Evaluation of the Kidney Function

Question 1

Define Renal Clearance

Answer 1

The renal clearance of a substance is the volume of plasma that is completely cleared of the substance by the
kidneys per unit time.

Question 2

The concept of renal clearance is very abstract explain why.

Answer 2

There is no single volume of plasma that is completely cleared of a substance.

Question 3

Define renal clearance in a mathematical expression

Answer 3

The clearance of a substance(Cs) = The urine concentration of the substance(Us) multiplied by the Urine flow rate(V) divided by the Plasma concentration of the the substance(Ps)

Thus, renal clearance of a substance is calculated from the:

Urinary excretion rate (Us × V) of that substance
divided by its plasma concentration.

Question 4

Explain the following alternative definition of clearance: “Renal clearance refers to the volume of plasma in
which the amount of substance that was excreted in the urine per time unit was originally dissolved in”.

Answer 4

There is no single volume of plasma that is completely cleared of a substance.

Question 5

List five aspects of urine production that can be determined using the principle of renal clearance.

Answer 5

Filtration rate (GFR)

Renal plasma flow (RPF)

Filtration fraction (FF)

Tubular reabsorption

Tubular secretion

Question 6

Why does the clearance criteria suite the determination of the GFR of the Kidneys

Answer 6

Freely filtered (filtered as freely as water)
Not reabsorbed by the renal tubules
Not secreted by the renal tubules

Question 7

List the substances which may be used for the criteria for the use in the determination of the GFR

Answer 7

Inulin

Creatinine

Question 8

List the Advantages and Disadvantages of using of Inulin

Answer 8

Not endogenous

IV administered to maintain constant concentration in
the plasma

Only filtered

Question 9

List the Advantages and Disadvantages of using of Creatinine

Answer 9

Endogenous from muscle metabolism

Relatively constant in the plasma and it is not necessary to administer

Filtered and a small amount is also secreted,
therefore an estimator if GFR

Question 10

Why does the clearance criteria suite the determination of the Renal Plasma Flow

Answer 10

Theoretically, if a substance is completely cleared from the plasma, the clearance rate of that substance is equal to the total renal plasma flow.

In other words, the amount of the substance
delivered to the kidneys in the blood (renal plasma flow × Ps) would be equal to the amount excreted in the urine (Us × V).

Question 11

List the substance(s) which may be used for the criteria for the use in the determination of renal plasma flow

Answer 11

PAH-Para-aminohippurate

Question 12

How can the total blood flow through the kidneys be determined

Answer 12

It can be determined by:

• Using PAH
• Determining C_PAH
• Determining haematocrit
• Calculating blood flow

One can calculate the total blood flow through the kidneys from the total renal plasma flow and hematocrit (the percentage of red blood cells in the blood). If the hematocrit is 0.45 and the total renal plasma flow is 650 ml/min, the total blood flow through both kidneys is 650/(1 to 0.45), or 1182 ml/min.

Question 13

What is the Mathematical definition of Filtration Fraction

Answer 13

The mathematical definition of the filtration fraction is FF = GFR/RPF.

Question 14

Which substance/s can be used to determine the Filtration Fraction

Answer 14

Inulin = GFR

PAH = RPF

Question 15

Explain how the filtered load and excretion rate of a substance tells whether it was reabsorbed,
secreted or just filtered

Answer 15

Rate of excretion < filtered load = reabsorption

Rate of excretion > filtered load = secretion
Explain the above-mentioned principle in terms of clearance rates

Question 16

Explain how the filtered load and excretion rate of a substance indicates whether if a substance was reabsorbed, secreted or just filtered and how does this relate in terms of Clearance Rates

Answer 16

If Cs = CIN then the substance is only filtered

If Cs< CIN then the substance is filtered and reabsorbed

If Cs> CIN then the substance is filtered and secreted

Question 17

Define the albumin/Creatinine ratio and discuss its practical importance

Answer 17

The concentration or dilution of urine varies throughout the day, subject to physiological controls that release more or less liquid, and consequently the concentration of protein in the urine may also vary.

A 24 hour urine sample will therefore be more representative of the proteinuria than a random one.

However, the albumin/creatinine ratio(ACR) in a random sample will give a more accurate indication of how much albumin is escaping into the urine(Albumin being the smallest of the plasmaproteins and may be detected in the very early stages of Kidney Disease.)

The basis of the ACR is that creatinine, a byproduct of muscle metabolism is normally released into the urine at a constant rate.

This enables the use of a random urine sample to measure protein loss in the urine.

Question 18

List the practical application of renography

Answer 18

Abnormalities of blood perfusion

Abnormalities of renal parenchyma diseases

Congenital renal diseases

Obstructive uropathy

Visual evaluation of renal function

Objective measurement of renal function

Evaluation of renal function after renal transplant