Physiology of the Gastrointestinal System I: Ingestion and Propulsion

Salivary glands, including the parotid, submandibular, and sublingual glands, release saliva containing salivary amylase and other enzymes. These enzymes initiate carbohydrate breakdown while mucin lubricates food to form a bolus. Saliva also contains lysozyme with mild antibacterial properties, protecting the oral cavity during ingestion.

During deglutition, receptors in the oropharynx trigger an involuntary reflex that raises the soft palate to seal the nasopharynx and blocks the trachea with the epiglottis. This coordinated response prevents food from entering the respiratory tract, ensuring safe passage of the bolus into the esophagus while maintaining airway protection.

Peristalsis involves coordinated rhythmic contractions of esophageal muscles that move the bolus downward toward the stomach. Secondary peristaltic waves ensure complete food movement into the stomach. The upper esophageal sphincter allows entry while the lower sphincter regulates stomach entry and prevents reflux.

Salivary secretion is controlled by the autonomic nervous system through two pathways. Parasympathetic stimulation produces watery, enzyme-rich saliva that aids digestion, while sympathetic stimulation produces more viscous saliva. This dual control allows the body to adjust salivary composition based on digestive needs and stress responses.

Swallowing comprises three phases: oral, pharyngeal, and esophageal. The oral phase is voluntary, involving the tongue pushing the bolus toward the pharynx. The pharyngeal and esophageal phases are autonomically controlled, with the medulla oblongata coordinating the swallowing reflex to ensure efficient and safe food transit through the digestive tract.

Different tooth types have specific roles in mastication. Incisors cut food, canines tear it, and molars and premolars grind it into smaller pieces. This mechanical breakdown increases the surface area available for enzymatic action, facilitating more efficient chemical digestion and nutrient absorption.

Esophageal mucous glands secrete mucus that lubricates the food bolus and protects the esophageal lining against abrasion and acid reflux. This protective layer is essential for maintaining esophageal integrity during food transit and prevents damage from stomach acid that may reflux into the esophagus.