Urinary System

• Urea


  Nitrogenous end product of protein metabolism, NH3
• Uric acid


  Crystals from nucleic acid metabolism, can cause gout
• Excretion is different from defecation as fecal material was never used by the body
• Kidneys


  • Paired structures
  • Depression - hilum with renal artery and renal vein
  • Ureters exit the kidney
• Ureters


  • Connect the kidneys to the bladder
  • Peristalsis
• Urinary Bladder


  • Stores urine
  • Expandable
  • Mucosal valves prevent backflow
  • Sphincters - internal and external
• Urethra


  • Removes urine from the body
  • Shorter in females, longer in males and shared with reproductive system
• Fibrous capsule


  • Dense irregular connective tissue
• Renal cortex


  • Mostly the outer layer, contains renal columns, renal pyramids, and renal papilla
• Renal sinus


  • Contains minor calyx, major calyx, and renal pelvis
• Arteries


  • Renal artery
  • Segmental arteries
  • Interlobar arteries
  • Arcuate arteries
  • Interlobular (cortical radiate)
• Veins


  • Interlobular (cortical radiate)
  • Arcuate
  • Interlobar
  • Segmental
  • Renal
• Vascular elements of a nephron


  • Afferent arterioles
  • Glomerulus
  • Efferent arteriole
  • Peritubular capillary network
  • Vasa recta
• Tubular components of a nephron

  • Bowman's capsule (renal corpuscle)
  • Proximal convoluted tubule
  • Loop of Henle
  • Distal convoluted tubule
  • Collecting ducts
• Filtration fraction


  Only 20% of the plasma that passes through the glomerulus is filtered
• Filtration barriers


  • Capillary endothelium
  • Basal lamina
  • Epithelium of the Bowman's capsule
• Net filtration pressure


  Hydrostatic pressure forces fluid into the Bowman's capsule (55 mm Hg), colloid osmotic pressure pulls fluid back into the capillaries (30 mm Hg), and hydrostatic fluid pressure in the Bowman's capsule resists filtration (15 mm Hg)
• Glomerular Filtration Rate


  180 L/day, entire volume 60/day, determined by renal blood flow and filtration coefficient
• Intrinsic regulation of GFR

  • Myogenic response - arterioles undergo vasoconstriction or vasodilation to maintain blood pressure
  • Tubuloglomerular feedback - paracrine signaling through the distal convoluted tubule
• Extrinsic regulation of GFR

  • Sympathetic nervous system - baroreceptors trigger vasoconstriction
  • Hormonal control - renin-angiotensin II pathway, aldosterone release, vasoconstriction
• Proximal Convoluted Tubule


  • About 65% of filtrate reabsorbed
  • Active transport of Na+ as primary driving force
  • Sodium-linked secondary active transport of glucose
  • Diffusion of urea as other solutes are reabsorbed
  • Receptor-mediated uptake of small proteins
  • Saturation of transporters
• A drop in blood pressure


  Triggers vasoconstriction
• Renin-angiotensin II pathway


  1. Granular cells

  2. Aldosterone release

  3. Vasoconstriction
• Proximal Convoluted Tubule


  • About 65% of filtrate reabsorbed
  • Active transport of Na+
  • Sodium-linked secondary active transport of glucose
  • Diffusion of urea
  • Receptor-mediated uptake of small proteins
  • Saturation of transporters
• Loop of the Nephron

  • About 25% of filtrate reabsorbed
  • Descending (thin) limb is permeable to water
  • Ascending (thick) limb is impermeable to water, Na+, Cl-, K+ actively transported out
  • Countercurrent multiplier maintains osmotic gradient
• Distal Convoluted Tubule


  • Variable absorption of filtrate influenced by hormones
  • Tubular Secretion of organic metabolites and xenobiotics
• Excretion


  • Excreted fluid is very different from the original filtrate, useful molecules reabsorbed, metabolic wastes added
  • Renal clearance is the rate of removal from the body
  • Passage of filtrate through collecting duct results in concentrated or dilute urine
• Vasopressin


  Released in response to increased osmolarity, by posterior pituitary, induces insertion of aquaporin-2 into collecting duct cells, increases water reabsorption
• Aldosterone


  Released in response to increased extracellular K+ and low blood pressure, by adrenal cortex, induces new ion channels in distal nephron, increases Na+ reabsorption and K+ secretion
• Atrial Natriuretic Peptide (ANP)


  Released in response to increased myocardial stretch from high blood pressure, by myocardial cells, increases GFR, decreases Na+ and water reabsorption, decreases blood pressure, inhibits aldosterone and vasopressin
• Micturition


  1. Urine from kidneys transported to bladder

  2. Bladder stores urine

  3. Internal sphincter contracts, external sphincter relaxes

  4. Micturition reflex responds to bladder stretch

  5. Micturition reflex can be inhibited

  6. Urine leaves bladder through urethra
• Pyelonephritis


  Infection of the kidney
• Cystitis


  Infection of the urinary bladder
• Symptoms of urinary infections

  • Dysuria
  • Pyuria

  
• Kidney Stones


  Granules in the renal pelvis
• Bladder Stones


  Result from poor urine flow, enlarged prostate
• Dialysis


  Removal of blood solutes