Process by which solutes are taken up from the tubular fluid back into the body
Reabsorptive mechanisms
Primary active transport
Secondary active transport
Carrier-mediated transport
Passive diffusion
Majority of reabsorption occurs in proximal tubule (~60-65%)
Transcellular pathway
Carrier-mediated reabsorption
Paracellular pathway
Passive diffusion reabsorption
Na+/K+-ATPase
Drives most solute reabsorption in proximal tubule
Renal threshold
Concentration in plasma below which none appears in urine, above which progressively larger quantities appear
Tubular maximum (Tm)
Maximum capacity of kidney to reabsorb a substance; highest rate at which tubules can transfer a substance before it may be excreted
If a substance is filtered beyond its Tm value it will be reabsorbed but will also be excreted in the urine
Normally glucose is filtered below its Tm, therefore all is reabsorbed but in diabetes mellitus, glucose is filtered more than its Tm, therefore excreted in the urine
Specific Na+ dependent transporters
Glucose - SGLT1, SGLT2
Amino acids - EAAT3, SIT1
Phosphate - NaPi2a, NaPi2c
Sulfate - NaS1
Citrate - NaDC1, NaDC3
HCO3- reabsorption
1. Na+ gradient drives Na+/H+ counter-transport via NHE3
2. H+ combines with filtered HCO3- to form H2O and CO2, catalyzed by carbonic anhydrase
3. CO2 enters facilitated by AQP1
4. Carbonic anhydrase catalyzes hydroxylation of CO2 to form H+ and HCO3-
5. HCO3- crosses basolateral membrane via NBCe1 and Na+-dependent HCO3-/Cl- exchanger
6. Majority of H+ transported into tubule fluid via NHE3 and H+-ATPase
Cl- reabsorption is also indirectly powered by the Na+/K+-ATPase pump and occurs via both paracellular and transcellular routes
~65% of filtered Ca2+ is reabsorbed, ~90% is paracellular due to favourable electrochemical gradient in late proximal tubule and solvent drag
Majority of K+ reabsorption occurs by passive mechanisms, mainly via the paracellular route
Peptide reabsorption
Hydrolyzed at luminal brush border, amino acids reabsorbed by co-transport with Na+
Low molecular weight protein reabsorption
Reabsorbed via endocytosis, then degraded by cellular lysosomes to amino acids
Amino acid transport
Move from cell to peritubular space by facilitated diffusion
Descending limb of loop of Henle
Low epithelium, few mitochondria and membranous infoldings, high permeability to H2O, little to no permeability to Na+, Cl- and urea
Ascending limb of loop of Henle
Taller epithelium, many mitochondria and basolateral infoldings, active transport of solutes, main site of Na+ reabsorption, ~25% K+ reabsorption, impermeable to water
Distal convoluted tubule
Even taller epithelium, dense array of mitochondria, ~80% of water reabsorbed, impermeable to water unless ADH present (late segment), reabsorbs Na+, Cl-, Ca2+