Renal plasma clearance

Question 1

What is renal plasma clearance?

Answer 1

The volume of plasma from which a substance is completely removed
by the kidney per unit time (minute)

Question 2

What does it mean by metabolic clearance of a substance?

Answer 2

Volume of plasma cleared in unit time
by any mechanism

Question 3

What percentage of cardiac output does the kidney receive?

Answer 3

20%

Question 4

What does renal plasma clearance depend on?

Answer 4

1. Glomerular filtration hence variables which affect GFR
2. Tubular reabsorption and secretion
3. Tubular catabolism

Question 5

What is the mass of insulin?

Answer 5

Insulin is a small peptide with a mass of 5.8 kDa

Question 6

Where is insulin filtered and what happens to it in tubular cells?

Answer 6

Filtered in the glomerulus, reabsorbed
and broken down in the amino acid
In tubular cells

Question 7

What are factors affecting GFR?

Answer 7

• Changes in renal blood flow
• Changes in systemic blood pressure
• Afferent and efferent arteriolar constriction
• Changes in hydrostatic pressure in the Bowman’s space
• Ureteral obstruction
• Oedema of kidney in the tight capsule
• Changes in concentration of plasma proteins (dehydration, hypoalbuminemia)
• Changes in filtration barrier
• Changes in effective filtration surface area

Question 8

What are the 3 layers that make up the glomerular filtration barrier?

Answer 8

1. Endothelium
2. Basement membrane
3. Podocytes

Question 9

What is the size of the endothelium in the glomerular filtration barrier?

Answer 9

Endothelial fenestration size (70-90 nm)

Question 10

What is the size of the podocytes in the glomerular filtration barrier?

Answer 10

Filtration slit (podocytes) (25 nm)

Question 11

What does glomerular filtration depend on?

Answer 11

Hence the filtration of a substance depends on size, charge and plasma free concentration

Question 12

What is the equation for urinary excretion rate?

Answer 12

Urinary excretion rate = GFR – reabsorption rate + secretion rate

Question 13

What is the rate of urine clearance of creatine?

Answer 13

1. Filtration only

Question 14

What is the rate of urine clearance of electrolytes?

Answer 14

1. Filtration
2. Partial reabsorption

Question 15

What is the rate of urine clearance for Glucose and amino acids?

Answer 15

1. Filtration
2. Complete reabsorption

Question 16

What is the rate of urine clearance for organic acids?

Answer 16

1. Filtration
2. Secretion

Question 17

What is inulin, not insulin?

Answer 17

An inert polysaccharide, MW ~5,000

Question 18

What is inulin’s filtration properties in the glomerular membrane?

Answer 18

Filters freely through the glomerular membrane

Question 19

Does anything else happen to Inulin during its clearance, apart from filtration?

Answer 19

No, not absorbed, secreted or metabolised

Question 20

What is the rate of filtration of inulin through glomerular membrane per minute equal to?

Answer 20

Rate of filtration through glomerular membrane per minute = Rate of entry into bladder per minute
therefore
Pin x GFR = Uin x V*

Question 21

What is the equation for rate of filtration for inulin?

Answer 21

Rate of filtration = [Pin] x GFR

Question 22

What is the equation for rate of entry into bladder?

Answer 22

[Uin] x urine flow rate (V*)

Question 23

What is clearance of a substance which is not secreted or reabsorbed in the tubule equal to and why?

Answer 23

Clearance of a substance which is not secreted or reabsorbed in the tubule is equal to glomerular filtration rate; its because the volume of plasma cleared is the volume filtered

Question 24

What are exogenous agents used to measure GFR?

Answer 24

A. 125I-iothalamate
B. 59Cr-EDTA
C. 99mTc-DTPA

Question 25

What are endogenous agents used to measure GFR?

Answer 25

A. Creatinine
B. Cystatin C