Mechanisms to adjust urine concentration

Question 1

Renal control of salt and water balance

-Normal function of these mechanisms allows?

Answer 1

• Water retention during dehydration
• Excretion of dilute urine when well hydrated
• Sodium excretion when blood pressure rises (Na retention when BP falls)

Question 2

Failure of renal control of salt and water balance can cause?

Answer 2

• Edema
• Hyper/hypokalemia
• Undesirable changes in BP
• Acid/base disorders
• Neurological problems-shrinking/swelling of brain

Question 3

Sodium reabsorption mechanisms

-Proximal tubule?

Answer 3

• Cotransport with glucose, aas, and phosphate

- Countertransport with H

Question 4

Sodium reabsorption mechanisms

-Thick ascending limb?

Answer 4

Na, K, 2Cl cotransport

Question 5

Sodium reabsorption mechanisms

-Early DCT?

Answer 5

-Na, Cl cotransport

Question 6

Sodium reabsorption mechanisms

-Late DCT, CD?

Answer 6

Luminal Na membrane channels

Question 7

Water and chloride follow sodium

-Water reabsorption?

Answer 7

• Always passive, can be transcellular or paracellular

- Follows osmotic gradients established by reabsorption of sodium

Question 8

Water and chloride follow sodium

-chloride reabsorption?

Answer 8

• Always linked, either directly or indirectly, to Na reabsorption (Cl can balance the pos and neg charges)
• Different mechanisms in different segments

Question 9

Loop of Henle-Descending limb

• Freely permeable to?
• Impermeable to?

Answer 9

• Freely permeable to water

- Impermeable to Na, Cl

Question 10

Loop of Henle-Ascending limb

• Always impermeable to?
• Thin segment?
• Thick segment?
• Referred to as the?

Answer 10

• Always impermeable to water
• Thin segment-NaCl reabsorption mechanism is controversial
• Thick segment-active Na, K, 2Cl cotransport
• Referred to as the “diluting segment”

Question 11

Positive potential in tubular urine

-Function?

Answer 11

Drives the reabsorption of solutes-if it becomes less positive, the solute reabsorption will decrease

Question 12

Late DCT and CD

-Major site of?

Answer 12

-Major site of physiological control of salt and water balance

Question 13

Late DCT and CD

-Aldosterone-function?

Answer 13

Stimulates Na reabsorption and secretion of K and H

Question 14

Late DCT and CD

-ANP-function?

Answer 14

Inhibits Na reabsorption (medullary CD)

Question 15

Late DCT and CD

-ADH-function?

Answer 15

Stimulates water reabsorption

Question 16

Water permeability of CD is physiologically controlled

-Well-hydrated individuals?

Answer 16

• CD is impermeable to water

- Water remains in tubular lumen, dilute urine excreted

Question 17

Water permeability of CD is physiologically controlled

-Dehydrated individuals?

Answer 17

• CD is highly permeable to water

- Water is reabsorbed, low volume of concentrated urine is excreted

Question 18

ADH MOA?

Answer 18

ADH increases water permeability (reabsorption) of late DT/CD via V2 receptors and insertion of aquaporin channels

Question 19

Solute concentrations in peritubular interstitium

-Inner medullary interstitial fluid?

Answer 19

Very high solute concentration

Question 20

Countercurrent multiplier mechanism

-Function?

Answer 20

• Concentrates solute in medullary interstitium
• High solute concentrations enables kidneys to excrete highly concentrated urine, conserve water during periods of dehydration

Question 21

Countercurrent Multiplier Mechanism

-Requires the integrated function of 3 components?

Answer 21

• Descending, ascending limbs of loop of Henle
• Vasa recta capillaries
• Collecting ducts

Question 22

Components of Countercurrent Multiplier

Answer 22

• Na gradient that the Na, K, Cl cotransporter can establish in TAL
• Interstitium becomes hyperosmolar and pulls water out of the descending limb
• Urine in descending limb is concentrated
• Process repeats

Question 23

Components of Countercurrent Multiplier

-Augmented by?

Answer 23

Augmented by action of ADH in CD

Question 24

Get half of the gradient from Na-K-Cl cotransporter

-The other half comes from?

Answer 24

Get the other half of the gradient from from reabsorption of urea

• Recycling concentrates urea in the inner medulla
• Reabsorption of urea is promoted by ADH from medullary CD

Question 25

Role of urea in the coutercurrent mechanism | -In the presence of ADH (cortical collecting tubule)?

Answer 25

- In the presence of ADH, water but NOT urea is reabsorbed in the cortical collecting tubule - Results in an increase in the tubular fluid urea concentration

Question 26

Role of urea in the coutercurrent mechanism | -In the presence of ADH (medullary collecting tubule)?

Answer 26

- In the presence of ADH, more water but NOT urea is reabsorbed in the medullary collecting tubule, further raising the concentration - The inner medullary collecting tubule is relatively permeable to urea - As a result, urea passively diffuses into the interstitium, increasing the interstitial osmolality

Question 27

Countercurrent multiplier - Vasa recta function? - What feature allows them to do this?

Answer 27

- Maintain solute gradient - Low blood flow allows them to equilibrate with surrounding interstitium-allows waste to be taken out but keep what the body needs - Water and NaCl are exchanged between descending and ascending limbs - Solute gradient is maintained while small amounts of NaCl and water are returned to systemic circulation

Question 28

Countercurrent mechanism review figure

Answer 28

slide 26

Question 29

Antidiuresis-high ADH - Effect on the collecting duct? - Characteristics of urine excreted?

Answer 29

- ADH makes the collecting duct epithelium highly water permeable - Water is reabsorbed in this segment and a LOW VOLUME, HIGHLY CONCENTRATED URINE is excreted

Question 30

Diuresis-Low ADH - Effect on the collecting duct? - Characteristics of urine excreted?

Answer 30

- Collecting duct epithelium is impermeable to water | - HIGH VOLUME OF DILUTE URINE is excreted

Question 31

Diuresis-Low ADH | -Solute concentrations in the medullary interstitium?

Answer 31

LOW

Question 32

Osmolar clearance

Answer 32

Cosm = (Uosm x V)/Posm

Question 33

Osmolar clearance | -What happens to Cosm, when kidneys excrete excess solute?

Answer 33

Cosm increases (decreases when solute is retained)

Question 34

* **Free water clearance*** | - Definition?

Answer 34

Excretion of water in excess of amount needed to excrete isosmotic urine, i.e. excretion of solute-free urine by the kidneys

Question 35

* **Free water clearance*** | - Equation?

Answer 35

Cwater = V - Cosm

Question 36

Free water clearance | -What is Cwater if Uosm is less than Posm?

Answer 36

Cwater is positive-pure water is cleared from the body (opposite if Uosm is greater than Posm)

Question 37

Does ADH affect Cwater? If so, how?

Answer 37

Yes, it would decrease free water clearance | -water reabsorption is increased

Question 38

Fractional excretion | -Definition?

Answer 38

The fraction (percentage) of the filtered load of a substance that is excreted in urine

Question 39

Fractional excretion | -Equation?

Answer 39

(Ux x Pcr)/(Px x Ucr)

Question 40

Normal fractional excretion?

Answer 40

1%

Question 41

Fractional excretion - If fractional excretion is below 1% what does it suggest about what the problem might be? - What does it say about sodium?

Answer 41

- It suggests the problem is prerenal and acute glomerulonephritis - Na avidly reabsorbed

Question 42

Fractional excretion | -If fractional excretion is above 2% what does it suggest about what the problem might be?

Answer 42

-Acute tubular necrosis, renal

Question 43

Case presentation

Answer 43

at the end

Question 44

Renal control of salt and water balance | -Crucial for regulation of?

Answer 44

- Blood pressure - Extracellular fluid concentration - Concentrations of Na and K in body fluids

Question 45

Why is the ascending limb referred to as the diluting segment?

Answer 45

Because it is impermeable to water so the water stays in the tubule while Na and Cl are pumped out

Question 46

Which part of the countercurrent multiplier mechanism is inhibited by loop diuretics?

Answer 46

The Na-K-Cl cotransporter

Question 47

What if a drug was given that increases blood flow to peritubular capillaries?

Answer 47

Concentrating ability of the kidneys is decreased (medullary interstitial osmolarity gradient is abolished)