Renal Flow and Glomerular Filtration

Question 1

What proportion of cardiac output normally perfuses the kidney?

Answer 1

Kidneys recieve 20% of cardiac output which is approx 5L/min so renal blood flow is around 1L/min

Question 2

What does the glomerular filtration rate depend on?

Answer 2

Glomerular capillary pressure
Plasma oncotic pressure
Tubular pressure
Glomerular capillary surface area

Question 3

What does freely filtered mean?

Answer 3

Equal concentration in filtrate and plasma

Question 4

What are the factors affecting filtration of substances in the glomerulus?

Answer 4

Severe haemorrhage - lower hydrostatic pressure GFR down
Obstruction in nephron tubule - more tubular pressure GFR down
Reduced plasma protein concentration - less osmotic pressure GFR up
Small increase in BP - compensated for, no change in GFR

Question 5

What is the glomerular filtration rate?

Answer 5

Amount of fluid filtered from the glomeruli into the bowmans capsule in ml/min

Question 6

What is the filtration fraction?

Answer 6

ratio between renal blood flow and amount of filtrate filtered by glomerulus

Question 7

What is the normal value for glomerular filtration pressures

Answer 7

10-20mmHg

Question 8

What is glomerular filtration?

Answer 8

Passive process with fluid driven through the fenestrated walls of the glomerular caillaries by heart hydrostatic pressure

Question 9

What is primary urine?

Answer 9

Clear fluid free of blood and proteins that contains electrolytes and small solutes at the same concentration as in the blood

Question 10

What is the ultrafiltration coefficient?

Answer 10

Kf, accounts for membrane permeability and surface area available for filtration

Question 11

What is the normal filtration fraction rate?

Answer 11

normally 20%

Question 12

What is the normal glomerular filtration rate?

Answer 12

approx 120ml/min

Question 13

What is the approximate value of renal blood flow and renal plasma flow?

Answer 13

Renal blood flow 1L/min

Renal plasma flow 0.6L/min

Question 14

What is renal clearance?

Answer 14

Number of litres of plasma that is completely cleared of substance x per unit time

Question 15

What is the use of renal clearance to asses renal function?

Answer 15

GFR can be calculated by measuring a molecule that is freely filtered and not absorbed eg inulin or creatine

Question 16

How to calculate renal clearance?

Answer 16

(conc substance in urine x rate of urine production) / conc of substance in plasma
in ml/min

Question 17

What is inulin?

Answer 17

Plant polysaccharide that is not found in mammals so is transfused

Question 18

What is special about creatine?

Answer 18

Amount released from body is constant, so clearance stable. Low clearance or high plasma creatine could be renal failure

Question 19

What is renal failure?

Answer 19

Abrupt fall in glomerular filtration

Question 20

How is renal plasma flow measured?

Answer 20

PAH - paraaminohippurate
Clearance 625ml/min
Filtered and actively secreted in one pass of the kidney so can be used to measure RPF

Question 21

Why must drug dose be altered when giving drugs to patients with renal failure?

Answer 21

Excretion of many drugs is impaired in patients with renal failure

Question 22

Difference in composition of glomerular filtrate and plasma?

Answer 22

Glomerular filtrate has everything in the plasma that is less than 50,000 Da in size