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Medicine Unit 10 UoS > Renal clearance and water balance > Flashcards

Renal clearance and water balance Flashcards

1
Q

What is renal clearance?

A

Volume of plasma completely cleared of a substance by the kidneys per unit time

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2
Q

What is the equation for GFR?

A
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3
Q

What is the equation for renal clearance?

A

c = [U]xXv / [P]x
C - clearance
[U]x - urine conc of substance x
v - urine flow rate (mL/min)
[P]x - plasma conc of substance x (mg/mL)

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4
Q

Why is albumin renal clearance 0?

A

It is not filtered across glomerular capillaries

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5
Q

Why is glucose renal clearance 0?

A

It is filtered and the completely reabsorbed back into the bloodstream

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6
Q

What substances are filtered and partially reabsorbed in the kidneys?

A

Na+
Urea
Phosphate
Cl-

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7
Q

What happens to inulin in the kidneys?

A

Freely filtered across glomerular capillaries
Neither reabsorbed or secreted > its clearance measures the GFR

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8
Q

What happens to organic acids and bases in the kidneys?

A

Highest clearances of all substances as they are both filtered and secreted

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8
Q

What is clearance ratio?

A

Inulin is the only substance which its clearance is exactly equal to the GFR
It is freely filtered across the glomerular capillaries
Neither reabsorbed nor secreted
Filtered inulin = inulin excreted
Therefore inulin is a reference substance - glomerular marker - clearance of any substance compared with clearance of inulin and expressed as clearance ratio

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9
Q

What substances will have a clearance ratio of less than 1 and why?

A

Albumin, glucose, urea
Either substance is not filtered or it is filtered and subsequently reabsorbed

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10
Q

What substances have a clearance ratio of more than 1 and why?

A

Organic acids and bases
The substance is filtered and secreted

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11
Q

What is the equation for the clearance ratio?

A

Clearance ratio = Cx/Cinulin

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12
Q

What is the response to water deprivation?

A

Plasma osmolarity increases (high conc of ions)
Stimulates osmoreceptors in anterior hypothalamus
> increases thirst, increases water
> increases ADH secretion from posterior pituitary
- increases water permeability of principle cells in late DT and CD
- increases water reabsorption
- increases urine osmolarity and decreases urine volume

Decreases plasma osmolarity towards normal

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13
Q

What is the response to water drinking?

A

Plasma osmolarity decreases (low conc of ions)
Inhibits osmoreceptors in anterior hypothalamus
> decreases thirst, decreases water
> decreases ADH secretion from posterior pituitary
- decreases water permeability of principle cells in late DT and CD
- decreases water reabsorption
- decreases urine osmolarity and increases urine volume

Increases plasma osmolarity towards normal

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14
Q

What is the corticopapillary osmotic gradient?

A

Gradient of osmolarity in the interstitial fluid of the kidney from the cortex to the papilla
Moving from the cortex to outer medulla, inner medulla and papilla - the interstitial fluid osmolarity progressively increases

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15
Q

What solutes contribute to osmotic gradient, and what mechanisms deposit these solutes in the interstitial fluid and concentrate urine?

A

NaCl and urea
- Countercurrent multiplication: function of the loop of Henle, which deposits NaCl in the deeper regions in the kidney
- Urea recycling: a function of the inner medullary collecting ducts, which deposits urea

16
Q

What is the countercurrent multiplication system in the loop of henle?

A

Thick ascending limb - Na+ pumped out and negative ions follow e.g. Cl-. Makes medulla concentrated - creates gradient
Water leaves passively from thin descending limb due to high salt conc of surroundings - increases conc of filtrate - to equilibrate between filtrate and medulla
As filtrate is flowing all the time, there is a gradient of increasing osmolarity into the medulla
Facilitates water moving out of CT and concentrating urine

17
Q

What transport mechanisms occur in the collecting duct?

A

In cortical and outer medullary CD, ADH increases water permeability, but not urea permeability = water is reabsorbed, urea is left behind
Causes urea concentration to increase
In inner medullary CD, ADH increases water permeability and transporter for fac diff of urea. Urea diffuses down its gradient into IF, adds to corticopapillary osmotic gradient

18
Q

What is counter-current exchange vs multiplication?

A

Multiplication is an active process that establishes the corticopapillary osmotic gradient
Exchange maintains it in a passive process

19
Q

What structure is involved in counter-current exchange?

A

Vasa recta - straight arterioles and venules of the kidneys
Freely permeable to small solutes and water
Blood flow through vasa recta is slow, so water and solutes can move in and out

20
Q

What are the three action of ADH on the renal tubule?

A

1) increases water permeability of principle cells in LATE DISTAL AND COLLECTING DUCTS by insertion of aquaporins
2) increases activity of the Na+/K+/2Cl- cotransporter of the THICK ASCENDING LIMB - enhancing counter-current multiplication and the size of the corticopapillary osmotic gradient
3) increases urea concentration in INNER MEDULLARY COLLECTING DUCTS, enhancing urea recycling and size of corticopapillary osmotic gradient

21
Q

What happens in the absence of ADH?

A

Principle cells in the late DCT and CD become impermeable to water

22
Q

What is hyperosmotic urine? When does this occur?

A

Concentrated urine with higher osmolarity than the blood
Produced when levels of ADH are high, as in water deprivation or in SIADH

23
Q

How is hyperosmotic urine created?

A

Glomerular filtrate has same osmolarity than blood as solutes and water are freely filtered - same in PCT - 300mOsm/L
Thick ascending limb of LOH NaCl reabsorbed due to Na+/K+/2Cl- cotransporter. Water reabsorption cannot happen - dilutes filtrate
Early distal tubule, NaCl is reabsorbed as well, and is also water impermeable, so dilutes
Late distal tubule - principle cells are permeable to water with ADH, water flows out of the tubule until isoosmotic with IF - 300mOsm/L
CD, principle cells water permeable with ADH. Fluid flows down the tubule - exposed to increasing IF conc - water reabsorbed until iso-osmotic with IF - 1200mOsm/L

24
Q

What is hypoosmotic urine?

A

Diluted urine has osmolarity lower than blood osmolarity
Produced when ADH levels are low (water drinking), or when ADH is ineffective (nephrogenic diabetes insipidus)

25
Q

How does the kidney produce hypoosmotic urine?

A

No ADH
Reabsorption in the proximal tubule is not affected in ADH, so fluid reabsorbed until isosmotically
Thick ascending limb of LOH - NaCl reabsorbed via Na+/K+/2Cl- cotransporter, water not reabsorbed due to impermeability, dilutes filtrate.
Early distal tubule - dilution continues
Late distal tubule and CD - without ADH principle cells impermeable to water. Cannot equilibrate with increasing osmolarity of IF
Dilute urine excreted

26
Q

What is free water and where is it produced?

A

Free water is distilled water free of solutes
In the nephron, free water is generated in the diluting segments, where solute is reabsorbed without water.
The diluting segments of the nephron are the water impermeable segments - thick ascending limb and early distal tubule

27
Q

What is free water clearance, why is it measured?

A

Measurement of free water clearance provides measurement for assessing ability of kidneys to dilute or concentrate urine

28
Q

What happens to free water in different states of the nephron?

A

When ADH levels are low, all free water generated in the thick ascending limb and early distal tubule is excreted - as cant be reabsorbed by CD - urine hypoosmotic, and free clearance is positive.
When ADH levels are high, all free water generated in thick ascending limb and early distal tubule is reabsorbed by late distal tubule and CD - urine is hyperosmotic and free water clearance is negative

Medicine Unit 10 UoS (26 decks)

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