Reabsorption & Secretion Of Proximal Tubule
REABSORPTION & SECRETION OF PROXIMAL TUBULE
- In proximal tubule, GFR & volume reduces to one-third.
- 60%-70% GFR reabsorbed.
- Both filtered solute & water.
- Solutes concentraion remains unchanged (As in GF).
- Fluid out of proximal tubule is isotonic with GR & plasma.
PROXIMAL REABSORPTION – OBLIGATORY TYPE:
- Due to 2 reasons,
- Independent of fluid & electrolyte status of body.
- Independent of hormonal control.
- Hence, “Obligatory reabsorption”.
REABSORPTION OF SUBSTANCES:
SODIUM RESORPTION:
- First mechanism activated in proximal tubule.
- Driven by Na2+-K+-ATPase located on basolateral membrane.
- Most other resorptions are secondary to Na2+ active transport.
EVENTS FOLLOWING Na2+ REABSORPTION:
- Na2+ reabsorption activates,
1. PASSIVE Cl– REABSORPTION:
- Due to trans-epithelial potential difference development.
2. WATER REABSORPTION:
- 60-70% of filtered water reabsorbed passively.
- Because of osmosis.
- Occurs through water channel aquaporin-1.
- Coupled mainly to sodium reabsorption.
3. GLUCOSE REABSORPTION:
- Absorbed completely (100%).
- Occurs by Na2+-Glucose cotransporter-1 (SGLT -1)
- Located in luminal membrane (brush border).
- Completely absorbed (100%).
- By sodium-amino acid cotransporter.
5. PHOSPHATE REABSORPTION:
- Co-transports with sodium.
6. HYDROGEN ION:
- H+ ions – Secreted into tubular fluid.
- By secondary active transport (antiport/countertransport).
- Mainly Na2+-H+ exchanger.
7. BICARBONATE REABSORPTION:
- Maximum (90%) bicarbonate absorption.
- H+ ions secretion by Na2+-H+ exchanger couples bicarbonate reabsorption.
8. OTHER REABSORPTION:
- 2/3rd filtered K+ reabsorbed in PCT.
- 40-50% urea reabsorbed.
- Glucose, amino acids & bicarbonate – Reabsorbed from filtrate in early/first part of PCT.
- Creatinine – Completely unabsorbed.
Exam Question
REABSORPTION & SECRETION OF PROXIMAL TUBULE
- GFR & its volume is reduced to 1/3rd (About 60-70% reabsorbed) in proximal tubule.
- Fluid out of proximal tubule is isotonic with GR & plasma.
- Primary active step for sodium reabsorption in proximal tubule involves Na+-K+-ATPase at baso-lateral membrane.
- About 60-70% of water reabsorbed in proximal tubules because of osmosis through aquaporin-1 channel.
- Passive water reabsorption is coupled mainly to sodium reabsorption.
- Glucose is absorbed completely (100%) in proximal tubule by Na2+-Glucose cotransporter-1 (SGLT -1) in luminal membrane & by glucose transporter GLUT-1 in basolateral membrane.
- Amino acids are completely reabsorbed in proximal tubules.
- H+ ions secreted into tubular fluid is mainly by Na2+-H+ exchanger.
- Phosphate reabsorption occurs along with sodium co-transport.
- Maximum (90%) bicarbonate absorption in proximal tubules is coupled with H+ ions secretion by Na2+-H+ exchanger.
- About 40-50% of urea reabsorbed.
- Glucose, amino acids, & bicarbonate are reabsorbed along with Na+ in early portion of proximal tubule.
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