Glomerular Filtration Notes

Lecture 3 & 4

Glomerular Filtration

• Introduction

  • Occurs at rate ~120ml/min

  • Urine output ~1ml/min (i.e. over 99% glomerular filtrate normally absorbed)

  • Varies with: gender, age, body size

  • Energy comes from hydrostatic pressure of blood (generated by heart beat)

  • No energy expenditure by kidney in filtering glomerular blood

• Glomerulus as a filtration barrier

  • Behaves as though perforated by ‘functional’ pores of the diameter ~8-10nm (1/1000 diameter of RBC)

  • Rate of filtration of given substance depends upon

    • Molecular weight

      • <10,000= freely filtered

      • >100,000= excluded from glomerular filtrate

      • Rate of filtration ~ inversely proportional to molecular weight

    • Shape

      • Long, thin molecules filtered more easily than spherical molecules of same molecular weight

    • Electrical charge

      • Ease of filtering

        • (+) charge >> Neutral >> (-) charge

  • Examples

    • Albumen

      • Molecular weight= ~69,000

      • (-) charge

      • Normally almost completely retained in blood of glomerular capsule

    • Free haemoglobin

      • Molecular weight= ~68,000

      • Not charged

      • Some leaks into Bowman’s space if in blood (e.g. IV haemolysis)

• Anatomical barriers in the Glomerulus and Bowman’s capsule (3)

  • Capillary endothelium

    • Has open pores (fenestrations)

    • ~100nm wide

    • Do not prevent plasma protein filtration

  • Basement membrane

    • Has fixed (-) charge

    • Gives the electrical charge selectivity of the filter

    • NEPHROTIC SYNDROME removes negative charge

  • Glomerular epithelium (podocytes)

    • Epithelial cells put out primary and secondary processes (podocytes)

    • These envelope glomerular capillaries

    • Molecules must get through gaps between podocytes

• The factors involved in glomerular filtration

  • GFR= K S [(P_GC - P_T) - (π_{GC -} π_T)]

  • K= Permeability of glomerular membrane

  • S= Surface area available for filtration

  • KS= Filtration coefficient

  • P_GC= Hydrostatic pressure in glomerular capillaries

  • P_T= Hydrostatic pressure in Bowman’s capsule/tubule

  • (P_GC - P_T)= Net hydrostatic pressure FAVOURING filtration

  • π_GC= Colloid osmotic (oncotic) pressure in glomerular capillaries

  • π_T= Colloid osmotic pressure in Bowman’s space/tubule

  • (π_{GC -} π_T)= Net colloid osmotic pressure OPPOSING filtration

• Hydrostatic and oncotic pressure changes during filtration

  • No reabsorption, just filtration

  • Afferent

    • Driving pressure for filtration ~(45-10) – (25-0)

    • Higher P_GC than elsewhere in body as it drains to efferent arterioles (not veins, so thicker, causing higher pressure)

    • 10mmHg

  • Efferent

    • Driving pressure for filtration ~(44-10) – (34-0)

    • π_GC risen because lots of fluid lost, but none of plasma protein. Low fluid, high protein

    • 0mmHg

• Filtration along a glomerular capillary

  • Blood coming out of efferent arteriole

    • High colloid osmotic (oncotic) pressure

    • High haematocrit

    • Very viscous

• Changes in blood hydrostatic & colloid osmotic pressure as blood flows through renal vessels

  • Hydrostatic pressure

    • Renal artery= high (90mmHg)

    • Afferent arteriole= decrease

    • Glomerular...

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