3_HST110 Clearance 2017 Flashcards
What are 2 nitrogenous wastes generated by metabolism and removed by the kidneys?
Urea and creatinine
What is the general relation between the ingestion/production and excretion of a substance for homeostasis?
Rate of excretion parallels rate of ingestion/production
What is it called when a substance is removed from the blood and excreted in the urine?
Renal clearance
What is the formal definition of renal clearance?
The volume of plasma per unit time from which ALL of a substance is removed by the kidneys
What are the units of clearance?
mL/min (volume per time) NOT amount/time
The rate at which the kidneys excrete a solute into the urine (X) the rate at which the solute disappears from the plasma provided that the kidney does not (Y) the solute.
X = equals Y = synthesize, degrade, or store
For any solute X that the kidney does not synthesize, degrade, or store, the only route of entry to the kidney is the (X), and the only 2 routes of exit are the (Y)
X = renal artery Y = renal vein and ureter
What is the relationship between arterial input, venous output, and urine output of the kidney based on the mass balance principle?
Arterial input of X =
Venous output of X +
Urine output of X
What is the clearance equation general mass formula in terms of concentrations and flow rates?
PRPF (artery) = PRPF(vein) + U*V’(urine)
mmol/L)(mL/min
What is the general clearance equation?
C_x = (U_x * V’)/(P_x)
Clearance rate from blood
To calculate clearance, you need to know only 3 variables:
- Concentration of X in the urine
- Concentration of X in the systemic blood plasma
- Volume of urine formed in a given time
Inulin is (X) filtered and is neither reabsorbed nor (Y) by the nephron. All inulin filtered is excreted into urine
X = freely Y = secreted
What is the relationship between filtered, secreted, reabsorbed, and excreted quantities in urine?
Amount filtered \+ Amount secreted = Amount reabsorbed \+ Amount excreted
What compound is used to determine GFR in the clinic?
Inulin. Inulin clearance (mL/min) = GFR since it is freely filtered
What are the 4 criteria for using the clearance of a substance to measure GFR?
- Substance must be freely filtered at the glomerulus.
- Substance must neither be reabsorbed nor secreted by the tubular system.
- Substance must not be synthesized, degraded, or stored by the kidney.
- Substance must not alter renal function.
What compound is used to estimate renal plasma flwo (RPF) due to the fact that it is freely filtered AND secreted by the tubular system?
PAH (Para-aminohippurate)
Clearance PAH > GFR due to secretion
If a filtered substance is completely removed from the nephron by either reabsorption or degradation, then none of the substance will be excreted in the urine, and therefore renal clearance is (X)
X = zero
What are some examples of 0 clearance solutes?
Insulin (filtered but degraded), Glucose, amino acids, Na+ (reabsorbed)
What are results of the comparisons of clearance of a substance X with GFR (inulin determined)
If CX = GFR -> No net reabsorption or secretion of X
If CX > GFR -> Net secretion of X
If CX < GFR -> Net reabsorption of X
What is the expression for fractional excretion of a solute?
Defined as the ratio of the amount excreted in the urine (UX x V’) to the filtered load (PX x GFR)
FE_x = (UX x V’)/(PX x GFR) = (C_x)/GFR = (C_x)/(C_inulin)
FE < 1 = net reabsorption
FE > 1 = net secretion
For routine GFR assessment, inulin clearance is invasive, so (X) clearance is used instead
X = creatinine
Creatinine is (X) filtered and is not reabsorbed, but it is secreted to a small extent by the (Y)
X = Freely Y = Proximal Tubule
Creatinine clearance (CrCl) overestimates GFR by what percentage in healthy individuals?
10-20%
What is used in common practice to determine GFR?
Plasma creatinine levels. Inverse relationship with clearnance
What is the normal Creatinine range?
0.7 - 1.3 mg/dL
