1. Nitrogenous Wastes Accumulation:

• Animals accumulate substances like ammonia, urea, uric acid, carbon dioxide, water, and ions (Na+, K⁺, Cl^–, phosphate, and sulfate) through metabolic activities or excess ingestion.

2. Major Forms of Nitrogenous Wastes:

• Ammonia: Highly toxic, and requires large water amounts for elimination.
• Urea: Less toxic, excreted by mammals, terrestrial amphibians, and marine fishes.
• Uric Acid: Least toxic, excreted by reptiles, birds, land snails, and insects.

3. Ammonotelism:

• Definition:
  • Excretion of ammonia, primarily in aquatic organisms.
• Excretion Process:
  • Diffusion across body surfaces or gill surfaces in fish.
  • Limited role of kidneys.

4. Ureotelic Animals:

• Examples:
  • Mammals, some terrestrial amphibians, and marine fishes.
• Excretion Process:
  • Ammonia is converted to urea in the liver.
  • Urea is released into the blood, filtered, and excreted by the kidneys.
  • Some urea retention for osmolarity maintenance.

5. Uricotelic Animals:

• Examples:
  • Reptiles, birds, land snails, insects.
• Excretion Process:
  • Excretion of nitrogenous wastes as uric acid in pellet or paste form.
  • Minimal water loss.

6. Excretory Structures in Animal Kingdom:

• Protonephridia or Flame Cells:
  • Found in Platyhelminthes (e.g., Planaria), rotifers, some annelids, and cephalochordates.
  • Primarily for ionic and fluid volume regulation (osmoregulation).
• Nephridia:
  • Present in earthworms and other annelids.
  • Involved in nitrogenous waste removal and maintaining fluid and ionic balance.
• Malpighian Tubules:
  • Excretory structures in insects, including cockroaches.
  • Assist in nitrogenous waste removal and osmoregulation.
• Antennal Glands or Green Glands:
  • Perform excretory function in crustaceans like prawns.

Human Excretory System

1. Components of the Human Excretory System:

• Kidneys:
  • Bean-shaped organs are located between the last thoracic and third lumbar vertebrae.
  • Measure 10-12 cm in length, 5-7 cm in width, and 2-3 cm in thickness.
  • Average weight: 120-170 g.
  • Each kidney has a hilum, a renal pelvis, and an outer capsule.
  • Divided into cortex (outer) and medulla (inner).
• Ureters:
  • A pair of tubes connecting kidneys to the urinary bladder.
• Urinary Bladder:
  • Storage organ for urine.
• Urethra:
  • The tube connecting the bladder to the external environment for urine expulsion.

2. Kidney Structure:

• Hilum:
  • The notch on the inner concave surface through which the ureter, blood vessels, and nerves enter.
• Renal Pelvis and Calyces:
  • Renal pelvis at the center, with calyces projecting from it.
• Capsule:
  • Tough outer layer of the kidney.
• Cortex and Medulla:
  • Outer and inner zones respectively.
  • The medulla has conical masses (medullary pyramids) extending into calyces.
  • Renal columns (Columns of Bertini) between pyramids.
• Nephrons:
  • Nearly one million complex tubular structures, the functional units.
  • Consists of glomerulus and renal tubule.
• Renal Corpuscle (Malpighian Body):
  • Includes glomerulus and Bowman’s capsule.
• Renal Tubule:
  • Begins with Bowman’s capsule and continues as the proximal convoluted tubule (PCT).
  • Henle’s loop (descending and ascending limbs) and distal convoluted tubule (DCT) follow.
  • DCTs of many nephrons open into a collecting duct, which converges into the renal pelvis.
• Types of Nephrons:
  • Cortical Nephrons:
    • The loop of Henle is short, extending minimally into the medulla.
  • Juxtamedullary Nephrons:
    • The loop of Henle is long, extending deep into the medulla.

3. Blood Circulation in Nephrons:

• Glomerulus:
  • A tuft of capillaries formed by the afferent arteriole.
  • Blood is carried away by the efferent arteriole.
• Peritubular Capillaries and Vasa Recta:
  • Capillary network around the renal tubule.
  • Vasa recta (U-shaped) runs parallel to Henle’s loop.
  • Absent or reduced in cortical nephrons.

Urine Formation in the Nephron

1. Processes Involved:

• Glomerular Filtration:
  • Occurs in the glomerulus.
  • Filtration of blood through the endothelium of glomerular blood vessels, Bowman’s capsule epithelium, and a basement membrane.
  • Podocytes in Bowman’s capsule leave filtration slits.
  • Ultrafiltration process, allowing most plasma constituents except proteins to pass into Bowman’s capsule.
  • Glomerular Filtration Rate (GFR): Approximately 125 ml/minute, 180 liters per day.
  • Juxta Glomerular Apparatus (JGA) regulates GFR by releasing renin.
• Reabsorption:
  • Nearly 99% of the filtrate (180 liters per day) is reabsorbed.
  • Tubular epithelial cells in nephron segments perform active or passive reabsorption.
  • Active reabsorption for glucose, amino acids, Na⁺, etc.
  • Passive reabsorption for nitrogenous wastes and initial segments of water.
  • Maintains essential substances in the body.
• Secretion:
  • Tubular cells secrete H⁺, K⁺, and ammonia into the filtrate.
  • Important for ionic and acid-base balance in body fluids.
  • Part of the urine formation process.

2. Regulation of Glomerular Filtration Rate:

• Juxta Glomerular Apparatus (JGA):
  • The sensitive region is formed by cellular modifications in the distal convoluted tubule and afferent arteriole.
  • Activated by a fall in GFR, releases renin.
  • Renin stimulates glomerular blood flow, restoring GFR to normal.

3. Comparison of Filtrate and Urine:

• Filtrate formed per day: 180 liters.
• Urine released: 1.5 liters.
• Signifies approximately 99% reabsorption of the filtrate by renal tubules.

Function of Renal Tubules in Urine Formation

1. Proximal Convoluted Tubule (PCT):

• Lined by simple cuboidal brush border epithelium.
• Reabsorption:
  • Absorbs nearly all essential nutrients.
  • Reabsorbs 70-80% of electrolytes and water.
• Maintains Balance:
  • Helps maintain pH and ionic balance.
  • Selectively secretes hydrogen ions and ammonia into the filtrate.
  • Absorbs HCO₃^–.

2. Henle’s Loop:

• Role in Osmolarity:
  • Minimum reabsorption in the ascending limb.
  • Maintains high osmolarity of medullary interstitial fluid.
• Water and Electrolyte Transport:
  • Descending limb: Permeable to water, almost impermeable to electrolytes.
  • Ascending limb: Impermeable to water, allows electrolyte transport.
  • Dilute the filtrate as it moves upward.

3. Distal Convoluted Tubule (DCT):

• Conditional Reabsorption:
  • Reabsorbs Na⁺ and water conditionally.
• Selective Secretion:
  • Secretes hydrogen ions, potassium ions, and NH₃.
  • Maintains pH and sodium-potassium balance in blood.
  • Reabsorbs HCO₃^–.

4. Collecting Duct:

• Water Reabsorption:
  • Extends from cortex to medulla.
  • Reabsorbs large amounts of water, producing concentrated urine.
• Urea Passage:
  • Allows passage of small amounts of urea into the medullary interstitium.
  • Maintains osmolarity.
• Ionic Balance:
  • Selective secretion of H⁺ and K⁺ ions.
  • Maintains pH and ionic balance in the blood.

Mechanism of Filtrate Concentration in Mammals

1. Counter-Current Mechanism in Henle’s Loop and Vasa Recta:

• Opposite Flow:
  • Filtrate in the two limbs of Henle’s loop flows in opposite directions.
  • Blood flow in the two limbs of the vasa recta also follows a counter-current pattern.
• Proximity and Exchange:
  • Proximity between Henle’s loop and vasa recta.
  • The counter-current exchange maintains increasing osmolarity from the cortex to the inner medulla.
• Osmolarity Gradient:
  • The gradient increases from 300 mOsmolL–1 in the cortex to about 1200 mOsmolL–1 in the inner medulla.
  • Mainly caused by NaCl and urea.

2. Role of NaCl and Urea:

• NaCl Transport:
  • The ascending limb of Henle’s loop transports NaCl.
  • Exchanged with the descending limb of the vasa recta.
  • The ascending portion of the vasa recta returns NaCl to the interstitium.
• Urea Transport:
  • Small amounts of urea enter the ascending limb of Henle’s loop.
  • Transported back to the interstitium by the collecting tubule.

3. Counter-Current Mechanism:

• Facilitated Transport:
  • Special arrangement of Henle’s loop and vasa recta.
  • The counter-current mechanism maintains a concentration gradient in the medullary interstitium.
• Interstitial Gradient:
  • Facilitates easy water passage from the collecting tubule.
  • Concentrates the filtrate (urine).
• Concentration Capability:
  • Human kidneys can produce urine nearly four times more concentrated than the initial filtrate.

Regulation of Kidney Function

1. Monitoring and Regulation:
   • Monitored by hormonal feedback mechanisms.
   • Involves the hypothalamus, juxtaglomerular apparatus (JGA), and to some extent, the heart.
2. Osmoreceptor Activation:
   • Activated by changes in blood volume, body fluid volume, and ionic concentration.
   • Excessive fluid loss activates osmoreceptors.
   • Stimulates the hypothalamus to release antidiuretic hormone (ADH).
   • ADH facilitates water reabsorption, preventing diuresis.
3. Feedback Mechanism:
   • An increase in body fluid volume suppresses osmoreceptors and ADH release.
   • ADH also affects kidney function through vasoconstriction, increasing blood pressure.
   • Elevated blood pressure enhances glomerular blood flow and glomerular filtration rate (GFR).
4. Juxtaglomerular Apparatus (JGA):
   • Monitors glomerular blood flow, blood pressure, and GFR.
   • Low GFR activates JG cells to release renin.
   • Renin converts angiotensinogen to angiotensin I and then to angiotensin II.
5. Renin-Angiotensin Mechanism:
   • Angiotensin II is a vasoconstrictor, increasing glomerular blood pressure and GFR.
   • Activates the adrenal cortex to release aldosterone.
   • Aldosterone promotes Na+ and water reabsorption, further increasing blood pressure and GFR.
6. Atrial Natriuretic Factor (ANF):
   • Released with increased blood flow to the atria of the heart.
   • Causes vasodilation, reducing blood pressure.
   • Acts as a check on the renin-angiotensin mechanism.

Micturition: The Release of Urine

1. Urine Storage:
   • Urine from nephrons is stored in the urinary bladder.
   • Storage continues until a voluntary signal is given by the central nervous system (CNS).
2. Initiation of Micturition:
   • CNS signal triggered by stretching of the bladder due to urine accumulation.
   • Stretch receptors on the bladder walls send signals to the CNS.
3. Motor Messages and Muscle Contraction:
   • CNS transmits motor messages to initiate smooth muscle contraction in the bladder.
   • Simultaneous relaxation of the urethral sphincter is induced.
4. Micturition Reflex:
   • The neural mechanism causes the release of urine.
   • Involves coordination between CNS, bladder stretch receptors, and smooth muscles.
5. Average Urine Production:
   • An adult human excretes 1 to 1.5 liters of urine per day.
   • Urine characteristics: light yellow, slightly acidic (pH-6.0), characteristic odor.
   • Approximately 25-30 gm of urea is excreted daily.
6. Clinical Significance:
   • Urine analysis aids in diagnosing metabolic disorders and kidney malfunctions.
   • The presence of glucose (Glycosuria) and ketone bodies (Ketonuria) in urine may indicate diabetes mellitus.

Role of Other Organs in Excretion

1. Lungs:
   • Remove large amounts of CO2 (approximately 200mL/minute).
   • Eliminate significant quantities of water daily.
2. Liver:
   • Largest gland in the body.
   • Secretes bile containing bilirubin, biliverdin, cholesterol, degraded steroid hormones, vitamins, and drugs.
   • These substances pass out along with digestive wastes.
3. Skin:
   • Sweat Glands:
     • Produce watery fluid containing NaCl, small amounts of urea, lactic acid, etc.
     • The primary function is to facilitate body surface cooling.
     • Contributes to the removal of certain wastes.
   • Sebaceous Glands:
     • Eliminate substances like sterols, hydrocarbons, and waxes through sebum.
     • Sebum provides a protective oily covering for the skin.
4. Overall Contribution:
   • Lungs, liver, and skin collectively aid in the elimination of excretory wastes.
   • Each organ plays a specific role in removing distinct types of waste products.
   • This comprehensive excretory process ensures the elimination of diverse metabolic by-products from the body.

Disorders of the Excretory System

1. Uremia and Hemodialysis:
   • Malfunctioning kidneys lead to urea accumulation in the blood, termed uremia.
   • Uremia may result in kidney failure.
   • Hemodialysis Process:
     • Blood is drained from an artery into an artificial kidney dialyzing unit.
     • Dialysing fluid with plasma-like composition except nitrogenous wastes surrounds a cellophane tube.
     • Cellophane membrane allows the movement of substances based on concentration gradients.
     • Nitrogenous wastes move out into the dialyzing fluid, clearing the blood.
     • After adding anti-heparin, cleared blood is pumped back into the body through a vein.
     • Hemodialysis is crucial for uremic patients globally.
2. Kidney Transplantation:
   • The ultimate method for correcting acute renal failures.
   • Functioning kidney transplanted from a donor, preferably a close relative, to minimize rejection chances.
   • Modern clinical procedures enhance the success rate of this intricate technique.
3. Renal Calculi:
   • Formation of insoluble crystallized salt masses (oxalates, etc.) within the kidney.
   • Also known as kidney stones.
4. Glomerulonephritis:
   • Inflammation of the kidney’s glomeruli.
5. Overall Impact:
   • Disorders like uremia, renal failure, kidney stones, and glomerulonephritis highlight the critical importance of kidney health.
   • Medical interventions such as hemodialysis and kidney transplantation provide vital solutions to these excretory system disorders.