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Short Notes Excretory Products and Their Elimination - Short Notes for NEET
Modes of Excretion
Based on Nitrogenous Waste Excreted
| Type | Main Waste Product | Characteristics | Examples |
|---|---|---|---|
| Ammonotelism | Ammonia (NH₃) | • Highly toxic • Very soluble in water • Requires large amounts of water for excretion • Direct diffusion across body surface | Aquatic animals: Bony fishes, aquatic amphibians, aquatic insects |
| Ureotelism | Urea | • Less toxic than ammonia • Moderately soluble • Requires less water • Formed in liver (ornithine cycle) | Mammals, terrestrial amphibians, sharks, some bony fishes |
| Uricotelism | Uric acid | • Least toxic • Poorly soluble; excreted as paste/pellet • Requires minimal water • Advantage in water conservation | Birds, reptiles, insects, land snails |
Other Excretory Products
- CO₂ and water: From cellular respiration (excreted via lungs)
- Bile pigments (bilirubin, biliverdin): From RBC breakdown (excreted in bile)
- Excess salts, vitamins: Excreted in urine and sweat
- Small amounts: Creatinine, uric acid (in humans)
Human Excretory System - Structure
| Organ/Part | Structure | Function |
|---|---|---|
| Kidneys | • Bean-shaped, reddish-brown • 10-12 cm long • Located between last thoracic and 3rd lumbar vertebra • Right kidney slightly lower than left | • Filtration of blood • Urine formation • Osmoregulation |
| Ureters | • Thin muscular tubes, 25-30 cm long • One from each kidney | • Transport urine from kidney to bladder |
| Urinary Bladder | • Muscular sac • Capacity: 700-800 mL | • Temporary storage of urine |
| Urethra | • Membranous tube • Originates from bladder | • Passage of urine to outside |
Kidney Structure (Internal)
- Hilum: Notch where ureter, blood vessels, and nerves enter
- Outer Cortex: Dark reddish-brown, contains nephrons
- Inner Medulla: Lighter color, divided into medullary pyramids (8-18)
- Renal Pelvis: Funnel-shaped space, receives urine
- Calyces: Cup-like projections that collect urine from pyramids
- Renal Columns of Bertini: Cortical extensions between pyramids
Nephron - Structural and Functional Unit
| Component | Structure Details |
|---|---|
| Glomerulus | • Tuft of capillaries formed by afferent arteriole • Drains into efferent arteriole • Covered by Bowman's capsule |
| Bowman's Capsule | • Double-walled cup-like structure • Inner visceral layer (podocytes) • Outer parietal layer |
| Proximal Convoluted Tubule (PCT) | • Highly coiled • Lined by brush border (microvilli) • Located in cortex |
| Henle's Loop | • Hairpin-shaped • Descending limb: Thin, permeable to water • Ascending limb: Thick, impermeable to water |
| Distal Convoluted Tubule (DCT) | • Highly coiled • Located in cortex • Opens into collecting duct |
| Collecting Duct | • Many nephrons drain into one • Extends from cortex to medulla • Opens at papilla |
Types of Nephrons
| Type | Location | Loop of Henle | Percentage |
|---|---|---|---|
| Cortical Nephrons | Cortex | Short loop, barely enters medulla | 85% |
| Juxtamedullary Nephrons | Near corticomedullary junction | Long loop, deep into medulla | 15% |
Urine Formation - Three Steps
1. Glomerular Filtration
- Site: Glomerulus and Bowman's capsule (Malpighian corpuscle)
- Process: Ultrafiltration - blood filtered under pressure
- Filtration membrane:3 layers
- Endothelium of glomerular capillaries (fenestrated)
- Basement membrane (negatively charged)
- Epithelium of Bowman's capsule (podocytes with filtration slits)
- Glomerular Filtration Rate (GFR): 125 mL/min or 180 L/day
- Filtrate composition: Water, glucose, amino acids, urea, uric acid, ions (plasma minus proteins and blood cells)
- Driving force:Glomerular hydrostatic pressure (~55 mm Hg)
- Opposing forces:
- Capsular hydrostatic pressure (~15 mm Hg)
- Blood colloidal osmotic pressure (~30 mm Hg)
- Net filtration pressure: 10 mm Hg (55 - 15 - 30)
2. Tubular Reabsorption
| Site | Substances Reabsorbed | Mechanism |
|---|---|---|
| PCT (Maximum reabsorption) | • ~70% water • 100% glucose • 100% amino acids • Na+, K+, Cl- • Uric acid • Bicarbonates | • Active transport • Passive transport • Facultative reabsorption |
| Descending Limb of Henle | • Water | • Passive (osmosis) • Permeable to water, not to salts |
| Ascending Limb of Henle | • Na+, Cl-, K+ | • Active transport • Impermeable to water |
| DCT | • Na+, Cl- • Water (conditional) | • Active transport • Regulated by hormones |
| Collecting Duct | • Water (large amounts) • Small amounts of urea | • ADH regulated • Obligatory reabsorption |
- Amount reabsorbed: 178.5 L out of 180 L (99%)
- Final urine: 1.5 L per day
3. Tubular Secretion
- Site: Mainly PCT, also DCT
- Substances secreted:
- H+ ions
- K+ ions
- NH3 (ammonia)
- Creatinine
- Drugs and toxins
- Significance:
- Maintains ionic and acid-base balance
- Eliminates unwanted substances
Osmoregulation
- Definition: Regulation of osmotic pressure of body fluids by controlling water and salt concentration
- Mechanism - Counter Current Mechanism:
- Counter Current Multiplier:Between ascending and descending limbs of Henle's loop
- Descending limb: Permeable to water, water moves out
- Ascending limb: Impermeable to water, actively transports NaCl out
- Creates osmotic gradient: 300 mOsmol/L (cortex) to 1200 mOsmol/L (inner medulla)
- Counter Current Exchange:In vasa recta (capillaries around Henle's loop)
- Maintains osmotic gradient
- Prevents washing away of gradient
- Counter Current Multiplier:Between ascending and descending limbs of Henle's loop
- Result: Concentrated urine production (up to 4 times more concentrated than blood)
Regulation of Kidney Function
1. Renin-Angiotensin-Aldosterone System (RAAS)
| Step | Process |
|---|---|
| 1 | Juxtaglomerular (JG) cells of kidney detect fall in blood pressure/GFR |
| 2 | JG cells secrete Renin |
| 3 | Renin converts Angiotensinogen (from liver) → Angiotensin I |
| 4 | ACE (Angiotensin Converting Enzyme) converts Angiotensin I → Angiotensin II |
| 5 | Angiotensin II: • Potent vasoconstrictor (increases BP) • Stimulates Aldosterone release from adrenal cortex |
| 6 | Aldosterone: • Increases Na+ and water reabsorption from DCT and collecting duct • Increases K+ secretion • Raises blood volume and BP |
2. Atrial Natriuretic Factor (ANF)
- Source: Atrial wall of heart (when stretched due to increased blood volume)
- Action:
- Dilates blood vessels (vasodilation)
- Decreases blood pressure
- Increases GFR
- Inhibits renin release
- Inhibits aldosterone release
- Increases Na+ excretion (natriuresis)
- Net effect: Decreases blood volume and BP (opposite to RAAS)
3. Anti-Diuretic Hormone (ADH) / Vasopressin
- Source: Posterior pituitary (synthesized in hypothalamus)
- Trigger: Osmoreceptors in hypothalamus detect increased blood osmolarity (dehydration)
- Action:
- Increases water reabsorption from DCT and collecting duct
- Makes collecting duct more permeable to water (by inserting aquaporins)
- Produces concentrated urine
- Vasoconstriction (increases BP)
- Result: Conserves water, decreases urine volume
Diabetes Insipidus
- Cause: Deficiency of ADH or kidney insensitivity to ADH
- Symptoms:
- Excessive, dilute urine production (polyuria) - up to 20 L/day
- Excessive thirst (polydipsia)
- Dehydration
- Note: Different from Diabetes Mellitus (no glucose in urine)
Role of Other Organs in Excretion
| Organ | Excretory Products | Additional Information |
|---|---|---|
| Lungs | • CO2 (~200 mL/min) • Water vapor • Small amounts of acetone | Major site for CO2 removal |
| Liver | • Bile pigments (bilirubin, biliverdin) • Cholesterol • Steroid hormones • Drugs and toxins | Detoxifies harmful substances; converts ammonia to urea |
| Skin (Sweat glands) | • Water • NaCl • Urea • Lactic acid • Small amounts of uric acid | Also involved in thermoregulation |
| Intestine | • Bile pigments • Salts of heavy metals • Undigested matter | Via feces |
Disorders of Excretory System
1. Uraemia (Uremia)
- Definition: Accumulation of urea and other nitrogenous wastes in blood
- Cause: Kidney failure (kidneys unable to filter waste)
- Symptoms: Nausea, vomiting, fatigue, metabolic acidosis, may lead to coma and death
2. Renal Failure
- Acute Renal Failure:
- Sudden loss of kidney function
- Causes: Injury, infection, toxins, sudden drop in blood flow
- Reversible if treated promptly
- Chronic Renal Failure:
- Gradual, progressive loss of kidney function
- Causes: Diabetes, hypertension, chronic glomerulonephritis
- Irreversible, may require dialysis or transplant
3. Renal Calculi (Kidney Stones)
- Definition: Stone/crystal formation in kidney
- Composition: Calcium oxalate, calcium phosphate, uric acid
- Causes: Dehydration, excess calcium/oxalate in diet, metabolic disorders
- Symptoms: Severe pain (renal colic), hematuria (blood in urine), nausea
4. Nephritis (Glomerulonephritis)
- Definition: Inflammation of glomeruli
- Causes: Infection (post-streptococcal), autoimmune disorders
- Symptoms:
- Hematuria
- Proteinuria (protein in urine)
- Edema (swelling)
- Hypertension
Hemodialysis and Artificial Kidney
- Indication: Renal failure when kidneys cannot function
- Hemodialysis:
- Blood removed from artery, passed through dialysis machine
- Dialyzer: Contains semipermeable cellophane membrane
- Dialysate: Dialyzing fluid with specific composition (similar to plasma minus waste)
- Principle: Diffusion - waste products move from blood to dialysate
- Cleaned blood returned to vein
- Frequency: 2-3 times per week, 4-6 hours per session
- Peritoneal Dialysis: Uses peritoneum as natural dialysis membrane (alternative method)
- Kidney Transplant: Permanent solution; donor kidney from living relative or cadaver
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FAQs on Short Notes Excretory Products and Their Elimination - Short Notes for NEET
| 1. What are the main excretory products in humans? |  |
Ans. The main excretory products in humans include urea, uric acid, creatinine, and ammonia. Urea is formed from the breakdown of proteins, uric acid from the metabolism of purines, creatinine from muscle metabolism, and ammonia is produced during amino acid deamination. These substances are eliminated from the body primarily through urine.
| 2. How does the kidney function in excretion? | |
Ans. The kidney functions in excretion by filtering blood through its nephrons, which consist of a glomerulus and a renal tubule. During filtration, waste products and excess substances are removed from the blood, forming urine. The kidney also regulates water balance, electrolyte levels, and blood pH, making it crucial for maintaining homeostasis.
| 3. What is the role of the ureters in the excretory system? | |
Ans. The ureters are tubes that transport urine from the kidneys to the bladder. They are approximately 25 to 30 cm long and use peristaltic movements to push urine downwards. This ensures that urine is stored in the bladder until it is ready to be expelled from the body.
| 4. What is the significance of the urinary bladder in excretion? | |
Ans. The urinary bladder is a muscular sac that stores urine until it is excreted from the body. It can hold approximately 400 to 600 ml of urine, and its walls stretch to accommodate increasing volumes. The bladder plays a vital role in controlling urination through the sphincters, allowing for voluntary expulsion of urine.
| 5. What are some common disorders related to the excretory system? | |
Ans. Common disorders related to the excretory system include urinary tract infections (UTIs), kidney stones, glomerulonephritis, and chronic kidney disease. UTIs occur due to bacterial infection, kidney stones form from crystallised minerals, glomerulonephritis is inflammation of the kidney's filtering units, and chronic kidney disease is a progressive loss of kidney function over time.
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