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Formation of Urine

Formation of Urine. 7.5. Three functions urine formation depends on:. Filtration: movement of fluids from blood into the Bowman’s Capsule Reabsorption : transfer of essential solutes and water from nephron back into blood Secretion: movement of materials from blood back into the nephron .

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Formation of Urine

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  1. Formation of Urine 7.5

  2. Three functions urine formation depends on: • Filtration: movement of fluids from blood into the Bowman’s Capsule • Reabsorption: transfer of essential solutes and water from nephron back into blood • Secretion: movement of materials from blood back into the nephron.

  3. Filtration • Each nephron has independent blood supply. • Afferent arteriole  glomerulus (high-pressure filter)  selected dissolved solutes pass into Bowman’s capsule • Normal Capilary bed pressure: 25 mm Hg • Pressure in glomerulus: 65 mm Hg

  4. Selectiveness of the Bowman’s Capsule • Plasma protein, blood cells, and platelets are too ___________.

  5. Reabsorption • 120 mL of fluid is filtered into the nephrons every minute • If reabsorption did not occur, we would form 120 mL of urine each minute. Need to consume at least 1 L of fluid every 10 minutes for homeostasis. • Only 1/120 mL of fluid forms urine. • Remaining 119 mL of fluids and solutes reabsorbed.

  6. Active Transport (selectivity) PROXIMAL TUBULE • Occurs by active transport. • Carrier molecules: move Na+ across cell membrane (out of proximal tubule) • Negative ions: Cl-, HCO3-, follow Na+ by charge attraction. • Glucose + AAs attach to specific carrier molecules, shuttling them out of the nephron. • Amount limited. • Threshold level: maximum amount of material that can be moved across a nephron (energy limitations/substrate saturation). • Excess NaCl remains in nephron: excreted with urine.

  7. Passive Transport (selectivity) • Actively transported solutes (out of nephron) create osmotic gradient. • Draws water from nephron. • Proteins not absorbed by nephron (still in blood) create osmotic force • Draw water from interstitial fluid into the blood. As water is reabsorbed FROM the nephron, solutes become more concentrated. Urea and uric acid may diffuse back into blood stream, but less than was originally filtered.

  8. Secretion (distal tubule) • Movement of wastes from blood into nephron. • Nitrogen-containing wastes, excess H+ ions, K+ ions. • Cells loaded with mitochondria line the distal tubule. • Similar to reabsorption: tubular secretion occurs by active transport • Different from reabsorption: molecules shuttled from blood into nephron.

  9. Summary: Urine Formation http://www.youtube.com/watch?v=vEXx5YLcGmQ&feature=related

  10. Seatwork/Homework • Page 352, #1-6.

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