1. Regulation of Body Fluid Osmolality – Regulation of Water Balance (DSA)

Question 1

Describe the permeability of the collecting duct.

Answer 1

1. Cortical collecting duct is impermeable to water AT ALL TIMES, unless ADH is present.
2. Medullary CD- Medullary CD actively reabsorbs NaCl is always permeable to water, despite if ADH is present or not. However, ADH makes it more permeable.
   
   1. ADH is controlled by osmoreceptors in the hypothlamus to plasma osmolality and volume.

Question 2

What factors contribute to making the osmotic gradient?

Answer 2

1. countercurrent multiplier
2. urea recycling

Question 3

What 2 sources created our medullary interstitial osmotic gradient?

Answer 3

1. AQP channels and no tight junctions in the thin descending limb allow water to move to the medulla without Na+.
2. Loop of Henle and the CD’s role in the countercurrent multiplier, which increase osmolality as the loop goes deeper into the medulla.

Question 4

Describe the 7 step process of urine concentration

Answer 4

1. As fluid enters the descending limb from the proximal tubule, it is isotonic with the medulla (300 mOsm).
2. Descending LoH becomes hypertonic until is reaches 1200-1400 at the bottom of the loop.

Question 5

___________ and the _________ always come to eqiullbrium, creating a concentration gradient that ranges from what to what?

Answer 5

Medulla and desending limb always come to equillibrium, creating a concentration gradient that ranges from 300-1200 mOsm/L.

Question 6

Concentration in the tubular fluid ________ in the ascending limb.

Why?

Answer 6

Decreases.

NaCl is pumped out via NKCC2, but water cannot follow to offset the gradient.

Question 7

Befores the tubular fluid leavs the ascending lumb to enter the distal tubule, what happens?

Answer 7

The tubular fluid will become even MORE hypotonic at 100mOsm/L.

Question 8

Describe the process of urea recycling and how it contribues to creating an osmotic gradient?

Answer 8

Urea recycling is controlled by ADH and contributes to osmotic gradient.

Occurs in inner medullary CD

Question 9

Urea is reabsorbed from the [nephron–> the medulla] at what part of the nephron?

This is accomplished via ________________

Answer 9

inner medullary collecting duct via urea transporters

[UT-A1 and UT-A3].

Question 10

What happens to the urea the exits the nephron through UT-A1 and UT-A3 transporters in the inner medullary CD?

Answer 10

UREA RECYCLING.

Some of the urea re-enters the thin descending loop of Henle via UT-A2 transporters, to flow back through the rest of the nephron.

Question 11

When does urea accumulation in the medulla occur most effectively?

Answer 11

When a hyperosmotic urine is excreted (antidiuresis).

Question 12

When dilute urine is made, osmolality of medullary interstitium declines almost entirely d/t _____.

Answer 12

Urea.

Question 13

High urea in the medulla allows what?

Answer 13

it can be excreted in small volumes in urine, limiting the amount of water we need to excrete the amount the body makes.

Question 14

What is the purpose of the vasa recta?

Answer 14

Vasa recta are capillaries that send blood to the medulla and is highly permeable to solute and water;

It is involved in the countercurrent exchange: remove solute and water that is continuously added to the medulla to help us maintain the gradient.

Question 15

Osmotic gradient is highest when: ___ ADH

Answer 15

High

Question 16

Increase in blood flow in vasa recta –>

Answer 16

decreases the medullary gradient;

because removing more NaCl than is being added

Question 17

Decrease in blood flow of vasa recta–>

Answer 17

1. Increase the gradient of the medulla and reduces O2 delivery to the nephron –>
2. decreases salt and solute transport by nephron segments in the medulla –>
3. reduces ability to concentrate urine.

Question 18

What are the main solutes in the medullary ISF?

Answer 18

1. NaCl

2. Urea

Question 19

Because water resbsorption is driven by osmotic gradient in the medulla, urine can/can never be more concentrated than the medulla.

Answer 19

CAN NEVER

Question 20

Can urea drive water reabsorption in the medulla?

Answer 20

No.

the inner medullary collecting duct is highly permeable to urea, especially in the presence of AVP, urea cannot drive water reabsorption across this nephron segment.

Urea in the tubular fluid and medulla equillbriate and a small amount of urine is made with a high concentration of urea.

Question 21

Cortex: _____ salt; ____ water

Medulla: _____ salt; ____ water

Answer 21

Cortex: low salt; high water

Medulla: high salt; low water

Question 22

In the countercurrent multiplier, as fluid moves around the hairpin loop and goes to the ascending limb, what maintains the ion concentration as they are pumped out?

Answer 22

Na/K ATPases.

Question 23

When water intake is low or water losses increase, how to the kidneys respond?

Answer 23

conserve water by producing a small volume of urine that is hyperosmotic with respect to plasma.

Question 24

When water intake is high, how to the kidneys respond?

Answer 24

A large volume of hypoosmotic urine is produced.

Question 25

If our water balance is fucked up, it is due to changes in ______________

Answer 25

Plasma osmolality.

Question 26

What is the major determinant of plasma osmolality?

Answer 26

Na+ (with its anions Cl- and HCO3-).

these disorders also result in alterations in the plasma [Na+]

Question 27

When plasma osmolality is reduced (hypoosmolality)–>

Answer 27

water moves from ECF–> cells, causing them to swell.

Question 28

When plasma osmolality is increased (hyperosmolality)–>

Answer 28

water is lost from cells, causing them the shrink

Question 29

Whether or not the kidney maintains water balance by excreting hypoosmotic (dilute) or hyperosmotic (concentrated) urine is determined by _____

Answer 29

ADH (vasopressin).

Question 30

What is ADH?

Answer 30

Acts on the kidneys to regulate the volume and osmolality of the urine.

Increasse the permeability (reabsorption) of the collecting duct to water by adding AQP 2 channels on the DCT and CD–> decreasing urine volume.

Question 31

High and low concentrations of ADH.

Answer 31

• Low levels of ADH–> CD is made impermeable to water–> large volume of dilute urine is excreted (diuresis).
• High levels of ADH–> insertion of AQP channels –> increasing water reabsorption in the small volume of concentrated urine is excreted (antidiuresis).

Question 32

Where is ADH made?

Answer 32

Supraoptic and PVN neurons in the hypothalamus.

The hormone is then stored in the neurohyophysis, located in the posterior pituitary.

Question 33

Secretion of ADH is influenced by what 2 primary mechanisms?

Answer 33

1. Osmolality of the plasma*

2. BP/BV

3. Nausea (+), Angiotensin II (+) and ANP (-), to a smaller degree

Question 34

What regulates ADH’s response to plasma osmolality?

Answer 34

Osmoreceptors:

• detect changes small changes in plasma osmolality RAPIDLY and regulate the activity of ADH-secreting neurons by either shrinking or swelling and
• causing thirst.

Question 35

Increase in effective osmolality of plasma–>

Answer 35

[pic]

Question 36

Decrease in effective plasma osmolality–>

Answer 36

[Pic]

Question 37

As we have said: activation of osmoreceptors cause the activates ADH pathway and thirst. What occurs first?

Answer 37

Activation of ADH pathway occurs rapidly. Thirst occurs later.

This allows us to not always have to search for water.

Question 38

When does ADH secretion occur?

Answer 38

The set point is the plasma osmolality value at which ADH secretion begins to increase.

• 275-290 mOsm/kh H2O.
• However, it will shift in response to BV, BP, pregnancy.

Question 39

How does BP and BV affect ADH secretion?

Answer 39

[pic]

Decrease in BP/BV–> + ADH secretion.

Increase in BP/BV–> - ADH secretion

Question 40

How do changes in BP/BV change plasma osmolality?

Answer 40

• Decrease in BV/BP–> set point is shifted to lower osmolality values. Thus, when circulatory collapse, the kidneys continue to conserve water, even though by doing so, they reduce the osmolality of the body fluids.
• Increase in BV/BP–> set point is shifterd to higher osmolality values (thus, a higher osmolality will trigger ADH release).

Question 41

What are the actions of ADH? 4

Answer 41

1. Primary action: increase the permeability (reabsorption) of the collecting duct to water by adding aquaporin 2 channels on the DCT and CD–> decreasing urine volume.
2. Increase the permeability of the medullary collecting duct to urea.
3. Stimulate NaCl reabsorption at the thick ascending LoH, distal tubule and CD via V2 receptors.

• This helps to maintain a hyperosmotic medullary interstitium when, at the same time, ADH is increasing collecting duct water reabsorption
  4. Vasoconstriction of vascular SM via V1 receptors–> increasing

BP–> increasing Na+ excretion (HENCE; VASOPRESSIN)

Question 42

As a result of these competing effects, ADH may either increase or decrease NaCl excretion. What determines which response predominates ____________.

Answer 42

Plasma levels of ADH.

• Low-moderate ADH levels; V2 mediated response predominates and NaCl excretion is decreased
• High ADH levels: V1 mediated response predominates and NaCl excreteion is increased.
  
  • Even when NaCl excretion is increased w/ high AVP levels, NaCl reabsorption by the thick ascending limb of Henle’s loop, the distal tubule, and the collecting duct is still stimulated. However, the resorptive capacity of these segments is overwhelmed by the NaCl delivered to these segments from the proximal tubule, and as result, NaCl excretion is increased.

Question 43

Late distal tubule and CD have what 2 cell types that do what?

Answer 43

1. Principal cells: reabsorb Na+, Cl- and H20 and secrete K+
2. Intercalated cells, which reabsorb K+ and secrete H+

Question 44

How does ADH act on principal cells of the late DCT and CD?

What happens to K and Na+?

Answer 44

1. High ADH/restriction of water–> inserts AQP2 channels apical membrane of the late DCT and CD–> + permeability to water–> + reabsorption of water –> creating a more concentrated urine.
2. No ADH/ingestion of large amount of water–> no AQP2 channels inserted –> DCT and CD are impermeable to water –> little H2O is reabsorbed–> creating a more dilute urine.
   - Na is reabsorbed via Na/K ATPases. K enters the cell from the BL membrane via Na/K ATPase and diffuses down concentration gradient –> apical membrane–> tubular fluid.

Question 45

Number of AQP channels–> affects the volume of water reabsorbed or secreted.

Which affects what?

Answer 45

1. Plasma osmolarity

2. Urine osmolarity

3. BP

Question 46

SIMPLE RULES FOR ADH

Overhydrated –> _____ ADH

Dehydrated –> _____ ADH

Answer 46

Overhydrated –> low ADH

Dehydrated –> high ADH

Question 47

Why are intercalated cells important?

Answer 47

Intercalated cells are important for acid-base balance.

+ Aldosterone–> + H+ secretion via H+-ATPase

Question 48

Aldosterone is often called ____________.

Where is it made and how is it released.

Answer 48

Aldosterone aka “salt-retaining hormone”.

Released from the adrenal CTX d/t

1. Indirectly via angiotensin II
2. Directly via increased plasma K+ levels.

Question 49

Aldosterone mechanism.

Answer 49

[pic]

Question 50

Decrease in plasma K+–> _____ aldosteroen

Answer 50

reduction in aldosterone secretion

Question 51

Plasma osmolality of a hydrated person–>

Plasma osmolality if a dehydrated person–>

Answer 51

Plasma osmolality of a hydrated person–> 275-295 mOsm/kg

Plasma osmolality if a dehydrated person–> >300 mOsm/kg

Question 52

Central Neurogenic Diabetes Insipidus occurs after

Answer 52

head trauma

brain neoplasma

infections

Question 53

Central Neurogenic Diabetes Insipidus effect:

Answer 53

Low ADH levels d/t abnormal synthesis and secretion of ADH

Question 54

Characteristcs of Central Neurogenic Diabetes Insipidus (3) and treatment

Answer 54

1. Poluria–> inadequate release of ADH from the posterior pituitary causes an excretion of a large volume of dilute urine. Leads to increased thirst, to compensate for too much water loss.

2. Dilute urine

3. Polydipsia–> Increase thirst leads to an increase in water consumption.

• Thus, a person with CDI must ingest a shit ton of water to maintain a constant body fluid osmolality.

-Thus: production of a large volume of dilute urine that can exceed 15 L/day.

Tx: exogenous ADH (desmopressin).

Question 55

Patients with Central Neurogenic Diabetes Insipidus exhibit polydipsia, an increase in thirst and consumption of what. What happens if water intake is restricted or the patient is unconscious because of a head injury?

Answer 55

Body fluids become serverely hyperosmotic and severe dehydration occurs.

Question 56

Nephrogenic Diabetes Insipidus effect

Answer 56

Effect: ADH levels are norma or elevated, however collecting ducts do not respond to ADH. Thus, cannot maximally concentrate urine.

• Thought to be d/t a failure of the countercurrent mechanism to create a hyperosmotic medulla or the distal and CD to respond to ADH.

Question 57

Nephrogenic Diabetes Insipidus characteristics (2) and tx.

Answer 57

1. Polyuria
2. Dilute urine
3. Polydipsia

Large volumes of dilute urine are made, causing dehydration unless fluid intake is increased by the same amount as urine volume is increased.

Tx:

Question 58

How can we distinguish Nephrogenic Diabetes Insipidus from Central Diabetes Insipidus?

Answer 58

-Administration of Desmopressin-

If the patient does not decrease urine volune and increase urine osmolarity within 2 hours after the injection–> nephrogenic DI.

Question 59

Nephrogenic Diabetes insipidus is also assx with what?

Answer 59

1. Hypernatremia (can be decreased with a low-Na+ diet and thiazide diuretic, with increases Na+ secretion)
2. Hypokalemia
3. Hypocalcemia
4. Low protein diet
5. Uretal obstruction

Question 60

SIADH is caused by what?

Answer 60

1. Infections and neoplasms of the brain,
2. drugs
3. pulmonary diseases
4. lung cancer.

Question 61

Effects of SIADH

Answer 61

Elevated plasma ADH levels above what would be expected for ones plasma osmolarity, BP and BV.

Question 62

Characteristic of SIADH (mechanism)

Answer 62

1. If plasma osmolality has dropped and ADH continues to be released–>
2. Increased aquaporin 2 channels on DCT and CD–>
3. Increase water reabsorption in blood (increasing ECF)–>
   
   1. Diluting blood (hypoosmotic) –>
   2. Dilutes Na+ and [takes up more space in blood vessels]–>
   3. Decrease in aldosterone, causing a decrease in plasma Na+ as Na+ is secreted –>
   4. Increases GFR
4. Creates a concentration gradient–>
5. Increased Na+ excretion and a shifting fluid into cells and causing them to swell to balance the concentration of fluid (water intoxication).
6. Normalizing fluid volume in blood

Thus, we are retaining water and removing Na+ from our body, which already has low Na+

Tx: non-peptide vasopressin antagonists.

Question 63

A patient with SIADH will develop what?

Answer 63

Thirst, dyspnea on exertion, vommiting, abdominal cramps, confusion, lethargy and hypnotremia.

Question 64

Diabetes Insipidus

____ urinary output

____ levels of ADH

____natremia

Overhydrated/dehydrated

Loses/retains too much fluid

_____ thirst

Answer 64

High urinary output

Low levels of ADH

Hypernatremia

Dehydrated

Loses too much fluid

Excessive thirst

Question 65

SIADH

____ urinary output

____ levels of ADH

____natremia

Overhydrated/dehydrated

Loses/retains too much fluid

_____ thirst

Answer 65

SIADH

Low urinary output

High levels of ADH

Hyponatremia

Overhydrated

Retains too much fluid

Excessive thirst

Question 66

Excreting Dilute Urine (Water diuresis) when ADH levels are low.

Steps:

Answer 66

1. Fluid entering the descending thin limb of the LoH is isosmotic with the plasma, reflecting the isosmotic nature in the proximal tubule.
2. Thin Descending LoH: water is reabsorbed (mostly in the outer medulla) via AQP1.
   
   1. This limits the amount of the amount of water added the deepest part of the inner medullary interstitial space, preserving the hyperosmolarity of this part of the medulla
3. Inner medulla of the terminal descending thin limb and the thin ascending limb is impermeable to water.
   
   1. However, they have CLCK1 Cl- channels, allowing Cl- to be reabsorbed and Na+ to be reabsorbed paracellular. This begins the dilution of the tubular fluid.
4. Thick ascending LoH: reabsorbs NaCl & impermeable to water, diluting the tubular fluid and creating a hypoosmotic setting.
   
   1. NaCl accumulates in the medullary interstitium and is necessary to make urine hyperosmotic to plasma because it creates an osmotic gradient for water reabsorption in the medullar collecting duct.
5. DCT and cortical portion of CD: actively reabsorb NaCl.
   
   1. • ADH–> not permeable to water–> osmolality of the tubular fluid is further reduced because NaCl is reabsorbed without water.
   2. Fluid in the cortical portion of the collective duct is hypoosmotic with the plasma.
6. Medullary CD: actively reabsorbs NaCl. Even without ADH, it is slightly permeable to water and some is reabsorbed.
7. Urine osmolality is as low as 50 mOsm/kg H2O and has low concentrations of NaCl.
   
   1. Volume can be as much as 18 L/day.

Question 67

Pt Johnny consumes a large amount of water. Excess H20 must be removed from the body without losing solutes that are critical for homeostasis.

Steps:

Answer 67

1. No ADH is secreted, so the DT and the CT are impermeable to water.
2. Tubular fluid entering the DT hypotonic (100 mOsm), having lost NaCl without H20 in the thick ascending LoH.
3. As it goes through the distal tubule and is CT, the medullary osmotic gradient does not effect tubular fluid because the late tubule because it is impermeable to H20.
4. Without ADH, 20% of the filtered fluid that reaches the distal tubule is not reabsorbed. However, excretion of wastes and other solutes is constant.
5. When we have excess water, urine flow rate increases and the concentration of the urea in the inner medullary CD is reduced, causing less diffusion of urea in the medulla
6. RESULT: large volume of dilute urine, which helps ride the body of excess H20.

Question 68

What happens when we have a H20 deficit? (* look at slide)

Answer 68

1. H20 deficit–> Increases ADH levels.
2. ADH stimulates NaCl reabsorption in the thick ascending LoH, which maintains the medullary gradient as water is being added from the medullary collecting duct, which would decrease the gradient.
3. Fluid reaching CD is hypoostmotic, compared to interstitialàcreates a concentration gradient. ADH inserts AQP2 channels into late DCT and CD–> water is reabsorbedàfluid osmolarity increases, concentrating the urine
4. As tubular fluid descends deeper into the medulla, water continues to be reabsorbed from the CDàincreasing tubular fluid osmolality to 1200 mOsm/kg H2O.
5. Urine made when ADH levels are high has an osmolality of 1200 mOsm/kg H20 and has a high concentration of urea + other solutes.
   
   1. Urine volune can be as low as 0.5 L/day.

Question 69

In the presence of ADH, recall that the intra-tubular fluid of the distal tubule can only reach, ____ mOsm / kg H20. Why?

Answer 69

300 mOsm/kg H20.

This is because the interstitial fluid of the cortex is isotonic.

Question 70

In situations where we have no ADH and high ADH levels, how much fluid is delivered to the late DT and CD?

Answer 70

In both situations, a constant volume of dilute tubular fluid is delivered to the ADH-late DT and CD. ADH levels will then determine the amount of water that we reabsorb.

• Low ADH–> small volume is reabsorbed and hypoosmotic urine is excreted.
• High ADH–> high volume is reabsorbed and a small volume of hyperosmotic urine is excreted.

Most H20 is reabsorbed in the distal tubular and cortical and outer medullary portions of the CD. Thus, only a small amount of urine will reach the inner medullar CD, where it is then reabsorbed.

Question 71

What is hyponatremia?

Answer 71

Hyponatremia–> high water, low Na+ in the plasma.

It usually occurs d/t high ADH decreasing water excretion.

Question 72

Very rarely is hyponatremia a consequence of what two things?

Answer 72

1. Insufficient solute consumption
2. Polydipsia.

Question 73

What is hypernatremia?

Answer 73

Hypernatremia–> High Na+, compared to water.

Often d/t

1. impaired thirst
2. decreased water consumption

Question 74

in diabetes insipidus, even in situations of hypernatremia, the urine is _____

Answer 74

dilute

Question 75

in salt intoxication, the concentration of sodium in the urine will be ______– as would be expected.

Answer 75

very high

Question 76

Values for

Polyuria

Oliguria

Anuria

Answer 76

1. Polyuria: >2.5L/day
2. Oliguria (output below minimum): 300-500 mL/day
3. Anuria (virtually no pee): <50mL/day

Question 77

Polyuria is usually associated with __________

Answer 77

polydipsia (water intake >100 mL/kg/day)

Question 78

4 mechanisms can cause polyuria

Answer 78

1. Increased intake of fluids.
2. Increased GFR (hyperthyroidism, fever, hyper metabolic states.)
3. Increased output of solutes (diabetes mellitus, hyperthyroidism, hyperparathyroidism, diuretic use)
4. Inability of the kidney to reabsorb water in the distal convoluted tubule (diabetes insipidus, drug use, chronic renal failure.)

Question 79

What is water diuresis?

Answer 79

An increase in water excretion, without an increase in Na+ excretion.

• Primary cause: increased intake in water (as in polydipsia and diabetes insipidus).

Question 80

What is solute (osmotic) diuresis?

Answer 80

Increased water excretion + increased NaCl excretion due to increase salt in fluid.

Examples include: IV, hyperglycemia, high protein intake, recovery from AKI.

Question 81

What is the obligatory urine volume (minimum amount of urine one must excrete in day)?

How many mOsm of solutes/day must a bedridden and active person excrete?

Answer 81

Obligatory urine volume –> minimum amount of urine one must excrete in day.

Bedridden: 700 mOsm

Active: 1000 mOsm.

—the kidney can concentrate up to 1200 mOsm/L a day—.

Question 82

How can we calculate obligatory urine volume?

Answer 82

(mOsm of solutes/day an individual of a certain weight must excrete)/ 1200 mOsm/L

An individual must excrete at least 600 mOsm of solutes per day, and the kidneys are capable of concentrating urine to a maximum of 1200 mOsm per liter. This means an individual must excrete at least .5 L/day of urine. We call this “Obligatory Urine Volume.”

Question 83

What is free water clearance?

Answer 83

Free water clearance–> rate at which solute free water is excreted from the kidney.

Free water clearance (C_H2O )= [urine flow rate - osmolar clearance]

if +, excess water is being excreted from the kidneys.

If -, excess solutes are being removed from the blood by the kidneys and water is being conserved.

Question 84

How to calculate osmolar clearance

Answer 84

C_osm= U_osm*V/P_osm

_{=urine osmolarity * urine flow rate/ plasma osmolarity}

Question 85

Why is the ratio of urine osmolarity (U_osm) to plasma osmolality (P_osm) useful?

Answer 85

It can tell us the ability of the kidney to concetrate or dilute urine.

Question 86

U_osm:P_osm > 1

tells us what?

Answer 86

The kidney can concentrate urine.

Question 87

U_osm:P_osm = 1

tells us what?

Answer 87

Water and solute are being excreted in a state that is iso-osmotic with plasma.

Question 88

U_osm:P_osm < 1

tells us what?

Answer 88

Kidneys are able to dilute urine.

Question 89

Look at last slides in the ppt

Question 90

State tubular fluid osmolalities during diuresis and antidiuresis at the distal end of each of the following tubular segments of juxtamedullary nephrons: the proximal convoluted tubule, descending limb of Henle’s loop, thick ascending limb of Henle’s loop, distal convoluted tubule, cortical collecting duct, and inner medullary collecting duct.

Answer 90

Proximal convoluted tubule:

“Reabsorption of solute in the proximal tubule results in the reabsorption of a proportional amount of water.” Therefore, the proximal tubule cannot be said to produce dilute or concentrated tubular fluid. Furthermore, fluid that enters the proximal convoluted tubule is isosmotic. Therefore, fluid that exits the proximal convoluted tubule must also be iso-osmotic.

Descending limb of the loop of Henle:

Isosmotic fluid enters the descending limb of the loop of Henle, and is rapidly concentrated. 1200 milliosmolar in the presence of ADH, 600 milliosmolar in the absence of ADH. The difference here is due to the increased amount of urea in the medullary interstitial space, increasing the osmotic pressure.

Ascending limb of the loop of Henle:

The ascending limb of the loop of Henle is not permeable to water, but it is permeable to solutes. Sodium chloride, for example, is pumped out very quickly. I like to think that the speed of this transporter is why we see an osmolality of 140 whether we are in the presence of ADH or not.

Distal convoluted tubule / Cortical collecting duct:

The distal tubule and cortical collecting duct reabsorb sodium chloride (NaCl goes into the interstitium). These sections are only permeable to water in the presence of ADH. Therefore, in the absence of ADH, the intertubular fluid here becomes even more hypoosmotic, because sodium chloride is reabsorb without water.

Inner medullary collecting duct:

The inner medullary collecting duct actively reabsorbs NaCl. Even in the absence of ADH, some water will leave the medullary collecting duct. This is when the difference between concentrated and un-concentrated urine is the most significant, and we see and osmolality of 1200 in the presence of ADH, and and osmolality of 50 in the absence of ADH.

Question 91

7. Describe the actions of diuretics on the ability of the kidneys to maximally concentrate and dilute urine.

Answer 91

Diuretics decrease the ability of the kidneys to maximally concentrate urine by inhibiting the sodium / potassium / chloride transporters that the nephron would use to establish an increase of osmolality of the medullary interstitium.

However, diuretics would increase the ability of the kidneys to dilute urine, as the activity of the solute transporters would be diminished, but the activity of ADH would be unaffected. This would be useful in cases where the patient needs to release water in greater amounts than the kidney is normally capable of, such as in cases of edema.