Glomerular Filtration and GFR Flashcards
How much of the CO is bloodflow to the kidney?
20% (1L/min)
What type of water does filtration of the blood occur in?
Plasma water only
What are the components of the glomerular filtration barrier?
- Endothelial cells of glomerular capillaries
- Glomerular basement membrane
- Epthelial cell foot processes including proteins of the podocytes and filtration slits
2/3 of what ions are reabs in the PT?
Na, K+, Cl-, H20
80% of what is reabs in the PT?
Bicarb and phosphate
What is almost all reabs in the PT?
Glucose and amino acids
What is the first barrier to filtration?
Endothelium
What are capillary fenestrations filled with?
Glycocalyx with various negatively charged proteoglycans that restrict filtration across endothelial layer
–>restrict albumin
What driving pressures push fluid out of the glomerulus capillary? (ie favor filtration)
Capillary hydrostatic pressure (constant in a normal kidney) and bowman’s oncotic pressure
What driving pressures oppose ultrafiltration?
Bowman’s hydrostatic pressure and capillary oncotic pressure (these are low in a normal kidney)
Describe Bowman’s space hydrostatic pressure
- Opposes filtration
- Low
- Entire capillary sees the same low pressure along its entire length
- Fxn of rate of urine formation and flow of urine into the PT
Describe Bowman’s oncotic pressure
- Favors filtration
- But is very small except under pathologic conditions since normally proteins are only minimally filtered
Describe glomerular capillary hydrostatic pressure
Nearly constant along the length of the glomerular capillaries due to regulation of resistances of afferent and efferent arterioles (which determine this pressure)
Describe glomerular capillary oncotic pressure
- Opposes filtration across the capillary
- Rises during transit along cap bed because of progressive extraction of water by ultrafiltration across the filtration barrier
How do we calculate the net driving force favoring ultrafiltration?
Difference between forces favoring filtration and those opposing
What is the formula for filtration fraction
GFR/renal plasma flow
Angio II effect on arteriolar tone?
Vasoconstriction (efferent>afferent)
Catecholamines effect on arteriolar tone?
Vasoconstriction (afferent>efferent)
Endothelin effect on arteriolar tone?
Vasoconstriction, both
Prostaglandins effect on arteriolar tone?
Vasodilation on efferent (NSAIDs inhibit these)
Describe autoregulation of GFR
Mostly a fxn of changes in afferent arteriolar tone/resistance
Maintained by a myogenic response, when afferent art are stretched, they respond with contraction and by tubulo-glomerular feedback system)
Where is the juxtaglomerular apparatus located?
Thick ascending limb of the loop of henle and the glomerulus between the afferent and efferent vessels
What are the cells of the thick ascending limb of the loop at the JGA?
macula densa cells adj to the hilum of the glomerulus
What do macula densa cells do?
Sense sodium and cause a signaling cascade that modulates afferent arteriolar resistance by causing juxtaglomerular cells in the walls of the afferent arteriole to synthesize renin
Tubulo-glomerular feedback
- Uptake via Na-K-Cl cotransporter on luminal membrane of the MD cells is driven by an increase in Cl concentration
- Increased production of adenosine which
- Triggers an increase in cytosolic calcium in the extraglomerular mesangial cells
- Granular cells containing renin and smooth muscle cells of afferent arterioles vasoconstrict and decrease renin production
—ie increase cl tells us that there is too much volume, pee stuff out please
How many liters per day of plasma water do the kidney’s filter?
180L/d
What is a normal GFR
120-125ml/min
What is clearance?
Urinary excretion rate (U*V)/Plasma concentration (P)=clearance
- net of filtration, reabsorption and secretion
- Not about how much is removed, but the volume of the plasma it was removed from
What are the characteristics of an ideal GFR marker?
- Inert/non-toxic
- Freely filtered
- Not metabolized or synthesized in the kidneys
- Not secreted
What is the equation for GFR?
(UV)/P of inulin or creatinine …but really we use BUN/Creatinine ratio to estimate the GFR because creatinine overestimates GFR and urea underestimates GFR
What happens to creat concentration in the urine as GFR decreases?
More is found in the urine. It is derived from creatinine secreted by the tubules rather than the glomerular filtrate
–>so it becomes less of a good measure of GFR at lower levels. At normal GFR, secretion accounts for 10-15% of creatinine in urine. At low GFRs it raises to 45%
–>organic cation transporters excrete creatinine
Drugs that inhibit creatinine secretion
inhibit the organic cation transporters that excrete it
- Cimetidine
- Trimethoprim (bactrim, septra)
- Probencid
Plasma creatinine does what as GFR declines?
Increases
What is a possible future substitute for creatinine measuring GFR?
Cystatin C
What is a fractional excretion?
Amt filtered/Amt in urine (UV/P*GFR) if freely filtered
