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2 SOLUTE REABSORPTION (Distal tubule segments (impermeable to water…

- - - - indirectly driven by Na+ gradient
      - transcellular
    - - indirectly powered by Na+,K+-ATPase pump
      - paracelullar & transcellular
    - - paracellular
  - - - 2 pathways
        
        transcellular pathway
        
        cross the apical plasma membrane
        
        discharge across basolateral plasma membrane into interstitial fluid on the blood side of cell
        
        paracelullar pathway
        
        from tubule fluid across zonula occludens
        
        passive diffusion or by solvent drag
        
        enter lateral intercellular space
    - - driven by Starling's forces
  - - - bile salts
        
        oxalate
        
        urate
        
        creatine
        
        Prostaglandin
    - - antibiotics
        
        diuretics
        
        analgesic morphine
        
        potent herbicide paraquat
    - - hormones & foreign substances as reflection of blood levels that may be only temporarily elevated
- - - - creating electrochemical gradient across apical plasma membrane
        
        drives Na+, K+, Cl- enter cell from luminal fluid through Na+, K+, 2Cl- co-transporter
        
        Cl- diffuses down its concentration gradient
        
        K+ leaves cell through both apical & basolateral K+ channel
  - - - fluid volume regulation
      - allow kidney excrete excess water without salt
        
        prevent water overload & plasma hypotonicity
  - - - aldosterone
        
        acts on connecting segment cells & principal cells
        
        enhance Na+ reabsorption
        
        enhances water reabsorption to correct perceived volume depletion
        
        stimulated by hyperkalemia (elevated plasma K+ level)
        
        enhance K+ secretion
      - angiotensin II
        
        directly enhances Na reabsorption in proximal tubule, thick ascending limb of Henle's loop , collecting duct
      - antidiuretic hormone
        (vasopressin)
        
        release when volume depleted, dehydrated / hypotensive
        
        enhances salt reabsorption from thick ascending limb & collecting duct
      - endothelin
        
        peptide hormone produce in kidney
        
        in collecting duct & thick ascending limb
        
        acts on specific receptors
        
        increases renal NaCl & water excretion
        
        by effects on epithelial transport & renal microcirculation
        
        mediated by nitric oxide & prostaglandin
      - atrial natriuretic peptide
        
        produced in cardiac atria
        
        enhances Na+ excretion in distal tubules & collecting duct
        
        by inhibiting aldosterone release & vasopressin effects in collecting duct
      - parathyroid hormone (PTH)
        
        stimulated by hypocalcemia (low plasma Ca2+ level)
        
        affects bone, intestines & kidneys to raise plasma Ca2+ level
        
        increase apical uptake of Ca2+ in cortical thick ascending limb, dct & connecting segment
        
        by increasing activity of apical Ca2+ channel
        
        in dct, Cl- increases at basolateral plasma membrane
        
        hyperpolarizes cells
        
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        inhibits inorganic phosphate reabsorption in proximal tubule
        
        internalization of NaPi-2
      - vitamin D3
        
        converted to active form in proximal tubule, stimulated by PTH
        
        receptors are located predominantly in dct & connecting segment
        
        increases cellular content of Ca2+ binding protein
        
        contributes to enhanced Ca2+ reabsorption
        
        increase expression of NaPi-2 in proximal tubule
        
        enhances phosphate reabsorption
      - calcitonin
        
        reduces serum Ca2+ concentration
        
        by stimulating Ca2+ deposition in bone
        
        enhances renal Ca2+ reabsorption in thick ascending limb, dct & connecting segment
- - - - remainder of cortical & outer medullary collecting duct cells
      - reabsorb potassium
    - - initial collecting duct
        
        cortical collecting duct
        
        outer medullary collecting duct
      - reabsorbs NaCl
        
        Na+ is actively reabsorbed through basolateral Na+, K+ -ATPase pump
        
        creating electrochemical gradient across apical plasma membrane
        
        lumen-negative electrical potential drives Cl- absorption through paracellular pathway
        
        K+ is pumped actively from intracellular fluid into cell
        
        intracellular K+ concentration increase
        
        intracellular K+ exits the cell (secretion)
        
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