Swallowing, also known as deglutition, is a complex physiological process that moves food and liquids from the mouth to the stomach. This intricate action involves the precise control of numerous muscles and nerves. Swallowing is fundamental for nutrition and hydration, enabling the body to take in sustenance. It is an often-unconscious process that plays a significant role in overall health.
The Oral Preparatory Phase
The journey of food begins with the oral preparatory phase, a voluntary stage focused on preparing food for safe passage. During this phase, chewing breaks down solid food into smaller, more manageable pieces. Saliva mixes with the food, moistening it and initiating carbohydrate breakdown through enzymes like amylase.
The tongue and cheeks actively manipulate the food, ensuring it is thoroughly mixed with saliva and formed into a soft, cohesive mass known as a bolus. This manipulation prevents food from prematurely entering the throat. The duration of this phase varies depending on the food’s texture and consistency, with tougher foods requiring more chewing.
The Oral Transit Phase
Following preparation, the oral transit phase marks the second voluntary step in the swallowing process. The tongue propels the bolus to the back of the mouth. The tongue tip rises to contact the hard palate, and a wave-like motion pushes the bolus backward towards the oropharynx.
This coordinated tongue movement is a conscious action. As the bolus reaches the posterior part of the oral cavity, it stimulates sensory receptors, triggering the involuntary swallow reflex. This brief, transitional phase quickly positions the bolus for the next, involuntary stages of swallowing.
The Pharyngeal Phase
Once the bolus reaches the oropharynx, the pharyngeal phase commences. This rapid, involuntary sequence directs food away from the airway and into the esophagus, often occurring within a second. To prevent food from entering the nasal cavity, the soft palate elevates, sealing off the nasopharynx.
Simultaneously, the larynx elevates, and the epiglottis closes over the trachea. The vocal cords also close tightly. These protective mechanisms prevent aspiration (entry of food or liquid into the lungs), and if they fail, can trigger a cough reflex. Pharyngeal muscles then contract in a wave-like motion, propelling the bolus downward towards the esophagus.
The Esophageal Phase
The final stage of swallowing is the esophageal phase, an involuntary process that transports the bolus from the pharynx to the stomach. As the bolus enters the esophagus, rhythmic, wave-like muscle contractions known as peristalsis begin. These contractions push the food downward through the muscular tube.
At the top and bottom of the esophagus are two sphincters: the upper esophageal sphincter (UES) and the lower esophageal sphincter (LES). The UES relaxes to allow the bolus to enter, then closes to prevent backflow. As the bolus approaches the stomach, the LES relaxes, permitting the food to pass into the stomach, before closing to prevent stomach contents from re-entering.