Tubular reabsorption and secretion - PCT Flashcards
How do reabsorption and secretion compare to filtration?
Reabsorption is very selective whereas filtration is not
How does the amount of reabsorption compare to the amount of excretion?
Much more reabsorption occurs than excretion
What are the different routes by which substances can be reabsorbed?
Transcellular uptake
Paracellular uptake
What is transcellular uptake?
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
What is paracellular uptake?
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
How are substances transported from the interstitial fluid, across the peritubular capillary membrane into the capillary?
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
Where does most reabsorption occur in the nephron?
Proximal convoluted tubule
What is the main primary active transporter in the proximal convoluted tubule?
Na+ K+ ATPase
Where is the Na+ K+ ATPase located on the epithelial cell in the proximal convoluted tubule?
Basolateral domain
What does the Na+ K+ ATPase in the proximal convoluted tubule to?
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
What is the electrochemical gradient of sodium ions in the proximal convoluted tubule?
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
What is the importance of the electrochemical gradient of sodium ions in the proximal convoluted tubule?
It is used in reabsorption of other substances by secondary active transport
What are the different parts of the proximal convoluted tubule?
S1 segment
S2 segment
S3 segment
What is reabsorbed in the S1 segment?
Sodium ions
Water
Bicarbonate ions
Glucose
Amino acids
Phosphate
How are bicarbonate ions reabsorbed in the S1 segment?
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
How is glucose reabsorbed in the S1 segment?
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
How much of the filtered glucose is reabsorbed in the proximal convoluted tubule?
100%
How are amino acids reabsorbed in the S1 segment?
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
How much of the filtered amino acids are reabsorbed in the proximal convoluted tubule?
100%
What is reabsorbed in the S2-S3 segments?
Sodium ions
Water
Chloride ions
How are chloride ions reabsorbed in the S2-S3 segments?
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
How are sodium ions reabsorbed in the proximal convoluted tubule?
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
How much of the filtered sodium ions are reabsorbed in the proximal convoluted tubule?
67%
How is water reabsorbed in the proximal convoluted tubule?
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
How much of the filtered water is reabsorbed in the proximal convoluted tubule?
65%
How does the concentration of bicarbonate ions, glucose and amino acids change along the length of the proximal convoluted tubule? Why?
Concentrations decrease
because they are reabsorbed more rapidly than water is
How does the concentration of sodium ions change along the length of the proximal convoluted tubule? Why?
Remains constant
because water reabsorption keeps up with sodium ion reabsorption
How does the osmolality change across the length of the proximal tubule? Why?
Remains constant
because sodium ion and water reabsorption occur together
move an isoosmotic solution from the tubular lumen into the interstitial fluid
