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3 Urinary Unit > (9) S5) Corticopapillary Gradient and Countercurrent Exchange > Flashcards

(9) S5) Corticopapillary Gradient and Countercurrent Exchange Flashcards

1
Q

Outline the effect of water balance on plasma osmolarity

A
  • Water intake < water excretion = plasma osmolarity ↑
  • Water intake > than water excretion = plasma osmolarity ↓
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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

Where is ADH produced?

A

Produced by neurosecretory cells in the hypothalamus

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6
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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7
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
  • Inhibited by decreased plasma osmolarity
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8
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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9
Q

Identify 2 forms of clinical management for low plasma ADH

A
  • ADH injections
  • ADH nasal spray treatments
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10
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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11
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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12
Q

Identify the different aquaporin channels and their location in the nephron

A
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13
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 urine produced
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14
Q

Describe the relationship between haemodynamic and osmotic changes

A
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15
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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16
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
17
Q

Where is the maximum osmolality reached in the nephron?

A

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

18
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
19
Q

What is the osmolality of the filtrate at DCT1?

A

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

20
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
21
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

22
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
23
Q

Why is the vertical osmotic gradient an essential mechanism?

A
  • Active NaCl transport in thick ascending limb
  • Recycling of urea (effective osmole)
24
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

25
Q

Why is urea an effective osmole in the kidney?

A

Urea is hydrophilic and does not readily permeate artificial lipid bilayers

26
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

27
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
28
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+

29
Q

What does the vasa recta do?

A

The vasa recta maintains the concentration gradient by acting as a counter-current exchanger

30
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

31
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)
32
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)
33
Q

What happens in the descending limb of the vasa recta?

A
  • Isosmotic blood in vasa recta enters hyperosmotic milieu of the medulla
  • Na+, Cl- and urea diffuse into the lumen of vasa recta
  • Osmolarity in vasa recta increases as it reaches tip of hairpin loop
34
Q

What happens in the ascending limb of the vasa recta?

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

3 Urinary Unit (23 decks)

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