12 Questions
How does alteration in the filtration area due to mesangial cell contraction affect the filtration coefficient (Kf)?
Decreases Kf
What happens to the filtration coefficient (Kf) when there is thickening of the capillary membrane?
Decreases
What is the relationship between Glomerular capillary hydrostatic pressure (PGC) and GFR?
Directly proportional
How does hyperproteinaemia affect Glomerular capillary oncotic pressure (πGC) and GFR?
Increases πGC but decreases GFR
How does strong sympathetic stimulation affect renal blood flow (RBF)?
Marked decrease in RBF
What effect does mild to moderate sympathetic stimulation have on renal blood flow (RBF)?
Mild increase in RBF
What is the normal glomerular filtration rate (GFR) in an average sized man?
125 mL/min
What does the filtration fraction (FF) represent?
The ratio of GFR to the renal plasma flow
What percentage of the renal plasma flow is actually filtered per minute under normal conditions?
20%
Which process involves ultrafiltration of plasma from the glomerular capillaries into the Bowman's capsule?
Glomerular filtration
How is the filtration load calculated?
GFR x Plasma concentration of substance
What is the main function of the glomerulus in kidney physiology?
Filtration of substances from blood to form the primary filtrate
Study Notes
Glomerular Filtration
- Glomerular filtration is the process of ultrafiltration of plasma from the glomerular capillaries into the Bowman's capsule.
- Glomerular filtration rate (GFR) is the rate of filtration across the glomerular membrane, expressed as ml per minute.
- Normal GFR in an average-sized man is about 125 mL/min (range 90–140 mL/min).
- 180 L/day of plasma is filtered at the glomerulus, with 99% or more reabsorbed and only 1% or less excreted as urine.
Filtration Fraction
- Filtration fraction (FF) is the ratio of GFR to renal plasma flow (RPF).
- FF is approximately 0.2 (125/650) at normal values of GFR 125 mL/min and RPF 650 mL/min.
- Only about 20% of the renal plasma flow is actually filtered per minute.
Filtration Load
- Filtration load is the amount of substances filtered by the kidneys per day, calculated as GFR x plasma concentration of substance.
Factors Influencing Glomerular Filtration
Filtration Coefficient (Kf)
- Kf is the product of permeability and filtration area of the glomerular capillary membrane.
- Kf is directly proportional to GFR.
- Permeability is increased in hypoxia and toxic agents, and decreased by thickening of the capillary membrane.
- Alterations in filtration area occur due to contraction or relaxation of mesangial cells.
Hydrostatic Pressure in Bowman's Space Fluid (PBS)
- Hydrostatic pressure inPBS opposes filtration and therefore GFR is inversely related to it.
- Decreased PBS leads to increased GFR, as seen in acute obstruction of the urinary tract (e.g., ureteric obstruction by stone).
Glomerular Capillary Hydrostatic Pressure (PGC)
- GFR is directly proportional to PGC.
- PGC is dependent on:
- Arterial pressure: GFR is autoregulated between 80–200 mm Hg, and increased above 200 mm Hg or decreased below 70 mm Hg affects GFR.
- Renal blood flow: GFR is directly proportional.
- Afferent and efferent arteriolar resistance.
Glomerular Capillary Oncotic Pressure (πGC)
- GFR is inversely proportional toπGC.
- Increased πGC in hyperproteinaemia and haemoconcentration leads to decreased GFR, while decreased πGC in hypoproteinaemia and haemodilution leads to increased GFR.
Sympathetic Stimulation
- Mild to moderate stimulation has mild effects on renal blood flow (RBF) due to autoregulation mechanism.
- Strong stimulation leads to a marked fall in RBF (10–30%) due to constriction of both afferent and efferent arterioles.
Test your knowledge on glomerular filtration rate (GFR), filtration fraction, factors influencing GFR, and variations in GFR. Learn important concepts in kidney function and renal physiology.
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