Water Reabsorption in the Kidneys

1. KEEP OFF YOUR MOBILE PHONES The Body Fluids and Kidneys Lecture 16  

2. Water Reabsorption • Passive / Osmosis • Mainly coupled to Sodium Reabsorption • Water channels made up of proteins called: AQUAPORINS • Paracellular + Transcellular

3. Water Reabsorption contd…….. • Proximal Tubule High Permeability Solvent Drag Distal Tubule ↓ Permeability i. Tighter junctions ii. ↓ Surface Area Ascending Loop of Henle Almost Zero Permeability More Distal Parts ADH

4. Reabsorption of Urea & Chloride

5. Proximal Tubule

6. Reabsorption in First half of Proximal tubule Active Transport: 1. Primary Active Transport Sodium( Sodium-Potassium ATPase) 2. Secondary Active Transport (Co-Transport) Glucose, Amino Acids, Organic Acids Passive Transport: Water, Urea? Chloride ?

7. Reabsorption in Second half of Proximal tubule Active Transport: 1. Primary Active Transport Sodium( Sodium-Potassium ATPase) 2. Secondary Active Transport (Co-Transport) Chloride Passive Transport: Water, Glucose, Amino Acids ? Urea reabsorption and secretion

8. Secretion in Proximal tubuleCounter Transport • H+ • Organic Acids + Bases e.g. Bile Salts Oxalate Urate Catecholamines Drugs Toxins PAH (Para-Amino- Hippuric Acid)

10. Reabsorption of various solutes in proximal tubule

11. Reabsorption in Loop of Henle 20% Urea reabsorbed Back in Thin Ascending Limb

12. Reabsorption in Loop of HenleThin descending segment • Structure • Thin epithelial membrane • No brush border • Few mitochondria • Function • Highly Permeable to water • Almost all water absorbed in Loop of Henle is absorbed here →↑osmolarity • Moderately permeable to most solutes, including Sodium and Urea • No active transport

13. Reabsorption in Loop of HenleThin ascending segment • Structure • (Same as descending limb) • Thin epithelial membrane • No brush border • Few mitochondria • Function • Impermeable to water • Permeable to NaCl • No active transport • Urea is reabsorbed back Juxta -medullary nephrons

14. Reabsorption in Loop of HenleThick ascending segment • Structure • Thick epithelial cells • Function • Na+-K+ ATPase in basolateral membrane i) Active reabsorption of Na+, K+, Cl- (about 20-25% of filtered load) (Secondary active transport) (1-Sodium, 2-chloride, 1-Potassium co-transporter) ii) Reabsorption of Ca++, HCO3--, Mg++ 2. Paracellular reabsorption of Ca++, K+, Mg++, Na+ 3. Na+ -H+ Counter-transport 4. Impermeable to water→↓osmolarity (dilute tubular fluid) • Loop diuretics

15. Reabsorption in Loop of Henle • 20% Water Reabsorbed in Loop of Henle • 25% Sodium Reabsorbed in Loop of Henle • some Passive Reabsorption of Urea • No Water Reabsorption in Ascending Limb • No Active Transport in Thin Limb

16. Transport in Thick Ascending Loop of Henle

17. NaCl transport in thick ascending limb of loop of Henle

18. Reabsorption in Distal Convoluted Tubule • Juxta Glomerular Apparatus • Early Distal Tubule Almost same as Thick Ascending Loop of Henle (5% sodium reabsorption in early DCT) Diluting Segment

19. Reabsorption in Distal Convoluted Tubule water Aldosterone Aldosterone H+ ATPase

20. Mechanism of Na+ Reabsorption in Distal Convoluted Tubule Early Late K+ sparing diuretics

21. Reabsorption in Second Half of Distal Convoluted Tubule • Water Reabsorbed from both cells in the presence of ADH (Anti-Diuretic Hormone) • Impermeable to urea • Na+ reabsorption and K+ secretion under influence of Aldosterone • Active secretion of H+

22. Cortical Collecting Tubule • Same as second Half of Distal Collecting Tubule

23. Medullary Collecting Duct

24. Tubular processing in Medullary Collecting Duct • Structure • Cuboid • Smooth surface • Few mitochondria • Function • Reabsorption of 10% of water and sodium • Final site of processing of urine • ADH • Permeable to urea • H+ secretion

25. Summary Contd…

26. Summary (ADH present)