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25 Cards in this Set

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What is normal renal blood flow?
about 1L/min
20-25% of cardiac output
What is the function of high renal blood flow?
Deliver enough plasma for high rates of glomerular filtration
To deliver nutrients and O2 to renal cells so they can function (very high basal metabolic rate in proportion to their mass)
What is the order of blood flow through the kidneys?
1. aorta
2. renal artery
3. segmental A
4. lobar A
5. interlobar A
6. arcuate artery
7. interlobular A
8. afferent arteriole
9. glomerules (capillaries)
10. efferent arteriole
11. peritubular capillaries and vasa recta
12. interlobular vein
13. arcuate vein
14. interlobar vein
15. renal vein
16. inferior vena cava
What are the 2 major sites of resistance control in renal blood flow?
afferent arterioles
efferent arterioles
What part of the nephron receives the majority of the renal blood flow and why?
cortex receives 90-95% of RBF
this maximizes flow dependent processes, such as glomerular filtration and tubular reabsorption
What part of the kidney is blood flow restricted? Why? What are the consequences of this?
Medullary blood flow is restricted
Due in part to high resistance of the descending vasa recta
Low flow is required to maintain the medullary osmotic gradient required to concentrate urine
medullary tissues are often hypoxic; the metabolically active cells of the late proximal tubule and TALH at risk with decreases in renal blood flow
What are the determinants of renal blood flow (RBF)?
RBF=change in pressure/vascular resistance
pressure change=renal artery pressure-renal vein pressure
resistance=sum of all resistances in renal vasculature (primarily due to afferent and efferent arterioles)
resistance factors-radius (most important factor) and length
What are the relative presssure of the glomerular capillaries and peritubular capillaries and why?
glomerular capillaries-pressure is high (promotes filtration)
peritubular capillaries-pressure is low (promotes reabsorption)
Describe the effect of either the afferent or efferent arterioles constricting independently of the other
A. Afferent constriction
1. renal plasma flow decreases
2. capillary pressure decreases (afferent arteriole acts as a dam)
3. filtration decreases (due to decreased capillary pressure)
B. Efferent constriction
1. flow decreases
2. capillary pressure increases (constriction past capillary->back up of pressure)
3. filtration
a. initially increases (due to increased capillary pressure)
b. decreases at high constriction levels (due to pronounced decrease of renal flow)
Why does the RBF remain relatively constant over a wide range of renal aterial pressures?
autoregulation of afferent arteriolar resistance (as renal pressure increases, so does afferent arteriole resistance to maintain constant RBF)
What are the mechanisms of autoregulation that occur in the leval of the kidney (separate from neural and hormonal mechanisms)?
arterial myogenic regulation
tubuloglomerular feedback
Describe the arteriolar myogenic response
increased arterial pressure and blood flow-->stretch of blood vessel-->increased cell permeability and inflow of Ca-->induces smooth muscle contraction-->increased vascular resistance-->decreased renal blood flow
Describe the tubuloglomerular feedback (TGF) response
An increase in renal arterial pressure will ↑ GFR
Through mechanisms associated with TGF, paracrines are secreted from the macula densa that will lead to constriction of the smooth muscle around the afferent arteriole
This will decrease RBF (and GFR)
What is the Juxta-Glomerular Apparatus (JGA) comprised of?
1. macula densa-modified renal tubular cells found near the junction of the TALH and the distal convoluted tubule; express NKCC in their apical membranes
granular cells-modified vascular smooth muscle cells surrounding the afferent arteriole and secrete renin
Mesangial cells
What neural and hormonal factors regulate renal blood flow?
Vasoconstrictors-catecholamines, angiotensin II, endothelin, AVP/ADH
these decrease RBF and glomerular filtration rate (GFR)
vasodilators-prostaglandins, nitrous oxide, bradykinin
these generally increase RBF and GFR
describe the hemodynamic actions of angiotensin II
increases afferent and efferent arteriolar resistance (especially efferent)-->decreases RBF but increases GFR (due to greater efferent effect)-->increases inward pressure gradient of peritubular capillaries-->increased Na reabsorption and decreased Na and H2O excretion
also decreases vasa recta blood flow-->increase in Na reabsorption
What is renal clearance?
The “clearance” of a solute is the virtual volume of blood that would be totally cleared of a solute in a given time
Cx=Ux * V/Px
clearance of the solute=concentration of the solute in the urine * urine flow rate/ concentration of solute in the plasma
What does renal clearance tell us?
Clearance compares the rate at which the glomeruli filter a substance (H2O or solute) with the rate that the kidneys excrete it into the urine (renal function)
If clearance (Cx)=GFR: no net secreation or reabsorption
Cx<GFR: tubular reabsorption of x
Cx>GFR: secretion of x
What is a GFR marker?
1. Solute x is freely filtered
2. Tubules do not reabsorb, secrete, synthesize or metabolize x
3. solute must be physiologically inert (i.e., not toxic and have no effect on renal function)
4. It must not undergo any extrarenal elimination
Thus, Filtered load (Px * GFR) = quantity excreted (Ux * V)
most common markers are inulin and cratinine
Describe inulin as a GFR marker
A. considered the most reliable method of measuring GFR
B. not useful for routine clinical use
1. requires constant IV infusion to get constant plasma levels
2. Chemical analysis of inulin in plasma and urine is technically demanding
C. still used to assess renal function in some cases (i.e., related to kidney transplant)
Describe creatinine as a GFR marker
A. Creatinine produced at approx. constant rate by skeletal muscle (steady state concentration in blood); this avoids IV infusion
B. Creatinine is secreted by tubules, so overestimates GFR by 20% in humans; However, colorimetric methods used to measure creatinine overestimate creatinine concentrations (errors cancel each other out)
C. Cheap, easy, reliable, easily used in clinic; Bottom line – Ccr is “good enough” of an estimate of GFR
Describe how to use plasma creatinine as an estimate of GFR in the stages of chronic kidney disease
creatinine levels may vary substantially between individuals, but change within an individual gives good indication of renal function
Graph appears as a hyperbola:
beginning stages of kidney disease-small changes in creatinine (Cr) levels-->big changes in GFR
end stage kidney disease-big changes in Cr levels only lead to small changes in GFR
What factors affect plasma creatinine levels?
decrease-age, females, hispanic/asian, muscle wasting, vegetarian diet
increase-blacks, muscular bodies, meat diets
What is p-AminoHippurate (PAH)?
organic acid that is not usually present in the body, so must give by IV infusion
kidneys completely clear PAH from plasma during a SINGLE PASSAGE through the kidneys (PAH is secreted from peritubular capillaries into renal lumen); In this case, renal clearance of PAH = arterial renal plasma flow; Thus, PAH clearance is a good estimate of renal plasma flow
most substances are NOT cleared completely – some x goes out in venous blood; Thus, the virtual volume cleared of x in given time is less than total renal plasma flow for most solutes
What is fractional excreation (FE), and how do you calculate it?
%FE = [quantity excreted / filtered load] * 100
=[(Ux * Pcr) / (Ucr * Px)] * 100