Filtration and Clearance Flashcards
What concentrations do organic and inoragnic anionic and cationic solutes exist at in plasma vs. ultrafiltrate?
They exist at the same concentration
What is GFR relative to the ECF? What is the numerical value of GFR per day? How often is the ECF filtered approximately?
It’s 10-fold ECF; 125 ml/min or 180L/day; every 2 hours
What does the rapid turnover of ECF through the kidneys ensure?
Maintain ECF volume and solute composition within narrow limits by rersponding to correct changes in ECF volume and solute composition
What is Kf for the glomerular capillary?
Filtration coefficient is the product of the capillary hydraulic conductivity and the surface area available for filtration
What is the difference in hydrostatic pressure?
PGC - PBS
What is the difference in oncotic pressure?
PiGC - PiBS
What does the PGC only decrease about 3 mmHg going from the beginning to the end of the glomerular capillary?
Because of post-capillary efferent arteriole constriction (does not happen in systemic capillaries)
What happens to oncotic pressure as you go through the glomerular capillary?
It increases because of plasma filtration and concentration of plasma protein
What is increased from its normal value in nephrotic syndrome?
PiBS, which is normally zero; plasma protein is filtered
What are the two forces driving filtration in the glomerular capillaries? What are the two forces driving reabsorption?
PGC and PiBS (approx 50 mmHg, PiBS 0); PBS and PiGC (PiGC rises from 25-35 and 10 for PBS)
What are the three glomerular barrier to filtration?
- endothelial cells of glomerular capillaries
- capillary basement membrane (restrict filtration of solutes greater than 1kDa; proteoglycans)
- visceral epi layer of Bowman’s capsule (podocytes with foot processes separated by filtration slits to restrict anionic protein filtration)
What determines the permselectivity of the glomerular barrier? What is freely filtered?
Size and charge of solute; water and solutes with diameter less than 4 nm freely filtered
Which area begins processing of the ultrafiltrate?
Proximal convoluted tubules in the tubule
What normally has soluteBS/solutePlasma around 1?
Water, glucose, NaCl, inulin
What happens with something like albumin and Hg with regard to the soluteBS/solutePlasma ratio?
They decrease steadily
What does inulin help measure?
Infused into bloodstream to measure glomerular filtration
What is the clearance ratio (solute/inulin) of anionic dextrans relative to cationic dextrans? Why?
Lower; neg charge on basement membrane and foot processes that impede passage of negatively charged solutes
What happens when you remove the negative charge of the glomerular barrier? When do you see this?
Increased filtration of plasma proteins (anions); nephrotic serum nephritis
What is the cardiac output/min? How much blood flow goes to the kidney/min? How much of the renal blood flow is plasma? What is the amount of RBF that is ultrafiltered (GFR)?
5-6 L; 1-1.2 L (20%); 600-720 ml (55%); 125 mL
How does GFR change relative to urine output? How does urine output change?
Remains constant; rate and volume of urine excretion varies depending on rate and volume of fluid consumed
How much of RPF is filtered at lower rates? Higher rates? What is FF equal to?
Greater fraction; smaller fraction;
GFR/RPF
As plasma flow increases through glomeruli, what allows for a max rate of glomerular filtration?
Larger surface area of glomerular capillaries until you have all the glomerular surface area filtering plasma
What happens to RPF and GFR with afferent, efferent, and both arteriolar vasoconstriction?
Decrease, decrease; decrease, increase; really decrease, no change
What happens to RPF and GFR with incresed, decreased plasma protein, obstruct ureter?
No change, decrease; no change, increase; no change, decrease
For filtration, when does the surface area of the glomerular capillary mediating filtration begin to decrease?
Low RPF flow rates below the normal value of 600 ml/min
In the peritubular capillaries, what are the numerical values of the four pressure forces? What force is favored? How does fluid get from the tubule back to this capillary?
PPC = 20 mmHg; PiPC = 35 mmHg; PIS = 6-10 mmHg; PiIS = 4-8 mmHg;
reabsorption;
Tubular fluid –> tubular epithelial cell –> interstitial fluid –> peritubular capillary
From the beginning to the end of the peritubular capillary, what force is favored, filtration or reabsorption?
Reabsorption (17 mmHg difference at beginning, 12 mmHg at end)
When can renal clearance of a solute be used to measure GFR (5)?
- freely filtered at glomerulus
- not reabsorbed along length of neprhon
- not secreted into tubular fluid along length of nephron
- not synthesized by kidney
- not metabolized by kidney
A solute with the five properties will have what equal to what?
What is excretion equal to?
amount of solute filtered/time = amount solute excreted/time;
Ps x GFR = Us x V;
Excretion (100%) = Filtration (100%)
What are exogenous and endogenous solutes that can measure GFR?
inulin (fructose polymer); creatinine (creatine phosphate metabolism in skeletal muscle)
Given that creatine production is constant, what does increased plasma creatinine indicate?
Decreased clearance of creatinine from the plasma, indicating decreased GFR and acute renal failure
What is the normal level of plasma creatinine? How much of the inulin if filtered into Bowman’s space?
1 mg/100ml; all is excreted in the urine
What is the clearance of solute equal to?
Cs = (Us X V)/Ps
What do Cs/CIn of less than or greater than one mean with respect to reabsorption and secretion of solute?
1 = net solute secretion
What is the fractional excretion of water? What is the denominator here equal to?
FEH20 = V/GFR; GFR = CIn = (UIn x V)/PIn
If inulin is 100-fold more concentrated in the urine than plasma…
Pin/Uin = 1/100 = .01, so 1% of the filtered water is eliminated in the urine
Is inulin reabsorbed or secreted into the tubular fluid? How does its concentration in the urine arise?
Neither;
arises directly from the amount of water reabsorption occurring in the nephron
What is fractional excretion of solute equal to? What can Ccreat be equal to?
FES = CS/Ccreat =
Us x V/Ps)/(Ucreat x V/Pcreat
In fractional reabsorption, what is it quantified as especially in water and Na balance? How much water and Na are excreted in water and Na balance?
1 - FE; approximately 99% of filtered Na and water would be reabsorbed;
FEH20 = FENa = 1%
What is RBF equal to?
RBF = renal AV BP/renal vascular resistance
