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Renal (Year 2) > Physiology - Glomerular Filtration > Flashcards

Physiology - Glomerular Filtration Flashcards

1
Q

What is filtration?

A

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

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2
Q

What is reabsorption and how quickly does it occur?

A

Substances that the body wants are reabsorbed, those it doesn’t want stay in the tubule and are excreted

  • Initial reabsorption begins immediately after secretion
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3
Q

What is secretion?

A

Substances may be specifically removed from the body is this way

  • Kidney is actively able to secrete
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4
Q

What is the normal GFR?

A
  • ~180l/day
    • Only ~2l/180l ends up in urine so almost all of it is reabsorped too

Kidney has ample opportunity to precisely regulate ECF volume and composition and eliminate “nasty” substances

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5
Q

Why are the kidney’s very vulnerable to damage by vascular disease?

A

As they recieve such a high BF

  • They have the highest BF/g of any tissue in the body
  • In <5mins a volume equal to total BV passes through renal circulation
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6
Q

What is the plasma filtration fraction at the glomerulus?

A
  • 1200ml/min of blood passes through
    • Of which 55% = 660ml/min is plasma
    • GFR of plasma is 125ml/min
  • Therefore 125/660 = ~19%
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7
Q

Explain the forces behind filtration?

A
  • Hydrostatic pressure favours filtration
  • Oncotic pressure favours reabsorption back into glomerular capillaries

For filtration to work capillary hydrostatic pressure (Pgc) must > fluid oncotic pressure

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8
Q

What factors influence filtration rate of a specific particle?

A
  • Size
  • Charge
  • Shape

Small particles like Na+/K+/Water are almost completely filtered out.

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9
Q

Describe the physical structure of the capillary walls that allows filtration?

A
  1. Fenestrated (pore) of glomerular endothelial cells: prevents filtration of blood cells but allow plasma to pass
  2. Basal lamina of glomerulus: prevents large proteins passing through
  3. Slit membrane between pedicals under the basal lamina prevent medium proteins passing
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10
Q

How is a high capillary Hydrostatic pressure maintained?

A
  • Afferent arterioles
    • Wide and short to put up little resistance
  • Efferent arterioles
    • Long and narrow to put up high resistance
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11
Q

What is the Golden rule of circulation?

A

If you have a high resistance, hydrostatic pressure upstream is increased, while the pressure downstream is decreased

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12
Q

Describe extrinsic control of Glomerular Capillary Pressure

A
  • Sympathetic vasoconstrictive nerves
    • Afferent and efferent constriction, greater sensitivity of afferent arteriole
  • Circulating catecholamines
  • Angiotensin II
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13
Q

How do circulating catecholamines affect glomerular Capillary Pressure (PGC)

A

Cause vasoconstriction primarily in the afferent arteriole.

  • Lowering renal blood flow and PGC to provide more blood to other tissues e.g. heart and muscle
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14
Q

How does angiotensin II affect PGC?

A
  • Constriction of the efferent at [low]
  • Constriction of both efferent/afferent at [high]
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15
Q

Renal vessels also have an intrinsic ability to control PGC. What is the purpose of this?

A
  • ​Autoregulation of the GFR
    • Keeps BF and GFR essentially constant
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16
Q

How do renal vessels autoregulate GFR?

A

Afferent arterioles constrict and dilate in response to changes in blood volume in order to main PGC.

17
Q

What happens if mean arterial P increases?

A

There is an automatic increase in afferent arteriolar constriction, preventing a rise in glomerular PGC

  • DIlatation occurs if P falls
18
Q

When would autoregulation of the GFR be overridden?

A

When blood volume faces serious problems, such as a large haemorrhage.

  • In order to liberate blood for more immediately essential tissues (Symp nerves and angiotensin II)
19
Q

What controls autoregulation of the GFR?

A
  • The renal vessels themselves
    • Its independant of nerves and hormones, occuring even in denervated and isolated kidneys
20
Q

How are the peritubular capillaries capable of reabsorping almost 180l a day?

A
  1. Extremely low hydrostatic pressure thanks to the high resistance efferent arteriole upstream
  2. High oncotic pressure in peritubular capillaries thanks to losing 20% of the plasma but still containing 100% of normal blood protein
    • Net starling forces are much more in favour of reabsorption leading to reabsorption of
      • 50% of urea
      • 99% H2O
      • 100% Glc
      • 99.5% of Na.
21
Q

What is the equation for workingout the amount of solute excreted (E)?

A

E = F - R + S

  • F (Amount Filtered)
  • R (Amount Reabsorbed)
  • S (Amount Secreted)

Renal (Year 2) (25 decks)

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