GLOMERULAR FILTRATION RATE| Determinants of GFR| Factors affecting GFR

GLOMERULAR FILTRATION RATE

GLOMERULAR FILTRATION RATE; It is the process by which filtration occurs in the kidney from the blood in the glomerular capillaries. Filtrate formed is called glomerular filtrate. Normal Glomerular filtration rate (GFR): 125 ml/min, or 180 L/day. 1^st Step in urine formation. Urine formation includes 3 processes:

• Glomerular filtration
• Tubular reabsorption
• Tubular secretion

Here I am explaining only on glomerular filtration.

Glomerular Capillary Membrane

• Endothelial Layer: Fenestrated, Diameter = 70-   90 nm, contains negatively charged Sialo Protein
• Basement Membrane: negatively charged Proteoglycans, does not contain visible gaps or pores
• Epithelial Cells (Podocytes): Foot like processes i.e. pseudopodia, interdigitate & form filtration slit, about 25nm wide

                                           

Glomerular Capillary Membrane

                   

Determinants of GFR

• Balance of hydrostatic and colloid osmotic forces in capillaries
• Capillary filtration coefficient (K_f) (Product of permeability and filtering surface area)
• GFR = K_f x Net Filtration Pressure (NFP)

Where, NFP = Total force favoring filtration – Total force opposing filtration.

Determinants of GFR

                               

•        Increased glomerular capillary filtration coefficient increases GFR

•        Increased Bowman’s capsule hydrostatic pressure decreases GFR

•        Increased glomerular capillary hydrostatic pressure increases GFR

•        Increased glomerular capillary colloid osmotic pressure decreases GFR

 

Permeability of Glomerular Membrane

•    Molecular Size:  Neutral substances that are <4 nm wide are freely filtered but >8 nm are not filtered.

•    Shape: Elongated particles can easily pass the membrane than the globular particles.

•  Electrostatic Charge: negatively charged large molecules are filtered less easily than positively charged molecules of same size.

Renal Autoregulation of GFR

Kidneys themselves help to maintain a constant GFR despite normal, everyday changes in blood pressure

Myogenic mechanism

• Systemic increase in BP also increases GFR.
• Increase to BP stretch the afferent arteriole.
• Smooth muscles fibers contract, reducing diameter of arterioles-decreases blood flow and GFR.

Tubular glomerular feedback

• Elevated systemic BP raises the GFR-blood flows too rapidly through the renal tubules and Na⁺, Cl^- and water are not reabsorbed quickly enough.

• Afferent arteriole constricts and reduce GFR.

 

                                

Tubular glomerular feedback

GFR measurement

Clearance: a mathematical concept

• Renal clearance of a substance is the volume of plasma that is completely cleared of the substance by the kidney per unit time.
• Calculated from:
• U = concentration of substance in the urine
• V = volume of urine per minute
• P = concentration of substance in the plasma

Clearance = U x V

                      P

Factors affecting GFR

• Renal plasma flow
• Afferent and efferent arteriole resistance
• Mesangial cell contraction
• Pressure in Bowman’s capsule
• Plasma protein concentration
• Permeability of capillary membrane
• Reduction in effective area of the filtration surface

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