Renal Excretory Function

Question 1

What is the glomerulus?

Answer 1

Structure enclosed by Bowman’s capsule filtering plasma

Question 2

What is the significance of the afferent and efferent arterioles?

Answer 2

To the glomerulus

• afferent arteriole has large diameter
• efferent has a smaller diameter
• this generates a filtration pressure
• means fluid can be forced through the endothelium

Question 3

What are podocytes?

Answer 3

cells that cover the capillaries of the glomerulus allowing filtration of selected material

Question 4

How is does selective filtration occur in the glomerulus?

Answer 4

• podocytes

- fenestrated capillaries

Question 5

What is proteinuria, how does it occur?

Answer 5

Proteins enter urine due to swollen podocytes

Question 6

What factors determine filtrate?

Answer 6

Net filtration pressure
Podocyte slit pores
Size of molecule
Charge of molecule

Question 7

How does charge and size of a molecule affect its filtration?

Answer 7

Negatively charged large proteins will not allowed through

Question 8

Which molecules can easily be filtered in the glomerulus?

Answer 8

• free movement of small solutes/molecules (water, electrolytes, urea, amino acid)
• restriction of larger solutes (proteins)

Question 9

What is GFR?

Answer 9

Total amount of fluid filtered through the glomerulus

• normally 120ml/min when CO is 1.2L/min
• parameter of kidney function

Question 10

How is GFR measured?

Answer 10

• measure creatinine clearance
• this is the amount of substance cleared from the plasma ending up in the urine and can be compared to plasma concentration as it is completely filtered and none is reabsorbed

Question 11

What is creatinine?

Answer 11

• breakdown product of creatine phosphate in muscle

- completely filtered and none reabsorbed but actively secreted = overestimation of 10-20%

Question 12

How is creatinine clearance calculated?

Answer 12

• (urine conc x urine volume)/plasma concentration

Question 13

What other methods of measuring GFR are there and why might you use them instead?

Answer 13

• creatinine clearance often takes place over a 24 hour period which can be difficult
• gold standard: nuclear medicine scan
• estimated GFR

Question 14

How does creatinine change GFR?

Answer 14

• small changes in creatinine lead to large changes in GFR

Question 15

Why may using creatinine to measure GFR be confusing?

Answer 15

• GFR fall decreases creatinine filtration so you get a proportional rise in creatinine
• misleading in muscular individuals (increased serum creatinine), malnourished individuals (low serum creatinine), drugs (inhibit tubular secretion of creatinine)

Question 16

What is the renal threshold?

Answer 16

concentration of substance dissolved in blood above which the kidneys begin to remove it from the urine
- if exceeded reabsorption in the PCT is incomplete so the substance remains in the urine

Question 17

What is the glucose threshold?

Answer 17

• greater than 10mmol/L
• appears in the urine as the amount of glucose exceeds reabsorption capacity of tubules and all glucose carriers are saturated

Question 18

What are the effects of glucose in the urine?

Answer 18

• induces osmotic diuresis
• water and urine attracted into collecting duct due to increased solute
• polyuria