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3 - Determinants of GFR


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On what two properties does ultrafiltration depend on?
1. Anatomical properties of the filtration barrier
2. Blood flow hemodynamics
What is the structure of the filtration barrier? What is the size of the holes in the barrier?
*capillary endothelium, basement membrane, podocytes with gaps between foot processes
*~40 angstroms
What property other than size may limit passage through the filtration barrier?
Charge - the membrane has a negative charge and thus exerts a repellant force on negatively-charged particles
Are cations and anions able to pass through the filtration membrane?
Yes - the small size of the anions negates the effect of the negative charge
How small do substances need to be in order to be freely filtered?
Less than 20 angstroms
What is increased GFR called? What might cause decreased GFR?
*a decrease in the number of functional glomeruli
Name two things whose presence in the urine might indicate a glomerular disease.
1.blood - hematuria
2.albumin - proteinuria
(3.glucose - glucosuria)
What occurs in proliferative diseases causing decreased GFR?
Mesangial cells proliferate and occlude blood flow.
How does membraneous disease cause a decrease in GFR?
By increasing the thickness of the filtration membrane.
What happens in necrotic disease of the glomeruli? What about sclerotic?
In necrotic disease, glomerular cells die. In sclerotic disease there is damage and fibrosis with excess collagen, which often occludes the glomerulus.
How is GFR altered in type I diabetes?
High concentration of glucose in the blood leads to abnormal glycation of proteins of the filtration membrane. This masks the normal negative charge of the membrane and alters its filtration characteristics.
Name two differences between glomerular and systemic capillary beds that allow net filtration in the glomeruli.
*hydrostatic pressure is much higher in the glomeruli and remains greater than colloid oncotic pressure for the length of the bed
*Kf is 100-200 times greater
What is the MFP? How is it calculated?
*mean filtration pressure
*MFP = (Pgc - Pbs) - mean COP
*mean COP = (COPaff + COPeff)/2
How can GFR be calculated from Kf?
GFR = Kf x MFP
Filtration pressure disequilibrium is the normal state for the glomerulus.
True - it is because hydrostatic pressure remains greater than COP that net filtration occurs.
How would an increase in GFR without an accompanying increase in RPF affect filtration?
Filtration would decrease as the increase in GFR would result in higher COP in the capillary. If COP increased to a value greater than hydrostatic pressure no net filtration would occur.
What are three factors that might alter GFR?
1.Alter Pgc
2.Alter arteriolar resistance
3.Alter COP
4.Alter Kf
Name two things that constrict glomerular arterioles.
1.Sympathetic innervation (afferent arterioles)
2.Angiotensin II (efferent arterioles)
Name three things that dilate glomerular arterioles.
2.Nitric oxide
4.decreased sympathetic tone
5.decreased angiotensin II
Do most mediators of glomerular blood flow affect afferent or efferent arterioles? Why?
Most affect afferent arterioles because they have thicker walls.
Describe the effect of increasing afferent resistance on Pgc, GFR, and RBF.
Decreases Pgc, GFR, and RBF.
Describe the effect of increasing efferent resistance on Pgc, GFR, and RBF.
Increases Pgc and GFR, but decreases RBF.

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