Excretion PDF

Introduction to Excretion: Animals accumulate ammonia, urea, uric acid, carbon dioxide, water, and ions (Na+, K+, Cl–, phosphate, sulphate) through metabolic activities or excess ingestion. These substances need to be removed either totally or partially. Types of Excretory...

Introduction to Excretion: Animals accumulate ammonia, urea, uric acid, carbon dioxide, water, and ions (Na+, K+, Cl–, phosphate, sulphate) through metabolic activities or excess ingestion. These substances need to be removed either totally or partially. Types of Excretory Products: - Ammonia: Most toxic, requires large amounts of water for elimination. - Urea: Less toxic than ammonia, conserves water, requires less water for elimination. - Uric Acid: Least toxic, minimal water loss, excreted in solid form. Mechanisms and Forms of Excretion: - Ammonotelism: - Excretion of ammonia. - Common in many bony fishes, aquatic amphibians, and aquatic insects. - Ammonia is excreted by diffusion across body surfaces or gill surfaces as ammonium ions. - Kidneys play an insignificant role. - Ureotelism: - Excretion of urea. - Common in mammals, many terrestrial amphibians, and marine fishes. - Ammonia produced by metabolism is converted into urea in the liver. - Urea is released into the blood, filtered, and excreted by the kidneys. - Some urea may be retained in the kidney matrix to maintain osmolarity. - Uricotelism: - Excretion of uric acid. - Common in reptiles, birds, land snails, and insects. - Uric acid is excreted in the form of pellet or paste, conserving water. Excretory Structures in Animals: - Invertebrates: - Protonephridia (Flame Cells): Found in Platyhelminthes (e.g., Planaria), rotifers, some annelids, and cephalochordates (e.g., Amphioxus). - Function: Ionic and fluid volume regulation (osmoregulation). - Nephridia: Found in earthworms and other annelids. - Function: Removal of nitrogenous wastes, fluid, and ionic balance. - Malpighian Tubules: Found in most insects, including cockroaches. - Function: Removal of nitrogenous wastes, osmoregulation. - Antennal Glands (Green Glands): Found in crustaceans like prawns. - Function: Excretion. Human Excretory System Component Details Excretory System Consists of a pair of kidneys, one pair of ureters, a urinary bladder, and a urethra. Kidney Shape & Color Reddish-brown, bean-shaped structures. Kidney Location Situated between the last thoracic and third lumbar vertebrae, close to the dorsal inner wall of the abdominal cavity. Kidney Dimensions Length: 10-12 cm, Width: 5-7 cm, Thickness: 2-3 cm, Weight: 120-170 g. Hilum A notch on the inner concave surface of the kidney through which the ureter, blood vessels, and nerves enter. Renal Pelvis A broad funnel-shaped space inside the hilum with projections called calyces. Outer Layer Tough capsule covering the kidney.  Kidney Zones Outer Cortex and Inner Medulla. Medullary Pyramids Conical masses in the medulla projecting into the calyces. Renal Columns (Columns Cortex extending between the medullary pyramids as renal columns. of Bertini) Nephrons Nearly one million complex tubular structures in each kidney, functional units. Nephron Parts Glomerulus and Renal Tubule. Glomerulus A tuft of capillaries formed by the afferent arteriole, a fine branch of the renal artery; blood is carried away by the efferent arteriole. Bowman’s Capsule Double-walled cup-like structure enclosing the glomerulus. Malpighian Body (Renal Combination of the glomerulus and Bowman’s capsule. Corpuscle) Proximal Convoluted Highly coiled network following Bowman’s capsule. Tubule (PCT) Henle’s Loop Hairpin-shaped structure in the renal tubule with descending and ascending limbs. Distal Convoluted Tubule Highly coiled tubular region following the ascending limb of Henle’s loop. (DCT) Collecting Duct The DCTs of many nephrons open into this straight tube, which converges and opens into the renal pelvis through medullary pyramids in the calyces. Types of Nephrons: - Cortical Nephrons: Nephrons where the loop of Henle is short and extends only slightly into the medulla. - Juxta Medullary Nephrons: Nephrons with a very long loop of Henle that runs deep into the medulla. Peritubular Capillaries: - Efferent arterioles forms a fine capillary network around the renal tubule called the peritubular capillaries. - Vasa Recta: A minute vessel from this network runs parallel to Henle’s loop, forming a 'U'-shaped structure. - Vasa Recta Presence: Absent or highly reduced in cortical nephrons. Urine Formation 1. Glomerular Filtration: - Function: Filtration of blood through three layers: - Endothelium of glomerular blood vessels - Basement membrane - Epithelium of Bowman’s capsule - Special Structures: - Podocytes: Epithelial cells of Bowman’s capsule, arranged intricately to create filtration slits. - Filtration Slits (Slit Pores): Allow the passage of plasma constituents except proteins into the lumen of Bowman’s capsule. - Filtration Rate (GFR): Normal GFR: ~125 ml/minute, or 180 liters/day. - Regulation of GFR: - Juxtaglomerular Apparatus (JGA): Sensitive region where the distal convoluted tubule contacts the afferent arteriole. - Function of JGA: Releases renin when GFR falls, stimulating blood flow and restoring GFR to normal. 2. Reabsorption: - Process: Reabsorption of nearly 99% of the filtrate by the renal tubules. - Location: Renal tubules, different segments of the nephron. - Mechanism: - Active Transport: For substances like glucose, amino acids, and Na+. - Passive Transport: For nitrogenous wastes and water (initial segments). 3. Tubular Secretion: - Process: Secretion of substances such as H+, K+, and ammonia into the filtrate by tubular cells. - Function: Maintains ionic and acid-base balance in body fluids. ``` Formation of urine (1.5 liters/day) Tubule Segment Structure/Characteristics Functions Proximal - Lined by simple cuboidal brush border - Reabsorbs nearly all essential nutrients and 70-80% of Convoluted Tubule epithelium electrolytes and water (PCT) - Maintains pH and ionic balance through selective secretion (H+, ammonia, K+) and reabsorption (HCO₃⁻) Henle’s Loop Descending Limb: Permeable to water, almost - Concentrates filtrate by reabsorbing water as it moves impermeable to electrolytes down Ascending Limb: Impermeable to water, allows - Dilutes filtrate by allowing electrolytes to pass into active/passive electrolyte transport medullary fluid Distal Convoluted - Capable of conditional reabsorption of Na⁺ and - Reabsorbs HCO₃⁻ and selectively secretes H⁺, K⁺, and Tubule (DCT) water NH₃ to maintain pH and sodium-potassium balance Collecting Duct - Extends from the kidney cortex to inner - Reabsorbs large amounts of water to produce medulla concentrated urine - Allows small amounts of urea to pass into medullary interstitium to maintain osmolarity - Maintains pH and ionic balance through selective secretion of H⁺ and K⁺ ions Mechanism of Concentration of Filtrate Concentration of Urine: Mammals have the ability to produce concentrated urine, primarily facilitated by the Henle's loop and vasa recta. Counter Current Mechanism: - Henle’s Loop:  - The filtrate flows in opposite directions in the descending and ascending limbs, creating a counter current. - Vasa Recta: - Blood flows in opposite directions in the descending and ascending limbs, also creating a counter current. - Proximity of Henle’s Loop and Vasa Recta: - The close arrangement and counter current flow between these structures help maintain an increasing osmolarity gradient in the medullary interstitium. - Osmolarity Gradient: - Osmolarity in Cortex: Starts at ~300 mOsmol/L. - Osmolarity in Inner Medulla: Increases to ~1200 mOsmol/L. - Substances Involved: - NaCl: - Transported by the ascending limb of Henle’s loop. - Exchanged with the descending limb of vasa recta. - Returned to the interstitium by the ascending portion of vasa recta. - Urea: - Small amounts enter the thin segment of the ascending limb of Henle’s loop. - Transported back to the interstitium by the collecting tubule. - Result: Human kidneys can produce urine that is nearly four times more concentrated than the initial filtrate. 1. Filtrate enters the Henle's loop ↓ 2. Descending limb of Henle's loop - Permeable to water, impermeable to electrolytes - Filtrate becomes more concentrated ↓ 3. Ascending limb of Henle's loop - Impermeable to water, actively transports NaCl - Filtrate becomes diluted ↓ 4. NaCl is exchanged with the descending limb of vasa recta - NaCl is returned to the interstitium by the ascending limb of vasa recta ↓ 5. Urea enters the thin segment of the ascending limb of Henle's loop - Urea is transported back to the interstitium by the collecting tubule ↓ 6. Counter current mechanism established by Henle's loop and vasa recta - Maintains osmolarity gradient in medullary interstitium (300 mOsmol/L to 1200 mOsmol/L) ↓ 7. Gradient facilitates water reabsorption from the collecting tubule - Concentrates urine, Human kidneys produce urine ~4 times more concentrated than initial filtrate Regulation of Kidney Function Regulatory Component Mechanism/Role Effects on Kidney Function Hypothalamus & ADH - Osmoreceptors: Activated by - ADH Release: Triggered by excessive fluid loss, leads to (Vasopressin) changes in blood volume, body fluid increased water reabsorption from the latter parts of tubules volume, and ionic concentration - ADH: Released from the - Prevents Diuresis: Reduces urine output by increasing neurohypophysis (posterior pituitary) water reabsorption - Blood Vessel Constriction: ADH causes vasoconstriction, increasing blood pressure and GFR Juxtaglomerular - Monitors glomerular blood flow, blood - Renin Release: Triggered by a fall in glomerular blood Apparatus (JGA) pressure, and GFR flow, blood pressure, or GFR - Renin-Angiotensin Mechanism: - Angiotensin II Formation: Renin converts angiotensinogen to angiotensin I, then to angiotensin II - Vasoconstriction: Angiotensin II increases glomerular blood pressure, thereby increasing GFR - Aldosterone Release: Angiotensin II stimulates adrenal cortex to release aldosterone - Na+ and Water Reabsorption: Aldosterone increases reabsorption in the distal tubules, raising blood pressure and GFR Atrial Natriuretic Factor - Released by the atria of the heart due - Vasodilation: ANF causes dilation of blood vessels, (ANF) to increased blood flow leading to decreased blood pressure - Check on Renin-Angiotensin Mechanism: ANF counters the effects of the renin-angiotensin mechanism, reducing GFR Micturition Signal Initiation: The signal for micturition is initiated when the urinary bladder stretches as it fills with urine. Stretch Receptors and CNS Response: - Stretch Receptors: Located on the walls of the bladder, these receptors send signals to the central nervous system (CNS) when the bladder stretches. - CNS Response: In response to the stretch signals, the CNS sends motor messages to the bladder. Micturition Process: - Bladder Contraction: The CNS initiates the contraction of smooth muscles in the bladder. - Urethral Sphincter Relaxation: Simultaneously, the CNS causes the relaxation of the urethral sphincter. - Urine Release: The contraction of the bladder muscles and relaxation of the sphincter leads to the release of urine. - Micturition Reflex: The neural mechanism that triggers the release of urine is known as the micturition reflex. Urine Characteristics: - Daily Excretion: An adult human excretes 1 to 1.5 liters of urine per day. - Appearance: Urine is a light yellow-colored watery fluid. - pH: Urine is slightly acidic with a pH of 6.0. - Odor: Urine has a characteristic odor. - Urea Content: On average, 25-30 grams of urea are excreted daily. Clinical Significance: - Urine Analysis: The analysis of urine is crucial in diagnosing metabolic disorders and kidney malfunctions. - Glycosuria: The presence of glucose in urine, indicative of diabetes mellitus. - Ketonuria: The presence of ketone bodies in urine, also associated with diabetes mellitus. Role of Other Organs in Excretion Lungs: - Primary Function: Removal of large amounts of carbon dioxide (CO2), approximately 200 mL/minute. - Secondary Function: The lungs also remove significant quantities of water each day. Liver: - Largest Gland: The liver is the largest gland in the body. - Secretion of Bile: The liver secretes bile that contains various excretory substances including: - Bilirubin and Biliverdin: Pigments derived from the breakdown of hemoglobin. - Cholesterol, Degraded Steroid Hormones, Vitamins, and Drugs: These substances are processed by the liver and eventually excreted along with digestive wastes. Skin: - Sweat Glands: - Sweat Production: Sweat is a watery fluid produced by sweat glands. - Composition of Sweat: Contains NaCl, small amounts of urea, lactic acid, and other substances. - Primary Function: Sweat helps in cooling the body by evaporation. - Secondary Function: It aids in the elimination of certain waste products. - Sebaceous Glands: - Sebum Production: Sebaceous glands secrete sebum, which contains sterols, hydrocarbons, and waxes. - Function: Sebum provides a protective oily covering for the skin and also helps in the excretion of certain substances. Saliva: - Minor Role: Small amounts of nitrogenous wastes can be excreted through saliva. Disorder Description Treatment/Management Uremia Accumulation of urea in the - Hemodialysis: Blood is filtered using a dialysing unit (artificial blood due to malfunctioning kidney). kidneys. Highly harmful and can lead to - Process: Blood is drained from an artery, mixed with an kidney failure. anticoagulant (like heparin), and passed through a dialyzing unit. - Dialysing Fluid: Composed like plasma but without nitrogenous wastes; clears the blood of wastes through a cellophane membrane. - Blood Return: Cleared blood is returned to the body through a vein after adding anti-heparin. Kidney Transplantation The ultimate treatment for acute - Transplantation: Involves transferring a functioning kidney from a renal failure. donor, preferably a close relative. Involves replacing a failed - Modern Procedures: Increased success rate with advanced kidney with a healthy one from a clinical techniques. donor. Renal Calculi Formation of stones or insoluble - Treatment Options: May include medical treatment, dietary masses of crystallized salts changes, or surgical removal depending on the size and type of (e.g., oxalates) within the kidney. stone. Glomerulonephritis Inflammation of the glomeruli in - Management: Treatment may involve addressing the underlying the kidneys, which can impair cause, using anti-inflammatory medications, or other medical kidney function. interventions.