Detailed notes 1-Chapter-16-Excretory Products And Their Elimination

EXCRETORY PRODUCTS AND THEIR ELIMINATION

Introduction

1. Types of Nitrogenous Wastes:

   • Ammonia: Highly toxic and soluble, requires large amounts of water for elimination. Excreted by diffusion across body surfaces or through gill surfaces in aquatic animals.
   • Urea: Less toxic than ammonia, produced in the liver of ureotelic animals (e.g., mammals) and excreted primarily by the kidneys.
   • Uric Acid: Least toxic form, excreted by uricotelic animals (e.g., reptiles, birds, insects) with minimal water loss.

2. Types of Excretory Adaptations:

   • Ammonotelism: Excretion of ammonia, common in aquatic organisms like bony fishes, amphibians, and aquatic insects.
   • Ureotelism: Excretion of urea, prevalent in mammals, some amphibians, and marine fishes, allowing for conservation of water.
   • Uricotelism: Excretion of uric acid, characteristic of reptiles, birds, land snails, and insects, minimizing water loss.

3. Excretory Structures:

   • Protonephridia: Found in various invertebrates like flatworms, rotifers, some annelids, and cephalochordates. Primarily involved in osmoregulation.
   • Nephridia: Tubular excretory structures in earthworms and other annelids, responsible for removing nitrogenous wastes and maintaining fluid and ionic balance.
   • Malpighian Tubules: Excretory organs in insects, including cockroaches, aiding in nitrogenous waste removal and osmoregulation.
   • Antennal Glands (Green Glands): Perform excretory functions in crustaceans such as prawns.

16.1 HUMAN EXCRETORY SYSTEM

1. Kidneys:

   • Paired organs located in the abdominal cavity between the last thoracic and third lumbar vertebrae.
   • Bean-shaped with dimensions of approximately 10-12 cm in length, 5-7 cm in width, and 2-3 cm in thickness.
   • Average weight ranges from 120 to 170 grams.
   • Notch called the hilum on the inner concave surface through which ureter, blood vessels, and nerves enter.

2. Renal Pelvis and Calyces:

   • Renal pelvis is a broad, funnel-shaped space towards the center of the inner concave surface of the kidney.
   • Contains projections called calyces.
   • Functions as the collecting system for urine formed in the kidney.

3. Outer Cortex and Inner Medulla:

   • Two distinct zones within the kidney.
   • Cortex is the outer layer, and the medulla is the inner layer.
   • Medulla is further divided into conical masses called medullary pyramids, which project into the calyces.

4. Capsule:

   • Outer layer covering the kidney.
   • Provides protection and structural support to the organ.

5. Ureters:

   • Paired muscular tubes connecting each kidney to the urinary bladder.
   • Transport urine from the kidneys to the bladder through peristaltic contractions.

6. Urinary Bladder:

   • Hollow, muscular organ located in the pelvic cavity.
   • Functions as a reservoir for urine until it is expelled from the body.

7. Urethra:

   • Tube connecting the urinary bladder to the external environment.
   • In males, also serves as the passage for semen during ejaculation.

1. Medullary Pyramids and Columns of Bertini:

   • Medullary pyramids are conical structures within the renal medulla.
   • They extend from the renal cortex towards the renal pelvis.
   • Renal columns, also known as Columns of Bertini, are extensions of cortical tissue that project into the medulla between the pyramids.

2. Nephrons:

   • Each kidney contains nearly one million nephrons, which are the functional units responsible for urine formation.
   • Nephrons consist of two main parts: the glomerulus and the renal tubule.

3. Glomerulus and Bowman’s Capsule:

   • Glomerulus is a tuft of capillaries formed by the afferent arteriole, a branch of the renal artery.
   • Bowman’s capsule is a double-walled cup-like structure that encloses the glomerulus.
   • Together, the glomerulus and Bowman’s capsule form the malpighian body or renal corpuscle.

4. Renal Tubule:

   • The renal tubule begins with Bowman’s capsule and continues as the proximal convoluted tubule (PCT).
   • Next is the hairpin-shaped Henle’s loop, which consists of a descending limb and an ascending limb.
   • The ascending limb leads to the distal convoluted tubule (DCT).
   • DCTs of multiple nephrons converge into a straight tube called the collecting duct.

5. Cortical and Juxta Medullary Nephrons:

   • Nephrons are categorized based on the length of their loops of Henle.
   • Cortical nephrons have short loops that extend minimally into the medulla.
   • Juxta medullary nephrons have long loops that extend deep into the medulla.

6. Peritubular Capillaries and Vasa Recta:

   • The efferent arteriole emerging from the glomerulus forms a capillary network around the renal tubule called the peritubular capillaries.
   • In juxta medullary nephrons, a specialized capillary network known as the vasa recta runs parallel to the Henle’s loop.
   • Vasa recta is either absent or highly reduced in cortical nephrons.

1. Glomerular Filtration:

   • Occurs in the glomerulus, where blood is filtered through three layers: endothelium of glomerular blood vessels, epithelium of Bowman’s capsule, and a basement membrane.
   • Podocytes, specialized epithelial cells of Bowman’s capsule, have intricate arrangements that leave minute spaces called filtration slits or slit pores.
   • Almost all constituents of plasma except proteins pass into the Bowman’s capsule, resulting in ultrafiltration.
   • Glomerular filtration rate (GFR) measures the amount of filtrate formed per minute, typically around 125 ml/minute in a healthy individual, equivalent to approximately 180 liters per day.

2. Regulation of GFR:

   • Juxta glomerular apparatus (JGA) regulates GFR. It consists of cellular modifications in the distal convoluted tubule and the afferent arteriole.
   • A decrease in GFR can activate JGA cells to release renin, stimulating glomerular blood flow and restoring GFR to normal levels.

3. Reabsorption:

   • Nearly 99% of the filtrate (180 liters per day) is reabsorbed by renal tubules.
   • Tubular epithelial cells perform reabsorption through active or passive mechanisms.
   • Substances like glucose, amino acids, and sodium are actively reabsorbed, while nitrogenous wastes are absorbed passively.
   • Water reabsorption also occurs passively in the initial segments of the nephron.

4. Secretion:

   • Tubular cells secrete substances like hydrogen ions (H+), potassium ions (K+), and ammonia into the filtrate.
   • Tubular secretion plays a crucial role in maintaining the ionic and acid-base balance of body fluids.

16.3 FUNCTION OF THE TUBULES

1. Proximal Convoluted Tubule (PCT):

   • Lined by simple cuboidal brush border epithelium, increasing surface area for reabsorption.
   • Reabsorbs nearly all essential nutrients and 70-80% of electrolytes and water.
   • Helps maintain pH and ionic balance by selectively secreting hydrogen ions and ammonia into the filtrate and absorbing bicarbonate ions (HCO3–).

2. Henle’s Loop:

   • Ascending limb: Minimal reabsorption, but crucial for maintaining high osmolarity of medullary interstitial fluid.
   • Descending limb: Permeable to water but almost impermeable to electrolytes, concentrating the filtrate as it descends.
   • Ascending limb: Impermeable to water but allows active or passive transport of electrolytes, resulting in dilution of the filtrate as it ascends.

3. Distal Convoluted Tubule (DCT):

   • Conditionally reabsorbs sodium (Na+) and water.
   • Capable of reabsorbing bicarbonate ions (HCO3–) and selectively secreting hydrogen and potassium ions and ammonia (NH3) to maintain pH and sodium-potassium balance in blood.

4. Collecting Duct:

   • Extends from the cortex to the inner medulla, allowing significant water reabsorption to produce concentrated urine.
   • Permits the passage of small amounts of urea into the medullary interstitium to maintain osmolarity.
   • Plays a role in maintaining pH and ionic balance by selectively secreting hydrogen (H+) and potassium (K+) ions.

16.4 MECHANISM OF CONCENTRATION OF THE FILTRATE

• Counter Current System:

  • Filtrate flows in opposite directions in the two limbs of Henle’s loop, creating a counter current.
  • Blood flow in the vasa recta also follows a counter current pattern.

• Proximity and Interaction:

  • The close proximity between Henle’s loop and vasa recta allows for efficient exchange of substances.
  • Counter current flow in both structures facilitates the maintenance of an increasing osmolarity gradient from the cortex to the inner medulla.

• Osmolarity Gradient:

  • The osmolarity gradient increases from approximately 300 mOsmolL–1 in the cortex to about 1200 mOsmolL–1 in the inner medulla.
  • This gradient is primarily established by the transport of NaCl and urea.

• Transport of Substances:

  • NaCl is transported by the ascending limb of Henle’s loop, exchanged with the descending limb of vasa recta, and returned to the interstitium by the ascending portion of vasa recta.
  • Small amounts of urea enter the thin segment of the ascending limb of Henle’s loop and are transported back to the interstitium by the collecting tubule.

• Counter Current Mechanism:

  • This coordinated transport of substances facilitated by the arrangement of Henle’s loop and vasa recta is termed the counter current mechanism.
  • It helps to maintain a concentration gradient in the medullary interstitium.

• Concentration of Urine:

  • The presence of the interstitial gradient allows for easy passage of water from the collecting tubule.
  • This facilitates the concentration of the filtrate (urine), enabling the human kidneys to produce urine nearly four times more concentrated than the initial filtrate formed.