Overview
The transcript explains how different nephron segments reabsorb molecules via membrane transporters, focusing on apical vs basolateral membranes and secondary active transport driven by the Na+/K+ ATPase.
Nephron Cell Architecture
- Apical side faces the lumen where filtrate flows; basolateral side faces interstitium and capillaries.
- Proximal tubule has a brush border (microvilli) increasing apical surface area.
- Peritubular capillaries surround tubules; reabsorbed solutes move into blood there.
Core Transport Principle
- Na+/K+ ATPase on basolateral membrane uses ATP to export Na+ and import K+.
- This lowers intracellular Na+, creating a Na+ gradient from lumen to cell.
- Transporters exploit this gradient to drive cotransport or exchange of other solutes.
- This coupling is called secondary active transport.
Proximal Convoluted Tubule (PCT)
- Major reabsorption site for glucose, amino acids, and Na+.
- Basolateral Na+/K+ ATPase maintains low intracellular Na+.
- Apical Na+-coupled symporters move Na+ with glucose or amino acids into the cell.
- Elevated intracellular glucose diffuses out basolaterally to blood when permeable.
Ascending Limb of Loop of Henle
- Focus on reabsorbing salts: Na+, K+, and Cl−.
- Basolateral Na+/K+ ATPase lowers intracellular Na+.
- Apical Na+-K+-2Cl− cotransporter (symporter) brings in Na+, K+, and Cl−.
- K+ channels are leaky; K+ can recycle, aiding transporter function.
- Cl− exits basolaterally; NaCl reabsorption contributes to medullary hypertonicity.
Distal Convoluted Tubule (DCT)
- Reabsorbs calcium via Na+-Ca2+ exchange at the basolateral side.
- Basolateral Na+/K+ ATPase keeps extracellular Na+ high relative to cell, enabling exchange.
- Apical membrane is permeable to Ca2+, allowing Ca2+ entry down its gradient.
- Basolateral Na+-Ca2+ antiporter exports Ca2+ to blood as Na+ enters the cell.
Key Terms & Definitions
- Apical membrane: Cell surface facing the tubular lumen.
- Basolateral membrane: Cell surface facing interstitium and capillaries.
- Peritubular capillaries: Blood vessels surrounding nephron tubules for reabsorption.
- Na+/K+ ATPase (sodium-potassium pump): ATP-driven pump exporting Na+ and importing K+.
- Symporter (cotransporter): Moves two or more solutes in the same direction.
- Antiporter: Exchanges solutes in opposite directions across a membrane.
- Secondary active transport: Using one solute’s gradient (often Na+) to drive another’s movement.
Transporters by Segment
| Segment | Apical Transporter(s) | Basolateral Transporter(s) | Primary Solutes Reabsorbed | Driving Mechanism |
|---|
| Proximal convoluted tubule | Na+-glucose and Na+-amino acid symporters | Na+/K+ ATPase; glucose-permeable pathways | Glucose, amino acids, Na+ | Na+ gradient from ATPase (secondary active) |
| Ascending limb (Loop of Henle) | Na+-K+-2Cl− symporter | Na+/K+ ATPase; Cl− exit pathways; leaky K+ channels | Na+, K+, Cl− | Na+ gradient from ATPase (secondary active) |
| Distal convoluted tubule | Ca2+ entry via apical permeability | Na+/K+ ATPase; Na+-Ca2+ antiporter | Ca2+ | Na+ gradient drives Ca2+ export (secondary active) |
Action Items / Next Steps
- Remember apical vs basolateral orientation and roles across segments.
- Link Na+/K+ ATPase activity to secondary active transport in each segment.
- Associate each segment with its characteristic transporter and reabsorbed solutes.