嚥下

Swallowing consists of three distinct stages: the buccal phase, pharyngeal phase, and esophageal phase. The buccal phase is voluntary and occurs in the mouth, where the tongue propels food backward. The pharyngeal phase is involuntary and involves the soft palate, uvula, and epiglottis protecting the airway. The esophageal phase begins when food enters the esophagus and peristalsis transports it toward the stomach.

During the buccal phase, the tongue moves upward and backward against the palate, pushing the bolus toward the oropharynx. This voluntary action propels food from the mouth into the throat, initiating the swallowing process. Other muscles simultaneously keep the mouth closed, preventing food from escaping during this initial stage.

During the pharyngeal phase, the soft palate and uvula move upward to close the nasopharynx, blocking food from entering the nasal cavity. Simultaneously, the epiglottis closes the larynx's entrance, preventing food from entering the respiratory tract. These protective mechanisms are controlled by the deglutition center in the medulla oblongata and lower pons.

Peristalsis is a series of coordinated muscular contractions that propels the bolus through the esophagus during the esophageal phase. The upper esophagus contracts while the lower part expands to receive food, and this action repeats sequentially. Peristalsis continues until the bolus reaches the lower esophageal sphincter, which then relaxes to allow food entry into the stomach.

Swallowing duration varies depending on food consistency. Liquids or very soft food take approximately one second to swallow, while semi-solid or solid food requires four to eight seconds. Saliva and mucus aid in lubricating food throughout this process, facilitating smoother transport from the mouth to the stomach.

The autonomic nervous system controls both the pharyngeal and esophageal phases of swallowing, unlike the voluntary buccal phase. The deglutition center in the medulla oblongata regulates these involuntary stages, coordinating the complex muscular actions required to safely transport food through the throat and esophagus.

When the upper esophageal sphincter relaxes, the bolus enters the esophagus and peristalsis begins. As the bolus approaches the stomach, the esophagus expands and triggers a reflex that relaxes the lower esophageal sphincter, allowing food to enter the stomach. Mucus secreted by the esophagus lubricates the bolus and reduces friction during this final stage.