Urinary System 3 - GFR and liver failure Flashcards Preview

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Flashcards in Urinary System 3 - GFR and liver failure Deck (29)
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1
Q

Define glomerular filtration

A
  • 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
2
Q

Define freely filtered

A

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

3
Q

What is ultrafiltrate?

A
  • A clear fluid free from blood and proteins, is produced containing electrolytes and small solutes
  • ‘primary urine’
4
Q

What is the amount excreted equal to?

A

Amount filtered + Amount secreted - Amount absorbed

5
Q

How is net ultrafiltration pressure calculated?

A

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

6
Q

What is the net ultrafiltration pressure?

A

10-20mmHg

7
Q

How is glomerular filtration rate calculated?

A

Ultrafiltration rate x Kf (ultrafiltration coefficient)

8
Q

List the factors affecting the ultrafiltration constant.

A
  • Kidney disease reducing functioning nephrons and therefore reducting SA
  • Dilation of glomerular arterioles by drugs/hormones will increase Kf
9
Q

Define glomerular filtration rate

A

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

10
Q

What is renal blood flow?

A

Approx 1L

11
Q

What is renal plasma flow?

A

Approx 0.6L

12
Q

What is the filtration fraction?

A
  • 0.2

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

13
Q

What is normal glomerular filtration rate?

A

Renal plasma flow x filtration fraction = 0.12

14
Q

What does glomerular filtration rate depend upon?

A
  • Glomerular capillary pressure
  • Plasma oncotic pressure
  • Tubular pressure
  • Glomerular capillary surface area or permeability
  • Altered by neural and hormonal input
15
Q

Describe the mechanism of autoregulation in the kidney.

A
  • 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)
16
Q

Define renal clearance

A
  • 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.
17
Q

What is the equation for clearance?

A

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

18
Q

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

A
  • 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
19
Q

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

A
  • 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
20
Q

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

A
  • 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
21
Q

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

A

120ml/min

22
Q

How is renal plasma flow measured?

A
  • 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
23
Q

Describe the pathway through the kidney of most solutes

A
  • Clearance less than 120mL/min, with reabsorption as well as secretion and excretion
  • Controlled excretion
24
Q

What is the cardinal feature of renal disease?

A
  • A fall in GFR

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

25
Q

Why is renal disease important to consider when administering drugs?

A

Doses need to be monitored in case of overdose

26
Q

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

A

Renal plasma flow rate x plasma PAH concentration

27
Q

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

A

Urine PAH concentration x urine flow rate

28
Q

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

A
  • 1mmol/L

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

29
Q

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

A
  • 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