Final answer:
Saliva becomes hypotonic as it travels through the salivary ducts because cells lining the ducts reabsorb sodium and chloride ions while secreting bicarbonate and potassium, resulting in a lower solute concentration in the saliva.
Step-by-step explanation:
How Saliva Becomes Hypotonic
As saliva is secreted by the salivary glands, notably the parotid, submandibular, and sublingual glands, it initially contains a high concentration of electrolytes, making it isotonic with body fluids. However, as it travels through the salivary ducts, cells lining these ducts actively reabsorb some of the electrolytes, notably sodium (Na+) and chloride (Cl-) ions, while secreting bicarbonate (HCO3-) and potassium (K+) into the saliva. This reabsorption of ions without a concurrent reabsorption of water leads to a reduction in the osmolarity of the saliva, rendering it hypotonic compared to plasma by the time it reaches the mouth.
The process of electrolyte reabsorption and secretion is influenced by parasympathetic nervous stimulation, which increases during eating, thereby increasing both the volume and the hypotonicity of saliva. The resulting hypotonic saliva serves several functions, including facilitating taste, initiating starch digestion through salivary amylase, providing lubrication for swallowing, and contributing to oral hygiene by its antibacterial properties.