Changes In Glomerular Filtrate In PCT And DCT

Changes In Glomerular Filtrate In PCT And DCT

Describe the changes undergone by the glomerular filtrate as it courses through the proximal and distal convoluted tubule.
Answer:

• When the glomerular filtrate flows through the tubular portion of nephron, both quantitative and qualititative changes occur.

Changes occurring in proximal convoluted tubule:

• The proximal convoluted tubule reabsorbs about 70 – 80% of the filtered and water and almost 100% of the filtered, amino acids and glucose.
• However, there is no change in the osmolarity of the filtrate.

1. Reasorption of sodium (Na+):

• About 80% of Na+ is actively reabsorbed in the proximal convoluted tubule, which is followed by a passive movement of H+.
• Initially Na+ is passively transported through apical membrane followed by active transport into the intercellular space.

2. Reabsorption of bicarbonate (HCO₃^–):

• About 90% of the filtered HCO₃^– is a reabsorbed in the proximal convoluted tubule.
• The secreted H⁺ reacts with filtered HCO₃ to form carbonic acid (H₂CO₃).
  H⁺+ HCO₃^– → H₂CO₃
• Now, carbonic acid dissociates into carbon dioxide and water in the presence of carbonic anhydrase.
• This carbon dioxide and water enters tubular cell where they form carbonic acid.
  CO₂+H₂O→H₂CO₃
• It immediately dissociated into hydrogen and bicarbonate ions.
  H₂CO₃→H⁺+ HCO₃^–
• This newly synthesized HCO₃^– is reabsorbed into the peritubular blood along with Na.
• H+ is secreted into the lumen.

3. Reabsorption of glucose:

• Glucose is completely reabsorbed.
• It is transported by sodium cotransport mechanism.
• Glucose and sodium bind to a common carrier SGLT-2 and is carried into the cell.
• The Na+ is then pumped out of the cell into the intercellular spaces and the glucose is moved into peritubular capillaries via GLUT-2.

4. Reabsorption of water:

• Proximal convoluted tubule is highly permeable to water.
• When sodium is reabsorbed from the tubule, the osmotic pressure decreases.
• It causes osmosis of water from tubule.
• Thus, water reabsorption occurs secondary to the sodium reabsorption, so called obligatory water reabsorption.

Changes occurring in distal convoluted tubule (DCT):

• Distal convoluted tubule is impermeable to water but permeable to solutes.
• The filtrate that enters the DCT is always hypotonic.

1. The early part of the DCT shows reabsorption of solutes like Na+ and CI.

• About 1820% of the filtered Na+ is reabsorbed in DCT.
• Out of this.
  1. 75% reabsorbed by chloride driven sodium transport.
  2. 25% reabsorbed by Na+ H+ exchange and Na+ K+ exchange.

2. The distal part of the DCT shows the actions of aldosterone and ADH.

1. Aldosterone causes Na+ reabsorption.
2. ADH.
   • In the presence of ADH, DCT becomes permeable to water resulting in water reabsorption.
   • About 5% volume reduction occurs and the filtrate becomes hypertonic.