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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