RBF, GFR, and Clearance Flashcards
Normal RBF
1.3 Liter/min
Normal Renal Plasma Flow (RPF)
0.65 Liter/min
Assuming RBF of 1.3 Liter/min and HCT = 50%
Normal GFR
130 ml/min
20% of RPF
What is the molecular size cut-off of the glomerular filtration apparatus?
~60,000 daltons
Starling Equation
GFR = K (Pg - Pt - PIgc)
Prostaglandins - 2 functions
Produced by medullary interstitial cells in response to Angiotensin II
Dilate afferent arteriole of the glomerulus, maintaining adequate RBF; blunts the effect of AgII on renal arteriole constriction, protecting against acute renal failure in hypovolemia
Glomerulotubular Balance
The ability of the obligatory reabsorption mechanisms in the proximal tubule to compensate for changes in the filtered load such that a fixed proportion of the the filtered load of water and NaCl is always reabsorbed (65%)
Tubuloglomerular Feedback
Increased GFR causes a rise in tubular fluid flow, which decreases the proportional reabsorption of NaCl; increased NaCl concentration is sensed by the Macula Densa at the beginning of the DCT, which paracrine signals the afferent arteriole to contract,, thus returning GFR to normal
Macula densa
A group of specialized, NaCl sensitive cells at the start of the distal tubule; they monitor the status of obligatory reabsorption just before the filtrate enters the fine tuning segments and can cause the afferent arteriole to dilate or constrict
Clearance - Definition & Equation
The volume of plasma from which all of substance X is removed per unit time
Clearance = U[X] x V / P[X]
Effects of NSAIDS on GFR
NSAIDS shut down the prostaglandin-mediated vasodilation of the afferent arteriole, causing vasoconstriction and potential renal failure
Effects of ACEIs/ARBs on GFR
ACEIs/ARBs shut down the Angiotensin II mediated vasoconstriction of the efferent arteriole in response to decreased MAP, causing reduced GFR
24 hour urine collection
Uses the formula ClCr = UCr x V / PCr
Clearance is calculated from the urine creatinine, plasma creatinine, and urine flow rate (volume of urine collected / 1,440 minutes)
BUN
Urea is a nitrogenous waste that is endogenously produced from protein; it is reabsorbed at several tubular sites and therefore underestimates GFR
Plasma concentration of BUN depends both on renal clearance and rate of protein catabolism (muscle mass)
Creatinine
Creatinine is a breakdown product of creatine kinase found in muscles; it is secreted to varying degrees by the renal tubule and therefore tends to overestimate GFR by 10-20%