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The Urinary System Chapter 25

The Urinary System Chapter 25. Kidney. Cortex Nephrons -structural and functional unit Medulla Medullary (renal pyramids) Renal Column Renal Pelvis. Nephron. Over 1 million within kidney Renal corpuscle Renal Tubule Filatrate -plasma derived fluid collected within Bowman's Capsule.

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The Urinary System Chapter 25

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  1. The Urinary System Chapter 25

  2. Kidney • Cortex • Nephrons -structural and functional unit • Medulla • Medullary (renal pyramids) • Renal Column • Renal Pelvis

  3. Nephron • Over 1 million within kidney • Renal corpuscle • Renal Tubule • Filatrate -plasma derived fluid collected within Bowman's Capsule

  4. Urine Formation • Glomerular Filtration • Tubular Reabsorption • Tubular Secretion

  5. Step 1. Glomerular Filtration

  6. Juxtaglomerular Apparatus

  7. Filtration Membrane

  8. Glomerular Filtration • Passive Process utilizing hydrostatic processes • High capillary pressure • High net filtration pressure • Colloid osmotic pressure

  9. Glomerular Filtration • Net Filtration pressure (NFP) -forces acting on glomerular bed. Responsible for filtrate formation • Glomerular Hydrostatic Pressure (HPg) -glomerular blood pressure. Pushes water and solutes across filtration membrane. Opposed by: • Colloid osmotic pressure of glomerular blood (OPg) • Capsular hydrostatic pressure (HPc) • OPg and HPc limit fluid loss from glomerular capillaries NFP= Hpg – (OPg + Hpc)=pressure responsible for urine formation

  10. Glomerular Filtration Rate (GFR) • Volume of filtrate formed per minute by the combined activity of all glomeruli of the kidney • Dependent on: • Total surface area available for filtration • Permeability of filtration membrane • Net filtration pressure • Small changes in glomerular pressure (18%) stop filtration • Glomerular filtration rate is directly proportional to net filtration pressure • GFR is tightly regulated

  11. GFR Regulation • Intrinsic controls • Renal autoregulation -regulation of blood flow resistance • Allows almost constant GFR • Mygenic mechanism and tubuloglomerular feedback mechanism • Extrinsic controls • Maintenance of systemic blood pressure • BP is back to haunt you again! • Other Factors: Prostoglandin, intrarenal angiotension, and adenosine

  12. Step 2. Tubular Reabsorption

  13. Tubular Reabsorption • Selective trans-epithelial process • Transcellular route • Paracellular route

  14. Tubular Reabsorption • Sodium reabsorption • Primary active transport via Na/K pump on basolateral membrane • Rapid flow due to high osmotic pressure • Secondary active transport of Na into tubule cells • Action of pump and leaky K channels maintain negative charge inside tubule cells • Nutrients, Water, Ion reabsorbtion • Secondary active transport • Transport maximum -when reached remainder excreted in urine • Passive tubular reabsorption • Obligatory water resorption

  15. Tubular Reabsorption • Reabsorptive capabilites of renal tubules and collecting ducts • See Drawing from lecture • See Table 25.1 page 977

  16. Step 3. Tubular Secretion

  17. Tubular Secretion • Disposing of substances bound to plasma proteins • Eliminating substances not reabsorbed by passive processes • Ridding the body of excess K+ • Controlling blood pH

  18. Tubular Secretion • Metabolic waste and drugs • Urea and uric acid • Excess K+ • Controlling blood pH

  19. Urine Concentration and Volume • Countercurrent multiplier • Urea recycling • Medullary osmotic gradient • Countercurrent exchanger • Urine formation

  20. Countercurrent Multiplier • Descending loop of Henle • Permeable to water • Impermeable to solutes • Ascending loop of Henle • Impermeable to water • Permeable to solutes • Na/Cl pumps create 200mOsm difference between the two • Filtrate hypo-osmotic (hypotonic) when it enters DCT

  21. Countercurrent Multiplier

  22. Urea Recycling and Medullary Osmotic Gradient • Urea enters tubules in ascending loop of Henle • Urea leaves tubules in medullary region • Establishes high osmolarity in medulla • ADH enhances urea transport

  23. Countercurrent Exchanger • Vasa recta maintain osmotic gradient • Exchanger • Permeable to Na and H2O • Slow blood flow • Blood in medulla loses water and gains NaCl • Blood in the cortex gains water and loses NaCl • Volume of blood is greater at end of capillary bed than in beginning

  24. Urine Formation • ADH controls reabsorption of H2O • Insertion of aquaporins within collecting duct • Dilute urine • Absence of ADH • Collecting ducts impermeable to H2O • Concentrated urine • Dependent on medullary osmotic gradient and ADH • Facultative water reabsorbtion

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