L5: Urinary Concentration and Dilution

Question 1

Describe the permeability of the descending limb of the LOH. What is happening to the concentration of the fluid inside the lumen? Is the fluid hyperosmotic or hyposmotic?

Answer 1

Very permeable to water (water leaving lumen to ISF) so water being reabsorbed
No active transport of NaCl
Fluid progressively concentrated as it moves down = hyperosmotic

Question 2

Describe the permeability of the ascending limb of the LOH. What is happening to the concentration of the fluid inside the lumen? Is the fluid hyperosmotic or hyposmotic?

Answer 2

Very permeable to NaCl (salt leaving lumen via active transport) so NaCl reabsorbed
Impermeable to water
Fluid progressively diluted as it moves up = hyposmotic

*remember it goes from thin to thick

Question 3

What is the largest osmotic gradient that can be maintained across the wall of the ascending limb?

Answer 3

200 mOsm/L

Question 4

Why is the LOH both countercurrent and a multiplier?

Answer 4

Countercurrent: fluid flows in opposing directions between 2 limbs
Multiplier: 200 mOsm/L gradient created by active transport has been multiplied to 1200 mOsm/L

Question 5

What is the primary transporter on the apical side of the thick ascending limb?

Answer 5

NKCC2 (all coming from lumen of nephron into cell)

Question 6

What are the 2 transporters on the blood (interstitium) side of the thick ascending limb?

Answer 6

Na/K ATPase [NaCl increasing on interstitial side)

KCl symporter

Question 7

What is the diuretic that blocks NKCC2? What stimulates NKCC2?

Answer 7

Diuretic: Furosemide (Lasix)

Stimulated by ADH

Question 8

What is occuring in the descending portion of the vasa recta? Ascending portion? Is this a passive or active process?

Answer 8

Descending: NaCl diffusing in; H20 diffusing out
Ascending: NaCl diffusing out; H20 diffusing in (makes the area very salty for the CD!)
Passive process! Protects ISF gradient for CD

Question 9

What is the primary transporter on the apical side of the early distal tubule?

Answer 9

NCC (NaCl transporter)

Question 10

What is the diuretic that blocks NCC?

Answer 10

Thiazide (chlorothiazide)

Question 11

Describe the permeability of the early distal tubule.

Answer 11

NOT permeable to water (referred to as diluting segment because water stays in fluid)

Question 12

Describe a principal cell. Where is this located?

Answer 12

Located in the late distal tubule and collecting duct

• Reabsorbs Na and secretes K
• Site of aldosterone action
• Site of ADH action

Question 13

Describe an alpha-intercalated cell. Where is this located?

Answer 13

Located in the late distal tubule and collecting duct
-Secretes H
-Reabsorbs K
*Have H-ATPase and H,K ATPase (H is moving into nephron lumen and K is being brought into cell or moving into ISF)
(remember how it kind of catches the K the principal cell got rid of)

Question 14

Na reabsorption and K secretion is dependent on _____.

Answer 14

Na/K ATPase in the basolateral membrane and apical channels for both ions

Question 15

What does aldosterone do?

Answer 15

In principal cell:

• Increases # of apical Na and K channels
• Increases Na/K ATPase activity

Question 16

What controls water and urea permeability in the collecting duct?

Answer 16

Vasopressin (ADH)

Question 17

Describe the permeability of the collecting duct.

Answer 17

Can be high OR low. Depends on:

• Aquaporins in apical membranes
• ADH causing insertion of aquaporins

Question 18

What does ADH do?

Answer 18

Increases permeability of apical membrane to water by inserting new aquaporins

Question 19

Describe what happens in the CD if there is a high level of ADH.

Answer 19

High ADH = response to body needing more water
Body keeps the water by opening the aquaporins and reabsorbing water
By the end of the CD, the fluid is at 1200 (more equilibrated to ISF) and very concentrated

Question 20

Describe what happens in the CD if there is a low level of ADH.

Answer 20

Low ADH = body has enough water, can pee some out
CD becomes impermeable to H20
By the end of the CD, the fluid is at 75 (compared to 600 in the ISF) and very dilute

Question 21

Where does 60-80% of NaCl and water reabsorption take place? Describe the osmolarity of the fluid in this area.

Answer 21

Proximal tubule

Fluid is isosmotic

Question 22

Describe the osmolarity in all parts of the nephron starting at the proximal tubule.

Answer 22

PT = Isosmotic
Descending limb = Hyperosmotic
Ascending limb = Hyposmotic
Descending limb = Remains hyposmotic because impermeable to water
Late distal tubule/CD = If (+)ADH, water reabsorption is HIGH and fluid is hyperosmotic; If (-)ADH, water reabsorption is inhibited and fluid becomes hyposmotic