GFR, RPF, RBF, Clearance Flashcards
Filtered, but neither reabsorbed nor secreted by renal tubules
inulin
GFR formula = C (inulin) =
[(urine concentration of inulin)(urine flow rate)] / (plasma concentration of inulin)
Effect of age on GFR?
decrease
effect of decreased GFR on BUN?
increase
effect of decreased GRF on plasma[creatinine]
increase
effect of decreased GFR on extracellular potassium?
increase
what three things does decreased GFR result in?
Increased BUN, creatinine, extracellular potassium
In prerenal azotemia (hypovolemia), BUN increases more than serum creatinine →
increased BUN/creatinine ratio (> 20:1)
GFR = Kf [ (HPgc – HPbs) – (OPgc – OPbs)]
Starling equation
Filtered Load (amount of substance Z filtered per unit time) =
GFR * [plasma concentration Z]
Excretion Rate =
V * Ux
Excretion rate also =
(filtered load) – (reabsorption rate) + (secretion rate)
When glomerular capillary oncotic pressure decreases, what happens to GFR?
Increase
A BUN/creatinine ratio > 20:1 is indicative of what condition?
Prerenal azotemia (hypovolemia)
What is the driving force for glomerular filtration?
net ultrafiltration pressure across the glomerular capillaries
Bowman’s space oncotic pressure
Is usually zero since only a small amount of protein is filtered
Means of increasing HPgc (glomerular capillary hydrostatic pressure)
Dilation of afferent arteriole or constriction of efferent arteriole will increase HPgc
Constriction of ureters will increase HPbs
HPbs – Bowman’s space hydrostatic pressure
Decrease in oncotic pressure in glomerular capillary –>
Increased GFR
OPgc decreases by decreases in capillary protein concentration
cirrhosis or nephrotic syndrome
Relative clearances
PAH > K > inulin > urea > Na > glucose, amino acids, and HCO3-
The ability to dilate urine
Free water clearance
Free Water Clearance (CH2O) =
V – Cosm
where V = urine flow rate (ml/min) and
Cosm = osmolar clearance (UosmV/Posm)
With ADH: CH2O < 0
(retention of free water)
Without ADH: CH2O > 0
(excretion of free water)
Isotonic urine: CH2O = 0
(seen with loop diuretics)
the volume of plasma cleared of a substance per unit time
Clearance
equation for clearance
C (x) = U (x) V / P (x)
RBF is directly proportional to
pressure difference between the renal artery and the renal vein
RBF is inversely proportional to
the resistance of the renal vasculature
Autoregulation of RBF is achieved by changing
renal vascular resistance; RBF remains constant over a range of arterial pressures
Myogenic mechanisms of RBF autoregulation:
afferent arterioles contract in response to stretch caused by increased arterial pressure
Tubuloglomerular feedback:
increased renal arterial pressure leads to ↑ delivery of fluid to the macula densa. The macula densa secretes paracrine signals that lead to constriction of the nearby afferent arteriole.
RBF is related to RPF (Renal Plasma Flow) by the expression:
RBF = [ RPF / (1-Hematocrit) ]
Renal plasma flow can be approximated by measuring the clearance of
PAH (paraaminohippuric acid)
Why is PAH clearance a good representation of effective renal plasma flow (RPF)?
PAH is both filtered and secreted by the renal tubules (ie, none is reabsorbed)
Why does PAH clearance underestimate the true RPF by about 10%?
it doesn’t measure RPF to regions of the kidney not involved in the filtration and secretion of PAH
Effective RPF:
C (PAH) = [Urine] PAH x V / [Plasma] PAH
Filtered load of glucose is directly proportional to
plasma [glucose]
What reabsorbs glucose?
Na-glucose cotransport in the proximal convoluted tubule (there are a limited number of these Na-glucose carriers)
lowest plasma [glucose] at which glucosuria occurs.
Threshold =
Threshold ~180mg/dL.
glucose reabsorption rate at which all Na-glucose carriers are saturated ∴
Transport maximum
Tm is directly proportional to the number of.
functioning glucose transporters
functioning nephrons.
Tm in adult males
~375mg/min, which corresponds to a plasma [glucose] of 300mg/dL if GFR is 1.25dL/min [ 300mg/dL x 1.25dL/min = 375mg/min ]
If plasma [glucose] < 180mg/dL →
plenty of Na-glucose carriers are available:
- all filtered glucose is reabsorbed
∴ excretion of glucose is zero.
If plasma [glucose] > 300mg/dL →
all Na-glucose carriers are saturated:
- reabsorption is saturated ∴ any additional ↑ in plasma [glucose] results in glucosuria
If plasma [glucose] is between 180mg/dL and 300mg/dL,
some Na-glucose carriers are saturated while others are not, and the degree of Na-glucose carrier saturation varies from nephron to nephron (splay)
“Splay” represents glucosuria before reabsorption is fully saturated, and can be explained by:
1) Nephron heterogeneity
2) Relatively low affinity of Na-glucose cotransporters
equation for filtration fraction
FF = GFR / RPF
the fraction of RPF (renal plasma flow) filtered across the glomerular capillaries. The amount filtered becomes the urine.
FF (filtration fraction)
Normal FF
20%
80% leaves through efferent arterioles to enter the peritubular capillaries
↑ in FF leads to
↑ protein concentration of peritubular capillary blood, which leads to increased reabsorption in the proximal tubule
↓ in FF leads to
↓ in the protein concentration of peritubular capillary blood and decreased reabsorption in the proximal tubule
↑ HPgc –>
↑ GFR
if pressure is increased inside the glomerular capillary, then there will be a greater driving force pushing plasma through the capillary wall.
↑ HPbs –>
↓ GFR
↑OPgc –>
↓ GFR
↓ HPgc –>
↓ GFR
