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SA urinary > S5) Corticopapillary Gradient and Countercurrent Exchange > Flashcards

S5) Corticopapillary Gradient and Countercurrent Exchange Flashcards

1
Q

Outline the effect of water balance of plasma osmolarity

A
  • Water intake < water excretion = plasma osmolarity/conc ↑
  • Water intake > than water excretion = plasma osmolarity/conc ↓
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2
Q

What is the value for normal plasma osmolarity?

A

280-310 mOsm/Kg (280-310 mmol/L)

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3
Q

Outline the 2 different mechanisms of regulating plasma osmolarity in the body

A
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4
Q

Which sensors in the body detect changes in plasma osmolarity?

A

Osmoreceptors – located in the Hypothalamus (OVLT)

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5
Q

How do hypothalamic osmoreceptors act?

A

Signal secondary responses leading to two different complimentary outcomes:

  • Concentration of urine
  • Thirst
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6
Q

Where is ADH produced?

A

Produced by neurosecretory cells in the hypothalamus

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7
Q

What does ADH do?

A

ADH acts on the kidney to increase the permeability of the collecting duct to water and urea

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8
Q

Describe the actions of ADH in terms of diuresis

A
  • Low plasma ADH = diuresis
  • High plasma ADH = anti-diuresis
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9
Q

Describe the events occurring in the efferent pathway: ADH

A
  • Released from posterior pituitary
  • Released in conditions of predominant H2O loss
  • Renal H2O excretion decreases
  • more urea and h20 reabsorption as more aquaporins open and increased UT1 transporters
  • increase the cortico-capillary gradient
  • Inhibited by decreased plasma osmolarity
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10
Q

Identify and describe 2 clinical conditions which result from problems with ADH secretion

A
  • Central diabetes insipidus: low plasma ADH levels
  • Nephrogenic diabetes insipidus: acquired insensitivity of the kidney to ADH
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11
Q

Identify 2 forms of clinical management for low plasma ADH

A
  • ADH injections
  • ADH nasal spray treatments
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12
Q

Identify 5 different causes of low plasma ADH

A
  • Damage to hypothalamus / pituitary gland
  • A tumour
  • An aneurysm
  • Sarcoidosis
  • Tuberculosis
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13
Q

What is Syndrome of Inappropriate ADH secretion?

A
  • SIADH is the excessive release of ADH from the posterior pituitary gland or another source e.g. small cell lung tumour
  • Dilutional hyponatremia results
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14
Q

Identify the different aquaporin channels and their location in the nephron

A
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15
Q

In terms of aquaporins, explain how a lack of ADH secretion leads to diuresis

A
  • No Aquaporin 2 in apical membrane
  • No AQP 3 and 4 on basolateral membrane
  • Limited water reuptake in DCT2 and collecting duct
  • Hyposmotic (dilute) urine produced
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16
Q

Describe the relationship between haemodynamic and osmotic changes

A
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17
Q

Large deficits in water are only partially compensated for in the kidney.

Describe the events occuring in the efferent pathway: thirst

A
  • Induced by increases in plasma osmolarity or by decreases in ECF volume
  • Thirst increases intake of free water
  • Stops when sufficient fluid has been consumed
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18
Q

How does the osmolarity of the nephron increase down the LoH?

A
  • Descending limb of LoH is highly permeable to H2O (AQP 1)
  • Descending limb is not permeable to Na+
  • Na+ remains in the lumen and filtrate concentration (osmolarity) increases
19
Q

Where is the maximum osmolality reached in the nephron?

A

Maximum osmolality is at apex of LoH = 1200 mOsm/Kg

20
Q

Explain how the thick ascending limb of the Loop of Henle generates the medullary gradient

A
  • Ascending limb actively transports NaCl out of tubular lumen into interstitial fluid
  • Ascending limb is impermeable to H2O
  • Filtrate is diluted and osmolarity of interstitium increases

– interstitial fluid becomes more salty

21
Q

What is the osmolality of the filtrate at DCT1?

A

Fluid entering the DCT has low osmolality = 100 mOsm/Kg

22
Q

What is the juxtamedullary nephron what does it do?

A
  • Juxtamedullary nephron is a long-looped nephron of 20% abundance
  • It has importance in establishing the medullary vertical osmotic gradient
23
Q

What is the vertical osmotic gradient?

A

The vertical osmotic gradient is a gradient of increasing osmolarity established in the interstitial fluid of the medulla

24
Q

Describe 2 features of the vertical osmotic gradient

A
  • Isotonic (300mOsm/Kg) at corticomedullary border
  • Hyperosmotic in medullary interstitium up to 1200 mOsm/Kg at papilla
25
Q

Why is the vertical osmotic gradient an essential mechanism?

A
  • Active NaCl transport in thick ascending limb
  • Recycling of urea (effective osmole)
26
Q

What is an ineffective osmole?

A

If the membrane allows a solute to freely cross it, then the solute is totally ‘ineffective’ at exerting an osmotic force across this membrane

27
Q

Why is urea an effective osmole in the kidney?

A

Urea is hydrophilic and does not readily permeate artificial lipid bilayers

28
Q

In which circumstance would urea be an ineffective osmole?

A

In the presence of urea transporters which facilitate urea diffusion across most cell membranes

29
Q

Describe the recycling of urea in the kidney under the influence of ADH

A
  • Urea is reabsorbed from the medullary CD
  • Cortical CD cells are impermeable to urea
  • Urea moves into interstitium and diffuses back in LoH to increase concentration gradient for H2O reabsorption
30
Q

What is the Counter Current Multiplication?

A

Counter Current Multiplication is the increased osmolarity of the interstitial fluid surrounding the LoH due active transport of Na+

31
Q

What does the vasa recta do?

A

The vasa recta is the blood supply surrounding the nephron and maintains the concentration gradient by acting as a counter-current exchanger as its constantly moving around and creating diffusion gradient

  • the vasa recta is an extension of the efferent arteriole, the efferent arteriole doesn’t enter the venous system, the efferent arteriole forms the van recta which loops all around the nephron and then after that loop it then enters the venous system
32
Q

How does the vasa recta act as a counter current exchanger?

A

Flow in vasa recta is in opposite direction to fluid flow in the tubule so the osmotic gradient is maintained

33
Q

Compare and contrast blood flow in the renal cortex and medulla

A
  • Renal cortex: blood flow is one of the highest per gram of any tissue in the body
  • Renal medulla: low blood flow (5-10% of total RPF)
34
Q

Explain why the renal medulla and cortex have contrasting blood flow

A
  • Need to deliver nutrients to renal cortex (high blood flow)
  • Need to maintain medullary hyper-tonicity (low blood flow)
35
Q

What happens in the descending limb of the loop of henley?

A

– water is being reabsorbed passively into vasa recta

→ due active reabsorption of Na, Cl and K in the ascending limb of loop of henley occurring at the same time

→ the interstitial is more salty so water diffuses out

36
Q

What happens in the ascending limb of the loop of henley?

A
  • Blood ascending towards cortex has a higher solute content than surrounding interstitium
  • Water moves into vasa recta from the descending limb of the LoH

– Isosmotic blood in vasa recta enters hyperosmotic milieu of the medulla

–

37
Q

how much Na reabsorption is the lop of henley responsible for?

A

25%

38
Q

what is the thin ascending limb permeable and impermeable to?

A

impermeable to water and so little Na and Cl transport

39
Q

what is the thick ascending limb impermeable to?

A

impermeable to water

40
Q

what in the thin descending limb permeable to?

A

water

(water reabsorption happens here and not in the ascending limb)

41
Q

what is the difference in osmolality compared to the the top in the loop of henley and the top?

A

300 msomol/Kg at the top

1200 mosmol/Kg at the bottom

42
Q

what is the difference between the concentration of the solute in the descending limb of the loop of henley and the ascending limb of vasa recta?

A

→ concentration increases in the LofH as it goes down due to reabsorption of water

→ the concentration decreases In the vasa recta because water is being reabsorbed and so the conc of solute will reduce

43
Q

what are the roles of the ascending and descending limb of the vasa recta

A

remember: ascending and descending are opposite to the loop of henley

– ascending reabsorbs water

– descending reabsorbs Na, Cl, K

44
Q

describe the process of urea recycling with percentages

A
  1. glomerulus: 100% is filtered out of glomerulus into nephron
  2. PCT: 50% goes back into the blood
  3. ascending limb and early DCT: impermeable to urea so no change
  4. collecting duct: 70% reabsorbed by blood and 40% excreted

LOOP: collecting duct and bottom of LofH are close

  1. bottom of LofH: 110% of urea inside the nephron, 70% is in blood and interstitial which surrounds the bottom of LOFh and it all gets put back into the urine due to conc difference

SA urinary (25 decks)

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