Sodium ion reabsorption

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  1. Anatomy of the nephron – See Figure 11:
    1. glomerulus and Bowman’s capsule where blood is filtered; proximal convoluted tubule to straight proximal tubule to descending and ascending loop of Henle then to distal convoluted tubule and medullar collecting duct
  2. Generalizations:
    1. reabsorption of Na is mainly a primary active process dependent on the Na, K ATPase pumps in the basolateral membrane
      1. Na passes across the lumen because the inside of the lumen cell is negatively charged with respect to the outside lumen and because the concentration of Na in the cell is low (because the Na/K ATPase pumps are always pumping Na into the interstitial fluid)
        1. Na/K ATPase is inhibited by digoxin
      2. Na may be reabsorbed with glucose, amino acids, chloride or as a countertransport with hydrogen ion
      3. In the loop of Henle, Na reabsorption is mainly passive and follows the active transport of Cl ions
      4. By the end of the ascending loop of Henle only about 10% of the filtered Na and Cl and 20% of the filtered water remains in the tubule, the rest having been reabsorbed by the proximal tubule and loop of Henle; Na and Cl reabsorption continues along the distal convoluted tubule and collecting duct system so that the final urine normally contains <1% of the total filtered Na and Cl
      5. Aldosterone (secreted by adrenal cortex) stimulates Na reabsorption in the connecting tubule and cortical collecting duct in exchange for K and H by acting on the principal cells (same cells activated by ADH)
        1. Total Na reabsoprtion dependent on the influence of aldosterone is 2% of the total amount filtered but it is the most important controller of Na reabsoprtion
        2. Aldosterone secretion is regulated by Na and K concentration (increased serum Na means less aldosterone and less Na absorption); ACTH from the anterior pituitary (increased ACTH as in trauma causes increase secretion of aldosterone and increases Na absorption); angiotensin II (most important) which in turn is controlled by the amount of renin secreted by the kidneys
      6. Decreased plasma volume causes an increase in output from sympathetic nerves which causes renal arteriolar constriction which drops the peritubular capillary hydraulic pressure and decreases renal interstitial volume which causes increased Na reabsorption and decreased Na excretion
    2. reabsorption of chloride may be passive or active, depending on the nephron segment but is always coupled with primary active reabsorption of Na
    3. reabsorption of water is passive and depends on solute reabsorption, particularly Na
      1. water reabsorption is due to the differences in osmolarity between lumen and interstitial fluid created by reabsorption of solute
      2. major determinant of water permeability in the collecting ducts is ADH that acts on the principal cells of the collecting ducts; ADH enhances the water permeability of the collecting ducts
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