Exam #4: Renal Clearance Flashcards

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1
Q

What is “clearance?” What is renal clearance?

A

Clearance= the rate at which a substance is removed from the plasma

  • Thus, renal clearance is the rate at which a substance is removed from the plasma by the kidneys
  • Specifically, clearance is the Volume of substance cleared/ time (C=mL/min)
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2
Q

Write the equation for renal clearance.

A

N/A (p. 249)

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3
Q

What is the renal clearance the ratio of?

A

Urinary excretion/plasma concentration

  • High clearance= High excretion, low plasma concentration
  • Low clearance= Low excretion, high plasma concentration
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4
Q

What is the renal clearance of albumin?

A

~0–it is not filtered across the glomerular capillary; thus, it is not cleared

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5
Q

What is the renal clearance of glucose?

A

~0–it is filtered but then completely reabsorbed

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6
Q

Why is the renal clearance of inulin a measure of GFR?

A

GFR= Glomerular Filtration Rate (rate of filtration across the glomerular capillaries)

  • Inulin is freely filtered across the glomerular capillaries
  • However, it is neither reabsorbed or secreted afterward i.e. ALL the inulin that is filtered appears in the urine

Thus, Inulin clearance= GFR i.e. inulin filtered is exactly equal to inulin excreted

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7
Q

What is creatinine?

A

Creatinine is the byproduct of muscle catabolism of creatine; it is released into systemic circulation at a fairly constant rate

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8
Q

Why is clearance of creatinine a reasonable estimate of GFR?

A
  • Creatinine is freely filtered & not reabsorbed
  • However it is slightly secreted

*Thus, creatinine results in a slightly elevated measure of GFR

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9
Q

Why is measuring the clearance of creatinine easier than measuring the clearance of inulin?

A

Determining renal clearance by inulin is cumbersome & requires:

1) Extended office visit with IV inulin infusion
2) Catheterization of bladder
3) Collection of urine for 2 hours

Creatinine is much more convenient:

  • No IV infusion
  • No catheterization of the bladder
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10
Q

Why does creatinine clearance as a measure of GFR become less accurate in patient with renal disease and low GFR? In such patient does the creatinine clearance over estimate or underestimate the GFR?

A
  • Normally, the secreted fraction of creatinine is 10-20% of what is excreted; thus, clearance overestimates GFR
  • In patients with v. low GFR; this fraction is considerably HIGHER
  • In patients with LOW GFR, creatinine clearance severely OVERESTIMATES GFR
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11
Q

Assuming creatinine production remains constant, how do changes in GFR affect serum creatinine level?

A

High GFR= low serum creatinine concentration
Low GFR= high serum creatinine concentration

Note, 125mL/min is a normal GFR, which corresponds to plasma creatinine between 0.7 & 1.5 mg/100mL

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12
Q

If serum creatinine is above normal, is GFR above or below normal?

A

GFR is below normal

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13
Q

What variables are used to estimate GFR using the Cockcroft-Gault equation?

A

1) Serum creatinine
2) Age
3) Lean body weight

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14
Q

How could you use the estimated GFR to help in determining dosages of drugs that are excreted in the urine?

A

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15
Q

What are the variables used to estimate GFR using the Modification of Diet in Renal Disease (MDRD) study?

A

1) Serum creatinine
2) Age
3) Sex
4) Race

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16
Q

What is the the effect of age on the number of functional nephrons, and GFR?

A

In most people there is a 10% decrease in the number of functional nephrons each 10 years after 40
- Progressive decrease in GFR after 40

17
Q

How do hypertension & DM affect renal function changes with age?

A

These pathologies accelerate the loss of nephrons

- GFR will decline at a faster rate in individuals with these conditions compared to those without

18
Q

The clearance of substance x is lower than the clearance of inulin. What does this mean?

A
  • Substance is NOT filtered at all, or

- Substance is reabsorbed

19
Q

The clearance of substance x is higher than the clearance of inulin. What does this mean?

A
  • Substance is filtered & secreted