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79 Cards in this Set

  • Front
  • Back
What do the overall kidney processes do?
Determines the composition of urine
4 kidney processes:
1. Filtration
2. Reabsorption
3. Secretion
4. Excretion
What are the forces that govern filtration by glomerular capillaries?
Starling's forces
Glomerular hydrostatic pressure:
60 all along its length
Glomerular oncotic pressure:
32 mm Hg
Bowman's space hydrostatic pressure:
18 mm Hg
Bowman's space oncotic pressure:
not there; no protein filtered
4 common factors that can alter the GFR:
1. Increased G-cap filtration coefficient - increases GFR
2. Increased G-cap hydrostatic pressure - increases GFR
3. Increased gcap oncotic pressure - decreases GFR
4. Increased Bowman's space hydrostatic pressure - decreases GFR
What does gcap oncotic pressure start out as at the beginning of the glomerulus?
28 mm Hg - same as in systemic capillaries
What is the final gcap oncotic pressure at the end of the glomerular capillary?
36 mm Hg
Why does gcap oncotic pressure increase so much as fluid moves through it?
B/c fluid is filtered and so the concentration of plasma protein increases.
What happens to the change in oncotic gcap pressure if the filtration fraction is increased?
The increase in gcap oncotic pressure goes from 28 to 40 mm Hg and plateaus.
What happens to the change in oncotic gcap pressure if the filtration fraction is decreased?
Oncotic pressure in gcap does not increase as much - only to about 32 instead of 36
What is an important effect of the increase in gcap oncotic pressure along the length of it?
It decreases the net filtration pressure.
What is the normal difference between the forces out and in at the gcap?
Forces out = 60
Forces in = 46
Net force out = 14 mm Hg
What is the filtration fraction equal to?
What happens to gcap oncotic pressure as you move along the length of the gcap?
It increases from 28 to 36
What happens to the increase in gcap oncotic pressure if you increase the filtration fraction?
Filtering more means the gcap oncotic pressure increases more for example from 28 to 40
What happens to the increase in gcap oncotic pressure if you decrease the filtration fraction?
It increases less; from 28 to only 32
What is filtration equilibrium?
What causes it?
When NFP = 0 due to the increase in gcap oncotic pressure; caused by a decreased renal plasma flow (RPF) which increases FF and gcap oncotic pressure faster.
What is filtration disequilibrium? What causes it?
When NFP always stays positive even to the end of the gcap; Caused by high RPF, which causes gcap oncotic pressure to increase less quickly.
What will GFR change in; filtration equilibrium or disequilibrium?
What is the concept that is the basis for quantifying renal function?
Mass balance
For the body what does dietary fluid intake equal?
-Excretion in urine and feces
-Insensible loss (sweat/breath)
What does renal arterial input equal?
-Renal venous output
-Urine output
-Lymphatic output
What is the mass balance equation for the kidney?
Filtration+secretion = Reabsorbed+Excreted
What is Inulin?
An exogenous substance that is freely filtered, not reabsorbed, and not secreted.
So what is inulin used for?
measuring the GFR which corresponds to clearance of inulin
Formula for GFR based on inulin clearance:
(UF x U(inulin)
GFR = ---------------
If the urine flowrate is 1 ml/min, and you collect a urine sample that is 125 mg/dl in inulin, and you gave the plasma 1 mg/ml inulin, what is the GFR?
GFR = 1x125/1 = 125ml/min
Define Clearance:
the volume of plasma from which a substance has been removed and excreted into the urine per unit of time
units of clearance
vol / time
How do you calculate the clearance of a substance X?
Urine flowrate x Urine conc X
Plasma conc X
What endogenous substance can be used as a marker of GFR?
How is GFR estimated clinically?
By considering urine flowrate and creatinine conc to be constant, then taking the inverse of plasma creatinine.
So what is the clinical index for % normal creatinine?
100/plasma creatinine
Why isn't the creatinine index such a good estimate of GFR?
Because it is not exactly produced at the same rate in everyone, and it is slightly secreted.
What happens to the serum concentration of creatinine if you decrease GFR by 50%? Why?
Serum levels rise by 2X over several days because the kidneys are filtering and excreting only half as much creatinine.
What happens to the urinary excretion rate initially after a 50% drop in GFR?
It decreases by 50%
What happens to the urinary excretion rate of creatinine over the next few days after GFR reduction?
It will rise back to normal to regain the balance between creatinine production and excretion.
What if the GFR is maintained at 50% reduction?
Plasma creatinine levels will increase and remain elevated at the same level once kidney excretion equals production.
What will happen to plasma creatinine levels if GFR falls to 1/8 its normal rate, and maintained?
Plasma levels will increase 8X
So what can you say about creatinine excretion rate always?
It always equals the rate of creatinine production, despite reductions in GFR. At the expense of increased plasma levels.
What is the relationship between GFR and plasma creatinine concentration?
As GFR decreases, plasma concentration increases inversely proportionally.
What substances will be affect most by a decreased GFR and thus accumulate in the plasma?
Urea and creatinine
What substances will be affect less by a decreased GFR and thus accumulate in the plasma later at more severely decreased GFRs?
-H+ ions
What substances plasma levels will be maintained virtually constant even with severe decreases in GFR?
Sodium and chloride
So what are the best substances for identifying a case of chronic renal failure?
Creatinine or urea - because they will be abnormal soonest.
What are 2 clinical estimates of the GFR that are better than creatinine?
-Cockroft-Gault formula
-MDRD (modific. of diet in renal disease)
What is PAH clearance used to measure?
Renal plasma flow
What makes PAH a good marker for renal plasma flow?
The fact that it is completely cleared from the plasma.
How does the kidney treat PAH?
-Freely filtered
-Rest is secreted
-None is reabsorbed
How do you calculate renal PAH clearance?
(Urine vol)(Urine PAH)
Plasma PAH
Is ALL PAH removed from the plasma?
Well no, .001 mg/ml stays in the renal vein; but it's still the best estimate of RPF.
Why can't you use creatinine to estimate RPF?
Because the GFR is only about 20% of total renal plasma flow.
What is the extraction ratio of PAH?
The difference between renal arterial PAH and renal venous PAH divided by arterial PAH.
How do you calculate renal plasma flow based on PAH clearane?
Clearance of PAH
RPF = ---------------------
PAH extraction ratio
Why must you take into account the extraction ratio for PAH?
Because not all PAH is extracted from the plasma, and if you didn't you would underestimate RPF.
How can you use RPF to calculate renal bloodflow?
RBf = ------
What are the 2 types of GFR and RBF regulation?
1. Acute regulation
2. Chronic regulation
What is the timeframe over which acute regulation of GFR/RBF occurs?
Seconds to minutes
What is the timeframe over which chronic regulation of GFR/RBF occurs?
Minutes to days
What does Acute regulation respond to?
Transient changes in arterial blood pressure
What IS acute regulation?
What does Chronic regulation respond to?
Changes in neural or hormonal input to the kidney
What types of neural or hormonal input stimulates chronic regulation of GFR and RBF?
Ang II
Renal nerves
What happens to GFR and RBF in the short-term as arterial pressure increases? Why?
They remain fairly constant up to a point; because urine output increases in proportion to MAP to bring it back down to normal.
What are the 2 mechanisms of GFR and RBF autoregulation?
1. Myogenic mechanism
2. Tubuloglomerular feedback
What is the myogenic autoregulation mechanism?
Stretch of vessels during increased arterial pressure stimulates contraction, increases vasc resistance, and maintains constant RBF & GFR.
What is the Tubuloglomerular feedback mechanism?
When renal perfusion pressure is high, more fluid is filtered; more NaCl passes the macula densa cells; they detect the increase and elicit an increase in vascular resistance.
What are macula densa cells? Where are they?
Specialized cells in the distal tubule near the afferent arteriole
What results from macula densa cells detecting increased NaCl?
Constriction of the afferent arteriole to decrease renal bloodflow.
What is the tubuloglomerular response to a decreased renal perfusion pressure?
1. Decreased pressure
2. Decreased filtration
3. Decreased NaCl detected by macula densa
4. Decreased afferent arteriole resistance
5. Increased renal perfusion
What are 3 hormones/autocoids that decrease the GFR?
How does angiotensin II affect the GFR?
It can go either way; it acts on both the afferent and efferent arterioles.
What does Ang II preferentially act on?
The efferent arteriole - causes vasoconstriction and thus reduces renal bloodflow and increases gcap hydrostatic pressure.
Under what circumstances is Ang II normally elevated?
Those where blood pressure needs to be raised, so the GFR would be decreased to promote fluid retention.
So what is Ang II's normal effect on the GFR?
It prevents a decrease in the GFR while the efferent constriction allows for increased Na reabsorption.
What are 2 hormones/autocoids that increase the GFR?
-Endothelial-derived Nitric oxide