H1iii: Describe renal tubular function & its regulation

  • The renal tubule is the part of the nephron which leads away from glomerulus → and empties into collecting duct
    • Kidneys produce 180L filtrate/day
    • At all points, tubule it is intimately assoc with PERITUBULAR CAPILLARIES → allows transfer of materials b/w tubule lumen & peritubular capillary blood

Mechianism of Material Transfer

Reabsorption

  • Reabsorbs 99% of Na+ & H2O
  • Almost complete absorption of essential nutrients (amino acids, glucose)
  • Incomplete absorption of waste products → for excretion

→ Movement across TUBULAR WALL

  • Paracellular
    • Across tight junctions
    • Diffusion or solvent drag
    • Requires electrochemical gradient
  • Transcellular
    • Across cytosol & PM
    • Diffusion or active transport
    • Via channels or transporters
    • Active transport mechanisms have a TMAX at which their transfer capacity is saturable

→ Movement across PERITUBULAR CAPILLARY WALL

  • Diffusion
    • Down electrochemical tradient
  • Bulk flow
    • Driven by Starling Forces (diff b/w hydrostatic & oncotic P)
    • Favours movement into caps because:
      1. Peritubular cap P is low
      2. Oncotic P of peritubule caps is v high cf. tubule fluid because ultrafiltrate should be relatively protein free
      3. Permeability (of Kf) is v high in these particular capillaries

Secretion

  • For a substance to be secreted it must be in Ultrafiltrate
  • Can be transcellular/paracellular diffusion or transcellular active transport

Metabolism

  • Metabolism by tubule cells for their own nutritional requirement
  • Metabolism of tubule fluid for urine/plasma composition (i.e. NH3 from glutamine, production of HCO3–)

Regulation of Tubule Function

  • Control of secretion/reabosprtion is by regulation of membrane proteins → neurotransmitters & hormones & membrane potential
  • Most bulk reabsorption occurs at PCT
  • LoH responsible for Countercurrent Multiplication
  • DCT & collecting ducts → fine tune urine & most regulation is exerted here

Division of Labour

  • PCT
    • Most reabsorption of ultrafiltrate
    • Most metabolically active
    • High O2 consumption
    • Complete glucose & amino acid reabsorption
    • Almost all HCO3–
    • 65% Na+ & H2O reabsorption
  • LoH
    • Strategically impermeable to solutes
    • Osmosis as part of countercurrent multiplier
  • DCT
    • Impermeable to H2O
    • Some solute reabsorbed by Na/Cl co-transporter → driven by Na/K/ATPase
    • Ca2+ reabsorption controlled by PTH
  • Collecting Duct
    • Fine tuning of urine composition
    • Apical Na+ channels → controlled by aldosterone
    • H2O reabsorption via Aquaporin channels → controlled by ADH
    • K+ & H+excretion