Final answer:
Saliva plays a crucial role in digestion, beginning in the oral cavity, where it moistens food forming a bolus, contains enzymes for chemical digestion, and helps direct the bolus into the esophagus while preventing entry into the trachea.
Step-by-step explanation:
Saliva moistens ingested food and helps it become a slick, semisolid bolus that is more easily swallowed. In addition to moistening, cleansing, and lubricating the oral cavity structures, saliva contains important components such as immunoglobulins, lysozymes, and enzymes like salivary amylase and lipase.
Salivary amylase initiates the breakdown of carbohydrates into maltose, a disaccharide, while lipase starts the digestion of fats. Saliva keeps the oral cavity moist and helps in the formation of the bolus, which the tongue then moves from the mouth into the pharynx. When swallowing, the epiglottis prevents food from entering the trachea, directing the bolus to the esophagus and then to the stomach.
The nasal cavity is not directly involved in the swallowing process but saliva also aids in keeping the nasal cavity moist and functional.