The physiology of the gastrointestinal system begins with ingestion as food enters the mouth.

1. Taste and Salivation: Ingestion is both mechanical and sensory. Taste buds on the tongue detect flavors, triggering brain signals that stimulate salivary glands to produce more saliva. This response is essential for appetite regulation and digestive enzyme secretion.
2. Role of Teeth in Mastication: Different tooth types have specific roles in mechanical digestion. Incisors cut, canines tear, and molars and premolars grind food, increasing the surface area for enzymatic action.
3. Composition of Saliva: Saliva contains salivary amylase, lysozyme (with mild antibacterial properties), and mucin (a glycoprotein that lubricates and protects oral cavity mucosal surfaces).
4. Control of Salivary Secretion: Salivary secretion is controlled by the autonomic nervous system. Parasympathetic stimulation produces watery, enzyme-rich saliva, while sympathetic stimulation produces more viscous saliva.
5. Swallowing Process and Esophageal Movement: Swallowing comprises oral, pharyngeal, and esophageal phases. The oral phase is voluntary, but the latter two are autonomically controlled. Peristalsis in the esophageal phase includes downward movement and secondary waves to ensure complete food movement into the stomach.
6. Neural Control of Swallowing and Peristalsis: The medulla oblongata in the brainstem coordinates the swallowing reflex, integrating sensory and motor functions for efficient and safe food transit.
7. Esophageal Glands: The esophagus has mucous glands that lubricate the food bolus and protect against abrasion and acid reflux.
8. Interactions with the Respiratory System: The digestive and respiratory systems' close anatomical relationship requires coordination during swallowing to prevent aspiration. The epiglottis, vocal cords, and reflexive airway closure are critical to this process.

Understanding these steps is crucial, as they lay the groundwork for further digestion and nutrient absorption in the gastrointestinal tract.