The glomerulus

Question 1

Which substances are recovered by the kidney?

Answer 1

• Water (<99%)
• Na+ and Cl+ (<99%)
• Bicarbonate (100%)
• Glucose and amino acids (100%)

Question 2

Describe the electrolyte composition of the extracellular fluid

Answer 2

• Low K+
• High Na+
• Main anions are Cl- and HCO3-
• Failure to control these will affect transport and electrical functions.

Question 3

Describe the electrolyte composition of intracellular fluid

Answer 3

• High K+
• Low Na+
• Many large organic anions

Question 4

What is the RBF?

Answer 4

• Renal blood flow = 1l of blood flowing through glomeruli per minute

Question 5

What is the RPF?

Answer 5

• Renal plasma flow = amount of plasma flowing through glomeruli at any one time
• 600 ml/min
• RPF = RBF x (1-haemoatocrit)

Question 6

What is the filtration fraction?

Answer 6

• Proportion of fluid reaching the kidneys that passes into the renal tubules.
• FF = GFR/RPF
• Normally = ~20%

Question 7

What is the composition of the glomerular filtrate?

Answer 7

• It is composed of mostly organic solutes with a low molecular weight and inorganic ions.
• Contains no platelets
• Contains virtually no proteins

Question 8

How can you distinguish between the PCT and the DCT in a histology specimen?

Answer 8

• PCT has a brush border to help increase reabsorption.
• DCT can be seen snuggled up to the glomerulus

Question 9

What is the function of the podocytes?

Answer 9

• They help create a selectively permeable barrier so that not all molecules can pass into the Bowman’s capsule.
• They have a negative charge which allows them to repel other negatively charged molecules like albumin.

Question 10

What results in proteinuria?

Answer 10

• The negative charge on the filtration barrier between the glomerulus and the Bowman’s capsule is lost.
• This means that proteins are more readily filtered.

Question 11

What molecular weight is too big to pass through the filtration barrier of the nephron?

Answer 11

• Anything bigger than 68000

Question 12

What is the GFR?

Answer 12

• Glomerular filtration rate is used to measure kidney filtration function.
• It is the amount of filtrate produced from the blood flow per unit time.
• Measured in mL/min
• Determined by the average volume of filtrate produced by each nephron.

Question 13

What does a decrease in GFR mean?

Answer 13

• Kidney function has worsened
• There has been a decline in number of nephrons or a decline in GFR within individual nephrons.
• Slow decline in kidney function can cause individual nephrons to hypertrophy, so kidney function may not fall until significant damage has occurred.

Question 14

What is renal clearance?

Answer 14

• A surrogate marker for GFR
• The volume of blood plasma that is cleared of a substance in a unit of time.

Question 15

What is the equation for renal clearance?

Answer 15

• Clearance = [urine concentration x urine flow rate] / plasma concentration

Question 16

What are the units for the equation for renal clearance?

Answer 16

• Clearance is measured in mL/minute
• Urine concentration of x = mg/mL
• Urine flow rate measured in mL/min
• Plasma concentration of x = mg/mL

Question 17

How is filtration rate calculated?

Answer 17

• Filtration rate = Plasma concentration x GFR

Question 18

How is excretion rate calculated?

Answer 18

• Excretion rate = Urine concentration x urine flow rate

Question 19

What are the factors that affect GFR?

Answer 19

• Young age - due to lower nephron numbers
• Old age - due to loss of functioning nephrons
• Pregnancy - causes GFR and kidney size to increase

Question 20

What are the ideal qualities of a substance that is used to measure kidney clearance?

Answer 20

• Be produced at a constant rate
• Be freely filtered across the glomerulus
• Not be reabsorbed in the nephron
• Not be secreted into the nephron
• E.g. inulin

Question 21

Why don’t we use inulin to measure renal clearance?

Answer 21

• It requires continuous IVI to maintain a steady state
• Requires catheter and timed urine collections

Question 22

What substance do we use to measure renal clearance?

Answer 22

• 51 Cr-EDTA
• A radioactive labelled marker
• Given as a timed injection with blood samples taken 2, 3 and 4 hours afterwards.

Question 23

What is creatinine?

Answer 23

• The end product of muscle breakdown
• Can be used to measure GFR (in pregnancy)
• Measured by urine creatinine over 24 hours or serum creatinine

Question 24

What are the disadvantages of using creatinine as a measure of GFR?

Answer 24

• Cumbersome - you have to carry a bottle of urine
• Frequently inaccurate
• Overestimates GFR by 10-20% due to creatinine secretion (i.e. creatinine is secreted into the nephron from the blood plasma).

Question 25

What is the normal value range for serum creatinine?

Answer 25

• 70-150 micromoles/L

Question 26

What factors may cause serum creatinine to increase?

Answer 26

• Large muscle bulk
• Young
• Male
• Creatine supplements
• High intake of meat
• Certain drugs e.g. trimethoprim

Question 27

What factors may cause serum creatinine to decrease?

Answer 27

• Reduced muscle mass
• Muscle cell breakdown
• Old
• Female
• Vegetarian

Question 28

What are some of the problems with estimating GFR?

Answer 28

• It’s inaccurate in mild kidney disease
• Reduced nephron number leads to nephron hypertrophy so there’s no change in GFR.
• Reduced filtration of creatinine causes increased secretion of creatinine into the tubule.