slit pores. - **Filtrate:** Almost all constituents of plasma except proteins pass into the Bowman's capsule. - **Glomerular Filtration Rate (GFR):** Amount of filtrate formed by kidneys per minute. Normal GFR is ~125 ml/minute or 180 liters/day. - **Juxta Glomerular Apparatus (JGA):** A sensitive region formed by cellular modifications in the DCT and the afferent arteriole at their contact. Regulates GFR. 2. **Reabsorption (Selective Reabsorption):** - Compares the 180 L of filtrate/day to the 1.5 L of urine/day. 99% of filtrate is reabsorbed. - **PCT:** Reabsorbs ~70-80% of electrolytes and water. Also reabsorbs glucose, amino acids. Secretes H+ and K+ ions, and ammonia to maintain pH and ionic balance. - **Henle's Loop:** Maintains high osmolarity of medullary interstitial fluid. - **Descending limb:** Permeable to water, almost impermeable to electrolytes. Filtrate becomes concentrated. - **Ascending limb:** Impermeable to water, permeable to electrolytes (active/passive transport). Filtrate becomes dilute. - **DCT:** Conditional reabsorption of Na+ and water (regulated by ADH and Aldosterone). Secretes H+ and K+ ions, and NH3 to maintain pH and Na-K balance. - **Collecting Duct:** Long duct extending from cortex to inner medulla. Reabsorbs large amounts of water to produce concentrated urine (regulated by ADH). Allows passage of small amounts of urea into medullary interstitium to maintain osmolarity. Secretes H+ and K+ ions. 3. **Tubular Secretion:** - Tubular cells secrete substances like H+, K+ ions, and ammonia into the filtrate. - Important for ionic and acid-base balance of body fluids. ### Counter Current Mechanism - The loops of Henle and vasa recta are involved in this mechanism. - The flow of filtrate in two limbs of Henle's loop is in opposite directions (counter current). - The flow of blood in two limbs of vasa recta is also in opposite directions (counter current). - Proximity between Henle's loop and vasa recta, and the counter current in them, helps maintain an increasing osmolarity gradient in the medullary interstitium (300 mOsmol/L in cortex to 1200 mOsmol/L in inner medulla). - This gradient is caused by NaCl (reabsorbed by ascending limb of Henle's loop and returned by ascending limb of vasa recta) and urea (entering collecting duct). - This mechanism helps in easy reabsorption of water from the collecting duct, leading to concentrated urine. ### Regulation of Kidney Function - **ADH (Antidiuretic Hormone)/Vasopressin:** - Released by hypothalamus (neurohypophysis). - Regulates water reabsorption from distal parts of tubule. - Release stimulated by increase in body fluid osmolarity (e.g., dehydration). - Increases permeability of DCT and collecting duct to water. - Alcohol inhibits ADH release, leading to diuresis (increased urine output). - **Renin-Angiotensin-Aldosterone System (RAAS):** - **JGA:** Releases Renin when GFR/blood pressure drops. - **Renin:** Converts Angiotensinogen (plasma protein) to Angiotensin I, then to Angiotensin II. - **Angiotensin II:** - A potent vasoconstrictor, increases GFR/blood pressure. - Activates adrenal cortex to release Aldosterone. - **Aldosterone:** Causes reabsorption of Na+ and water from DCT and collecting duct, leading to increased GFR/blood pressure. - **ANF (Atrial Natriuretic Factor):** - Released by atrial wall of heart when blood pressure increases. - Causes vasodilation, thereby decreasing blood pressure. - Acts as a check on the RAAS mechanism. ### Micturition - The process of release of urine. - **Micturition reflex:** Neural mechanism. - Stretch receptors on the bladder wall send signals to CNS. - CNS initiates motor messages to contract bladder smooth muscles and relax urethral sphincter. - Voluntary control over urination develops with age. ### Role of Other Organs in Excretion - **Lungs:** Excrete CO2 (~200 ml/min) and water. - **Liver:** Excretes bile pigments (bilirubin, biliverdin), degraded steroid hormones, drugs, excess vitamins. - **Skin:** - **Sweat glands:** Produce sweat containing NaCl, small amounts of urea, lactic acid. Primary function is cooling. - **Sebaceous glands:** Eliminate sterols, hydrocarbons, waxes through sebum. ### Disorders of the Excretory System - **Renal calculi (Kidney stones):** Stone or insoluble mass of crystallised salts (oxalates, etc.) formed within the kidney. - **Glomerulonephritis:** Inflammation of the glomeruli of the kidney. - **Uremia:** High concentration of urea in blood due to kidney malfunction. Can lead to kidney failure. - **Treatment:** Hemodialysis (artificial kidney) - blood is drained from a convenient artery, mixed with an anticoagulant (heparin), passed through a dialysing unit (cellophane tube surrounded by dialysing fluid without urea), and returned to a vein. - **Kidney transplantation:** The ultimate method in correction of acute renal failures. Close genetic match is desirable to minimise rejection.