🏛️Epithelial Transport
Epithelial cells form barriers between body compartments and mediate the selective transport of ions, water, and nutrients. Understanding epithelial physiology is essential for comprehending kidney, gut, and lung function.
🔑 Key Concepts
Transcellular Transport
Substances cross both apical and basolateral membranes, passing through the cell.
- • Requires asymmetric transporter distribution
- • Regulated by hormones
- • Example: Na⁺ absorption in kidney
Paracellular Transport
Substances pass between cells through tight junctions.
- • Depends on tight junction permeability
- • Passive, driven by gradients
- • Claudins determine selectivity
🏛️Epithelial Na⁺ Transport Circuit Model
Transport Rates
Transcellular Na⁺:
50.0
Paracellular Na⁺:
10.0
Total Absorption:
60.0
Clinical: Amiloride blocks ENaC (K⁺-sparing diuretic). Aldosterone increases ENaC expression (Na⁺ retention).
📊Epithelial Classification by Permeability
Tight Epithelia
- Resistance: High (500-5000 Ω·cm²)
- Examples: Collecting duct, bladder, frog skin
- Function: Generate large gradients
- Transport: Primarily transcellular
Leaky Epithelia
- Resistance: Low (10-100 Ω·cm²)
- Examples: Proximal tubule, small intestine, gallbladder
- Function: Bulk transport, isotonic absorption
- Transport: Significant paracellular
💧Aquaporins: Water Channels
Aquaporins are integral membrane proteins that facilitate rapid water transport across cell membranes. Peter Agre received the 2003 Nobel Prize for their discovery.
| Aquaporin | Location | Regulation | Clinical |
|---|---|---|---|
| AQP1 | Proximal tubule, RBC, endothelium | Constitutive | — |
| AQP2 | Collecting duct (apical) | ADH/AVP | Diabetes insipidus |
| AQP3, AQP4 | Collecting duct (basolateral), brain | Constitutive | Neuromyelitis optica |