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Urine formation

Urine formation. Section 7.5 Page349. Recall. The urinary system Urine formation occurs in the kidney. The functional unit of the kidney is the nephron . This is where all the action happens. Renal artery. The nephron A series of tubules where urine is formed.

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Urine formation

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  1. Urine formation Section 7.5 Page349

  2. Recall... The urinary system Urine formation occurs in the kidney. The functional unit of the kidney is the nephron. This is where all the action happens. • Renal artery

  3. The nephron • A series of tubules where urine is formed. • The nephron contacts the circulatory system where the glomerulus meets the Bowman’s capsule. • The afferent arteriole supplies blood to the glomerulus, and the efferent arteriole carries away filtered blood. • The efferent arteriole branches into a network of peritubular capillaries that are intertwined with the nephron tubules.

  4. Urine formation: Strategy • Everything but cells and protein gets filtered into Bowman’s capsule. • Filtrate in the nephron has the same substances at same concentrations as plasma. • Further along tubule, substances are removed (reabsorption) or added (secretion).

  5. Accomplished by three steps: • Filtration – Solutes enter nephrons from the blood • Reabsorption – Some solutes are selectively reabsorbed by the blood • Secretion – Additional solutes are secreted for elimination

  6. Filtration • Blood is carried to the glomerulus by the afferent arteriole • Pressures in the glomerulus are very high • 65 mm Hg, as compared with normal 25 mm Hg • Solutes are filtered through glomerulus walls into Bowman’s capsule

  7. What materials are filtered? Materials that aren’t filtered are too big to get through the membranes

  8. Reabsorption • Solutes and water are selectively reabsorbed into blood • Transport (both active and passive) out of the nephron, into the interstitial fluid, and back into blood • Reabsorption occurs until a maximum amount (the threshold level) has been transported

  9. Reabsorption and water balance • Very important that water be reabsorbed • Under ideal conditions, the body should produce urine that is very concentrated • Lots of solute excreted • Not a lot of water excreted

  10. Water is reabsorbed by osmosis • About 85% of filtered water is reabsorbed What drives osmosis? • Solute concentration gradients are established by active transport, and water follows passively

  11. Reabsorption: The illustrated guide

  12. Reabsorption • Active transport of NaCl from the ascending limb of the LoH • Osmosis of water from the descending LoH causes concentration of filtrate • Further active transport of Nacl from ascending LoH does two things: • Decreases filtrate solute concentration • Increases strength of osmotic gradient to pull H2O from descending LoH Therefore, the Loop of Henle actually achieves very little concentration of filtrate

  13. The main function of the loop of Henle is to build up solute concentration in the interstitial space • Concentration gradients drive the osmosis of water from the collecting duct • The bulk of water reabsorption occurs here • Concentrated urine is produced • Unfiltered proteins in the bloodstream also contribute to osmotic gradient that pulls H2O from nephrons

  14. What solutes are actively reabsorbed?

  15. Secretion • Occurs in the distal tubule • Waste solutes move from blood into the nephron • Nitrogen-containing wastes, excess H+ and K+

  16. Recap: Urine formation

  17. Homework • Pg. 352 #2-7

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