The Two-Stage Model of Secretion
This model applies to the salivary, sweat, and pancreatic glands.
- Stage 1: Acinar Secretion (Primary Fluid)
- The acinar cells at the base of the gland secrete the initial fluid.
- This primary fluid is always isotonic with plasma. It’s created by secreting Cl⁻ ions into the lumen, which electrically attracts Na⁺ to follow, and water then follows this osmotic gradient.
- Its composition is similar to plasma: high in Na⁺ and Cl⁻.
- Stage 2: Ductal Modification
- As this isotonic fluid flows down the ducts toward the lumen, the ductal cells modify it.
- This stage is where the key difference lies. The final tonicity (hypotonic vs. isotonic) depends entirely on what the ductal cells do and whether they are permeable to water.