Outline the factors that determine the composition and volume of glomerular filtrate in a normal person.

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College Answer

The volume and composition of the glomerular filtrate are best explained by referring to the
equation: Glomerular Filtration = Kf x net filtration pressure. Then describing the factors that
affect each part of the equation e.g. hydrostatic pressure, oncotic pressure, factors that
influence the filtration coefficient e.g. surface area
Glomerular filtrate is an ultrafiltrate of plasma and factors that affect the passage of proteins
and other molecules should be discussed
Extra marks were given for an explanation of filtration fraction and short facts about GFR in
paediatric patients and the elderly.
No marks were given for any discussion of drugs.
Syllabus D2b
Reference; Power and Kam 1st edition p 197-199

Discussion

Glomerular ultrafiltration is described by the Starling equation:

GFR = Kf [ (Pgc - PBC) - σ(Πgc - Πi) ]

where

  • GFR is the glomerular filtration rate, 
  • K is the filtration coefficient of the glomerular filtration surface,
    which is itself a product of:
    • k, the hydrostatic permeability constant of the membrane, and
    • S, the surface area of the glomerular filtration surface, which can be affected by the contraction of glomerular mesangial cells
  • Pgc  is the glomerular capillary hydrostatic pressure
  • PBC  is the hydrostatic pressure of fluid in Bowman's capsule
  • σ is the reflection coefficient for blood protein
  • Πgc is the oncotic pressure in the glomerular capillary blood,  
  • ΠBC is the oncotic pressure of the fluid in Bowman's capsule (usually zero)

Glomerular filtration of solutes is affected by:

  • Molecule size: glomerular size barrier resists the passage of large molecules (>7000 Da)
  • Molecule charge:​ anionic charge of the glomerular filtration surface may resist the passage of anioninc molecules​​​​​​

References

Renkin, Eugene M., and Roscoe R. Robinson. "Glomerular filtration." New England Journal of Medicine 290.14 (1974): 785-792.

Pollak, Martin R., et al. "The glomerulus: the sphere of influence.Clinical journal of the American Society of Nephrology 9.8 (2014): 1461-1469.

Horster, M., and K. Thurau. "Micropuncture studies on the filtration rate of single superficial and juxtamedullary glomeruli in the rat kidney." Pflüger's Archiv für die gesamte Physiologie des Menschen und der Tiere 301.2 (1968): 162-181.

Deen, William M., et al. "Dynamics of glomerular ultrafiltration in the rat. IV. Determination of the ultrafiltration coefficient." The Journal of clinical investigation 52.6 (1973): 1500-1508.

Hoang, Khoi, et al. "Determinants of glomerular hypofiltration in aging humans." Kidney international 64.4 (2003): 1417-1424.

Deen, William M., Matthew J. Lazzara, and Bryan D. Myers. "Structural determinants of glomerular permeability." American Journal of Physiology-Renal Physiology 281.4 (2001): F579-F596.