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URINARY > 1. Glomerular filtration > Flashcards

1. Glomerular filtration Flashcards

1
Q

what are the three basic renal processes

A
  1. filtration
  2. reabsorption
  3. secretion
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2
Q

what is filtration

A

the formation of an essentially protein-free filtrate of plasma at the glomerular capillaries

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3
Q

what percentage of plasma is filtered out at one time

A

~20%

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4
Q

what is the glomerular filtration rate (GFR)

A

around 180 l/day - very high so allows opportunity to precisely regulate ECF volume and composition and eliminate “nasty” substances

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5
Q

what happens during reabsorption

A

substances the body wants are retaken up and those it doesnt want stay in the tubule and are excreted

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6
Q

where does reabsorption take place

A

proximal or distal tube

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7
Q

what/what percentage of plasma is reabsorbed/filtered back

A

sugars, amino acids, Na, water, etc are reabsorbed

around 99% filtered back

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8
Q

what happens in secretion/excretion

A

substances may be specifically removed from the body

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9
Q

what 2 things is excretion particularly important for

A

drug metabolites

proton excretion for acid base balance

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10
Q

in basic terms how are drug metabolites excreted in the kidneys

A

drug metabolites broken down and processed by the liver to convert to water soluble substances – can then be actively excreted by the kidney

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11
Q

what percentage of total cardiac output do the kidneys receive

A

20-25% (~1200mls/min)

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12
Q

how long does it take for a volume of blood equal to total BV to pass through the renal circulation

A

<5 mins

as they get 20-25% of blood flow per minute

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13
Q

why is kidney haemorrhage particularly dangerous

A

due to high BV passing though - can bleed out in less than 5 mins

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14
Q

what components of plasma are excluded from filtration through the bowmens capsule

A

all cells - RBC< WBC, platelets, etc

*also only a tiny fraction of plasma gets filtered thorough the bowmens capsule

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15
Q

what percentage of total blood volume is plasma

A

~55%

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16
Q

what is the renal plasma row

A

around 660mls/min

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17
Q

what is the Glomerular Filtration Rate

A

around 125mls/min

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18
Q

what is the filtration fraction

A

GFR/renal plasma flow x100
= 19%

ie ~20% of the renal plasma become glomerular filtrate

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19
Q

what 2 forces is glomerular filtration dependent on

A
  1. hydrostatic forces from blood stream - pressure on top of fluids to drive through filter - favour filtration
  2. oncotic pressure forces (starlings forces) - osmotic effects - favour reabsorption
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20
Q

what is the main criteria for how effectively substances are filtered out

A

size - if small the filtrate ratio is 1 = concentration infiltrate is the same as plasma concentration
= get completely filtered out

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21
Q

apart from size what can also have an effect on how effectively substances are filtered out

A

charge of molecule and basement membrane that mediates the filtration process

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22
Q

how are different substances filtered out

A

by a “sandwich” system of different membranes

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23
Q

what are the three membranes that are part of the filtration process and what passes through them

A
  1. fenestrated glomerular endothelium - lets everything pass through except cells
  2. Basal lamina of glomerulus - prevents filtration of larger proteins
  3. slit membrane - prevents filtration of medium sized proteins
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24
Q

why is glomerular capillary pressure higher than most other capillaries in the body

A

because afferent arterioles are short and wide - little resistance to flow - lots of blood - blood arriving still has high hydrostatic pressure

25
Q

how do the efferent arterioles allow for a build up of pressure in the glomerulus and thus aid filtration

A

they are long and narrow - increase friction and resistance - build up glomerular pressure - inhibit free outflow from glomerulus - push fluid through

26
Q

what is the golden rule off circulation

A

high resistance =

upstream pressure INCREASED

downstream pressure DECREASED

27
Q

what is the relation between hydrostatic pressure and oncotic pressure at the glomerular capillaries

A

hydrostatic pressure always exceeds oncotic pressure at glomerular capillaries

BUT only at glomerular capillaries

28
Q

what happens to the plasma concentration as it gets filtered

A

become more concentrated

29
Q

what would happen if oncotic pressure exceeded hydrostatic pressure

A

filtration would reverse

30
Q

what happens when the tubes start to fill with filtered fluid

A

start to exhibit hydrostatic pressure themselves - makes water want to go back from tubes to bloodstream

31
Q

why does the fluid NOT go back to the blood stream despite increased hydrostatic pressure and oncosmotic pressure from the tubes

A

NET filtration is 10mmHg - still in favour of filtration

15 mmHg out
30 mmHg out
BUT
55mmHg in

32
Q

what is the primary factor affecting glomerular filtration rate

A

glomerular capillary pressure (PGC)

33
Q

what affects PGC

A

afferent and efferent arteriolar diameter and therefore balance of resistance between them

34
Q

what are three EXTRINSIC controls that can affect GFR

A
  1. sympathetic vasoconstrictory nerves - afferent and efferent constriction, greater sensitivity of afferent arteriole
  2. circulating catecholamines - constriction primarily afferent
  3. angiotensin II - constriction of efferent at [low]
    constriction of both afferent and efferent at [high]
35
Q

what happens when you constrict afferent arterioles

A

reduce blood flow to kidney and glomerulus – pressure in glomerulus drops – glomerular filtration goes down – as a result increased blood flow to other organs - can be important in preserving organ function if blood volume drops

36
Q

what happens when you constrict efferent arterioles

A

overall bloodflow decreased – but pressure in glomerulus goes up – upstream constriction = increased pressure – despite reduced blood flow to kidney

37
Q

what happens when you dilate afferent arterioles

A

decrease resistance in afferent – blood flow increased – efferent unchanged - pressure in glomerulus goes up – GFR goes up also

38
Q

how is BF and GFR kept constant in the kidneys

A

auto regulation of renal vasculature from intrinsic ability to adjust resistance in response to changes i arterial BP

39
Q

what is the range of mean blood pressure in man that the kidneys can auto regulate BF and GFR over

A

when mean blood pressure is from 60-130mmHg

40
Q

how is a rise in glomerular capillary pressure prevented

A

If mean arterial P increases, there is an automatic increase in afferent arteriolar constriction, preventing a rise in glomerular PGC . Dilatation occurs if P falls.

41
Q

what happens if mean blood pressure falls below the effective range for self regulation

A

If goes below effective range – filtration stops – pressure not big enough to drive filtration – movement of fluid can even reverse

*filtration ceases when MBP = 50mmHg

42
Q

how can glomerular filtration rate be maintained if blood volume if low

A

constrict afferent arterioles

43
Q

in what situations can autoregulation be overridden

A

in situations when BV/BP face serious compromise e.g. haemorrhage

44
Q

how can auto regulation be overridden

A

activation of sympathetic vasocontrictory nerves and angiotensin II can override to liberate more blood for immediately important organs e.g. brain

45
Q

what happens if there is prolonged reduction in renal BF

A

can lead to irreparable damage which may then lead to death because of disruption of the kidney’s role in homeostasis.

46
Q

how do marathons affect the kidneys

A

Endurance training for very long events involves adaptation of skeletal muscle so that its requirement for blood flow becomes more efficient and therefore there is less need to take it from the kidneys

47
Q

what is the % plasma volume entering the afferent arteriole

A

100%

48
Q

what is the % plasma volume filtered through the bowman capsule

A

20%

49
Q

what is the % plasma volume reabsorbed along remainder of nephron

A

> 19%

50
Q

what is the % plasma volume that enters back into circulation after entering the kidney

A

> 99% goes into systemic circulation

51
Q

what is the % plasma volume that is excreted to the external environment

A

<1%

52
Q

what capillaries are responsible for reabsorption

A

peritubular capillaries

53
Q

what does resistance along the entire length of peritubular capillaries mean pressure wise

A

causes a large drop in pressure so that hydrostatic pressure is very low - ~15mmHg

54
Q

how do pressure changes allow for reabsorption in the peritubular capillaries

A

Very low hydrostatic pressure in peritubular capillaries but high osmolarity (due to high concentrations due to fluid having been take out) – allows for reabsorption

ie balance of starling forces in favour of reabsorption

55
Q

what % of water is reabsorbed within the tubule

A

99%

56
Q

what % of glucose is reabsorbed within the tubule

A

100%

57
Q

what % of Na+ is reabsorbed within the tubule

A

99.5%

58
Q

what % of urea is reabsorbed within the tubule

A

50%

URINARY (27 decks)

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