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Renal > Physiology of Proximal Tubule and Loop of Henle > Flashcards

Physiology of Proximal Tubule and Loop of Henle Flashcards

1
Q

What is the normal GFR in the kidneys?

A

125 ml/min (180 l/day)

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2
Q

How many times is plasma filtered in a day?

A

65 times a day

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3
Q

What substances are reabsorbed by the kidneys?

A

99% of fluid, 99% of salt, 100% of glucose, 100% of amino acids, 50% urea, 0% of creatine

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4
Q

How is reabsorption different from filtration?

A

Reabsorption is specific

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5
Q

What is the glomerular filtrate?

A

Modified filtrate of blood

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6
Q

How much fluid is reabsorbed in the proximal tubule?

A

About 80 ml/min

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7
Q

What property does the fluid reabsorbed from the proximal tubule have?

A

Iso-osmotic with the filtrate

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8
Q

What substances are reabsorbed in the proximal tubule?

A

Sugars, amino acids, phosphate, sulphate and lactate

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9
Q

What substances are secreted from the proximal tubule?

A

H+, hippurates, neurotransmitters, bile pigments, uric acid, drugs and toxins

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10
Q

What is the flow of transcellular reabsorption in the proximal tubule?

A

Tubular lumen - tubular epithelial cell - lateral space - interstitial fluid - peritubular capillary

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11
Q

Where does paracellular reabsorption of the proximal tubule occur?

A

Between tubular epithelial cells

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12
Q

What are some forms of carrier-mediated transport?

A

Primary active transport, secondary active transport, facilitated diffusion

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13
Q

What occurs in primary active transport?

A

Energy is directly required to operate the carrier and move the substrate against its concentration gradient

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14
Q

What occurs in secondary active transport?

A

Carrier molecule is transported coupled to the concentration gradient of an ion (usually Na+)

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15
Q

What occurs in facilitated transport?

A

Passive carrier-mediated transport of a substance down its concentration gradient

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16
Q

What is essential for Na+ reabsorption in the proximal tubule?

A

An energy dependent Na+/K+-ATPase transport mechanism at the basolateral membrane

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17
Q

How is iso-osmotic fluid reabsorbed in the proximal tubule?

A

Reabsorbed across the leaky epithelium = due to standing osmotic gradient and oncotic pressure gradient

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18
Q

How is water reabsorbed in the proximal tubule?

A

Passively down the NaCl osmotic gradient

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19
Q

What kind of drag exists in the peritubular plasma of the proximal tubule?

A

Oncotic drag

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20
Q

What ion is Na+ exchanged for in the proximal tubule?

A

H+ by co-transport

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21
Q

How much glucose is reabsorbed from the filtrate in the proximal tubule?

A

100% = by co-transport with Na+ and facilitated diffusion

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22
Q

What is the transport maximum (Tm)?

A

Point at which increases in concentration of a substance don’t result in an increase in movement of a substance across a cell membrane

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23
Q

How is the Tm of glucose expressed?

A

By the maximum transporting capacity of the SGLT transport system

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24
Q

How is the excretion of PAH calculated?

A

Excretion = filtration + secretion

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25
Q

What do the protein channels in the proximal tubule do?

A

Either reabsorb glucose or secrete PAH

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26
Q

What is the maximum reabsorption of glucose in healthy people?

A

About 375 mg/min

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27
Q

How is glucose excretion calculated?

A

Excretion = filtration - reabsorption

28
Q

What direction is PAH transport in the proximal tubule?

A

From the peritubular capillaries to the lumen = opposite direction from glucose

29
Q

What happens to clearance once Tm is reached?

A

Clearance of reabsorbed/secreted substance isn’t constant

30
Q

How is high [PAH] obtained?

A

By infusion or injection

31
Q

What does Na+ reabsorption in the proximal tubule drive?

A

Cl- reabsorption through the paracellular pathway

32
Q

What property does the tubular fluid have once it leaves the proximal tubule?

A

It is iso-osmotic

33
Q

What is the function of the loop of Henle?

A

Generates a cortico-medullary solute concentration gradient, enabling the formation of hypertonic urine

34
Q

How does fluid flow occur in the loop of Henle?

A

Opposing flow in the two limbs = countercurrent flow

Entire loop functions as a countercurrent multiplier

35
Q

What do the vasa recta and loop of Henle establish collectively?

A

A hyperosmotic medullary interstitial fluid

36
Q

What are some features of the ascending limb of the loop of Henle?

A

Na+ and Cl- are reabsorbed along entire length

Relatively impermeable to water = little/no water reabsorbed

37
Q

What are some features of the descending limb of the loop of Henle?

A

Doesn’t reabsorb NaCl and is highly permeable to water

38
Q

How is NaCl absorbed in the ascending loop of Henle?

A

Thick (upper) ascending limb absorbs NaCl by active transport
Thin (lower) ascending loop does this passively

39
Q

What do the selective permeabilities of the ascending and descending limbs of the loop of Henle enable?

A

The establishment of an osmotic gradient in the medulla

40
Q

What is the function of the TALH triple co-transporter in the loop of Henle?

A

Pumps ions out of the lumen = moves Na+, Cl- ad K+ out

K+ recycling means that NaCl is absorbed into the interstitial fluid

41
Q

What drug class block the triple co-transporter in the loop of Henle?

A

Loop diuretics

42
Q

What occurs when the triple co-transporter pumps solute from the thick ascending limb?

A

Solute removed from lumen of ascending limb = water can’t follow
Tubular fluid is diluted and osmolality of interstitial fluid is raised
Interstitial solute can’t enter the descending limb

43
Q

How does water leave the descending limb of the loop of Henle?

A

By osmosis

44
Q

What property does the fluid in the descending limb of the loop of Henle have?

A

It is concentrated

45
Q

What happens once fluid enters the descending limb of the loop of Henle?

A

Is concentrated and moves into the ascending limb

Hypotonic fluid then moves into the distal tubule from the ascending limb

46
Q

How does pumping occur in the loop of Henle?

A

Solute pumped out of the ascending limb
Osmolality of the interstitial fluid rises
Passive water efflux from descending limb
Flow occurs, moving everything on as before

47
Q

What does countercurrent multiplication allow?

A

A steady state to be reached = horizontal gradient has been multiplied into a large vertical gradient due to countercurrent flow

48
Q

What property does the fluid entering and leaving the loop of Henle have?

A

Iso-osmotic fluid leaves the proximal tubule and enters the loop of Henle, hypo-osmotic fluid leaves the loop to enter the distal tubule

49
Q

What contributes to half of the medullary osmolality?

A

The urea cycle

50
Q

How do the kidneys interact with urea?

A

Distal tubule not permeable to urea
Collecting duct absorbs 50% of urea
Urea adds solute to the interstitium
Urea diffuses passively into the loop

51
Q

What promotes the absorption of urea from the collecting ducts?

A

ADH

52
Q

What mechanism concentrates the medullary interstitial fluid?

A

Countercurrent multiplication

53
Q

How does countercurrent multiplication concentrate the medullary interstitial fluid?

A

Enables the kidneys to produce urine of a different volume and concentration according to the amounts of circulating ADH

54
Q

Where do the vasa recta run?

A

Run alongside the long loop of Henle to juxtamedullary nephrons

55
Q

What does capillary blood equilibrate with across the leaky endothelium of the vasa recta?

A

Interstitial fluid

56
Q

What happens to blood osmolality as it dips down into the medulla?

A

It rises = water is lost, solute is gained

57
Q

What happens to blood osmolality as it rises back up into the cortex?

A

It falls = water is gained, solute is lost

58
Q

What forms the countercurrent system?

A

The loop of Henle and the vasa recta

59
Q

What effect does essential blood flow through the medulla have on solutes?

A

Tends to wash away NaCl and urea

60
Q

How is the removal of NaCl and urea from the medulla by blood overcome?

A

Vasa recta capillaries follow hairpin loops
Vasa recta capillaries freely permeable to NaCl and H2O
Blood flow to vasa recta is low

61
Q

Why is the blood flow to the vasa recta low?

A

There are few juxtamedullary nephrons

62
Q

What preserves the medullary gradient?

A

Passive exchange across the endothelium = blood equilibrates at each layer to ensure solute isn’t washed away

63
Q

What creates the medullary osmotic gradient?

A

Countercurrent multiplier and the urea cycle

64
Q

What preserves the medullary osmotic gradient?

A

The countercurrent exchanger

65
Q

What does high medullary osmolality allow?

A

The production of hypertonic urine in the presence of ADH

66
Q

What is the only part of the nephron that doesn’t absorb sodium?

A

Descending limb of the loop of Henle

Renal (36 decks)

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