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Proximal Tubule

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  • Proximal (Convoluted) Tubule 
    • First segment the filtrate traverses after glomerular filtration 
    • Two major roles
      • Reabsorption of solutes
        • Majority of solute reabsorption occurs in early proximal tubule
        • Na+
          • Major ion undergoing reabsorption by Na+/K+ ATPase and Na/H antiporters
          • Always coupled to chloride and water reabsorption 
        • Water
          • Water is reabsorbed paracellularly by passive diffusion through tight junctions or by facilitated diffusion via aquaporins 
        • Important nutrients
          • Includes amino acids, vitamins, salts, glucose, bicarbonate, water
      • Secretion of waste
        • Commonly secreted solutes include H+, urea, ammonia, K+ 
    • Overall effect:
      • Amount of filtrate in nephron is reduced 
      • Solute composition in nephron is altered
      • But overall concentration of solutes does not change