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:
- Peritubular cap P is low
- Oncotic P of peritubule caps is v high cf. tubule fluid because ultrafiltrate should be relatively protein free
- 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