Mayank AgarwalMayank AgarwalY. Ashutosh Y. Ashutosh BhardwajBhardwaj

What Is Glomerular What Is Glomerular Filtration?Filtration?

• First step of urine formation• Filtration of fluid from glomerular capillary network into Bowman’s capsule

GlomerulGlomerulusus

about 200 μm in diameter

Glomerular Capillary Glomerular Capillary MembraneMembrane

Mesangial cells are similar to cells called pericytes, which are found in the walls of capillaries elsewhere in the body

Mesangial cells are contractile and play a role in the regulation of glomerular filtration

Relation of mesangial cells and podocytes to glomerular capillaries

Capillary endothelium is perforated by thousands of small holes (70–90 nm in diameter) called fenestrae

Basement membrane consists of a meshwork of collagen and proteoglycan fibrillae having negative charges

Podocytes have numerous pseudopodia that interdigitate to form filtration slits (25nm)

Total area of glomerular capillary endothelium across which filtration occurs in humans is about 0.8 m2

Glomerular membrane permits the free passage of neutral substances up to 4 nm in diameter and almost totally excludes those with diameters greater than 8 nm

Charge on molecules as well as their diameters affects their passage into Bowman’s capsule

Albumin (6 nm diameter) is restricted from filtration because of its negative charge

Glomerular filtrate has same composition as plasma except: Proteins Certain substances with low molecular weights such as fatty

acids and calcium which remain bound to proteins

Determinants Of GFRDeterminants Of GFR

• Kf : capillary filtration coefficient (product of the permeability and surface area of the capillaries), 12.5 mL/min/mm Hg

• PG : glomerular hydrostatic pressure, 60 mm Hg

• PB : hydrostatic pressure in Bowman’s capsule, 18 mm Hg

• πG : colloid osmotic pressure of glomerular capillaries, 32 mm Hg

• πB is the colloid osmotic pressure in Bowman’s capsule, 0 mm Hg

GFR =Kf × Net filtration pressure

Net filtration pressure = (PG − PB− πG + πB)

GFR = Kf x Net Filtration Pressure= 12.5 mL /min /mm Hg x 10 mm Hg= 125 mL /min= 180 L /day

Filtration Fraction = GFR ÷ Renal plasma flow = 125 mL/min ÷ 625 mL/min = 0.16 to 0.20

(RE=Efferent arteriolar resistance)

(AP=arterial pressure)

(RA=Afferent arteriolar resistance)

Mesangial cells contraction reduced filtration surface area Kf GFR

Podocytes possess actin like filaments, whose contraction or relaxation can, respectively, decrease or increase the number of filtration slits open in the inner membrane of Bowman’s capsule

The more open slits, the greater the permeability

Autoregulation of Autoregulation of GFR GFR

Myogenic mechanismTubuloglomerular feedback

GFR EstimationGFR Estimation

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BibliographyBibliography

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