Reabsorption & secretion

Tubular renal cells

  • polar

  • Apical / tubular cells: urine side

  • basolateral cells: interstitial side and peritubular capillaries

  • ­Maintains a concentration gradient for movement of solutes

  • ­Inside of cell = low concentration of Na

 Paracellular transport

  • in between cells

  • channels

Transcellular transport

  • moves across cell membrane

  • simple diffusion

    • CO2/ water

  • transporters

Active transport

  • low to high

  • primary

    • uniport / cotransport

    • FORMS GRADIENT

    • BASOLATERAL SIDE

    • pump mediated

    • ATP requires

  • secondary

    • antiport / symport

    • APICAL / URINE SIDE = REABSORBSION

      • can be on basolateral as well

    • carrier mediated

    • facilitated by concentration gradient of carrier

      • carrier = Na

Passive transport

  • high to low

  • Facilitated diffusion

    • carrier mediated

    • channel mediated

  • simple diffusion

    • water and Urea

Diuretics

  • do not cross glomerulus

  • theyre secreted

Amino acids

  • reabsorbed in PCT

  • ­Carrier mediated w Na+ or H+

  • Apical and basolateral are both secondary active transport

Intact proteins

  • reabsorbed in PCT

    • rare in filtrate but can happen

  • endocytosis to get in and broken down into AA then back to blood

Metabolic intermediates

  • reabsorbed in PCT via cotransport w Na on Apical side

    • exchanged on basal side for anions

      • secondary active transport

PCT cation/ anion secretion

  • tertiary active transport

  • low specificity, high rate (100%)

 Glomerular-tubular balance

  • ­Ability to adjust reabsorption rate to match the filtered load that comes across the glomeruli, and through the peritubular capillaries

  • Excretion > filtered load = net secretion

  • Excretion < filtered load = net reabsorption

Glucose

  • 100% reabsorbed in PT SHOULD NOT BE IN URINE

  • carrier mediated transport

  • ­Apical transport:

    • secondary active, coupled to sodium

    • SGLT2 in early PCT

    • SGLT1 in late PCT

  • Basolateral transport:

    • is facilitated diffusion

    • GLUT2 in early and GLUT1 in late

  • Both are insulin-insensitive transporters

Transport maximum

  • excess glucose youll reach trans max, and cannot reabsorb anymore glucose so its excreted in the urine (not good)

  • reabsorption matches filtered load

  • transports are 100% saturated at blood sugar of 350

  • excretion raises linearly w filtered load

PAH

  • also has transport max

  • filtered and secreted 100% but NOT REABSOBED

  • when transport max is reached = saturation = no more secretion and its excreted linearly w filtered load

Creatinine

  • not 100% filtered

  • about 10% is secreted in PCT

  • H2 blockers can block secretion

    • competes w creatine for secretion channels

    • increases serum creatinine in blood

Fanconi syndrome

  • deficit in PCT reabsorption

  • causes lot of solutes that should have been reabsorbed to be excreted in the urine