Kidney Anatomy: Distal and Collecting Tubules

Distal Convoluted Tubule and Collecting Tubule

• The distal convoluted tubule transitions gradually into the cortical collecting duct, and the connecting tubule is not clearly defined due to cell intermingling
• Two distinct cell types are present in the collecting tubules: principal cells (majority) and intercalated cells (minority)
• Principal cells are responsible for water reabsorption and are responsive to antidiuretic hormone (ADH)
  • They have fewer and smaller microvilli
  • They express aquaporin-2 channels
  • They have channels and transporters for sodium and water reabsorption
• Intercalated cells regulate acid-base balance by secreting H+ and K+
  • They have more and larger microvilli
  • They do not respond to ADH
  • They do not express aquaporin-2 channels
  • They have channels and transporters for hydrogen ion and potassium secretion

Renal Papilla

• The renal papilla is the site where urine in the collecting ducts empties into the renal pelvis
• The renal pelvis is the initial dilated portion of the ureter

Urinary Bladder

• The urinary bladder is a musculo-membranous organ with varying size and shape depending on urine content
• The bladder has three main parts:
  • Apex (cranial)
  • Body
  • Neck (continuous with urethra)
• The median and lateral vesical ligaments attach the urinary bladder to the pelvic cavity
  • The median ligament contains the urachus in the fetus, and collapses at birth
  • The lateral ligaments contain the umbilical arteries in the fetus, and collapse at birth to form the round ligaments

Urinary Bladder Histology

• The bladder wall has three layers:
  • Mucosa: lined by transitional epithelium with three cell layers
    • Cuboidal basal cells
    • Intermediate cells
    • Superficial cells
  • Muscularis: three layers with connective tissue separating muscle fiber bundles
    • Innermost and outermost layers: longitudinal muscle fibers
    • Intermediate layer: circular muscle fibers
  • Serosa: formed by the visceral peritoneum

Urethra

• Glomerulus Composition:
  • Glomerular Endothelial Cells:
    • Fenestrated capillaries with large, flat cells and large round fenestrations
  • Podocytes (Glomerular Epithelial Cells):
    • Cover the outer surface of the glomerular basement membrane
    • Have pedicels (finger-like projections) that form filtration slits
  • Intraglomerular Mesangial Cells:
    • Contractile cells located in the glomerulus center
    • Support glomerular capillaries and regulate blood flow
  • Parietal Epithelial Cells:
    • Line the outer layer of Bowman's capsule
    • Continuous with the proximal convoluted tubule

Filtration Barrier

• The filtration barrier consists of:
  • Fenestrated endothelium
  • Glomerular basement membrane
  • Glomerular visceral epithelium (podocyte pedicels)

Juxtaglomerular Apparatus

• Located at the vascular pole of the glomerulus
• Has two components:
  • Vascular Component:
    • Juxtaglomerular cells in the afferent arteriole terminal portion
    • Modified smooth muscle cells that release renin
    • Extend processes into the efferent arteriole initial portion
  • Tubular Component:
    • Macula densa cells in the distal convoluted tubule
    • Specialized epithelial cells that sense sodium concentration in the tubular fluid
  • Function: regulate blood pressure and filtration rate

This quiz covers the structure and function of the distal convoluted tubule and collecting tubules in the kidney. It highlights the roles of principal and intercalated cells, focusing on their functions in water reabsorption and acid-base balance. Test your knowledge on kidney anatomy and physiology!