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The Renal System

Learn the five key processes of the urinary system: filtration, reabsorption, secretion, excretion, and micturition. Discover how fluid moves through the nephron, mechanisms of reabsorption, and regulation of blood pressure. Understand the clinical importance of GFR and clearance in kidney function assessment.

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The Renal System

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  1. The Renal System

  2. Five Processes of Urinary System • Filtration, • Reabsorption, • Secretion, • Excretion • Micturition 180 L / day filtered, >99% reabsorbed, 1.5 L/day excreted

  3. 1) Filtration Movement of fluid from blood to lumen of nephron

  4. 1) Filtration: Passage across 3 Barriers • Capillary endothelium is fenestrated • Basal lamina • Filters proteins • Negative Charges • Bowman’s capsule epithelium (visceral layer), including podocytes

  5. Cause of Filtration • Three types of pressures are at work: • Hydrostatic pressure in capillaries (see exchange in tissues) • Osm. Pcapillaries > Osm. P Bowman’s capsule • Hydrostatic fluid P from presence of fluid in Bowman’s capsule • Netdriving pressure: ~ 10 mmHg

  6. 2) Tubular Reabsorption (99% of filtrate) Mostly transepithelial transport (examples: Sodium and glucose) Reabsorption may be active (Na+, glucose) or passive (urea)

  7. 2) Tubular Reabsorption (99% of filtrate) • Active Transport • Na+ /K+ ATPase pumps • Passive • concentration and osmotic gradients

  8. Na+ Reabsorption in PCT Apical: Na+ movement down conc. gradient. Basolateral: Na+/K+ ATPase.

  9. Na+ Linked Glucose Reabsorption Basolateral:Glucose diffusion down conc. gradient Apical:Na+-glucose cotransport

  10. Saturation of Renal Transport Saturation = Maximum rate of transport (tm) • Same 3 characteristics as discussed in mediatedtransport • Transport maximum determined by • Saturation  Renal Threshold • Specificity • Competition

  11. Clinical Importance of GFR and Clearance • GFR is indicator for overall kidney function • Clearance→ non-invasive way to measure GFR • Inulin (research use) • Neither secreted nor reabsorbed • Creatinine (clinically useful) • If a substance is filtered and reabsorbed but not secreted  clearance rate < GFR • If a substance is filtered and secreted but not reabsorbed  clearance rate > GFR

  12. Regulation of Blood Pressure via Tubuloglomerular Feedback Decrease blood pressure in afferent arteriole juxtaglomerular cells release Renin Renin Angiotensinogen  Angiotensin I ACE Angiotensin I  Angiotensin II Aldosterone/ ADH secretion

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