3 - Glomerular Filtration Rate and Renal Blood Flow Flashcards
How is renal function quantified for the body? How would you calculate dietary intake?
By utilizing mass balance: amount in = amount out.
Dietary intake = excretion in urine and feces + insensible loss (sweat, respiration).
How would you calculate renal function for the whole kidney?
in = out
Renal arterial input = urine output + renal venous and lymphatic output.
How would you calculate renal function for the nephron? What is one way this is done clinically?
Filtered + secreted = reabsorbed + excreted
Inulin is a polysaccharide that is filtered but not secreted or reabsorbed so filtration of inulin = excretion of inulin.
What does the properties of inulin allow us to infer?
That the amount of inulin filtered is equal to the glomerular filtration rate (GFR) multiplied by the plasma concentration of inulin (Pin)
What is the amount of inulin secreted equal to?
The urine flow rate (UF) multiplied by the urine concentration of inulin (Uin).
Using what we know about inulin, what is one way to calculate GFR?
Urine flow rate X Urine concentration of inulin / Plasma inulin concentration
GFR = (UF X Uin)/ Pin
What is renal clearance? How is the clearance of substance X (Cx) calculated?
The volume of plasma from which a substance has been removed and excreted into the urine per unit of time.
Cx = (UF X Ux) /Px
Where UF = urine flow rate
Ux = urine concentration of substance x
Px= plasma concentration of x
What is a second way to assess renal clearance? Which way is preferred?
Creatine clearance.
Inulin is the best way though.
How do you calculate creatine clearance?
Ccreat = GFR = (Ucreat X UF)/Pcreat
How is GFR-1 estimated clinically?
By using the reciprocal of serum creatinine.
In a steady state, urinary excretion of creatine can be considered a constant K = Ucreat x UF
Then GFR = K/Pcreat
Therefore GFR ~ 1/Pcreat
What occurs with creatinine when the GFR is reduced by 50%?
The filtered load, and therefore excretion, of creatinine is instantly reduced by 50%.
Since production of creatinine stays constant, the steady state is not maintained b/c production of creatinine exceeds the excretion.
There will therefore be a retention of creatinine in the body until a steady state is reached.
What is the relationship between plasma creatinine and GFR?
As GFR increases, plasma creatinine decreases.
Graph has a characteristic slope downward due to the fact that creatinine is entirely excreted from the body through the kidneys by glomerular filtration.
What is another solute whose plasma concentration is dependent on GFR for secretion similarly to creatinine.
Urea, which is very poorly regulated when GFR is decreased.
How is GFR related to plasma concentrations of bicarb, calcium, and phosphate?
Bicarb, calcium, and phosphate plasma levels are less dependent on GFR for excretion, and are also poorly regulated (but not as poor as creatinine and urea).
This is because tubular handling of these substances can also be altered to maintain plasma levels.
What is the relationship between plasma sodium, potassium, and water to GFR?
Sodium, potassium, and water concentrations in the plasma are well regulated until GFR becomes very low.
