1. Overview of Excretory Products-

  • Animals produce ammonia, urea, uric acid, carbon dioxide, water, and ions (e.g., Na⁺, K⁺, Cl⁻, phosphate, sulfate).
  • These substances are excreted to maintain homeostasis.

2. Nitrogenous Wastes And Their Mode of Excretion-

  • Ammonotelism –
    • Ammonia is the most toxic nitrogen waste and requires a lot of water to eliminate.
    • Organisms that excrete ammonia are called ammonotelic, and the elimination process is ammonotelism.
    • Examples: Some fish, aquatic amphibians, and aquatic insects.
    • Ammonia passes through body surfaces or gills, with kidneys playing a minor role.
  • Ureotelism-
    • Urea is a less toxic form of nitrogenous waste, requiring less water for elimination.
    • Organisms that excrete urea are called ureotelic, and the process is known as ureotelism.
    • E.g.- mammals, terrestrial amphibians, marine fishes, etc.
  • Uricotelism
    • Uric acid is the least toxic nitrogenous waste, needing the least water for elimination.
    • Those organisms that excrete uric acid are called uricotelic and the process of eliminate uric acid is called Uricotelism.
    • Uric acid is excreted as paste or pellets, conserving water.
    • E.g.- reptiles, birds, insects, land snails etc.

3. Excretory Structures Across Animals-

  • Invertebrates:
    • Protonephridia/Flame Cells: Found in flatworms, rotifers, some annelids, and Amphioxus (osmoregulation).
    • Nephridia: In earthworms, removes nitrogenous wastes and balances fluids.
    • Malpighian Tubules: In insects (e.g., cockroaches), excretes wastes and regulates osmoregulation.
    • Antennal/Green Glands: In crustaceans (e.g., prawns).
  • Vertebrates:
    • Complex tubular kidneys are the primary excretory organs.

Excretory System In Humans-

In human excretory system have 4 organs, kindeys, ureters, a urinary bladder and urethra.

  • KIDNEY-
    • There are two kidneys in the human body.
    • The kidneys are reddish-brown, bean-shaped organs located between the thoracic region and the third lumbar vertebra.
    • Each kidney measures about 10-12 cm in length, 5-7 cm in width, and 2-3 cm in thickness. They weigh on average 120-170 grams (approximately 135 grams in females and 150 grams in males).
    • Kidneys develop from the mesoderm.
    • The kidneys are positioned beneath the diaphragm on both sides of the body. The right kidney is typically lower than the left due to the presence of the liver on the right side.
    • The external structure of the kidney includes-
      • The outer surface is convex, while the hilum is the inward surface of both kidneys.
      • Blood enters and exits through the hilum, where the renal artery and renal vein connect, along with the ureter and nerves supplying the kidneys.
      • There are three protective layers covering the kidneys: the renal fascia (outermost layer), the adipose layer, and the renal capsule (innermost layer), which protect the kidneys from external impacts and injuries.
    • The internal structure of the kidneys consists of-
      • The renal pelvis, a funnel-shaped area where the ureter begins.
      • The major calyces (singular: calyx) and minor calyces are hollow spaces within the renal pelvis.
      • Each kidney contains approximately 1 million tiny tube-like structures called nephrons.
      • The nephron is known as the functional unit of the kidney.
  • NEPHRONS-
    • Each nephrons have mainly 2 parts-
    • 1. Malpighian Body-
      • Also called Renal Corpuscle.
      • The bowman ‘s capsule and glomerulus are collectively known as malpighian body.
      • They filter large solutes from blood and dilever small solutes to the renal tubule for modification.
      • Glomerulus- It is a network of thin walled capillaries which formed by branching of afferent arteriole.
      • The afferent arteriole is a well branch of renal artery and it is short and wide which supply blood to the glomerulus.
      • The efferent arteriole is narrow(or thin) and long which carry blood form glomerulus. (the different between afferent and efferent arteriole in questions section or below there).
      • Bowmans ‘s Capsule-
        • It is also called glomerular capsule.
        • It is di-walled, cup-like structure which surround by glomerulus.
        • The outer parietal wall of glomerulus is made from flattened cells while inner visceral wall is made from a special flattened cell called podicytes.
    • 2. Renal Tubules-
      • It connects to Bowman’s capsule.
      • It has long, thin tubes with three sections.
        • (i). Proximal Convoluted Tubules (PCT) – Located in the kidney, it forms a tightly coiled structure and is found in the outer part of the kidney.

Urine Formation

1. Steps in Urine Formation

  1. Glomerular Filtration:
    • Blood is filtered through glomerulus and Bowman’s capsule.
    • Filtration rate: ~125 mL/min or 180 liters/day.
    • Proteins are not filtered due to size.
  2. Reabsorption:
    • Most filtrate (99%) is reabsorbed in the renal tubules.
    • Active transport for glucose, amino acids, Na⁺; passive for water.
  3. Tubular Secretion:
    • Adds H⁺, K⁺, and NH₃ to the filtrate to maintain pH and ion balance.

2. Functional Roles of Tubule Segments

  • PCT: Reabsorbs 70-80% water, nutrients, and electrolytes. Regulates pH and ions.
  • Henle’s Loop:
    • Descending limb: Permeable to water (concentrates filtrate).
    • Ascending limb: Transports ions (dilutes filtrate).
  • DCT: Conditional reabsorption of Na⁺ and water. Maintains pH and ion balance.
  • Collecting Duct: Reabsorbs water, allows urea passage for medullary osmolarity, and regulates pH.

Key Processes and Terms for Competitive Exams

  • Ultrafiltration: Fine filtration through glomerulus and Bowman’s capsule.
  • GFR (Glomerular Filtration Rate): 125 mL/min in healthy individuals.
  • Juxtaglomerular Apparatus (JGA): Regulates GFR via renin release.
  • Vasa Recta: Maintains osmolarity; prominent in juxtamedullary nephrons.

Additional Information for Competitive Exams

  • Kidney Disorders:
    • Renal failure, kidney stones, glomerulonephritis.
  • Osmoregulation: Balancing water and ions (important in aquatic and desert animals).
  • Ammonia Toxicity: Requires large water volumes for excretion (adaptive constraint for terrestrial life).

Mechanism of Concentration of Filtrate

  1. Concentrated Urine Formation:
    • Mammals can produce concentrated urine.
    • The Henle’s loop and vasa recta are crucial for this process.
  2. Counter Current Mechanism:
    • Henle’s loop: Filtrate flows in opposite directions in its ascending and descending limbs.
    • Vasa recta: Blood flows in opposite directions in its two limbs.
    • This counter-current system maintains a gradient in the medullary interstitium, increasing osmolarity from 300 mOsmol/L in the cortex to 1200 mOsmol/L in the medulla.
  3. Role of NaCl and Urea:
    • NaCl: Transported by the ascending limb of Henle’s loop; exchanged and recycled in the vasa recta.
    • Urea: Recycled between the collecting tubule and ascending limb of Henle’s loop, enhancing the osmolar gradient.
  4. Outcome:
    • The concentration gradient facilitates water reabsorption from the collecting tubule.
    • Human kidneys can concentrate urine up to four times the initial filtrate.

Regulation of Kidney Function

  1. Hormonal Feedback Mechanisms:
    • Involves the hypothalamus, juxtaglomerular apparatus (JGA), and the heart.
  2. Role of ADH (Antidiuretic Hormone):
    • Activated by osmoreceptors detecting fluid loss or blood volume changes.
    • Released by the hypothalamus to reabsorb water in later nephron segments, reducing water loss (diuresis).
    • ADH also increases blood pressure by constricting blood vessels.
  3. Renin-Angiotensin Mechanism:
    • Low blood pressure/GFR: Activates JGA to release renin.
    • Renin converts angiotensinogen to angiotensin II (a vasoconstrictor), increasing blood pressure and stimulating aldosterone release.
    • Aldosterone: Promotes Na⁺ and water reabsorption, raising blood pressure and GFR.
  4. Atrial Natriuretic Factor (ANF):
    • Released from the heart during high blood flow.
    • Causes vasodilation, lowering blood pressure and countering the renin-angiotensin mechanism.

Micturition (Urine Excretion)

  1. Urine Storage and Release:
    • Stored in the bladder until the CNS signals for its release.
    • Stretch receptors in the bladder wall activate the micturition reflex.
  2. Characteristics of Urine:
    • Light yellow, slightly acidic (pH ~6.0).
    • Adults excrete 1–1.5 liters/day with ~25–30g urea.
  3. Clinical Significance:
    • Conditions like glycosuria (glucose in urine) and ketonuria (ketones in urine) indicate diabetes mellitus.

Other Excretory Organs

  1. Lungs:
    • Remove CO₂ (~200 mL/min) and water vapor.
  2. Liver:
    • Eliminates bile pigments, cholesterol, and waste products.
  3. Skin:
    • Sweat glands: Excrete NaCl, urea, and lactic acid.
    • Sebaceous glands: Release sterols and waxes in sebum.
  4. Saliva:
    • May excrete small amounts of nitrogenous wastes.

Disorders of the Excretory System

  1. Uremia:
    • High urea levels due to kidney failure.
    • Managed via hemodialysis or kidney transplantation.
  2. Renal Calculi:
    • Formation of kidney stones (crystallized salts).
  3. Glomerulonephritis:
    • Inflammation of kidney glomeruli.

Key Points for Competitive Exams

  • Filtration Rate (GFR): ~125 mL/min.
  • Main Reabsorption Site: Proximal Convoluted Tubule (PCT).
  • Osmolar Gradient in Kidney: 300–1200 mOsmol/L.
  • ADH Function: Reduces diuresis, increases water reabsorption.
  • Renin-Angiotensin System: Regulates blood pressure and filtration.
  • Sweat vs. Sebum: Sweat aids cooling and waste excretion; sebum protects skin.
  • Hemodialysis: Artificial filtration using a dialyzer.

These all are the notes of chapter 16. And important questions are below HERE. *#THANKS FOR VISITING, VISIT AGAIN#* 😊

1. What are the different types of excretory products produced by animals?

Answer:
Animals produce several waste products like ammonia, urea, uric acid, carbon dioxide, water, and ions (like sodium, potassium, chloride, phosphate, sulfate). These waste products are excreted from the body to maintain balance or homeostasis, keeping the body in a stable condition.

2. What are nitrogenous wastes, and how are they different?

Answer:
Nitrogenous wastes are the by-products of protein metabolism. The three main types are:

  • Ammonia: Highly toxic and needs a lot of water to be removed from the body.
  • Urea: Less toxic than ammonia and requires less water to be excreted.
  • Uric Acid: Least toxic and is excreted with minimal water loss, making it suitable for animals living in dry environments.

3. What are the different modes of excretion?

Answer:

  • Ammonotelism: Animals like bony fish and aquatic amphibians excrete ammonia, which diffuses through their skin or gills.
  • Ureotelism: Animals like mammals and terrestrial amphibians convert ammonia to urea in their liver, which is then filtered by the kidneys.
  • Uricotelism: Animals like birds, reptiles, and insects excrete uric acid as a paste or solid, conserving water.

4. What are the excretory structures in different animals?

Answer:

  • Invertebrates:
    • Protonephridia (Flame Cells): Found in flatworms and help in osmoregulation.
    • Nephridia: Found in earthworms, remove nitrogenous wastes and balance fluids.
    • Malpighian Tubules: Found in insects, excrete waste and regulate water.
    • Antennal Glands: Found in crustaceans like prawns.
  • Vertebrates:
    • Kidneys: The primary excretory organs in vertebrates, with complex structures to filter and eliminate waste.

5. What are the components of the human excretory system?

Answer:
The human excretory system consists of:

  • Kidneys: Filter blood and form urine.
  • Ureters: Carry urine from the kidneys to the bladder.
  • Urinary Bladder: Stores urine.
  • Urethra: Carries urine out of the body.

6. What are nephrons, and what do they do?

Answer:
Each kidney contains about one million nephrons, which are the functional units of the kidney. A nephron consists of:

  • Glomerulus: A network of capillaries that filters blood.
  • Renal Tubule: Where the filtrate is processed (contains Bowman’s capsule, PCT, Henle’s loop, DCT, and collecting duct).

7. How is urine formed in the kidneys?

Answer:
Urine formation involves three main steps:

  1. Glomerular Filtration: Blood is filtered in the glomerulus, and substances like water, glucose, and urea enter the nephron.
  2. Reabsorption: Most of the water, nutrients, and electrolytes are reabsorbed back into the blood in the renal tubules.
  3. Tubular Secretion: Additional wastes like hydrogen ions, potassium, and ammonia are added to the filtrate.

8. What is the role of different parts of the nephron in urine formation?

Answer:

  • Proximal Convoluted Tubule (PCT): Reabsorbs most of the water, nutrients, and electrolytes.
  • Henle’s Loop: The descending limb concentrates the filtrate, while the ascending limb dilutes it by transporting ions.
  • Distal Convoluted Tubule (DCT): Reabsorbs sodium and water, and helps maintain pH and ion balance.
  • Collecting Duct: Reabsorbs more water, regulates pH, and helps in water conservation.

9. How does the kidney concentrate urine?

Answer:
Kidneys use a countercurrent mechanism in the Henle’s Loop and vasa recta to create a concentration gradient, which helps in reabsorbing water from the filtrate. This process allows mammals to produce concentrated urine and conserve water, especially in dry conditions.

10. What are the hormonal feedback mechanisms that regulate kidney function?

Answer:

  • Antidiuretic Hormone (ADH): Increases water reabsorption in the kidneys, reducing urine output when the body is dehydrated.
  • Renin-Angiotensin Mechanism: Increases blood pressure by releasing renin, which helps in sodium and water reabsorption.
  • Aldosterone: Promotes reabsorption of sodium and water, increasing blood pressure.
  • Atrial Natriuretic Factor (ANF): Lowers blood pressure by reducing sodium and water reabsorption.

11. What is micturition?

Answer:
Micturition is the process of urine excretion. Urine is stored in the bladder and released when the brain sends a signal, triggering the micturition reflex. This process helps regulate the body’s waste removal.

12. What are some disorders of the excretory system?

Answer:

  • Uremia: High levels of urea in the blood due to kidney failure.
  • Renal Calculi (Kidney Stones): Formation of solid crystals in the kidneys.
  • Glomerulonephritis: Inflammation of the kidney’s glomeruli, leading to kidney damage.

13. What is the function of other excretory organs like the lungs, liver, and skin?

Answer:

  • Lungs: Remove carbon dioxide and water vapor.
  • Liver: Eliminates bile pigments and waste products.
  • Skin: Sweat glands excrete water, salts, and urea, while sebaceous glands secrete oils to protect the skin.

14. What is hemodialysis?

Answer:
Hemodialysis is a medical procedure used for kidney failure, where a machine filters waste and excess fluid from the blood, acting as an artificial kidney.