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Urinary > Week 5 - Plasma Osmolarity > Flashcards

Week 5 - Plasma Osmolarity Flashcards

1
Q

If water intake>water excretion then plasma osmolarity…

A

…decreases

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2
Q

How much urine is produced on average per day?

A

-1.5L

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3
Q

What senses changes in plasma osmolarity?

A

-Hypothalamic osmoreceptors in OVLT

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4
Q

What two pathways are activated through the hypothalamic osmoreceptors?

A
  • ADH

- thirst

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5
Q

What is the result of ADH secretion?

A

-Decreased renal water excretion

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6
Q

What is the result of activating thirst pathway?

A

-Increased water intake

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7
Q

How are hypothalamic osmoreceptors exposed to plasma?

A

-Fenestrated leaky endothelium which senses changes in plasma osmolarity directly

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8
Q

Besides from increased plasma osmolarity, what else activates ADH and thirst pathways?

A

-Decreased ECF volume

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9
Q

Which efferent pathway of the hypothalamic osmoreceptors is most sensitive?

A
  • ADH (occurs at 1% change)

- Thirst occurs at 10% change)

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10
Q

How is ADH secretion controlled?

A

-Negative feedback loop

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11
Q

What is the ultimate effect of ADH on the urine?

A

-Produces a low volume of concentrated urine

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12
Q

What effect does ADH have on the CD?

A

-Increases permeability to water and urea

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13
Q

What happens if plasma osmolarity decreases?

A

-No ADH stimulation -> CD is impermeable to water -> diuresis (high volume hypotonic urine)

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14
Q

Why is the increasing gradient in the interstitium of the kidney essential?

A

-Allows water to be drawn out when ADH is present producing concentrated urine

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15
Q

What is the mechanism of ADH?

A

-Causes aquaporin 2 channels to be inserted into the apical membrane of CD and late DCT so water can be resorbed

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16
Q

What happens to the permeability of CD and late DCT when ADH removed?

A

-AQP2 channels retrieved by endocytosis so permeability is removed

17
Q

Which AQP channels are always present in basolateral membrane?

A

-AQP 3+4

18
Q

What other effects does ADH have besides AQP channels?

A
  • Causes vasoconstriction of glomerulus to decrease the effective filtering surface area
  • Increases Na, K+ and Cl- resorption in ascending limb due to urea -> causes a more hypotonic filtrate at top of loop
  • Increases K+ secretion in cortical CD
19
Q

Describe the effect on the response to plasma osmolarity when there is a change in blood volume/pressure

A
  • Decreased blood volume -> lower osmolarity is tolerated as you need to maintain blood volume and therefore tolerate the lower osmolarity in order to increase volume
  • Increased blood volume -> Higher osmolarity tolerated as you want to reduce your blood volume so excrete more water
20
Q

What is diabetes insipidus?

A
  • A condition caused by the pituitary gland not producing enough ADH or insensitivity of the kidney to ADH
  • More water is excreted than wanted resulting in polyuria and polydipsia, with a possible resulting hyponatraemia
21
Q

What is syndrome of inappropriate antidiuretic hormone secretion (SIADH)?

A
  • A syndrome characterised by excessive release of ADH from the posterior pituitary or another source (small cell carcinoma of lung)
  • Results in dilutional hyponatraemia as the total body fluid is increased
22
Q

What is the corticopapillary osmotic gradient?

A

-The increasing osmotic gradient from isotonic to hypertonic between the cortex and the renal papilla

23
Q

What 3 factors are the main contributors to the corticopapillary gradient?

A
  • Active transport of NaCl in thick ascending limb
  • Urea recycling
  • Vasa recta which maintains the gradient
24
Q

What is an effective osmole?

A
  • Any solute which increase the osmolarity of a solution

- To effect the osmolarity a solute must not be able to freely diffuse across the membrane or be transported

25
Q

Why is urea an ineffective osmole in the body but an effective osmole in the kidney?

A
  • In the body urea is transported across membranes ie an ineffective osmole
  • In the kidney the tubule is impermeable to urea making it an effective osmole
26
Q

Why is the thick ascending limb crucial in the generation of the corticomedullary gradient?

A

-Actively transports salts out of the filtrate into the interstitium without water following therefore increasing the osmolarity of the interstitium

27
Q

Explain how urea contributes to the corticopapillary gradient and how its contribution differs in the presence of ADH

A
  • Urea is normally reabsorbed from the medullary CD into the interstitium
  • This causes an increase in the osmolarity of the interstitium
  • In the absence of ADH, the filtrate in the CD is hypotonic and thus there is no driving force for urea
  • When ADH is present, water has been resorbed and the concentration of urea in the medullary CD is high, generating a large driving force for urea resorption. Also ADH makes the medullary CD more permeable to urea. These factors together contribute to the corticopapillary gradient
28
Q

Explain urea recycling

A
  • As the concentration of the urea in the medullary interstitium increases, the driving force for passive urea secretion into the thin ascending limb of henle is increased.
  • This serves to increase the urea concentration in the medullary CD which in turn increases the driving force for urea reabsorption to contribute to the medullary corticopapillary gradient
29
Q

Explain how the ascending limb of loop of henle sets up the corticopapillary gradient (counter-current multiplier)

A
  • Ascending limb impermeable to H2O but permeable to NaCl and Urea
  • Urea secreted into the thin ascending loop from the interstitium
  • Na is pumped out of the ascending loop, raising the osmotic pressure outside and lowering it inside (max gradient is 200mosm/L)
  • Water diffuses out of descending limb into interstitium and thus the filtrate in descending limb has higher osmolarity
  • Fresh filtrate enters and pushes the concentrated fluid into the ascending limb
  • Na is pumped out of the concentrated filtrate into the interstitium, but osmolarity cannot reach as low as 1st round as started from a more concentrated filtrate
  • Pumping of Na, osmotic flow and filtration flow all occur simultaneously
  • The third round of Na pumping raises the interstitium more and so on
  • The final gradient is limited by diffusional processes
30
Q

How is the counter-current multiplier maintained?

A

-Vasa Recta flow in the opposite direction to fluid flow in the tubule maintaining the osmotic gradient by maximally reabsorbing any water which diffuses out of the tubule

31
Q

What features of the vasa recta allow maximal reabsorption of water?

A
  • Flows in opposite directions to maximise gradients
  • Slow flowing which means the blood can absorb some of the NaCl pumped out of the loop and mimic its surroundings -> as the interstitium becomes concentrated so does the vasa recta -> flows past CD -> water reabsorption
  • As it passes through the concentrated interstitium past the descending limb it reabsorbs the water
  • ie it mops up dilutional water and as it enters the cortex the blood is isotonic and fast flowing again

Urinary (21 decks)

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