Renal Filtration Notes

Anatomy for Filtration

  • Kidney Anatomy

    • Renal Medulla
      • 10-18 Renal Pyramids
      • Renal Papilla (Nipples)
    • Renal Pelvis
    • Major Calyces
    • Minor Calyces
    • Renal Cortex
      • Outer Cortical Zone
      • Inner Juxtamedullary Zone
    • Renal Columns
    • Renal Lobes

  • Renal Blood Supply

    • Renal Artery
      • Segmental Artery
      • Interlobar Artery
      • Arcuate Artery
      • Cortical Radiate Artery
    • Afferent Arteriole
      • Glomerulus (Capillaries)
    • Efferent Arteriole
      • Peritubular Capillaries
    • Venule (Not to Veins)

Filtration Membrane Layers

  • Endothelium:

    • Part of the capillary.

  • Basement Membrane:

    • Separates the Epithelium and the endothelium.

  • Epithelium:

    • Made of Podocytes.
      • Wraps around the basement membrane like tentacles.
      • Has gaps called Filtration Slits that allows passage of small molecules.

  • Juxtaglomerular complex

Anatomy of the Filter

  • Fenestrated Endothelia

    • Size: 70nm to 100mm
    • Part of the capillaries.
    • Filters out cells.

  • Basement Membrane

    • Made of Collagen (Type IV)
      • Collagen is negatively charged.
      • Repels/Filters out negatively charged proteins.
      • Lamina Densa is the \"Filter\" part of the basement membrane.

  • Podocytes (Epithelium)

    • Size: 10nm to 40nm
    • Has gaps called Filtration Slits.
      • Wraps around the basement membrane like tentacles.
      • Has Finger-like arms called Pedicels.
    • Filtered molecules size: ≤ 4 nm

Glomerular Filtration

  • If 100% is the volume of the plasma in the afferent arteriole, then 20% passes through the filter membrane to form the initial filtrate. However, 19% is reabsorbed, so the volume of urine is always much lower than 1% of the afferent arterial volume.

  • Mesangial cells role:

    • Controls resistance on afferent and efferent arterioles.
    • Helps communicate between macula densa cells and juxtaglomerular cells.

  • Positively and negatively charged protein molecules cannot pass through the filter, but medium sized molecules with a Positive charge can. Very small molecules even if they have a negative charge can pass.

  • Proteoglycans role:

    • Make up the basement membrane making it filter negative and big molecules

  • The principal force favouring filtration out of the capillary is Glomerulus Hydrostatic Pressure which has a pressure of 60mmHg. Opposing this are two pressures:

    • Bowman’s Capsule Pressure: 16mmHg
    • Glomerulus Colloid Osmotic Pressure: 34mmHg

  • Net filtration pressure (NFP)

    • NFP = Pressure\ causing\ filtration - Pressures\ resisting\ filtration
    • NFP = Glomerulus\ Hydrostatic\ Pressure – ( Pbc + Pgco)
    • NFP = 60mmHg – (16mmHg + 34mmHg)
    • NFP = 10mmHg

  • Glomerular filtration rate (GFR)

    • On average, 125ml/min which is 180L/day
    • GFR is used by ward doctors are the main indicator of Kidney Function
    • The usual way that the kidney reduces the amount of filtrate produced is by altering the glomerular Hydrostatic Pressure is by increasing the resistance in the Afferent
    • In contrast, increased resistance in the efferent Increases Glomerular Hydrostatic Pressure, therefore Increase GFR

  • Volume of Blood Filtered be unit of time