6 – Reabsorption in Different Segments of a Nephron

Question 1

Reabsorption in proximal tubule:

Answer 1

-around 65% of Na+, water, chloride, bicarbonate, and potassium are reabsorbed
-epithelial cells are adapted for high reabsorption capacity

Question 2

How are the epithelial cells of the proximal tubules adapted for high reabsorption capacity?

Answer 2

1. Large number of mitochondria (energy for active mechanisms)
2. Enhance SA (extensive brush borders)
3. Abundance of carrier molecules

Question 3

Reabsorption in 1st half of proximal tubule:

Answer 3

-glucose and AA co-transported with sodium

Question 4

Reabsorption in 2nd half of proximal tubule:

Answer 4

-more chloride is reabsorbed due to increased Cl concentration
>paracellular diffusion

Question 5

Sodium and chloride are reabsorbed in proximal tubule but the concentration in the tubule remains the same, why?

Answer 5

-because water is also highly reabsorbed in proximal tubule
*absorbed at the same rate=osmolarity remains the same

Question 6

Organic acids consist of:

Answer 6

-bile salts
-oxalate
-urate
-catecholamines

Question 7

Reabsorption/secretion in proximal tubule of organic acids:

Answer 7

-secreted into proximal tubules
-removed quickly

Question 8

What are some other things that are also secreted in the proximal tubules?

Answer 8

-drugs
-toxins

Question 9

How does the concentration of waste products (creatine and urea) change in the proximal tubule?

Answer 9

-get more concentrated due to LOW or NO reabsorption and reduction in water volume

Question 10

What are the 3 segments in the loop of Henle?

Answer 10

1. Thin descending
2. Thin ascending
3. Thick ascending

Question 11

What do thin segments (descending and ascending) of the loop of Henle contain?

Answer 11

-thin epithelium
-no brush borders
-few mitochondria

Question 12

Reabsorption in the thin descending segment of Loop of Henle:

Answer 12

-highly permeable to water (20% water reabsorption)
-medium permeability to solutes (including Na)
-no calcium, magnesium, and bicarbonate reabsorption

Question 13

Reabsorption in the thin ascending segment of Loop of Henle:

Answer 13

-no permeability to water
-low calcium, magnesium, and bicarbonate reabsorption

Question 14

What are the ‘characteristics’ of thick segment of Loop of Henle:

Answer 14

-thick epithelium
-high metabolic activity

Question 15

Reabsorption in the thick ascending segment of Loop of Henle:

Answer 15

-no permeability of water
-Na, K, and Cl reabsorption
-high calcium, magnesium and bicarbonate reabsorption
*where tubular content becomes diluted=dilutes urine (due to do reabsorption of water and only reabsorption of water)

Question 16

What is the reabsorption capacity of the 3 segments of the Loop of Henle?

Answer 16

-thick ascending > thin ascending > thin descending

Question 17

What are the transporters on the luminal side in the thick ascending segment?

Answer 17

-co-transporter for 1Na+, 1K, 2Cl- (leaving the lumen)
-H+ secretion via counter transporter of H+ and Na+ (H+ into lumen, Na+ out of lumen)

Question 18

What are the transporters and channels on the basolateral side in the thick ascending segment?

Answer 18

-K channels (also on luminal side=contributes to slight positive charge in tubular lumen)
-Cl channel (K+ out)
-Na/K ATPase (Na out, K in)

Question 19

Slight positive charge on tubular side compared to interstitial fluid in the thick ascending segment:

Answer 19

*facilitates diffusion of Ca and Mg through paracellular space
>also other solutes with a positive charge (ex. Na, K)

Question 20

What are ‘loop’ diuretics?

Answer 20

-inhibit Na-Cl-K cotransporter=inhibits their reabsorption in this segment
>leads to less water being reabsorbed so more water is excreted
Ex. furosemide

Question 21

Early distal tubule reabsorption:

Answer 21

-contains macula densa (regulating GFR)
-similar to thick ascending loop of Henle
>water impermeable
>reabsorption of Na, K, Cl, Ca, Mg
*Na/Cl cotransport is important (about 5% of Na-Cl reabsorbed here)

Question 22

Late distal tubule and collecting tubule contains 2 main cell types:

Answer 22

1. Principal cells
2. Intercalated cells

Question 23

Reabsorption in late distal tubule and collecting tubule:

Answer 23

-principal cells: reabsorb Na and secrete K
-intercalated cells: reabsorb K and HCO3 AND secrete H+

Question 24

What controls the function of principal cells?

Answer 24

-aldosterone
-K+ concentration in body fluids

Question 25

Aldosterone:

Answer 25

-stimulated by hyperkalemia
-receptors on principal cells
*increases activity of Na/K pumps=more K+ secreted into tubule

Question 26

Aldosterone antagonists:

Answer 26

-interfere with Na reabsorption in late distal tubules and cortical collecting tubules
*more Na and water is excreted

Question 27

ADH:

Answer 27

-antidiuretic hormone (aka vasopressin)
-controls permeability of late distal and cortical collecting tubules to water
*important for controlling dilution and/or concentration of urine

Question 28

Reabsorption/secretion in collecting duct:

Answer 28

-less than 10% of Na and water reabsorption
>important since it is the final stage
-less metabolically active cells
-water permeability regulated by ADH
-cells are permeable to urea
-bicarbonate reabsorbed
-secretes H+