Reabsorption & Secretion: A Trip Down The Tubule

Lecture given:


Describe how reabsorption of solute and water from the tubule into the bloodstream and secretion of solutes from the bloodstream into the tubule are key aspects of nephron function

  • Ultrafiltrate has similar content as plasma but without protein
  • Normally, ~150-180 litres/day of ultrafiltrate is formed, but we need to conserve essential nutrients and solutes while removing toxic wastes
  • >99% of filtered sodium salts and water are reabsorbed
Substance Urine/Plasma Ratio
Glucose 0
Sodium ~1
Urea ~60
Creatinine ~150

List the various components of the renal tubule and how they function to help generate urine

Na/K/ATPase H2O Ions Glucose, AA, protein Uric acid, Creatinine, drugs Diuretic Histology Mutation
PCT yes reabs reabs reabs secrete CI 15mm; Dark cytosol, small lumen Fanconi's
Thin loop (D) little highly reabs little no no N/A 2-14mm; epithelial cells like sunny side up eggs
Thin loop (A) yes NO passive reabs no no N/A
TAL yes NO NaCl reabs, X2+ reabs no no Loop 12mm; Thicker cells, larger nucleus Bartter's
DCT yes no? Ca2+ reabs, Cl reabs no no Thiazide 5mm; lighter cytosol, larger lumen Gitelman's
CD (p) yes (aldo) yes (ADH + Aldo) reabs K or Cl no no K sparing 20mm; Cell divisions visible due to interdigitation
CD (α) yes no H secretion no no N/A
  • glomerular capillaries have fenestrations that make them ~5000 times more permeable than brain
  • ~25% of cardiac output are diverted to renal blood flow (RBF)
  • renal plasma flow = (1-hct)(RBF)
  • GFR = ~20% of renal plasma flow
  • Most of the reabsorption/secretion is driven by gradients and Na/K/ATPase

Out of lumen; Into lumen

Proximal Tubule

  • Symporter: HCO3-, only place for Glucose, AA, Phosphate, and vitamins absorption
  • Anteporter: H+, NH4+
  • Important process: H2O -> H+ + OH-; OH- + CO2 --Carbonic Anhydrase--> HCO- (vasa recta)
  • No anteporter/symporter: Organic cations and anions (e.g. urate, creatinine, drugs)

Thin Loop of Henle

  • 2-14 mm long depending on location of the nephron
  • Descending: very little Na/K/ATPase, so very little salt movement, but highly permeable to water
  • Ascending: Impermeable to water, highly permeable to salt

Thick Ascending Limb

  • 12 mm long, pass near afferent arteriole where the macula densa form the juxtaglomerular apparatus
  • Symport: Na, K, Cl (loop diuretics)
  • Channel: K
  • NO water absorption
  • Absorbs ~30% of filtered NaCl
  • K back leak causing urine to be positively charged, and will push X2+ and cations through paracellular spaces
  • Therefore, loop diuretics will cause calcium loss in the urine

Distal Convoluted Tubule

  • ~5mm long
  • Symporter: Cl (Thiazides)
  • Channels: Ca -> Ca+Vit D
  • Basal side has Na/Ca anteporter to put Ca into vasa recta
  • Major site of Ca absorption via calcium channel, and combine with Vitamin D-induced Ca binding protein
  • Cl sensor in cell to control activity of symporter
  • If lots of Cl in cell, means there's lots of Na in cell too through the symporter. Ca will be repelled and not come through the channel as easily.
  • Thiazides can be used to treat kidney stones by inhibiting the symporter to increase Ca absorption

Collecting Ducts

  • Coalescence of many DCTs
  • ~20 mm, emput into renal pelvis
  • Principal cells:
    • Na move through channels will repel K to move through channels, and Cl can move paracellularly
    • Aquaporins can be inserted via ADH to modify water content of urine
    • Aldosterone binds to Aldosterone receptors in the nucleus by entering from the basal side (Spironolactone) and increase the activity of the Na channel. ANP decreases its activity
  • α-intercalated cells: H+ channels and K & H anteporters are important for acidfying urine (pH ~6)
    • Movement of Na from principal cells out of the urine will attract H+ into the urine
  • Things interfering with principal cell function will cause hyperkalemia AND renal tubular acidosis

Counter-current Exchange

  • The descending and ascending limbs of Henle are next to each other
  • From the PCT, the filtrate is still pretty dilute, but the interstitium is already concentrated due to vasa recta carrying away the water
  • Since the descending limb is impermeable to salt, it lets only water out into the concentrated interstitium
  • When the filtrate gets to the bottom of the loop, the osmolality can be as high as 1200 mOsm/L, as concentrated as the interstitium (diffusion)
  • Then the ascending limb will allow salt to move into the interstitium (first passively, then active secretion)
  • By the time the filtrate gets to the collecting duct, the filtrate can be only 50 mOsm/L
  • In the collecting duct, urea can also move into the interstitium to concentrate urine, and urine can be concentrated or diluted depending on the presence of aquaporins
  • So, the urine can have a range of concentration 50-1200 mOsm/L


Unless otherwise stated, the content of this page is licensed under Creative Commons Attribution-ShareAlike 3.0 License