Renal V: Tubular Functions Along Nephron

Question 1

Where is the only place in the nephron in which sodium is NOT reabsorbed?

Answer 1

thin descending limb

Question 2

Where is the majority of Na+ reabsorbed in the nephron?

Answer 2

• PT (67%)

- TAL (25%)

Question 3

Excretion of Na+ is ____% of filtered load

Answer 3

less than 1

Question 4

How are the mechanisms for Na+ reabsorption in early and late proximal tubules different?

Answer 4

• early: Na+ reabsorbed primarily w/ HCO3- and organic solutes (ex: glucose, amino acids)
• late: Na+ reabsorbed primarily w/ Cl-

Question 5

Moving along the proximal tubule, why do the values for [TF/P]Na+ and [TF/P]osmolarity both remain at 1?

Answer 5

Na+ and total solute are reabsorbed in proportion to water (isosmotic reabsorption)

Question 6

How do [TF/P]glucose, [TF/P]aa’s, and [TF/P]HCO3- change along the early proximal tubule?

Answer 6

they fall below 1 because reabsorption of these substances exceeds water reabsorption

Question 7

Describe glomerulotubular balance.

Answer 7

It is the major regulatory mechanism of the PT, and it describes the balance b/t filtration (at glomerulus) and reabsorption (at PT). It ensures that a constant fraction of the filtered load is reabsorbed by the PT despite variation in GFR. It occurs by mechano-transduction of the microvilli of PT cells.

Question 8

What is the normal fraction of the filtered load that is reabsorbed by the PT?

Answer 8

67%

Question 9

Do the thin limbs have active or passive permeability properties?

Answer 9

passive

Question 10

Describe the permeability of the thin descending limb.

Answer 10

passive H2O reabsorption, therefore Na+ and Cl- impermeable

Question 11

Describe the permeability of the thin ascending limb.

Answer 11

passive Na+ and Cl- reabsorption, therefore H2O impermeable

Question 12

Does the thick ascending limb have active or passive permeability properties?

Answer 12

active

Question 13

Describe the permeability of the thick ascending limb.

Answer 13

It reabsorbs ~25% of the filtered Na+ via active transport, and it is impermeable to water.

Question 14

Why is the thick ascending limb called the diluting segment?

Answer 14

NaCl is reabsorbed but water is not reabsorbed with it, as the thick ascending limb is impermeable to water. Thus, there is more H2O and less Na+ in the thick ascending limb, so the tubular fluid is more dilute.

Question 15

Which part of the nephron is referred to as the diluting segment?

Answer 15

thick ascending limb

Question 16

What is the site of action of loop diuretics?

Answer 16

thick ascending limb

Question 17

Do loop diuretics act on the PT? Why or why not?

Answer 17

No, because we don’t want to interfere with the 67% reabsorption that happens at the PT (and loop diuretics inhibit reabsorption)

Question 18

What effect do loop diuretics (like furosemide) have?

Answer 18

They act at the thick ascending limb and inhibit Na+ reabsorption, resulting in excretion of 25% of the filtered Na+.

Question 19

Describe the permeability of the early distal tubule.

Answer 19

It is impermeable to water (like the thick ascending limb), and it reabsorbs ~5% of the filtered Na+.

Question 20

How much filtered sodium do the late distal tubule and collecting duct reabsorb?

Answer 20

only 3%

Question 21

What are the major cell types that line the late distal tubule and collecting duct?

Answer 21

principal cells and intercalated cells (alpha and beta)

Question 22

What are some main differences b/t the early DT and the thick ascending limb?

Answer 22

The early distal tubule is…

• a NaCl co-transporter (transports 1 Na+ and 1 Cl-, so 2 ions that make the process electroneutral)
• inhibited by thiazide diuretics (organic acids)

Question 23

The luminal membrane of principal cells contain which type of channels?

Answer 23

ENaC (epithelial Na channels)

Question 24

Where are principal cells found?

Answer 24

the late distal tubule and collecting duct

Question 25

ENaC’s are only found in ________.

Answer 25

principal cells

Question 26

Why does the late distal tubule and collecting duct have a high negative potential on the luminal side?

Answer 26

this is due to reabsorption of Na+ (Na+ is pretty much entirely depleted from the tubule by this point)

Question 27

Principal cells are involved with K+ ______, while alpha-intercalated cells are involved with K+ _______.

Answer 27

secretion; reabsorption

Question 28

What do K+ sparing diuretics do?

Answer 28

They inhibit K+ secretion by principal cells (due to inhibition of Na+ reabsorption).