Urinary system

• Urinary System

  Consists of: Paired kidneys, Paired ureters, Urinary bladder, Urethra
• Functions of the urinary system

  • Paired kidneys: conserve fluid & electrolytes, remove metabolic waste; endocrine organ
  • Paired ureters: transport urine to the bladder
  • Urinary bladder: temporary storage
  • Urethra: transport urine to the exterior
• Kidney - general structure

  • Renal capsule
  • Hilum & renal pelvis,
  • Parenchyma (Cortex = renal corpuscles + associated tubules & medullary rays; Medulla = renal columns, selected tubules arranged in pyramids)
  • Interstitial cells,
  • Lobes & lobules
• Nephron

  Structural & functional unit that consists of:

  • Renal corpuscle: glomerulus surrounded by Bowman's capsule (creates ultrafiltrate)
  • Proximal thick segment: proximal convoluted & straight tubule (thick segment loop of Henle)
  • Thin segment: thin part of loop of Henle
  • Distal thick segment: distal straight & convoluted tubule

  
• Renal corpuscle - filtration

  • Consist of glomerular capillaries + Bowman’s capsule
  • Filtration apparatus (glomerular filtration barrier): endothelium of the glomerular capillaries, glomerular basement membrane & visceral layer of Bowman's capsule (Podocytes)
• Ultrafiltrate

  Urinary space is the receptacle for the ultrafiltrate,

  • It is continuous with the proximal convoluted tubule
  • Encased by the parietal layer of Bowman’s capsule
• Clinical correlation of Renal corpuscle - Filtration apparatus

  • Albuminuria: Presence of significant amounts of albumin in the urine which indicates damage to the glomerular basement membrane
  • Diabetic nephropathy: Number of anionic sites in the glomerular basement membrane can be significantly reduced
• Renal corpuscle - poles

  • Vascular pole: afferent & efferent arterioles, Mesangial cells
  • Urinary pole: urinary space continuous with proximal convoluted tubule
• Juxtaglomerular apparatus

  • Juxtaglomerular cells: modified cells of the afferent arteriole that contain secretory granules
  • Macula densa: terminal portion of distal straight tubule; monitor Na+ concentration and regulate glomerular filtration rate & release of renin
  • Extraglomerular mesangial cells: contribute to regulation of glomerular distension
• Proximal tubule

  >Proximal convoluted tubule

  • Initial & major site of reabsorption,
  • Specialised epithelial cells: Cuboidal cells with a brush border, junctional complex with tight junctions & zonula adherens, lateral membrane folds (plicae) & basal striations with mitochondria
• Thin segment of the Loop of Henle

  Varies depending on the location of the nephron. Four types of epithelial cells:

  • Type I & Type IV (Simple squamous epithelium),
  • Type II (Low cuboidal epithelium with blunt microvilli),
  • Type III (Simple squamous epithelium with few microvilli)
• Distal tubule

  • Distal straight tubule: thick ascending part of the loop of Henle),
  • Distal convoluted tubule: cuboidal cells with lack of brush border & mitochondria found basal striations)
• Collecting tubules and Collecting ducts

  Squamous to cuboidal to columnar epithelium as ducts increase in size, can be distinguished from PT/DT by clear cell boundaries

  Two cell types:

  • Light cells: pale-staining with basal infoldings. Single cilium, few short microvilli.
  • Dark cells: few, with cytoplasmic apical folds and numerous mitochondria
• Excretory passages

  Collecting ducts exit at the area cribrosa

  E.g Urerthra

  1. Male ~20cm in 3 segments:

  • Prostatic urethra lined with transitional epithelium

  • Membranous urethra lined with stratified or pseudostratified columnar epithelium

  • Penile urethra lined with pseudostratified that becomes stratified squamous epithelium

  
  

  2. Female ~3-5cm: lined with pseudostratified epithelium that becomes stratified squamous epithelium
• Renal corpuscle - filtration apparatus

  >Endothelium of the glomerular capillaries

  • Fenestrations without diaphragms

  >Glomerular basement membrane

  • Joint product of the endothelium & visceral layer of Bowman’s capsule
  • Type IV collagen, laminin, proteoglycans, glycoproteins …

  >Visceral layer of Bowman’s capsule

  • Cells are termed podocytes
  • Podocytes extend processes around capillaries with secondary processes termed pedicels
• Proximal Tubule

  >Proximal straight tubule

  • Similar, shorter, less well developed brush border
  • Fewer lateral and basolateral processes
  • Not as specialised for reabsorption