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Urinary system > Renal blood flow and glomerular filtration > Flashcards

Renal blood flow and glomerular filtration Flashcards

1
Q

What does an abrupt fall in glomerular filtration rate mean?

A

kidney failure

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2
Q

What is glomerular filtration?

A

Formation of an ultrafiltrate of plasma in the glomerulus

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3
Q

Is glomerular filtration active or passive?

A

Passive process with fluid forced through the fenestrated walls of the glomerular capillaries by hydrostatic pressure

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4
Q

How does filtrate pass through the podocytes?

A

Filtrate moves down fenestra in podocytes foot processes

Fluids and small solutes are freely filtered meaning there is an equal concentration in filtrate and plasma here

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5
Q

What is the relation between the concentration of substances in blood and filtrate and why?

A

Fluids and small solutes are freely filtered meaning there is an equal concentration in filtrate and plasma here

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6
Q

What is primary urine?

A

Clear fluid, free of blood and proteins that contains electrocytes and small solutes at the same concentration as the blood

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7
Q

What is the filtration barrier permeable to?

A

fluids

small solutes

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8
Q

What is the filtration barrier impermeable to?

A

cells
proteins
drugs bound to protein

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9
Q

Are the rates of different substance excretion the same and why?

A

NO, the rate at which substances are excreted various in the body. Useful substances like proteins are reabsorbed and so the ultimate concentration of this will change

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10
Q

What is the amount of a substance excreted equivalent to?

A

amount excreted = amount filtered + amount secreted - amount absorbed

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11
Q

What other than filtration, absorption and secretion affect solute concentration in urine?

A

The volume of urine produced

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12
Q

Why is it not good to measure substances in urine to determine kidney function?

A

The concentrations will be so variable depending on the volume of urine produced

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13
Q

What is the main driving force of glomerular filtration?

A

hydrostatic pressure

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14
Q

What other pressure drive glomerular filtration?

A
  • Hydrostatic pressure of the tubule (higher pressure make it harder to filter)
  • Osmotic pressure in the capillary
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15
Q

How is net ultrafiltration pressure calculated?

A

Puf = Pgc - Pt - Pigc

Puf = net ultrafiltration pressure
Pgc = hydrostatic pressure
Pt = hydrostatic pressure of tubule lumen
pi gc = osmotic pressure of glomerular plasma proteins in capillary

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16
Q

What is normal GFP value?

A

10-20mmHg

17
Q

What is glomerular filtration rate and how is it calculated?

A

The amount of fluid filtered from the glomeruli into the bowman’s capsule (ml/min)

GFR = GFP* Kf

Kf = ultrafiltration coefficient which accounts for membrane permeability and surface area available for filtration

18
Q

How does kidney disease affect Kf and GFR?

What dilates the glomerulus and what does this do to Kf and GFR?

A

Kidney disease = reduce number of functioning glomeruli = reduce surface area = reduce Kf = reduce GFR

Dilation of glomerular arterioles by drugs/hormones = more surface area = increase Kf = increase GFR

19
Q

What does renal blood flow deliver to the kidneys?

A

Renal blood flow delivers oxygen, nutrients and substances for excretion.

20
Q

What is renal blood flow equivalent to?

A

Renal blood flow (RBF) = approx 1L/min

one fifth of cardiac output

21
Q

What is renal plasma flow?

A

Renal plasma flow (RPF) = approx. 0.6L/min

plasma is 60% of blood

22
Q

What is the filtration fraction?

A

Filtration fraction (FF) = 0.2 (ratio between RPF and amount of filtrate filtered by glomerulus, which is normally 20%)

23
Q

What is GFR?

A

RPF x FF
approximately 120ml/min
filtrate formed in 1 minute

24
Q

What is cardiac output?

A

5L/min

25
Q

What are the mechanisms of autoregulation to ensure that when blood pressure increases, GFR is constant?

A
  1. Myogenic Mechanism:
    Intrinsic vasoconstriction in response to stretch in the afferent arteriole due to high pressure , so blood flow reduces– keeps the GFR constant when BP rises
  2. Tuboglomerular Feedback:
    NaCl concentration in DCT sensed by macula densa in juxta-glomerular apparatus.
    Macula densa releases ATP which signals vasoconstriction
26
Q

What would be the effect of a severe haemorrhage, obstruction in the nephron tubule, reduced plasma protein concentration and small increase in blood pressure do to GFR?

A

SH - decreases (lower hydrostatic pressure)
OB - decreases (more tubular pressure)
RPP - increase (less osmotic pressure)
SBPI - no change (compensated for)

27
Q

What is renal clearance?

A

Number of litres of plasma that is completely cleared of substance-x per unit time.
(Each substance is filtered a different amount)

28
Q

What is the equation for renal clearance?

A

C = U*V/P ml/min

U= concentration of substance in urine
V =  rate of urine production
P = concentration of substance in plasma
29
Q

What does a molecule have to be in order to be used in the estimation of GFR using clearance?

A

If a molecule is freely filtered and not reabsorbed or secreted in the nephron, the amount filtered = amount excreted.

30
Q

How can inulin be used to measure GFR using clearance?

Use of creatine to measure GFR by measuring clearance?

A
  • GFR can be measured by a molecule that is freely filtered and not absorbed:
    E.G. Inulin (plant polysaccharide, non toxic, measurable in plasma and urine) clearance = 120ml/min.

Creatine – amount released constant, clearance stable. So low clearance or high plasma creatine could be renal failure.

31
Q

Give an example of a substance with low clearance value

A

Clearance rates for values like sodium is low so reabsorbred from urine

32
Q

What is PAH used for and why?

A
  • To measure renal plasma flow
  • It is freely filtered and all remaining is secreted so everything entering the kidney is filtered
  • Clearance = 625ml/min.
  • All of the PAH is removed passing through the kidney so clearance of it equals the RPF
33
Q

Renal diagnostics and pharmacokinetics

A
  • A fall in GFR is the cardinal feature of renal disease
  • If GFR falls, excretory products will build up in the plasma and a raised plasma concentration of creatine is diagnostic of renal disease.
  • Pharmacogenetics: excretion of many drugs is impaired in renal failure so this needs to be taken into account when calculating drug doses to give to patients with renal failure.

Urinary system (12 decks)

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