Urinary System

• Urinary system


  Rids the body of waste products
• Urinary system


  • Kidneys also play important roles in blood volume, pressure, and composition
  • Closely associated with the reproductive system
  • Shared embryonic development and adult anatomical relationship
• Urogenital (UG) system


  Collectively called the urinary and reproductive systems
• Major organs of the urinary system


  • Kidneys
  • Ureters
  • Urinary bladder
  • Urethra
• Kidney functions

  • Filter blood plasma
  • Excrete toxic wastes
  • Regulate blood volume, pressure, and osmolarity
  • Regulate electrolytes and acid-base balance
  • Secrete erythropoietin, which stimulates the production of red blood cells
  • Help regulate calcium levels by participating in calcitriol synthesis
  • Clear hormones from blood
  • Detoxify free radicals
  • In starvation, they synthesize glucose from amino acids
• Excretion


  Separating wastes from body fluids and eliminating them
• Waste


  • Any substance that is useless to the body or present in excess of the body's needs
• Body systems that carry out excretion


  • Respiratory system
  • Integumentary system
  • Digestive system
  • Urinary system
• Treatments for uremia


  • Hemodialysis
  • Organ transplant
• Zones of renal parenchyma


  • Outer renal cortex
  • Inner renal medulla
• Other kidney structures


  • Renal columns
  • Renal pyramids
  • Minor calyces
  • Major calyces
  • Renal pelvis
  • Ureter
• Kidney anatomy


  • Retroperitoneal
  • Left kidney about 2 cm higher than the right
  • Drop about 3 cm when going from lying down to standing up
• Renal fraction


  Kidneys are only 0.4% of body weight, but receive about 21% of cardiac output
• Nephron


  Each kidney has about 1.2 million nephrons
• Parts of the nephron

  • Renal corpuscle
  • Glomerulus
  • Glomerular capsule (AKA Bowman's capsule)
  • Renal tubules
  • Proximal convoluted tubule (PCT)
  • Nephron loop (AKA loop of Henle)
  • Descending limb
  • Ascending limb
  • Distal convoluted tubule (DCT)
  • Collecting duct
• Nephron loop


  • Long U-shaped portion of renal tubule
  • Descending and ascending limbs
  • Thick segments have simple cuboidal epithelium, heavily engaged in active transport of salts
  • Thin segment has simple squamous epithelium, very permeable to water
• Collecting duct


  Receives fluid from the DCTs of several nephrons as it passes back into the medulla, numerous collecting ducts converge toward the tip of the medullary pyramid, papillary duct formed by merger of several collecting ducts
• Renal plexus


  Nerves and ganglia wrapped around each renal artery, carries sympathetic innervation that reduces glomerular blood flow and rate of urine production, also receives parasympathetic innervation
• Urine formation


  1. Glomerular filtration

  2. Tubular reabsorption

  3. Tubular secretion

  4. Water conservation
• Glomerular filtrate


  Fluid in the capsular space, similar to blood plasma except almost no protein
• Tubular fluid


  Fluid from the proximal convoluted tubule through the distal convoluted tubule, substances have been removed or added by tubular cells
• Urine


  Fluid that enters the collecting duct, undergoes little alteration beyond this point except for changes in water content
• Filtration membrane barriers


  • Fenestrated endothelium of glomerular capillaries
  • Basement membrane
  • Filtration slits formed by podocyte cell extensions
• Filtration membrane


  • Almost any molecule smaller than 3 nm can pass freely through
  • Some substances of low molecular weight are bound to plasma proteins and cannot get through
• Proteinuria (albuminuria)


  Presence of protein in urine, can indicate kidney infections or trauma damaging the filtration membrane
• Hematuria


  Presence of blood in the urine, can indicate kidney infections or trauma
• Distance runners and swimmers often experience temporary proteinuria or hematuria due to reduced kidney perfusion from prolonged, strenuous exercise
• Filtration pressures


  Depend on hydrostatic and osmotic pressures on each side of the filtration membrane
• Net filtration pressure = 60 mm Hg (outward) - 18 mm Hg (inward) - 32 mm Hg (inward) = 10 mm Hg (outward)
• Glomerular filtration rate (GFR)


  Amount of filtrate formed per minute by the two kidneys combined
• Filtration pressure depends on hydrostatic and osmotic pressures on each side of the filtration membrane
• Blood hydrostatic pressure (BHP)


  High in glomerular capillaries (60 mm Hg compared to 10 to 15 in most other capillaries)
• Hydrostatic pressure in capsular space

  18 mm Hg due to high filtration rate and continual accumulation of fluid in the capsule
• Colloid osmotic pressure (COP) of blood


  About the same here as elsewhere: 32 mm Hg
• Glomerular filtrate


  Almost protein-free and has no significant COP
• Net filtration pressure: 60 out - 18 in - 32 in = 10 mm Hg out
• GFR = 125 mL/min. or 180 L/day (male), GFR = 105 mL/min. or 150 L/day (female)
• Net filtration pressure (NFP)
• Filtration coefficient (Kf)


  Depends on permeability and surface area of filtration barrier
• Total amount of filtrate produced per day equals 50 to 60 times the amount of blood in the body