Renal Excretory Function

Question 1

What factors determine the glomerular filtrate composition?

Answer 1

Net filtration pressure (determined by blood pressure), podocyte slit pores, size of the molecule, charge of the molecule and negative charge of glomerular basement membrane glycoproteins

Question 2

What molecules can freely move into the glomerular filtrate?

Answer 2

Water, electrolytes (sodium, potassium, chloride, phosphate and glucose), urea and amino acids

Question 3

What molecules cannot pass into the glomerular filtrate ordinarily?

Answer 3

Albumin and red blood cells

Question 4

What is the glomerular filtration rate?

Answer 4

GFR is the total amount of fluid that is filtered through the glomerulus per unit of time and it is the most important parameter of kidney function.

Question 5

How accurate is creatinine clearance for estimating GFR?

Answer 5

It overestimates the actual GFR by 10-20% as creatinine is also actively secreted by the peritubular capillaries

Question 6

How do you calculate creatinine clearance?

Answer 6

Creatinine clearance = (urine concentration x urine volume)/plasma concentration

Question 7

What issues are there with using creatinine clearance to calculate GFR?

Answer 7

• Creatinine is produced by muscle, so can be easily influenced by an individual’s activity and therefore the determined GFR with be an underestimate
• Malnourished individuals have low serum creatinine levels and therefore the GRF will be overestimated
• Some drugs inhibit the tubular secretion of creatinine (such as trimethoprim) leading to a raised plasma creatinine level even though GFR may be unchanged.

Question 8

What is the ‘gold standard’ for measuring GFR?

Answer 8

Using a nuclear medicine scan for lothalmatate excretion

Question 9

What is the MDRD equation?

Answer 9

An equation that is used to estimate GFR

Question 10

What variables affect the eGFR from the MDRD equation?

Answer 10

Age, creatinine levels, sex and race

Question 11

What happens if there is a defect in the apical Na+/cysteine cotransporter in the proximal tubule of the kidney?

Answer 11

Cysteinuria

Question 12

What happens if there is a defect in the apical Na+/glucose cotransporter in the proximal tubule of the kidney?

Answer 12

Glycosuria

Question 13

What happens if there is a defect in the basolateral Na+/HCO3- cotransporter in the proximal tubule of the kidney?

Answer 13

Proximal renal tubular acidosis

Question 14

What happens if there is a defect in the apical Na+/K+/2Cl- cotransporter in the thick ascending loop of Henle?

Answer 14

Bartter type 1

Question 15

What happens if there is a defect in the apical Na+/Cl- cotransporter in the distal tubule of the kidney?

Answer 15

Gitelman’s syndrome

Question 16

What causes Bartter type 1 syndrome?

Answer 16

Defect in the Na+/K+/2Cl- con transporter in the thick ascending limb of the LoH

Question 17

What causes Gitelman’s syndrome?

Answer 17

defect in the apical Na+/Cl- cotransporter in the distal tubule of the kidney

Question 18

Outline the mechanism of action of loop diuretics

Answer 18

The apical Na+/K+/2Cl- tranporter in the cells of the thick ascending limb is inhibited by loop diuretics which leads to decreased reabsorption and a greater osmotic gradient into the renal tubule water loss. This is accompanied by the loss of Na+, Ca2+, Mg2+ and K+.

Question 19

Outline the mechanism of action of thiazide diuretics

Answer 19

The Na+/Cl- symporter on the apical surface of the distal tubule cells is inhibited by thiazide diuretics and this leads to the increased presence of these electrolytes in the nephron and therefore increased water loss by osmosis.