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renal > Renal Tubular Function 1 L05 > Flashcards

Renal Tubular Function 1 L05 Flashcards

1
Q

What percentage of the proximal tubule is convoluted?

A

60% convoluted *convoluted becomes straight for the other 40%

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2
Q

How is the surface area of the proximal tubule increased?

A

presence of microvilli brush border on apical membrane.

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3
Q

What is the difference between primary and secondary active transport?

A

primary - relies on ATP

secondary - relies on concentration gradient of another ion e.g. Na+

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4
Q

define transcellular and paracellular.

A

transcellular - across cells

paracellular - between cells

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5
Q

Give examples of secondary active transporters

A

ion channels, co-transporters, exchangers

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6
Q

give examples of primary active transporters

A

protein pumps

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7
Q

In which direction is the Na+ gradient in the PCT and what is this important for?

A

high concentration in filtrate&raquo_space; low concentration in epithelial cells *strong electrochemical gradient

  • secondary active transport of glucose, AA, etc.
  • creates an osmotic gradient for the movement of water into cells (water follows movement of Na+)
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8
Q

How is a low concentration of Na+ maintained inside epithelial cells in PCT?

A

Na+/K+ ATPase on BASOLATERAL membrane moves Na+ out of cell and into the interstitial space

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9
Q

What percentage of water is reabsorbed from PCT?

A

70%

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10
Q

Through which routes is water reabsorbed from the PCT?

A

transcellular and paracellular routes

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11
Q

describe the transcellular route of water absorption in PT

A

water moves by osmosis through AQP1 water channels present on both apical and basolateral membrane of epithelial cell

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12
Q

What is solvent drag?

A

when the paracellular absorption of water (across tight junctions) by osmosis facilitates the absorption of other solutes e.g. K+, Mg2+, Ca2+. These solutes are carried across the membrane with the movement of water.

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13
Q

Is Inulin absorbed from the PT? How is its concentration at the end of the tubule compared to at the start?

A
  • not absorbed (remember L03)

- concentration higher at the end of the tubule *due to LESS WATER

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14
Q

Is urea absorbed from the PT? How is its concentration at the end of the tubule compared to at the start?

A
  • is absorbed weakly *at a slower rate than water

- concentration at the ends of tubule is HIGHER *due to LESS WATER

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15
Q

How quickly is Cl- reabsorbed from the PT in comparison to water? Is the concentration of Cl- higher or lower at the end of the tubule?

A
  • absorbed weakly/at a slower rate than water

- therefore concentration is HIGHER at the end of the tubule *due to LESS WATER

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16
Q

How quickly is Na+ reabsorbed from the PT in comparison to water? Is the concentration of Na+ higher or lower at the end of the tubule?

A
  • absorbed at the same rate as water *remember Na+ movement aids water movement
  • concentration of Na+ is the same at the start and end of PT
17
Q

How quickly is K+ reabsorbed from the PT in comparison to water? Is the concentration of K+ higher or lower at the end of the tubule?

A
  • absorbed at the same rate as water

- concentration of K+ is the same at the start and end of PT

18
Q

How quickly is HCO3- reabsorbed from the PT in comparison to water? Is the concentration of HCO3- higher or lower at the end of the tubule?

A
  • absorbed at a faster rate than water

- concentration of HCO3- is LOWER at the end of PT *due to MORE WATER than ion

19
Q

How quickly are amino acids reabsorbed from the PT in comparison to water? Is the concentration of amino acids higher or lower at the end of the tubule?

A
  • absorbed more strongly than water

- concentration of amino acids much LOWER at end of PT

20
Q

How quickly is glucose reabsorbed from the PT in comparison to water? Is the concentration of glucose higher or lower at the end of the tubule?

A
  • absorbed more strongly than water

- concentration of amino acids much LOWER at end of PT

21
Q

By what route is glucose reabsorbed from PT?

A

transcellularly

22
Q

Describe how glucose is reabsorbed from PT.

A
  • secondary active transport across apical membrane
  • facilitated diffusion across basolateral membrane

early PT (90%):

  • apical membrane: SGLT2 cotransporter transports glucose and Na+ in 1:1 ratio into cell
  • basolateral membrane: GLUT2 channel transports glucose into cortical interstitial space

late PT (10%):

  • concentration of glucose in filtrate diminishing, so more energy required to actively transport it into cell against electro-chemical gradient
  • apical membrane: SGLT1 cotransporter transports glucose and Na+ in 1:2 ratio into cell
  • basolateral membrane: GLUT1 channel transports glucose into cortical interstitial space
23
Q

what is tubular maximum (Tm) transport and its value for glucose?

A

The maximum rate of reabsorption (for glucose) . After this limit is reached, rate of glucose excretion begins to rise as not all of the glucose can be reabsorbed > glucosuria. This occurs when plasma concentration/filtrate concentration of glucose is around 12mM.
2.1mmol/min or 380mg/min.

24
Q

One of the symptoms of diabetes mellitus in glucosuria. Why does this occur?

A

Occurs as the glucose concentration in the filtrate exceeds Tm.

25
Q

What drugs can be given for decreasing plasma glucose in diabetes mellitus and how do they work?

A

canagliflozin, dapagliflozin

SGLT2 inhibitors > inhibit reabsorption of glucose > increased glucosuria > decreased blood-glucose concentration

*side effects include UTIs

26
Q

What is the normal plasma amino acid concentration?

A

2.5-3.5mM

27
Q

How are amino acids reabsorbed from the PT?

A
  • variety of cotransporters transporting different families of AA (e.g. different charges)
  • similar to glucose transport - secondary AT
  • use Na+ gradient to drive transport
  • transport is Tm limited
28
Q

Describe how HCO3- is reabsorbed from filtrate into interstitial space.

A

apical membrane:

  • HCO3- reacts with excess H+ that has been transported back into the filtrate
  • H+ moves out of cell through Na+/H+ exchanger (1:1)
  • HCO3- + H+ –> H20 + CO2 *mediated by CARBONIC ANYHDRASE
  • H20 and CO2 can readily diffuse across membrane into epithelial cell
  • through CA (same reaction) –> HCO3- and H+
  • H+ recycled through exchanger

basolateral membrane:

  • HCO3- moves into interstital space through Na+/3HCO3- cotransporter
  • HCO3- moves DOWN electro-chemical gradient while Na+ moves AGAINST electro-chemical gradient.
  • few other transporters: Na+/K+ pump and K+ channels to mediate Na+ levels inside cell
29
Q

How does acetazolamide affect HCO3- and water absorption?

A

inhibits CA
> HCO3- (and Na+) stay in filtrate (cannot be converted to CO2 and H20 - cannot cross membrane)
> decrease water potential of filtrate
> less water reabsorbed

30
Q

Why is acetazolamide only classed as a weak diuretic?

A

although it decreases water reabsorption in PT, the body compensates for this in DT.

31
Q

What is the drug acetazolamide used for?

A

mountain sickness, prophylaxis (preventing disease), glaucoma

32
Q

How might acetazolamide affect pH? How might this help with mountain sickness?

A

decreases blood pH - metabolic acidosis
increases urine pH - alkaline urine

mountain sickness due to hypoxia at high altitudes > increase in respiratory rate > respiratory alkalosis (CO2 conc drops) > therefore provides metabolic acidosis > normal blood pH

33
Q

Describe transcellular and paracellular movement of Cl- in the PT.

A

transcellular:
apical membrane:
- active transport of Cl- through antiporter for other anions (e.g. HCO3-, HCOO-)
basolateral membrane:
- Cl- channels - moves down electrochemical gradient

  • at start of PT, Cl- absorbed weakly/slower rate than water
  • therefore Cl- concentration increases in filtrate along tubule
  • eventually becomes high enough to drive paracellular PASSIVE Cl- movement into the interstitial space.
34
Q

Very little albumin enters the filtrate. How is albumin in the filtrate reabsorbed?

A

binds to tubule membrane > endocytosed > catabolised > AA > reabsorbed into the body

35
Q

Describe the secretion pathway for PAH- in the PT.

A

basolateral membrane:

  • organic ion transporter (OAT) exchanges PAH- with other anions such as alpha-ketoglutarate
  • PAH- actively moved into cell from interstitial space

apical membrane:

  • multidrug resistance-associated protein (MRP) exchanges PAH- with other anions such as Cl- or OH-
  • PAH- actively secreted into filtrate.

*body uses secretion pathway to remove many different anions from the blood

renal (16 decks)

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