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Urinary System > 3. Renal Blood Flow and Glomerular Filtration > Flashcards

3. Renal Blood Flow and Glomerular Filtration Flashcards

1
Q

What is glomerular filtration?

A

Formation of an ultrafiltrate of plasma in the glomerulus

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2
Q

What is a cardinal sign of renal failure?

A

Sharp drop in GFR

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3
Q

Why is glomerular filtration described as a passive process?

A

Fluid is ‘driven’ through the fenestrated walls of the glomerular capillaries into the Bowmans capsule space by the hydrostatic pressure of heart.

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4
Q

What is the fenestrated endothelium of capillaries and semipermeable Bowman’s capsule highly permeable to?

A

Fluids

Small solutes

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5
Q

What is the fenestrated endothelium of capillaries and semipermeable Bowman’s capsule impermeable to?

A

Cells
Proteins
Drugs etc. carried bound to protein

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6
Q

What is the result of glomerular filtration?

A

A clear fluid (ultra filtrate) completely free from blood and proteins, is produced containing electrolytes and small solutes = ‘primary urine’

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7
Q

Why do many people with cardiovascular disease have renal problems?

A

CO creates the pressure which drives glomerular filtration

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8
Q

What does ‘freely filtered’ mean?

A

A substance that has the same concentration in the filtrate as in the plasma (e.g. small solutes)

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9
Q

Concentration of solute does not change with glomerular filtration but may do further along nephron

A

Further down in the tubule, substances that are freely filtered can be reabsorbed or secreted
Components that are valuable to body e.g. ions are reabsorbed, so their concentration in the urine is quite low
If body wants to get rid of components, secreted from plasma into urine, so higher concentration in urine

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10
Q

Equation for amount excreted

A

Amount filtered + Amount secreted - Amount absorbed

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11
Q

What is the driving force for ultrafiltration?

A

It is a passive process driven by the hydrostatic pressure of the heart

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12
Q

What are the 2 forces opposing the hydrostatic pressure

A

Hydrostatic pressure of the tubule

Osmotic pressure of the plasma proteins in glomerular capillaries

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13
Q

What is the net ultrafiltration pressure?

A

Puf = 10 - 20 mm Hg

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14
Q

Define Glomerular Filtration Rate.

A

The amount of fluid filtered from the glomeruli to the Bowman’s capsule per unit time (ml/min)

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15
Q

What is the equation for glomerular filtration rate?

A 
GFR = Puf x Kf 
Kf = ultrafiltration coefficient (based on membrane permeability and SA)
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16
Q

Explain, using the equation, how renal disease can cause a decrease in GFR.

A

Lose nephrons
Hence lose SA
–> decrease in Kf
–> decrease in GFR

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17
Q

What effect do drugs/hormones have on Kf?

A

Increase Kf

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18
Q

How does inflammation affect Kf?

A

Inflammation causes increase in membrane permeability.
Increases Kf
Check for protein in urine= indicates inflammation of kidney

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19
Q

What proportion of cardiac output goes to the kidneys?

A

20%

~ 1 L/min

20
Q

What is filtration fraction? Give an approximate value.

A

The ratio between the renal plasma flow and the amount of filtrate filtered by the glomerulus (usually ~ 20%)

21
Q

What is RPF?

A

Renal plasma flow

~ 0.6 L/min

22
Q

State the equation for GFR using filtration fraction.

A

GFR = RPF x FF

23
Q

Approximate value of GFR

A

120 ml/min

= volume of filtrate formed in 1 minute

24
Q

Concentration of a small solute is 1mm in plasma, 20% is filtered by glomerulus: what is its concentration after filtration?

A

Still 1mm as it is freely filtered

25
Q

State 4 factors that affect GFR?

A

Glomerular capillary pressure
Plasma oncotic pressure
Tissue pressure
Glomerular capillary SA or permeability (Kf)

26
Q

State the 2 mechanisms involved in autoregulation of GFR.

A

Myogenic Mechanism

Tubuloglomerular Feedback

27
Q

Describe the myogenic mechanism.

A

CO increases
Afferent arteriole is stretched
Vascular smooth muscle constricts when stretched
Reduces flow and thus pressure in bowman capsule
This keeps GFR constant when BP rises

28
Q

How would severe haemorrhage effect GFR?

A

Decrease GFR

29
Q

How would obstruction in nephron tubule effect GFR?

A

Decrease GFR

as opposing pressure is increased

30
Q

How would reduced plasma protein concentration effect GFR?

A

Increase GFR

as opposing pressure is decreased

31
Q

How would small increase in BP effect GFR?

A 
No effect
(because of myogenic mechanism)
32
Q

Define clearance.

A

The number of litres of plasma that are completely cleared of the substance per unit time

33
Q

State the equation for clearance.

A 
C = (U X V) / P
V = rate of urine production 
U = concentration of substance in urine 
P = concentration of substance in plasma
34
Q

What will be the clearance rate of something that is actively reabsorbed from the urine?

A

Low clearance rate (as much of it is reabsorbed back into the plasma)

35
Q

What are the properties of a substance used to measure GFR? Give 2 examples

A

Inulin and Creatinine:

Freely filtered and is neither reabsorbed nor secreted

36
Q

Inulin

A

Plant polysaccharide
Measurable in urine and plasma
Not found in mammals so needs to be transfused

37
Q

What is used practically to measure GFR?

A

Creatinine:
Naturally produced waste product of creatine in muscle metabolism
Released at a fairly constant rate in the urine
If renal function is stable then the amount of creatinine in the urine is stable.

38
Q

How does rate of creatinine clearance help us?

A

Rate at which it is cleared is equal to GFR

39
Q

What happens to creatinine levels if GFR drops?

A

Clearance of creatinine drops

Plasma concentration of creatinine increases

40
Q

What would a high plasma concentration of creatinine indicate?

A

Renal problem
High muscle mass
Intake of protein shakes

41
Q

Values for renal clearance

A

Everything below the renal clearance rate of creatinine/ inulin (120 ml/min) is retained to some extent by the body

42
Q

What is used to measure Renal Plasma Flow Rate?

A

PAH: para aminohippurate

43
Q

What properties does PAH have that makes it appropriate for measuring RPF?

A

It is totally cleared from the plasma when it passes through the kidneys.
Normal glomerular filtration removes 20% of the total PAH and then secretion from the blood into the tubules removes the other 80%
There is no PAH in the renal vein.

44
Q

What is clearance of PAH equal to?

A

Renal plasma flow

45
Q

What are the 3 process substances can undergo during filtration?

A

Some substances are freely filtered, ultimate amount cleared can be significantly lower as can be actively reabsorbed from urine back into plasma

Some substances are freely filtered, then neither reabsorbed nor secreted

PAH is completely secreted by kidney so clearance of that = renal plasma flow

46
Q

What change in creatinine concentration indicates renal disease?

A

Increase

Cretinine is building up because the kidneys are unable to remove it

47
Q

How could a fall in GFR cause an overdose?

A

Excretion of drugs would be impaired.
If GFR fell to 30 ml/min, concentration of the drug will increase proportionally (4 fold)
A drug with a narrow therapeutic window could be fatal.

Urinary System (8 decks)

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