Urinary System

• Urinary system


  The system that regulates water volume and composition, balances electrolytes, salts, acids and bases, regulates blood pressure and pH, produces hormones, and disposes of metabolic waste

  
• Goals for understanding the urinary system

  • Anatomy of the urinary system
  • Filtration process between glomerulus and Bowman's capsule
  • Selective reabsorption of solutes by the nephron
  • Mechanism of production of hyperosmotic urine
  • Relationship between urine production and blood volume/solute concentrations
  • Hormonal control of urine output

  
• Hyponatremia is the condition of too much water
• Hyponatremia


  Condition caused by drinking too much water, where water rushes into cells including brain cells, causing them to swell and press against the skull, which can be fatal
• There are no reported cases of dehydration causing death in the history of world running, but there are plenty of cases of people dying of hyponatremia
• New York City Marathon tells participants to drink no more than eight ounces of water every 20 minutes
• Functions of the urinary system

  • Regulate water volume and composition
  • Balance electrolytes, salts, acids and bases
  • Regulate blood pressure and pH
  • Produce hormones (renin, calcitriol, erythropoietin)
  • Dispose of toxic byproducts of metabolism
  • Gluconeogenesis during prolonged fasting
  • Activation of vitamin D
• Ureters


  Transport urine from the kidneys to the urinary bladder

  
• Urinary bladder


  Stores urine and expels it into the urethra
• Urethra


  Discharges urine from the body
• Structure of the kidneys

  • Lie on either side of the vertebral column between the peritoneum and back wall of the abdominal cavity
  • Enclosed in a renal capsule surrounded by adipose tissue
  • Consist of renal cortex, renal medulla, renal pyramids, renal columns, major calyces, minor calyces, and a renal pelvis

  
• Kidneys filter 200 liters of blood daily, allowing toxins, metabolic wastes, and excess ions to leave the body in urine

  
• Osmosis


  Movement of water across a selectively permeable membrane
• Osmolarity


  Total solute concentration expressed as moles of solute per liter of solution
• Isoosmotic


  No net water movement by osmosis
• Hyperosmotic


  Solution with greater solute concentration or osmolarity
• Hypoosmotic


  Solution with less solute concentration or osmolarity
• Water flows from hypoosmotic solutions to hyperosmotic solutions
• Human blood is ~300 mosm/L
• 80% of the extracellular fluid is part of the body's interstitial fluid
• Most of the body's water comes from ingested liquid and foods
• Normally, most of the body's water is lost through excretion of urine
• Nephron


  The functional unit of the kidney, consisting of a renal corpuscle and a renal tubule

  
• Structure of the nephron

  • Renal corpuscle (glomerulus and glomerular capsule)
  • Proximal convoluted tubule
  • Descending limb of nephron (Loop of Henle)
  • Ascending limb of the nephron loop
  • Distal convoluted tubule

  
• Glomerular filtration


  The process where the glomerular endothelium and podocytes form a leaky filtration membrane that permits the passage of water and solutes from the blood into the capsular space
• Blood cells and most plasma proteins remain in the blood because they are too large to pass through the filtration membrane
• Glomerular filtration rate (GFR)


  The amount of filtrate that forms in both kidneys every minute

  
• Atrial natriuretic peptide (ANP)

  Increases GFR
• Sympathetic (F/F) stimulation


  Decreases GFR
• Substances reabsorbed by tubular reabsorption

  • Water
  • Glucose
  • Amino acids
  • Ions such as sodium, potassium, chloride, bicarbonate, calcium, and magnesium
• Renin-Angiotensin-Aldosterone System (RAAS)


  Enhances reabsorption of sodium and chloride, and stimulates the adrenal cortex to release aldosterone, which further stimulates reabsorption of sodium and chloride and secretion of potassium
• Atrial Natriuretic Peptide (ANP)


  Inhibits reabsorption of sodium, chloride and water by the renal tubules, reducing blood volume

  
• Antidiuretic Hormone (ADH)


  Regulates reabsorption of the remaining water in the last part of the distal convoluted tubule and collecting duct, adjusting blood osmolarity

  
• Descending nephron loop


  • Reabsorbs water as filtrate passes from cortex into medulla, which is hyperosmotic
  • Aquaporins speed water reabsorption
  • Impermeable to NaCl and solutes
  • Decreases urine volume and increases [NaCl]
• Ascending nephron loop


  • Permeable to salt, but not water (no aquaporins)
  • NaCl exits as filtrate moves back to renal cortex, decreasing [NaCl] in filtrate and making it more dilute
  • Active transport in cortex region removes more NaCl
  • NaCl loss contributes to high osmolarity of renal medulla

  
• Collecting duct


  As urine passes into the collecting duct, it passes from the renal cortex into the hyperosmotic renal medulla, which is high in urea

  
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  3/20/2020
• Descending Nephron Loop


  1. Reabsorbs water as filtrate passes from cortex into medulla

  2. Medulla interstitial fluid is hyperosmotic

  3. Aquaporins speed water reabsorption

  4. Impermeable to NaCl and solutes

  5. Decreases urine volume and increases [NaCl]
• Ascending Nephron Loop


  1. Permeable to Salt, but not Water. No Aquaporins

  2. NaCl exits as filtrate moves back to renal cortex decreasing the [NaCl] in filtrate, making it more dilute

  3. Active transport in cortex region of Ascending loop removes more NaCl

  4. NaCl loss contributes to high osmolarity of renal medulla

  
• Collecting Duct


  1. As urine passes into the collecting duct, it passes from the renal cortex into the hyperosmotic renal medulla

  2. The renal medulla is high in Urea (CH3-CO-CH3) and is hyperosmotic to the tissues around it

  3. Water exits the collecting duct and is reabsorbed

  4. This loss of water from the urine causes urine that is excreted to be isosmotic to the renal medulla and hyperosmotic to the plasma