ch. 25 - urinary

• Kidneys


  Major excretory organ that maintains the body's internal environment
• Functions of kidneys


  • Excreting metabolic wastes, toxins, drugs
  • Regulating total water volume, total solute concentration in water, ion concentrations in extracellular fluid (ECF)
  • Ensuring long-term acid-base balance
  • Producing erythropoietin and renin
  • Activating vitamin D
  • Carrying out gluconeogenesis, if needed
• Parts of the urinary system

  • Kidneys
  • Ureters
  • Urinary bladder
  • Urethra
• Kidney location and external anatomy


  • Retroperitoneal, in the superior lumbar region
  • Right kidney is lower than left due to liver
  • Adrenal gland sits atop each kidney
  • Convex lateral surface
  • Concave medial surface with vertical renal hilum

  
• Layers surrounding the kidney

  • Renal fascia
  • Perirenal fat capsule
  • Fibrous capsule
• Regions of the internal kidney

  • Renal cortex
  • Renal medulla
  • Renal pelvis

  
• Renal cortex


  • Granular-appearing superficial region
• Renal medulla


  • Deep to cortex, composed of cone-shaped medullary (renal) pyramids
• Renal pelvis


  • Funnel-shaped tube continuous with ureter
• Parts of the renal pelvis

  • Minor calyces
  • Major calyces

  
• Urine flow


  Renal pyramid → minor calyx → major calyx → renal pelvis → ureter → urinary bladder → urethra (prostatic → spongy) in males
• Blood supply to the kidney

  • Renal arteries deliver about one-fourth (1200 ml) of cardiac output to kidneys each minute
  • Arterial flow: renal → segmental → interlobar → arcuate → cortical radiate (interlobular)
  • Venous flow: cortical radiate → arcuate → interlobar → renal veins
• Nerve supply to the kidney

  • Via sympathetic fibers from renal plexus
• Nephrons


  Structural and functional units that form urine
• Parts of a nephron

  • Renal corpuscle
  • Renal tubule
• Renal corpuscle


  • Consists of glomerulus and glomerular (Bowman's) capsule
• Glomerulus


  • Tuft of capillaries composed of fenestrated endothelium
• Glomerular (Bowman's) capsule


  • Cup-shaped, hollow structure surrounding glomerulus
  • Consists of parietal layer (simple squamous epithelium) and visceral layer (branching epithelial podocytes with foot processes)
• Parts of the renal tubule


  • Proximal convoluted tubule
  • Nephron loop
  • Distal convoluted tubule
• Proximal convoluted tubule


  • Cuboidal cells with dense microvilli that form brush border
  • Functions in reabsorption and secretion
  • Confined to renal cortex
• Nephron loop


  • U-shaped structure consisting of descending limb and ascending limb
• Distal convoluted tubule


  • Cuboidal cells with very few microvilli
  • Functions more in secretion than reabsorption
  • Confined to renal cortex
• Collecting ducts


  • Receive filtrate from many nephrons
  • Run through medullary pyramids
  • Consist of principal cells and intercalated cells
• Classes of nephrons


  • Cortical nephrons
  • Juxtamedullary nephrons
• Cortical nephrons


  • Make up 85% of nephrons
  • Almost entirely in cortex
• Juxtamedullary nephrons


  • Long nephron loops deeply invade medulla
  • Ascending limbs have thick and thin segments
  • Important in production of concentrated urine
• Nephron capillary beds


  • Glomerulus
  • Peritubular capillaries (Cortical nephrons)
  • Vasa recta (Juxtamedullary nephrons)
• Glomerular capillaries


  • Specialized for filtration
  • Fed and drained by arterioles
  • Blood pressure in glomerulus is high
• Peritubular capillaries


  • Low-pressure, porous capillaries adapted for absorption of water and solutes
  • Arise from efferent arterioles
  • Cling to adjacent renal tubules in cortex
• Vasa recta


  • Long, thin-walled vessels parallel to long nephron loops of juxtamedullary nephrons
  • Function in formation of concentrated urine
• Juxtaglomerular complex (JGC)


  • Involves modified portions of distal portion of ascending limb of nephron loop and afferent (sometimes efferent) arteriole
  • Important in regulating rate of filtrate formation and blood pressure
• Cell populations in JGC

  • Macula densa
  • Granular cells (juxtaglomerular, or JG cells)
  • Extraglomerular mesangial cells
• Glomerular filtration


  Produces cell- and protein-free filtrate
• Tubular reabsorption


  Selectively returns 99% of substances from filtrate to blood (capillaries) in renal tubules and collecting ducts
• Tubular secretion


  Selectively moves substances from blood to filtrate in renal tubules and collecting ducts
• Glomerular filtration


  • Passive process, no metabolic energy required
• Layers of the filtration membrane

  • Fenestrated endothelium of glomerular capillaries
  • Basement membrane
  • Foot processes of podocytes with filtration slits
• Macromolecules "stuck" in filtration membrane are engulfed by glomerular mesangial cells
• Filtration membrane allows molecules smaller than three nm to pass, while plasma proteins remain in blood to maintain colloid osmotic pressure
• Pressures affecting filtration


  • Outward pressures: Hydrostatic pressure in glomerular capillaries
  • Inward pressures: Hydrostatic pressure in capsular space, Colloid osmotic pressure in capillaries