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Year 2 > Glomerular filtration and renal blood flow > Flashcards

Glomerular filtration and renal blood flow Flashcards

1
Q

What is glomerular filtration?

A

Process whereby a proportion of the plasma enters the bowman’s capsule and forms the initial tubular fluid

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2
Q

Which structures compose the glomerular epithelium? What does each form a barrier to?

A

Capillary endothelium - RBC
Basal lamina/basement membrane - proteins
Slit processes of podocytes - proteins

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3
Q

Across which structure does the process of filtration occur?

A

Glomerular epithelium

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4
Q

What feature of the basement membrane allows it to act as a barrier to plasma proteins?

A

Negatively charged glycoproteins repel negatively charge plasma proteins

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5
Q

Which (starling) forces comprise net filtration pressure?

A

Capillary hydrostatic pressure
Bowman’s oncotic pressure
Capillary oncotic pressure
Bowman’s hydrostatic pressure

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6
Q

Which of the forces involved in net filtration favour it?

A

Capillary hydrostatic pressure

Bowman’s oncotic pressure

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7
Q

Which of the forces involved in net filtration oppose it?

A

Bowman’s hydrostatic pressure

Capillary oncotic pressure

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8
Q

Is glomerular filtration an active or passive process?

A

Passive

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9
Q

What is oncotic pressure?

A

The pressure of plasma proteins (drags fluid towards it)

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10
Q

Why is capillary blood pressure constant throughout the nephron?

A

Because of the back pressure created by the afferent arteriole being of larger diameter than the efferent arteriole

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11
Q

Why is bowman’s oncotic pressure 0 in a healthy adult?

A

Plasma proteins should be unable to cross the glomerular membrane

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12
Q

What is GFR?

A

Glomerular filtration rate - the rate at which protein free plasma is filtered from the glomeruli into the bowman’s capsule per unit time

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13
Q

Write the equation to calculate GFR

A

GFR = filtration coefficient x net filtration pressure

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14
Q

What is the filtration coefficient?

A

A measure of how holey (i.e permeable to substances) the glomerular membrane is

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15
Q

What is the average GFR in a healthy adult?

A

125ml/min

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16
Q

Which starling force is the major determinant of GFR?

A

Capillary hydrostatic (blood) pressure

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17
Q

How is GRF regulated?

A 
Extrinsic mechanisms (sympathetic activity)
Intrinsic mechanisms/autoregulation (myogenic, tubuloglomerular feedback)
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18
Q

How does arterial blood pressure affect GFR?

A

Increase in BP –> increase in capillary hydrostatic pressure –> increase in net filtration pressure –> increase in GFR

(and vice versa)

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19
Q

How does vasoconstriction and vasodilation affect GFR respectively?

A

Vasoconstriction - decreased blood flow –> decreased GFR

Vasodilation - increased blood flow –> increased GFR

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20
Q

Do changes in systemic arteriolar pressure always cause changes in GFR?

A

No - autoregulation helps prevent short term changes in mean arterial BP from affecting renal blood flow and GFR

21
Q

How is mean arterial blood pressure calculated?

A

1/3 pulse pressure plus diastolic pressure

22
Q

How is autoregulation of GFR protective?

A

Helps prevent serious electrolyte imbalance

23
Q

Explain myogenic autoregulation

A

Vascular smooth muscle within the kidneys contracts when stretched (i.e by increased blood pressure) thereby limiting blood flow

24
Q

Explain tubuloglomerular autoregulation

A

Increased GFR causes increased salt flowing through the tubules which is detected by the macula densa and results in the constriction of afferent arterioles

25
Q

How does the macula densa cause constriction of afferent arteriole?

A

Release of vasoactive mediators which cause vasoconstriction

26
Q

When does extrinsic control of GFR override intrinsic control?

A

When extrinsic changes are severe enough

27
Q

How does increased hydrostatic pressure within the Bowman’s capsule change GFR? In which pathology might this be seen?

A

It decreases it

Kidney stones

28
Q

How does increased capillary oncotic pressure change GFR? In which pathology might this be seen

A

It decreases it

Diarrhoea (dehydration causes proportional increase in plasma proteins)

29
Q

How does decreased capillary oncotic pressure change GFR? In which pathology might this be seen?

A 
It increases it
Severe burns (due to leakage of plasma proteins from the damaged skin)
30
Q

How does a decrease in the filtration coefficient change GFR?

A

It decreases it (due to decreased surface area for filtration)

31
Q

What is plasma clearance?

A

The efficiency of the kidneys at clearing the plasma of a substance (the volume of plasma completely cleared of a substance per minute)

32
Q

What is the value of kidney plasma clearance?

A

Plasma clearance is a value unique to each substance being cleared

33
Q

What are the units of plasma clearance?

A

ml/min

34
Q

Write the equation used to calculate plasma clearance

A

Plasma clearance = rate of excretion / plasma concentration

i.e plasma clearance = [urine concentration x urine output]/plasma concentration

35
Q

For which substances does plasma clearance equal GFR?

A

Inulin

Creatinine (almost)

36
Q

Why are substances which have a plasma clearance equal to GFR useful?

A

They provide a clinical measure of GFR

37
Q

Why is creatinine used as a measure of GFR over inulin?

A

Creatinine is endogenous and therefore does not need to be infused

NB - creatinine is secreted a little bit so doesn’t give 100% accuracy

38
Q

Which important substance is filtered and completely reabsorbed (i.e clearance = 0) but not secreted?

A

Glucose (in a healthy adult)

39
Q

Which importance substance is filtered and partially reabsorbed (i.e clearance

A

Urea

40
Q

Which important substance is filtered and completed secreted (i.e clearance > GFR)?

A

Hydrogen ions

41
Q

How can we tell if a substance is reabsorbed, secreted or neither using clearance and GFR values?

A

Clearance GFR - secreted

NB - GFR is taken to be 125ml/min

42
Q

Which substance is used clinically to measure renal plasma flow?

A

Para-amino hippuric acid (PAH)

43
Q

How is PAH treated by the kidneys?

A

Filtered
Completely secreted
Not reabsorbed (i.e completely cleared from plasma)

44
Q

What is creatinine?

A

A muscle metabolite produced at a nearly constant rate

45
Q

Which three properties should substances used as clearance markers have?

A 
Non-toxic
Inert
Easy to measure 
GFR - not absorbed or secreted
RPF - not absorbed but completely secreted
46
Q

What is the filtration fraction?

A

Fraction of plasma flowing through the tubules that gets filtered

47
Q

Write the equation used to calculate filtration fraction

A

Filtration fraction = GFR / RPF

48
Q

Write the equation used to calculate renal blood flow

A

Renal blood flow = renal plasma flow x 1/(1-haematocrit)

Year 2 (72 decks)

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