Lecture #17: Urine Concentration and Dilution

Question 1

What is the maximum urine concentration that the kidneys can produce?

Answer 1

The kidney can produce a maximal urine concentration of 1200 to 1400 mOsm/L.

Question 2

What is required by the kidney tubules for the formation of a concentrated urine?

Answer 2

Requirements for forming a concentrated urine:
> presence of ADH
> high osmolarity of renal medullary interstitial tubule
- establishes osmotic gradient necessary for water reabsorption to occur.

Question 3

Explain why there is an obligatory volume of excreted urine of 0.5 liters per day.

Answer 3

> A normal 70-kg human must excrete about 600 mOsm of solute each day in order to get rid of waste products of metabolism and ions that are ingested.

> Maximal urine concentrating ability = 1200 mOsm/L

> 600 mOsm / day / 1200 mOsm/L = 0.5 L/day

**refer to text regarding determination of urine specific gravity, figure 29-3…….if needed.

Question 4

How do we excrete a dilute urine?

Answer 4

Ascending thick limb of Henle:
- sodium, potassium, chloride are avidly reabsorbed

 - this segment is impermeable to water 
 - tubular fluid becomes more dilute as it flows up the ascending loop of henle
 - osmolarity is about 100 mOsm/L at the early distal tubular segment
 * * note that whether ADH is present or not does not matter at this point!

Late distal convoluted tubule:
- additional reabsorption of sodium chloride

 - impermeable to water in absence of ADH 
 - osmolarity reaches 50 mOsm/L

Question 5

Where in the kidney tubule are most of the filtered electrolytes reabsorbed?

Answer 5

Proximal tubule reabsorbs about 65% of filtered electrolytes.

Question 6

Is the proximal tubule permeable/impermeable to water, is sodium chloride reabsorbed or not, permeable/impermeable to urea, and is it affected by ADH?

Answer 6

> Highly permeable to water

> sodium chloride is reabsorbed

> NOT affected by ADH

> permeable to urea

Question 7

What is the tubular osmolarity in the proximal tubule?

Answer 7

Tubular Osmolarity = 300 mOsm/L

Question 8

Is the descending loop of Henle permeable/impermeable to water, is sodium chloride reabsorbed or not, permeable/impermeable to urea, and is it affected by ADH?

Answer 8

> Permeable to water

> Reabsorbs sodium chloride

> Urea also diffuses into the ascending limb
- comes from urea absorbed into interstitium from collecting ducts.

> NOT affected by ADH

Question 9

Is the thick ascending loop of Henle permeable/impermeable to water, is sodium chloride reabsorbed or not, permeable/impermeable to urea, and is it affected by ADH?

Answer 9

> impermeable to water

> large amounts of sodium chloride, potassium, and other ions are actively transported from tubule into medullary interstitium

> impermeable urea

> NOT affected by ADH

Question 10

True or False:

The tubular fluid becomes more dilute in the thin ascending loop of Henle.

Answer 10

True

Question 11

What is the concentration in the thick ascending loop of Henle?

Answer 11

100 mOsm/L

Question 12

What is the tubular fluid concentration of filtrate in the early distal tubule?

Answer 12

50 mOsm/L

Question 13

What hormone determines the osmolarity of fluid in the late distal tubule and cortical collecting tubule?

Answer 13

Osmolarity of fluid depends on ADH

Question 14

Is urea very permeable in the late distal tubule and cortical collecting tubule?

Answer 14

Urea is not very permeable in the late distal tubule and cortical collecting tubule.

Question 15

What does the osmolarity of fluid depend on in the inner medullary collecting duct?

Answer 15

Osmolarity of fluid depends on ADH and surrounding intersitium osmolarity.

Question 16

What osmolarity is the concentration gradient limited to in the countercurrent multiplier system?

Answer 16

concentration gradient is limited to 200 mOsm/L because of paracellular diffusion of ions back into the tubule.

Question 17

True or False:

Descending tubules are impermeable to water and ascending tubules are permeable to water.

Answer 17

False - descending tubules are permeable to water and ascending tubules are impermeable to water

Question 18

Is the ascending loop of Henle and distal cortical collecting tubule impermeable to urea?

Answer 18

Yes

Question 19

What is the result if ADH concentration is increased in the cortical collecting tubule?

Answer 19

> water is reabsorbed from cortical collecting tubule

> urea is not very permeant here and becomes more concentrated in the tubule

Question 20

What is the response in the medullary collecting duct if ADH concentration is increased?

Answer 20

> more water is reabsorbed, resulting in a higher concentration of urea

> higher concentration of urine results in diffusion of urea out of duct into interstitial fluid

> facilitated by UT-A1 and UT-A3 (ADH-activated) transporters

Question 21

Explain how a high concentration of urea can be maintained in the collecting ducts even though urea is being reabsorbed.

Answer 21

> Role of Urea in Concentrating Urine

Simultaneous movement of water and urea out of the inner medullary collecting ducts maintains a high concentration of urea in the tubular fluid and, eventually, in the urine, even through urea is being reabsorbed.

Question 22

What does the countercurrent exchange in the vasa recta prevent from being dissipated?

Answer 22

Vasa recta do not create the medullary hyperosmolarity but they prevent it from being dissipated.

Question 23

During the formation of concentrated urine when ADH levels are high, what is reabsorbed in the inner medullary collecting ducts?

Answer 23

Water

Sodium Chloride

Urea

Question 24

During the formation of dilute urine when ADH levels are low, what is reabsorbed in the inner medullary collecting ducts?

Answer 24

Sodium Chloride

Question 25

Which areas of the tubule system is the absorption of water dependent on the presence of ADH?

Answer 25

> Late distal tubule

> Cortical portion of collecting tubule and duct

> Medullary portion of collecting tubule and duct

Question 26

Where specifically is ADH formed?

Answer 26

ADH is formed in magnocellular neurons in:
> supraoptic nuclei
> paraventricular nuclei

Question 27

What is the function of the osmoreceptor cells?

Answer 27

Osmoreceptor cells are in the vicinity of the AV3V region.
- anterior region of third ventricle.

** The osmoreceptor-feedback mechanism is for regulating the extracellular fluid.

Question 28

Listed the events that happen in the osmoreceptor-feedback mechansim if a water deficit is detected.

Answer 28

Water deficit

increase extracellular osmolarity

increase ADH secretion (from posterior pituitary)

increase plasma ADH

increase water permeability in distal tubules, collecting ducts

increase water reabsorption

decrease water excreted (which inhibits water deficit in beginning of mechanism).

Question 29

What happens in isotonic volume depletion?

Answer 29

The osmolarity remains the same but volume decreases

Question 30

What happens during isovolemic osmotic increase?

Answer 30

Volume remains the same but osmolarity increases.

Question 31

Compare and contrast the proximal tubule during the formation of concentrated urine when ADH levels are high and the formation of dilute urine when ADH levels are low.

Answer 31

Formation of Concentrated Urine when ADH levels are High:
–> sodium chloride, water, and urea are reabsorbed in the proximal tubule during high levels of ADH

Formation of Dilute Urine when ADH levels are Low:
–> sodium chloride and water are reabsorbed in the proximal tubule during low levels of ADH

Question 32

Compare and contrast the reabsorption in the distal tubule during the formation of concentrated urine when ADH levels are high and the formation of dilute urine when ADH levels are low.

Answer 32

Concentrated Urine - High ADH:
–> water and sodium chloride are reabsorbed

Dilute Urine - Low ADH
–> sodium chloride is reabsorbed

Question 33

Objective 3:

Describe how the counter current mechanism concentrates solute in the interstitium of a kidney’s medulla and the importance of this action.

Answer 33

Look over the Powerpoint presentation for lecture 17.

Question 34

How much water can be excreted by the kidneys per day when there is a large excess of water in the body?

Answer 34

When there is a large excess of water in the body, the kidneys can excrete as much as 20 liters per day with a concentration as low as 50 mOsm/L.