Urinary System 3 - GFR and liver failure

Question 1

Define glomerular filtration

Answer 1

• Formation of an ultrafiltrate of plasma in the glomerulus via a passive process
• Blood is driven through semi-permiable walls of glomerular capillaries into the bowmans capsule space by hydrostatic pressure of the heart

Question 2

Define freely filtered

Answer 2

Same concentration of a solute in the filtrate and in the plasma

Question 3

What is ultrafiltrate?

Answer 3

• A clear fluid free from blood and proteins, is produced containing electrolytes and small solutes
• ‘primary urine’

Question 4

What is the amount excreted equal to?

Answer 4

Amount filtered + Amount secreted - Amount absorbed

Question 5

How is net ultrafiltration pressure calculated?

Answer 5

Hydrostatic pressure in glomerular capillaries (Pgc) - Hydrostatic pressure of tubule (Pt) - Osmotic pressure of plasma proteins in glomerular capillaries (πgc)

Question 6

What is the net ultrafiltration pressure?

Answer 6

10-20mmHg

Question 7

How is glomerular filtration rate calculated?

Answer 7

Ultrafiltration rate x Kf (ultrafiltration coefficient)

Question 8

List the factors affecting the ultrafiltration constant.

Answer 8

• Kidney disease reducing functioning nephrons and therefore reducting SA
• Dilation of glomerular arterioles by drugs/hormones will increase Kf

Question 9

Define glomerular filtration rate

Answer 9

The amount of fluid filtered from the glomeruli into the Bowmans capsule per unit of time (ml/min).

Question 10

What is renal blood flow?

Answer 10

Approx 1L

Question 11

What is renal plasma flow?

Answer 11

Approx 0.6L

Question 12

What is the filtration fraction?

Answer 12

• 0.2

- Ratio between renal plasma flow and the amount of filtrate filtered by the glomerulus

Question 13

What is normal glomerular filtration rate?

Answer 13

Renal plasma flow x filtration fraction = 0.12

Question 14

What does glomerular filtration rate depend upon?

Answer 14

• Glomerular capillary pressure
• Plasma oncotic pressure
• Tubular pressure
• Glomerular capillary surface area or permeability
• Altered by neural and hormonal input

Question 15

Describe the mechanism of autoregulation in the kidney.

Answer 15

• Myogenic mechanism
• Vascular smooth muscle contracts when stretched, keeping GFR constant when blood pressure rises
• Tuboglomerular feedback (macula densa cell response to high sodium concentration)

Question 16

Define renal clearance

Answer 16

• As substances in the blood pass through the kidney they are filtered to different degrees.
• The extent to which they are removed from the blood is called clearance.
• Clearance is the number of litres of plasma that are completely cleared of the substance per unit time.

Question 17

What is the equation for clearance?

Answer 17

concentration of substance in urine x rate of urine production / concentration of substance in the plasma

Question 18

How can GFR be measured using clearance? Name two substances that can be used

Answer 18

• Clearance can be measured if a molecule is freely filtered and neither reabsorbed nor secreted in the nephron
• Amount filtered = amount excreted
• Inulin, a plant polysaccharide, can be used
• So can creatinine

Question 19

List the properties of inulin that make it an accurate measure of GFR, and why it is not desirable

Answer 19

• Plant polysaccharide - freely filtered, neither reabsorbed nor secreted
• Not toxic
• Measurable in the urine and plasma
• However, not made by humans so must be transfused with catheterisation

Question 20

List the properties of creatinin that make it good and bad for GFR use

Answer 20

• Waste product made in muscle metabolism
• Constant amount released
• If there is stable renal function, amount of creatinine in the urine is stable
• Low values both low and high values indicate renal failure
• Not as accurate as inulin

Question 21

What is the normal value of renal clearance of inulin and creatinine?

Answer 21

120ml/min

Question 22

How is renal plasma flow measured?

Answer 22

• Measured by PAH (625l/min)
• This is because all RPF is removed from the blood by the renal system, first by filtration then actively secreted at the tubule
• Clearance of PAH matches renal plasma flow

Question 23

Describe the pathway through the kidney of most solutes

Answer 23

• Clearance less than 120mL/min, with reabsorption as well as secretion and excretion
• Controlled excretion

Question 24

What is the cardinal feature of renal disease?

Answer 24

• A fall in GFR

- Excretory products build up in plasma, so creatinine concentration is raised

Question 25

Why is renal disease important to consider when administering drugs?

Answer 25

Doses need to be monitored in case of overdose

Question 26

What is the equation you would use to calculate the rate PAH enters the kidneys per minute in renal arteries?

Answer 26

Renal plasma flow rate x plasma PAH concentration

Question 27

What is the equation you would use to measure rate of excretion of PAH in urine?

Answer 27

Urine PAH concentration x urine flow rate

Question 28

If the plasma arterial inulin concentration is 1mmol/L, what is the plasma inulin concentration in the efferent arteriole?

Answer 28

• 1mmol/L

- 20% filtered from blood, so 20% less water and 20% less inulin. Therefore, concentration remains the same

Question 29

If the plasma arterial inulin concentration is 1mmol/L, what will the inulin concentration be in the renal vein? Why?

Answer 29

• 0.8mmol/L
• Though 20% water enters the filtrate, it is reabsorbed into the renal vein. Inulin is not reabsorbed, so the concentration of inulin decreases by 20%
• Say initial water in the blood is 1L and 200ml enters filtrate, that 200ml is reabsorbed into the renal vein.
• 0.8mmol of inulin remains in the blood from the efferent arteriole. 0.8/1=0.8mmol/L