Renal II Flashcards

1
Q

Define: Clearance

A

the volume of plasma from which a substance is completely cleared ,by the kidneys per unit time

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
2
Q

What does the principle of renal clearance emphasize?

A

the excretory function of the kidneys

it considers only the rate at which a substance is excreted into the urine and not its return to the systemic circulation through the renal vein

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
3
Q

Basic Clearance Formula

A

Cx = (UxV)/Px

Ux = urine concentration of x (mg/ml)

V = urine volume (mL/min)

Px = Plasma concentration of x (mg/mL)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
4
Q

If a substance is present in urine at concentration of 100 mg/mL and urine flow rate is 1 mL/min and the substance has a concentration of 1 mg/mL in plasma, what is the clearance of the substance?

A

(100 mg/mL X 1 mL/min)/ 1 mg/ml = 100 ml/min

100 mL of plasma is completely cleared of the substance/min

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
5
Q

Equation to find GFR using Clearance

A

GFR = (UwV)/Pw

urinary clearance of w is equal to the GFR

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
6
Q

What criteria needs to be met in order for GFR = (UwV)/Pw to be valid?

A
  • W must be
    • freely filtrable
    • not reabsorbed
    • not secreted
    • not synthesized by tubules
    • not broken down by tubules
How well did you know this?
1
Not at all
2
3
4
5
Perfectly
7
Q

What fits the required criteria and can be used to determine GFR using clearance?

A

Inulin → a polysaccharide

GFR = renal clearance of inulin

Clearance of inulin (Cin) = GFR

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
8
Q

What is the issue with using inulin to measure GFR?

A

inulin is not naturally occurring

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
9
Q

What is used in place of inulin in clinical settings? why?

A

Creatinine

creatinine is freely filterable and not reabsorbed like inulin

there is a direct relationship between plasma creatinine and GFR within limits

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
10
Q

What is the relationship between Creatinine and GFR?

A

a small amount of creatinine is secreted → clearance of creatinine is slightly higher than real GFR

if plasma creatinine is raised, it indicates that renal function, especially GFR, is impaired

GFR decreases → plasma creatine increases

large increase in creatinine → small drop in GFR

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
11
Q

What is normal plasma creatinine?

A

10 mg/L

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
12
Q

What happens to plasma creatinine if GFR decreases 50%?

A

plasma creatinine rises but stabilizes at 20 mg/L

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
13
Q

Para-aminohippurate (PAH)

A

filtered at glomerulus, not reabsorbed, almost totally secreted

PAH approximates renal plasma flow

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
14
Q

Equation: Clearance of PAH

A

CPAH = (UPAHV)/PPAH = ERPF

ERPF = effective renal plasma fluid

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
15
Q

What happens to 10-15% of total renal plasma flow?

A

it supplies non-filtering and non-secreting portions of the kidneys such as (peripelvic fat), and this plasma cannot lose its PAH by secretion

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
16
Q

ERPF

A

0.9RBF

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
17
Q

Equation: RBF

A

RBF = RPF/(1-hematocrit)

hematocrit = % of blood cells

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
18
Q

Define: Diffusion

A

occurs from an area of higher concentration to one of lower concentration (down concentration gradient)

affected by the electrical potential difference across the cell membranes of the renal tubule cells

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
19
Q

Define: Facilitated Diffusion

A

depends on interaction of solute with a specific protein in the membrane that facilitates its movement across the membrane

utilizes transporters to move proteins

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
20
Q

Define: Primary Active Transport

A

the movement of molecules through a mechanism which is directly coupled to energy derived from metabolic processes (consumes ATP)

occurs against the concentration gradient (low to high)

utilizes ATP-dependent transporters or through endocytosis

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
21
Q

Define: Endocytosis

A

transport process where there is invagination of a part of the cell membrane until it completely pinches off and forms a vesicle in the cytoplasm

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
22
Q

Define: Secondary Active Transport

A

occurs when the energy from the downhill movement of a solute provides energy for the uphill movement of another solute

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
23
Q

Define: Solvent Drag

A

occurs when water is reabsorbed and solutes which have been dissolved in the water is reabsorbed with water

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
24
Q

Two types of transport mechanisms in reabsorption

A

Paracellular

Transcellular

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
25
Q

Define: Paracellular

A

movement that goes between two cells

can be by diffusion or solvent drag

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
26
Q

Define: Transcellular

A

movement that goes across the tubular cell → has to pass through the luminal and basolateral membranes

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
27
Q

How can lipid soluble substances transverse both membranes?

A

by diffusion and net passive reabsorption occurring by the transcellular route

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
28
Q

How can poorly lipid-soluble substances transverse both membranes?

A

using active transport

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
29
Q

What criteria must be met for a substance to be reabsorbed transcellulary?

A

1 of its 2 movements has to be active

and the second mode of transport must be different

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
30
Q

The generalization for net transcellular reabsorption of a substance requires…

A

That the luminal and basolateral membranes be asymmetrical for that substance (contain dif channels and/or transporters for the two membranes)

That energy be used for the movement of the substance either from lumen into cell or from cell into interstitial fluid

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
31
Q

Define: transport maximum

A

many active reabsorptive systems in the renal tubule have a limit to the amounts of material they can transport per unit time

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
32
Q

What causes transport maximum of reabsorptive systems in the renal tubule?

A

membrane proteins responsible for the transport become saturated

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
33
Q

What does transport maximum represent/refelct?

A

The maximal transport capacity of both kidneys

In turn it represents the sum of transport capacities of individual functional nephrons

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
34
Q

What is the relationship between the amount of substance filtered and the amount reabsorbed?

A

linear

As Tm is approached, some nephrons have reached their capacity and some substance appears in the urine

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
35
Q

Define: Renal Threshold

A

the plasma concentration where the substance occurs in the urine

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
36
Q

What happens as more nephrons exceed their capacity?

A

the relationship between amount filtered and reabsorbed is not linear (the splay)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
37
Q

Glucose Reabsorption in the Renal System

A

Glucose is completely reabsorbed → reabsorption rate follows filtered load

renal clearance of glucose is 0 → urinary concentration of glucose is 0

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
38
Q

What has to happen to plasma glucose to reach renal threshold?

A

Plasma glucose has to rise more than double normal levels before renal threshold is reached

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
39
Q

What happens to glucose when renal threshold is met?

A

reabsorption stops and glucose starts appearing in the urine

renal threshold is reached slightly before Tm

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
40
Q

Why is renal threshold met slightly before Tm?

A

as one approaches Tm, there are so few transporters left, that there is a greater chance NOT to get reabsorbed, so glucose molecules are not reabsorbed and pass through nephron and appear in urine

excretion rate now rises in parallel with filtered load

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
41
Q

What do tubular secretory processes do?

A

They transport substances across the tubular epithelium into the lumen

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
42
Q

Tubular Secretory Process

A

begins with diffusion out of peritubular capillaries into the interstitial fluid from which it makes its way into the lumen by crossing either tight junctions (paracellular transport) or in turn, the basolateral luminal membranes of the cell (transcellular)

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
43
Q

What is the direction of flow for secretion?

A

From the peritubular capillary → across tight junctions (paracellular) or membranes (transcellular) → into lumen

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
44
Q

What happens to filtered substances when secretion stops?

A

it is excreted

excretion = secretion + filtered

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
45
Q

How are most substances transported?

A

bidirectionally

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
46
Q

How does bidirectional transport work?

A

active reabsorptive processes tend to establish a concentration lower in the lumen than in the interstitial fluid → concentration difference favors passive paracellular secretion → creates a “pump-leak” system where active transport creates a diffusion gradient that opposes its own action by favoring back diffusion

How well did you know this?
1
Not at all
2
3
4
5
Perfectly
47
Q

Where is bidirectional transport particularly prevelant?

A

the proximal tubule

can also occur if the tubular segment contains opposing pathways

48
Q

Due to bidirectional transport, reabsorption and/or secretion are…

A

net processes

49
Q

Important characteristics of the Proximal Tubule

A
  • Luminal membrane has microvilli which increases surface are available for reabsorption
  • Tight junction is relatively leaky.
    • leakiness allows some reabsorption to occur passively down osmotic gradient through tight junction
  • Na+-K+-ATPase pump in basolateral membrane has greater activity than in most other nephron segments
50
Q

Reabsorption of sodium is….

A

mainly an active transcellular process

51
Q

Reabsorption of chloride is…

A

both passive (paracellular diffusion) and active (transcellular) but it is either directly or indirectly coupled with sodium reabsorption

52
Q

Reabsorption of water is…

A

by diffusion (osmosis) and is secondary to reabsorption of solutes particularly sodium

53
Q

Low GFR means,

A

less filtered, less reabsorbed

54
Q

% of Filtered Load Reabsorbed by Kidneys: Water

A

99.2%

55
Q

% of Filtered Load Reabsorbed by Kidneys: Na+

A

99.4%

56
Q

% of Filtered Load Reabsorbed by Kidneys: Glucose

A

100%

57
Q

% of Filtered Load Reabsorbed by Kidneys: Urea

A

50%

58
Q

Cell model for Proximal Tubule Transport

A
  • Two membranes with different permeability characteristics
    • luminal membrane with microvilli
    • basolateral membrane
  • intercellular space between adjacent cells which is open at capillary end and to a lesser degree at the luminal end across from a tight junction
59
Q

Reabsorption of all organic solutes are coupled by….

A

Na reabsorption

60
Q

Na+-K+-ATPase

A

primary event in the Na+-K+-ATPase is the basolateral membrane

Pump maintains low intracellular Na concentration

steep concentration allows Na to be transported rapidly into cell

61
Q

Early Portion of Proximal Tubule

A

movement of Na is linked to co-transport of glucose, amino acids, and phosphate into cell

Na reabsorption occurs via countertransport with hydrogen ions

with each transporter, the favorable inward gradient for Na is sufficient to drive the transport of other solute (20 active transport)

62
Q

Early Portion Proximal Tubule: How are glucose, amino acids, and phosphate reabsorbed on the basolateral membrane?

A

via various facilitated diffusion mechanisms

63
Q

How is Na reabsorption accomplished in later proximal tubule?

A

via Na/H countertransport mechanisms.

64
Q

Isoosmotic reabsorption

A
  • in the proximal tubule, sodium and water reabsorption occur to the same degree
    • as Na is being reabsorbed (increase concentration of Na) water is also reabsorbed
65
Q

What is the driving force of water reabsorption?

A

transtubular osmotic gradient established by solute reabsorption

water reabsorption follows solute reabsorption in proximal tubule

66
Q

What is the effect of reabsorption of solutes on osmolarity in the proximal tubule?

A

reabsorption of solutes reduces osmolarity of the tubular fluid and increases osmolarity of the interstitial spaces

osmolarity remains relatively constant throughout the proximal tubule

67
Q

Why is the proximal tubule highly permeable to water?

A

because of the expression of aquaporin-1 water channels

68
Q

How is water reabsorbed in the proximal tubule?

A

by osmosis down the concentration gradient

paracellularly → tight junctions are permeable to water

69
Q

Proximal Tubule: Accumulation of solutes and fluid in interstitial space….

A

increases hydrostatic pressure in this compartment and fluid and solutes are forced into peritubular capillaries

70
Q

2 mechanisms for movement of fluid into peritubular capillaries

A

Net diffusion

Bulk Flow of interstitial Fluid

71
Q

Net filtration pressure across the peritubular capillaries always favors….

A

net movement into the capillaries

72
Q

Why does Net filtration pressure across the peritubular capillaries always favor net movement into the capillaries?

A

Because the peritubular capillary hydrostatic pressure is usually quite low (~13 mmHg)

oncotic pressure of the plasma entering the peritubular capillaries is higher than that of the arterial plasma (plasma proteins are concentrated by loss of protein-free filtrate during passage through the glomerular capillaries)

73
Q

Relationship between interstitial fluid hydrostatic pressure and oncotic pressure in the proximal tubule?

A

interstitial fluid hydrostatic pressure usually equals interstitial fluid oncotic pressure in the proximal tubule

74
Q

Due to the fact that interstitial fluid hydrostatic pressure usually equals interstitial fluid oncotic pressure in the proximal tubule, Reabsorption of fluid from the interstitial space is mostly dependent on…

A

forces in the peritubular capillary → specifically peritubular capillary oncotic pressure drives reabsorption from the interstitial space into the capillaries, while the capillary hydrostatic pressure opposes this

75
Q

Reabsorption of Bicarbonate in proximal tubule

A

bicarbonate is reabsorbed indirectly

  • secreted H+ (in exchange for Na) reacts with bicarbonate in filtrate
  • carbonic acid forms
  • carbonic acid dehydrates to carbon dioxide in presence of carbonic anhydrase
  • carbon dioxide (and water) diffuse through the luminal membrane into cell
  • CO2 is rehydrated to carbonic acid
    • (by intracellular carbonic acid anhydrase)
  • intracellular carbonic acid dissociates to HCO3- and H+ ion
  • HCO3- exits cell across the basolateral membrane by facilitated diffusion

Net absorption of HCO3- by above mechanism is important for the net reabsorption of Na+

76
Q

What does reabsorbing Na by the Na+-K+ ATPase pump do to the cell?

A

it makes it basic

77
Q

Chloride Reabsorption

A

a large amount of chloride reabsorption in the proximal tubule occurs by a passive mechanism of bulk flow of water and solute through the tight junction (paracellular reabsorption)

78
Q

Mechanism of Chloride Reabsorption in Proximal Tubule

A
  • Proximal tubule reabsorbs most of the filtered glucose, amino acids, and bicarbonate by coupled transport with sodium, but a smaller amount of chloride
    • chloride ion concentration increases within the proximal tubule compared to plasma
  • Tight junction is permeable to chloride
    • chloride moved through the tight junction down a concentration gradient
      • Na follows to maintain electroneutrality
  • Chloride behaves as a permeant ion → luminal fluid has a lower effective osmotic pressure than the intracellular space
    • imbalance between osmotic forces across the tight junction causes water to move from lower osmotic pressure of the tubule lumen into high osmotic pressure of the intracellular space
    • maintains isosmotic reabsorption of proximal tubule
79
Q

Chloride reabsorption linked to Formate

A
  • In late proximal tubule, specialized transport devices account for reabsorption of chloride ion
    • mechanism is in exchange for formate ion
  • formate ion is converted to formic acid (HF) by secreted hydrogen ion
  • Formic acid diffuse across the luminal membrane → b/c it is uncharged
  • formic acid dissociates to F- and H+ in cell interior
    • cell pH is higher than luminal pH
  • F- moves back across the luminal membrane in exchange for chloride
    • Hydrogen is secreted across the luminal membrane in exchange for sodium
80
Q

Filtration Fraction

A

GFR/RBF

81
Q

Effect of Filtration Fraction on Fluid Reabsorption in the Peritubular Capillaries

A

Increased sympathetic outflow and/or AII levels → increased arteriolar vasoconstriction → decreases RBF more than GFR → increases filtration fraction → increases peritubular capillary oncotic pressure (w. decreased hydrostatic pressure) → increases fluid movement (reabsorption) into peritubular capillaries

82
Q

Henle’s Loop

A

reabsorbs approximately 25% of filtered NaCl and 15% of filtered water

taken as a whole → Henle’s Loop always reabsorbs more sodium and chloride than water

83
Q

Descending Limb of Henle’s Loop

A

does not reabsorb sodium but is highly permeable to water → reabsorbs water

84
Q

Ascending Limb of Henle’s Loop

A

reabsorbs sodium and chloride but impermeable to water

85
Q

What does the movement of sodium and chloride out of ascending limbs of Henle’s Loop into interstitial fluid do?

A

It raises the osmolarity at this location which causes water reabsorption by diffusion from the water permeable descending limb

86
Q

What is the major transport mechanism in the ascending limb of Henle’s Loop?

A

Na+-K+-2Cl- cotransporter

87
Q

Why is Henle’s Loop called the diluting segment?

A

because of the reabsorption of sodium and water, the fluid that enters distal convoluted tubule is hypoosmotic compared to plasma

88
Q

Distal Convoluted Tubule and Collecting-Duct System

A

Distal convoluted tubule and collecting duct reabsorb ~ 10% of the filtered NaCl

secretes variable amounts of K+ and H+

reabsorb a variable amount of water

89
Q

What kind of cotransporter is found in the initial part of the distal convoluted tube?

A

Na-Cl

90
Q

Initial portion of Distal Convoluted Tubule

A

water reabsorption is low

fluid in this segment becomes more hypoosmotic

91
Q

Collecting Duct: Principal Cells

A

reabsorb sodium through Na+ channels.

secrete K+

92
Q

Collecting Duct: α - intercalated cells

A

H+ is secreted through an H+-ATPase as well as an H+-K+-ATPase counter transporter

93
Q

Collecting Duct: β-intercalated cell

A

secrete HCO3- in exchange for Cl-

reabsorb H+ and Cl-

94
Q

Water permeability in the collecting ducts

A

can be very high or very low

under the influence of Vasopressin or Antidiuretic Hormone (ADH)

95
Q

What effect does ADH have on collecting ducts?

A

water permeability increases dramatically

there is rapid reabsorption of water by diffusion down its concentration gradient becoming isoosmotic compared to cortical plasma

96
Q

What happens to water in the medullary collecting duct?

A

water reabsorption continues

luminal fluid becomes hyperosmotic

produces hyperosmotic urine

97
Q

What effect does aldosterone have on collecting ducts?

A

it is secreted by the adrenal cortex

acts in a classic steroid hormone fashion by stimulating protein synthesis in the cell

aldosterone binds to cellular receptors and through the increased synthesis of specific proteins, increases the number of sodium channels in the luminal membrane of the late distal tubule collecting duct

increases activity of the Na+-K+ ATPase pump

98
Q

What is the effect of reabsorption of Na+ in the late distal tubule and collecting duct?

A

it generates a lumen negative voltage across the luminal membrane which is the driving force of paracellular reabsorption of Cl-

99
Q

What is the purpose of the countercurrent multiplier system?

A

to produce concentrated urine

takes place in the Loops of Henle

100
Q

What is the Countercurrent Multiplier System dependent on?

A
  • Descending limb of the Loop does not absorb sodium or chloride but reabsorbs water
    • high permeability to water but low permeability to sodium and chloride
  • Ascending limb reabsorbs sodium and chloride but not water
    • segments are permeable to sodium and chloride but less to water
101
Q

Countercurrent Multiplier System

A

Step 1: Na, Cl, K are reabsorbed from the thick ascending limb

Step 2: hypertonic interstitium stimulates the reabsorption of water from the thin descending limb

Step 3: flow occurs within the loop so that more isotonic fluid enters the loop from the proximal tubule, and hyperosmotic fluid generated in the thin descending limb is delivered to the ascending limb

Steps 1-3 are repeated until you get the end result of a large vertical osmotic gradient generated but a smaller gradient exists horizontally

THIS IS HOW URINE IS CONCENTRATED

102
Q

What effect does the Countercurrent Multiplier System have on the medullary interstitium?

A

it becomes very hyperosmotic

103
Q

How is the Countercurrent Multiplier System maintained?

A

by the blood flow in the vasa recta which preserves the hyperosmotic interstitium of the Loop

104
Q

How can water be reabsorbed in the collecting duct?

A

by going down its osmotic gradient due to the very high medullary interstitial osmolarity

105
Q

Rate limiting step in the Countercurrent Multiplier System

A

Na-K-Cl cotransporter in the thick ascending limb

106
Q

What happens if the Na-K-Cl cotransporter in the Countercurrent Multiplier System is inhibited?

A

less NaCl and K is reabsorbed from the thick ascending limb

medullary interstitium will be less hyperosmotic and less water will be reabsorbed from the thin descending limb

more water will remain in the loop → less hyperosmotic

In the collecting tubule/ducts, less water will be reabsorbed because the medullary interstitium is not as hyperosmotic

107
Q

Role of Interstitial Urea

A

Urea is important in creating hyperosmolarity in tubular fluid

after filtration, luminal urea concentration rises progressively along the cortical collecting ducts and the outer medullary collecting ducts is reabsorbed

High urea concentration in the inner medullary collecting duct then drives reabsorption in the segment under the influence of ADH

108
Q

Why is Urea not reabsorbed in the cortical collecting ducts?

A

tubular segments are impermeable to it

109
Q

What does simultaneous movement of water out of the inner medullary collecting ducts do?

A

It maintains high urea concentration within the ducts → net result is that the urea concentration in the inner medulla equilibrates across the interstitial fluid and lumen of duct

concentration in the lumen helps create the high osmolarity of concentrated urine

110
Q

Sodium reabsorption in the nephron

A

Proximal tubule reabsorbs 65% of filtered sodium

Thick ascending limbs of Henle’s Loop together - 25%

Distal Convoluted Tubule and Collecting Ducts - remaining 10% so that final urine volume contains less than 1% of total filtered sodium

111
Q

Essential Event for Transcellular Sodium Reabsorption in the Nephron

A

primary active transport of sodium from cell to interstitial fluid by the Na-K-ATPase pumps in the basolateral membrane

on the luminal membranes there are many dif transport mechanisms which help reabsorb sodium in dif segments of the nephron

112
Q

Chloride reabsorption in the nephron: Paracellular transport in the proximal tubule:

A

Critical step for paracellular reabsorption of Cl is between lumen and interstitium

luminal membrane processes which reabsorb Na, HCO3, phosphate, glucose, amino acids achieve a high enough luminal chloride concentration to cause downhill chloride movement out of the lumen to interstitium

113
Q

Chloride reabsorption in the nephron: Transcellular transport in the TALH, Distal Convoluted tubule, Collecting Duct:

A

Most of the transport mechanism for Cl are coupled with Sodium (directly or indirectly)

Cl/bicarbonate transport system is independent of sodium

114
Q

Where does Cl/bicarbonate transport system occur?

A

occurs in collecting duct

independent of Na

115
Q

Where does chloride transport occur?

A

type B intercalated cells

116
Q

Where does sodium transport occur?

A

principal cells

117
Q

Water reabsorption in nephrons

A

occurs in the proximal tubule (65% of filtered water)

thin descending limb of Henle’s Loop (10%)

collecting duct system (a few percent to > 24%; depending on ADH levels)