Tubular reabsorption and secretion - PCT

Question 1

How do reabsorption and secretion compare to filtration?

Answer 1

Reabsorption is very selective whereas filtration is not

Question 2

How does the amount of reabsorption compare to the amount of excretion?

Answer 2

Much more reabsorption occurs than excretion

Question 3

What are the different routes by which substances can be reabsorbed?

Answer 3

Transcellular uptake

Paracellular uptake

Question 4

What is transcellular uptake?

Answer 4

Need to be transported from the tubular lumen, across the apical domain, into the epithelial cell

Then from the cell, across the basolateral domain into the interstitial fluid

Then from the interstitial fluid, across the peritubular capillary membrane, into the peritubular capillary

Question 5

What is paracellular uptake?

Answer 5

From the tubular lumen, between the epithelial cells across the tight junctions, into the lateral intercellular spaces, into interstitial fluid

Then from the interstitial fluid, across the peritubular capillary membrane, into the peritubular capillary

Question 6

How are substances transported from the interstitial fluid, across the peritubular capillary membrane into the capillary?

Answer 6

By the high hydrostatic pressure in the interstitial fluid pushing it into the peritubular capillary

Also by the high oncotic pressure of the peritubular capillary drawing fluid into it

Question 7

Where does most reabsorption occur in the nephron?

Answer 7

Proximal convoluted tubule

Question 8

What is the main primary active transporter in the proximal convoluted tubule?

Answer 8

Na+ K+ ATPase

Question 9

Where is the Na+ K+ ATPase located on the epithelial cell in the proximal convoluted tubule?

Answer 9

Basolateral domain

Question 10

What does the Na+ K+ ATPase in the proximal convoluted tubule to?

Answer 10

Actively transports 3 sodium ions from the epithelial cell into interstitial fluid

Actively transports 2 potassium ions from the interstitial fluid into the epithelial cell

Question 11

What is the electrochemical gradient of sodium ions in the proximal convoluted tubule?

Answer 11

High concentration of sodium ions in tubular lumen, positive charge in tubular lumen

Low concentration of sodium ions in epithelial cell, negative charge in epithelial cell

Question 12

What is the importance of the electrochemical gradient of sodium ions in the proximal convoluted tubule?

Answer 12

It is used in reabsorption of other substances by secondary active transport

Question 13

What are the different parts of the proximal convoluted tubule?

Answer 13

S1 segment

S2 segment

S3 segment

Question 14

What is reabsorbed in the S1 segment?

Answer 14

Sodium ions

Water

Bicarbonate ions

Glucose

Amino acids

Phosphate

Question 15

How are bicarbonate ions reabsorbed in the S1 segment?

Answer 15

Sodium bicarbonate in the tubular lumen dissociated into sodium ion and bicarbonate ion
NHE on apical domain of epithelial cell
Moves sodium ions down their concentration gradient from the lumen into the cell, whilst moving hydrogen ions against their concentration from the cell into the lumen, so is secondary active transport
Hydrogen ions in the lumen combine with bicarbonate ions in the lumen to form carbonic acid
Carbonic anhydrase converts carbonic acid into carbon dioxide and water
Carbon dioxide and water move from the lumen into the cell by simple diffusion
Carbonic anhydrase converts carbon dioxide and water into carbonic acid
Carbonic acid dissociates into bicarbonate ion and hydrogen ion
Na-HCO3- on basolateral domain of epithelial cell
Moves sodium ion and bicarbonate ion fromthe cell into the interstitial fluid

Question 16

How is glucose reabsorbed in the S1 segment?

Answer 16

SGLT on apical domain of epithelial cell
Moves sodium from tubular lumen into cell down its concentration gradient, also moves glucose from lumen into cell but against its concentration gradient, so is secondary active transport
GLUT2 transporter on basolateral domain of cell
Moves glucose from cell into intersitital fluid down its concentration gradient, so is facilitated diffusion

Question 17

How much of the filtered glucose is reabsorbed in the proximal convoluted tubule?

Answer 17

100%

Question 18

How are amino acids reabsorbed in the S1 segment?

Answer 18

Sodium-dependent amino acid trasporter on apical domain of epithelial cell
Moves sodium from tubular lumen into cell down its concentration gradient, also moves amino acid from lumen into cell but against its concentration gradient, so is secondary active transport
Amino acid transporter on basolateral domain of cell
Moves amino acid from cell into interstitial fluid down its concentration gradient, so is facilitated diffusion

Question 19

How much of the filtered amino acids are reabsorbed in the proximal convoluted tubule?

Answer 19

100%

Question 20

What is reabsorbed in the S2-S3 segments?

Answer 20

Sodium ions

Water

Chloride ions

Question 21

How are chloride ions reabsorbed in the S2-S3 segments?

Answer 21

Sodium ion reabsorption in S1 segment makes tubular lumen more negatively charged in S2-S3 segments
Ions and molecules and water reabsorption in S1 segment makes chloride ions more concentrated in lumen in S2-S3 segments
Chloride ions move through paracellular route from lumen to interstitial fluid, down its electrochemical gradient, so by passive diffusion

Question 22

How are sodium ions reabsorbed in the proximal convoluted tubule?

Answer 22

In the S1 segment of the proximal convoluted tubule
NHE, SGLT, sodium-dependent amino acid transporters on apical domain of epithelial cells
Move sodium from tubular lumen into cell down its concentration gradient

In the S2-S3 segments of the proximal convoluted tubule, moves with chloride ions through paracellular route from lumen to interstitial fluid, down its concentration gradient, so by passive diffusion

Question 23

How much of the filtered sodium ions are reabsorbed in the proximal convoluted tubule?

Answer 23

67%

Question 24

How is water reabsorbed in the proximal convoluted tubule?

Answer 24

As sodium ions move from the tubular lumen into the epithelial cell into the interstitial fluid and then into the peritubular capillary
The osmolality increases in the direction of movement of sodium ions
Higher osmolality draws water in by osmosis
Water moves from the tubular lumen into the interstitial fluid by the paracellular route mostly

But aquaporin1 channel on apical domain of epithelial cell
Water moves through it from tubular lumen into the epithelial cell
Aquaporin1 channel on apical domain of epithelial cell
Water moves through it from the cell into the interstitial fluid

Question 25

How much of the filtered water is reabsorbed in the proximal convoluted tubule?

Answer 25

65%

Question 26

How does the concentration of bicarbonate ions, glucose and amino acids change along the length of the proximal convoluted tubule? Why?

Answer 26

Concentrations decrease

because they are reabsorbed more rapidly than water is

Question 27

How does the concentration of sodium ions change along the length of the proximal convoluted tubule? Why?

Answer 27

Remains constant

because water reabsorption keeps up with sodium ion reabsorption

Question 28

How does the osmolality change across the length of the proximal tubule? Why?

Answer 28

Remains constant
because sodium ion and water reabsorption occur together
move an isoosmotic solution from the tubular lumen into the interstitial fluid