Lecture 3: Adjusting Urine Concentration

Question 1

Osmolarity of the urine depends on what 2 factors?

Answer 1

1) Water content in the body

2) ADH (vasopressin)

Question 2

What are the 2 cells types found in the distal tubule and collecting duct?

Answer 2

1) Principal Cells

2) Intercalated Cells

Question 3

Principle cells reabsorb and secrete what?

Answer 3

Reabsorb: Na+, Cl- and H2O

Secrete: K+

Question 4

Intercalated cells reabsorb and secrete what?

Answer 4

Reabsorb: K+

Secrete: H+

Question 5

How is Na+, Cl-, and H2O reabsorbed by Principal cells?

Answer 5

• Na+ passively diffuses across apical membrane and then is actively transported by the Na-K+ ATPase
• Cl- occurs passively via the paracellular pathway
• H2O reabsorption occurs in response to the effect of ADH on the principal cells.

Question 6

What stimulates the intercalated cells to increase H+ secretion?

Answer 6

Aldosterone by stimulating the H+-ATPase

Question 7

How much and through what method is urea reabsorbed in the proximal tubules?

Answer 7

5% of the filtered urea is reabsorbed passively

Question 8

The proximal straight tubule, descending thin segment, and ascending thin segments of nephron receive urea how?

Answer 8

By diffusion (tubular secretion) from the interstitium of renal medulla in which urea is present in high concentrations

Question 9

What areas of the nephron are impermeable to urea?

Answer 9

Thick ascending limb and DCT

Question 10

Where are large amount of urea reabsorbed and through which method?

Answer 10

Inner medullary collecting duct (IMCD) by utilizing a specialized urea transport protein (UT-A)

Question 11

What stimulates the UT-A and therefore increases the permeability of the IMCD to urea?

Answer 11

ADH

Question 12

The thick ascending limb can maintain a difference of what osmolarity between the tubular fluid and interstitium at any point along its length?

Answer 12

200 mOsmol/kgH2O

Question 13

What is the tonicity of the fluid leaving the loop of Henle?

Answer 13

Hypotonic

Question 14

4 major factors that contribute to the buildup of solute concentration into the renal medulla are?

Answer 14

1) Ions actively transported from collecting duct –> interstitium
2) Facilitated diffusion of large amounts of urea from inner medullary collecting ducts —> interstitium
3) Only a small amount of water diffuses into the interstitium
4) Active transport of Na+ and co-transport of K+, Cl-, and other ions out of the ascending limb into the interstitium

Question 15

Explain how to osmolarity is able to increase and then decrease through the loop of henle?

Answer 15

The descending portion is permeable to water so as water diffuses out and ions stay in tubule the osmolarity increases to its maximum near the inner most part of the medulla. Ascending portion is permeable to ions, but not water so the osmolarity decreases as you go back toward the cortex.

Question 16

How does the countercurrent multiplier work?

Answer 16

1) Fluid first moves through all w/ 300 mOsM, but this is not ideal for creating concentrated urine
2) The ascending part will create difference of 200 mOsm by pumping NaCl out, making the interstitial fluid hypertonic
3) Now water in the descending part will move passively toward the higher osmolarity of the interstitium, equilibrating the descending and interstitium.

Question 17

Give the most basic explanation the countercurrent multiplier?

Answer 17

1) Ascending portion creates a 200 mOsM difference
2) Descending portion equilibrates its concentration w/ that of interstitium
3) Ascending portion pumps out more NaCl creating a difference, establishing a gradient
4) Cycle continues until concentration in the descending limb reaches max concentration of 1200-1400 mOsm/L

Question 18

How does the renal medullary blood flow (vasa recta) contribute to the preservation of the high solute concentration?

Answer 18

Medullary blood flow is low, and this sluggish blood flow is sufficient to supply the metabolic needs of the tissue and helps to minimize solute loss from the medullary interstitium

Question 19

What is the function of the vasa recta?

Answer 19

Serves as countercurrent exchangers, minimizing washout of solutes from the medullary interstitium

Question 20

How does osmolarity of fluid change from the beginning of the distal tubule to the end of the distal tubule?

Answer 20

Start: Is hypotonic (about 100 mOsM/L)

End: Reaches 300 mOsM/L

Question 21

What happens to filtrate as it moves from the end of the distal tubule into the collecting duct?

Answer 21

The tubular fluid is exposed to even higher osmolarity in the surrounding medullary, causing the loss of more H2O as it progresses

Question 22

How does vasopressin affect the tubular fluid osmolarity?

Answer 22

The tubular fluid will lose more H2O and can become concentrated up to 1200 mOsm/L by the end of the collecting duct

Question 23

How does water excess affect the concentration of tubular fluid as it moves from the distal tubule into the collecting duct?

Answer 23

In water excess there will be no vasopressin present, so water will not be able to move out of the distal tubule or the collecting duct, but the excretion of wastes and urinary solutes will remain constant. The net result is a large volume of dilute urine. Urine osmolarity may be as low as 100 mOsm/L the same as it was in the distal tubule.

Question 24

Obligatory urine volume calculation for 70kg human?

Answer 24

• 70kg human must excrete 600 mOsm of solutes/day
• Using maximum concentrating ability of 1200 mOsm/L

600 mOsm/Day / 1200 mOsm/L = 0.5 L/Day

Question 25

If less than 0.5 L/day is excreted this is called? What is the range per hour for infants, children, and adults?

Answer 25

Oliguria

<1 mL/kg/h in infants
<0.5 mL/kg/h in children
<0.3 mL/kg/h in adults

Question 26

Fractional excretion of Na+ calculation

Answer 26

FENa+ = urinary sodium excretion rate / the filtered load

• FENa+ = (200/24,100) x 100 = 0.83%

Question 27

The kidney must typically excrete how much Na+/day?

Answer 27

200 mEq/day

Question 28

Discuss Uosm:Posm >1, 1, and <1

Urine osmolality:Plasma osmolality

Answer 28

> 1 = kidneys are able to concentrate urine

1 = water and solute are being excreted in a state iso-osmotic with plasma

<1 = kidneys are able to dilute urine

Question 29

What does a positive and negative free water clearance indicate?

Answer 29

Positive = excess water is being excreted by kidneys

Negative = excess solutes are being removed from the blood by the kidneys and water is being conserved

Question 30

Whenever urine osmolarity is greater than plasma osmolarity, what will the free water clearance be?

Answer 30

Negative! Indicating water conservation

Question 31

Calculation for free water clearance?

Answer 31

V - (Uosm x V)/(Posm)