Flashcards in Renal Blood Flow and GFR Deck (40):
The fluids that gets across the glomerulus into Bowman's space.
Very little protein, but lots of ions, glucose, etc.
What does (re)absorption refer to?
Movement of water and solutes from the lumen of tubules into the interstitial space/ blood.
What does secretion refer to?
Direct movement from tubule basolateral membrane -> lumen. (without being filtered)
What does excretion refer to?
Actual loss of water or solutes into the outside world.
What does oncotic pressure refer to?
Osmotic pressure resulting from the presence of large macromolecules (esp. albumin).
Review: Which nephrons have loops of Henle that go deep?
About what percentage of CO (at rest) goes to the kidneys?
Is renal blood flow (RBF) really what matters for filtration rate?
No, it's really the acellular plasma blood flow (PBF) that counts.
How is PBF derived from RBF?
PBF = RBF(1 - HCT)
What are the typical units of glomerular filtration rate (GFR)?
(often normalized to body surface area)
What's a normalish GFR for a young adult?
(or 100ml/min/1.73 m^2)
Do blood and protein get into the filtrate?
Nope. Not much anyway, in normal kidneys.
What ratio expresses how permeable the glomerulus is to a particular substance?
Ultra filtrate / plasma ratio (UF/P).
Na+, K+, glucose have UF/P = 1.
Albumin's UF/P is close to 0.
Okay, so actually there are 4 forces that affect glomerular filtration. What are they?
Glomerular capillary hydrostatic pressure.
Glomerular capillary oncotic pressure.
Bowman's space hydrostatic pressure.
Bowman's space oncotic pressure. (but this last one is normally close to 0)
Which of the 4 forces affecting glomerular filtration varies along the length of the glomerular capillary bed?
Capillary oncotic pressure increases downstream. (Water has been removed from the serum, leaving a higher concentration of osmotically active macromolecules behind.)
Thus the driving force for filtration diminishes downstream.
Where does "normal" sit on the graph of RPF (renal plasma flow) vs. GFR?
Right before the shoulder/plateau of the relationship - such that increase in RPF won't change GFR much, but decrease in RPF leads to rapid decline of GFR.
How is glomerular capillary pressure kept fairly constant despite changes in arterial pressure?
By regulation of constriction/dilation of the afferent and efferent arterioles.
What is the filtration factor?
What does a higher one mean for capillary oncotic pressure?
GFR / RPF.
Higher filtration factor -> increased capillary oncotic pressure.
The relationship between afferent/efferent arteriole constriction, capillary pressure, and GFR is pretty obvious.
Constrict the afferent -> decreased capillary pressure -> decreased GFR.
Constrict the efferent -> increased capillary pressure -> increased GFR.
What effect does angiotensin II have on GFR? How?
Angiotensin II increases GFR.
It constricts both afferent and efferent arterioles, but has a much greater effect on efferents -> net increase in capillary pressure.
(thus ACE inhibitors can decrease GFR)
What effect do catecholamines (epi and NE) have on GFR? How?
Both decrease GFR by causing constriction of the afferent arteriole.
What effect does endothelin have on GFR? How?
Endothelin reduces GFR because both afferent and efferent arterioles are constricted.
Effect of PGE2 and prostacyclin on GFR? How?
Increases GFR by causing dilation of afferent arteriole.
What does the intrinsic myogenic reflex refer to?
The afferent arteriole constricts or dilates in response to the pressure it experiences.
Review: Where are the macula densa cells?
Macula densa cells are in the thick ascending limb
Review: Where are the juxtaglomerular cells? What do they produce?
Juxtaglomerular cells are in the efferent arteriole.
These cells produce renin.
What does the macula densa actually detect?
What molecule is key for this?
Effectively it assesses tubuloglomerular flow (TGF), but it actually senses the amount of Cl- that enters it.
Cl- enters via the NKCC2 symporter.
What is the response to increased tubuloglomerular flow (TGF)?
Renin release -> A-II -> constriction of afferent arteriole to reduce flow.
For a solute that is not synthesized or metabolized by the kidney... how do arterial input, venous output, and urinary output of the substance relate?
Arterial input = venous output + urinary output
What is the expression for the filtration rate of a substance in the plasma that is freely filtered?
How does this relate to urine content of the substance (assuming no reabsorption or secretion)?
amount filtered (per unit time) = GFR * Px
Where Px is the plasma concentration of substance "x".
GFR * Px = Ux * V
(where Ux = [x] in urine, V = urine volume; assuming no reabsorption or secretion)
What molecule is an "ideal" marker of GFR?
Inulin - freely filtered, not secreted or absorbed, non-toxic, not metabolized
Thus P(in) * GFR = U(in) * V ... or GFR = ( U(in)*V ) / P(in)
U(in) and V is measured by 24 hour urine collection.
More generally, what does Ux*V / P represent?
(for inulin, clearance = GFR)
What endogenous substance has clearance almost equal to GFR (like inulin)?
Creatinine (though it's not perfect)
Is creatinine clearance equal to, greater than, or less than GFR?
It's a little greater than GFR, due to tubular secretion of creatinine.
Additionally, the less GFR there is, the more tubular secretion of creatinine happens (masking effects of declines in GFR).
Why might creatinine rise despite no change in GFR? 3 drugs that cause this?
Inhibition of creatinine secretion will cause increased plasma creatinine without reduced GFR.
For clinical purposes, what can you measure to get a pretty good easy estimate of GFR?
Take the average of creatinine clearance (overestimate) and urea clearance (underestimate).
Can creatinine clearance be estimated without a 24 hour urine collection?
Yes.. there are several equations that produce an estimate from serum creatine (based on age, race, sex, etc.)
What substance present in the serum might be used to calculate GFR in the near future?
(it's an endogenous protease that's produced at constant rate, 100%ish absorbed in the proximal tubule, and 100%ish metabolized there. So plasma levels correlate pretty well with GFR)
What is fractional excretion? What does it tell you?
FE = Clearance / GFR = U*V/(P*GFR)
A low FE tells you the body is avidly holding on to that substance (assuming it is freely filtered).