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GERR > Lecture 37: Glomerular Filtration Rate > Flashcards

Lecture 37: Glomerular Filtration Rate Flashcards

1
Q

what is glomerular filtration

A

process by which water and some solutes pass from glomerular capillary into Bowman’s capsule

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2
Q

what are the 3 barriers to filtration

A
  • capillary endothelium
  • podocyte foot processes
  • basement membrane
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3
Q

what are the 2 parts of the nephron

A
  • renal corpuscle (glomerulus and Bowman’s capsule

- renal ducts (incl. CD)

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4
Q

how much cardiac output do the kidneys receive

A

25%

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5
Q

what is renal blood flow

A

volume of blood delivered to the kidneys every minute

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6
Q

why is renal circulation unique

A

has 2 sets of capillary beds

  • one in glomerulus
  • other around the tubules and LoH
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7
Q

describe the structure of the glomerulus in terms of bv

A
  • arterioles either side of cap bed rather than venule after cap bed
  • arterioles defined by muscular layer and pressure within
  • caps very tightly spread
  • cap function is NOT to deliver O2 and nutrients
  • afferent arteriole is larger in diameter than efferent arteriole which adds to pressurisation of glomerulus
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8
Q

describe the specialisation vasa recta in the nephron

A

peritubular caps (which supply tubules) have additional branches that lead towards LoH

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9
Q

describe the overall structure of the glomerulus

A
  • vascular component (aff. and eff. arteriole) and capsular structure (Bowman’s capsule)
  • in junction between aff. eff. and DCT lies an arr. of cells called juxtaglomerular apparatus
  • caps (cap bed) lies within glomerulus itself
  • caps surrounded by epi cell called podocyte which has branching arms called end-feet which encapsulate caps in highly specialised arr.
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10
Q

what is the role of juxtaglomerular apparatus cells

A
  • cells on the vascular side sense BP changes and respond to it
  • cells on tubule side sense changes in [Na+] and respond to adjust it
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11
Q

what is important about the gaps between end-feet of podocytes in glomerulus

A
  • allow for passage of water and solutes out of blood and into filtrate i.e. glomerular filtration
  • lined w/ proteins jutting out into slit which act to control movement of small prots out of blood
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12
Q

outline how the glomerulus is specialised for filtration

A
  • caps are fenestrated with glycocalyx
  • podocytes with gaps between end-feet
  • basement membrane between caps and podocytes
    (these 3 things provide a selectivity filter)
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13
Q

describe role of glycocalyx

A

pecialised prots within fenestrae gaps that act as a barrier to loss of large molecules and cells from blood

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14
Q

describe role of basement membrane

A
  • relatively thick layer of filamentous proteins e.g. collagen, laminin and proteoglycans
  • prevent movement of smaller proteins out of blood
  • BM provides structural integrity
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15
Q

describe the filtrate composition

A
  • water
  • glucose
  • electrolytes e.g. Na+, K+, Ca2+,
  • amino acids
  • fatty acids
  • urea
  • protein smaller than 3.5nm

all above can pass easily across filtration barrier
all below are too big to pass through

  • RBC
  • WBC
  • platelets
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16
Q

what is proteinuria

A

protein present in urine

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17
Q

what is haematuria

A

blood in urine

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18
Q

under what circumstances might haematuria and proteinuria occur

A
  • diabetes
  • damage to glomerulus
  • hypertension
  • kidney disease
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19
Q

under what circumstances might leucocytes be present in urine

A

infection e.g. UTI

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20
Q

what is the equation of net filtration pressure in general capillaries

A

NFP = (HPc - HPif) - (πc – πif)

  • HP is the hydrostatic pressure
  • c is the capillary (HPc = BP)
  • if is the interstitial fluid
  • π is the osmotic pressure

(πc is the osmotic pressure exerted by blood contents and πif is osmotic pressure exerted by interstitial fluid contents)

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21
Q

describe the cap NFP at arterial end vs venous end

A

arterial end
- +ve NFPout

venous end
- -ve NFPin

22
Q

give the glomerular NFP equation

A

NFP = (HPgc - HPbc) - (πgc – πbc)

23
Q

compare HPgc to HPc

A

HPgc > HPc

24
Q

what is HPgc determined by

A
  • arterial pressure

- eff > aff. arterial resistance

25
Q

what is HPbc determined by

A
  • pressure of filtrate in capsule, tubules and CD (HPbc < HPgc)
26
Q

what is πgc determined by

A

prots in plasma

27
Q

what is πbc determined by

A

prots in filtrate

28
Q

what is GFR determined by

A

NFP and filtration co-efficient

29
Q

give the GFR equation

A

GFR = NFP x Kf

30
Q

what is Kf in the GFR equation

A

product of hydraulic conductivity (measure of ease w/ which a fluid can move through a pore) and glom cap SA

31
Q

give the appx GFR in males and females

A

males
~125mL/min

females
~105mL/min

GFR also expressed as mL/min/1.73m2

32
Q

what can make urine appear fizzy

A

excess proteins

33
Q

give the equation for clearance rate of a substance from the blood into the urine when any substance (X) is freely filtered and is neither reabsorbed nor secreted by the kidneys

A

GFR = Urine [X] × Urine flow

Plasma [X]

34
Q

give examples of substances that can be used to measure clearance rate

A
  • insulin (invasive and inaccurate in practice due to incomplete bladder voiding)
  • creatinine (naturally occurring) –> clearance based on serum creatinine level are used to estimate GFR
35
Q

outline how creatinine clearance is used to estimate GFR and indicate liver function

A
  • creatinine is breakdown product of creatinine phosphate found in muscle
  • freely filtered in glomerulus but also actively secreted by peritub. caps. in very small amounts
  • so creatinine clearance over estimates actual GFR by 10-20%
  • dec. clearance = ^ plasma [creatinine] = dec. renal function
36
Q

list what factors affect GFR and how

A
  • kidney disease (reduction)
  • hypertension (reduction due to glomeruli damage)
  • sleep (hormonal and oncotic variations)
  • exercise (blood flow diverted to muscles)
  • renal blood flow (reduced blood flow = reduced GFR e.g. in renal artery stenosis)
37
Q

what is one of the main factors controlling renal blood flow

A

contractile state of renal arterioles

38
Q

describe glomerulus HP and GFR if aff. arteriole constricts

A
  • vol. of blood flowing to glomerulus (and renal blood flow) dec.
  • HP drops because aff. arteriole constricts but eff. arteriole doesn’t
  • so NFP and thus GFR both dec.
39
Q

what is the importance of myogenic constriction in kidney

A

blood flow auto regulation in kidney which keeps GFR fairly constant over normal range of BP b/c renal blood flow stays fairly constant

40
Q

when might aff. arteriole constrict butt not eff.

A

when ADH is released

41
Q

describe glomerulus HP and GFR if eff. arteriole constricts

A
  • renal blood flow still decreased
  • HP actually rises
  • blood backs up in glomerulus
  • so GFR ^
42
Q

when might constriction of eff. but not aff. arteriole occur

A

when ang2 is released

43
Q

describe glomerulus HP and GFR if both aff. and eff. arteriole constrict

A
  • renal blood flow dec.
  • HP largely maintained around normal levels
  • thus GFR maintained
  • blood can be diverted to other parts of the body
44
Q

when might both aff and eff arterioles constrict

A
  • flight or fight response

- moderate SNS activity

45
Q

what effect does increased arterial BP have on glomerulus

A
  • ^ pressure in aff. arteriole
  • ^ flow through glom.
  • ^ HP in glom
  • ^ GFR
46
Q

what response occurs due to ^ aff. arteriole pressure and what is its effect

A
  • reflex myogenic vasoconstriction
  • reduces blood flow through aff. arteriole
  • dec. glom HP and GFR
  • once aff. BP dec. then myogenic vasoconstriction is relieved and vessel relaxes
  • blood flow ^
47
Q

what occurs at the glomerulus if BP drops significantly

A
  • sensed by juxtaglomerular apparatus in aff. arteriole
  • renin released
  • activates ang2
  • acts on eff. arteriole to ^ vasoconstriction
  • ^ HP in glom
  • maintaining filtration at reasonably normal levels
48
Q

how is renal blood flow measured

A

clearance:
- vol. of plasma which is cleared of a particular substance in 1 min calibrated for amount of blood that is plasma vs RBC

49
Q

what is an important factor when measuring renal blood flow

A

choice of substance:

  • needs to be cleared in 1 circuit through kidney
  • needs to be filtered and secreted as this will take into account tubular blood flow
50
Q

give example of substance commonly used to measure renal blood flow and why it is good

A

PAH - para-aminohippuric acid

  • > 90% extracted by kidneys in one circuit
  • not manufactured/metabolised by body (IV infused)
  • doesn’t affect kidney function
51
Q

what is the issue of using PAH to measure renal blood flow and how is that issue overcome

A
  • PAH confined to plasma
  • doesn’t directly measure renal blood flow

Clearance of PAH = plasma vol. delivered to kidneys every min = renal plasma flow

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

GERR (46 decks)

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