Swallowing is the process in which the food taken via mouth is mechanically broken down, made into a bolus, passed on to the pharynx and send down the esophagus into the stomach. All these actions are taken by sealing off the nasal passages as well as the entrance to the airways of the lungs. Therefore, the process of swallowing or else ‘deglutition’ as it is medically known, is simply beyond a simple mechanical process but is a complex neuro-muscular event.
The process of swallowing includes three phases.
Oral phase Pharyngeal phase Esophageal phase
The foods which are ingested will undergo initial transformation through various processes. The mastication by the teeth and the jaw muscles will mechanically break down the foods into minute fragments and with the support of saliva will facilitate the formation of a bolus. The phase is also the initial part of the enzymatic digestive process as both starch and fats are broken down by enzymes.
In the latter part of the oral phase, the food particles are formed in to a bolus and pushed towards the pharynx. The process is voluntary in nature and the tongue will fold back to facilitate the task. At the same time the uvula will be lifted by the styloglossal and palataglossal muscles along with the posterior tongue which effectively seals of the nasopharyngeal entrance.
Once the food reaches the pharynx, a reflex mechanism will take over and the bolus will be passed on to the esophagus through the sequential contraction of the constrictor muscles. At the same time, due to the pulling of the larynx and the hyoid bone upwards and to the front towards the epiglottis, the entrance to the airways will passively close along with relaxing the cricopharyngeus muscle that will allow the food bolus to shunt towards the esophagus. The sealing of the airways is further supported by the constriction of the two vocal cords thus narrowing the gap between the two.
In the pharyngeal phase, the person would not be able to breathe for a fraction of a second as the reflex mechanism inhibits the respiratory center in the medulla.
The phase initiates by relaxing the upper esophageal sphincter and allowing the bolus to be transferred from pharynx to the esophagus. The sequential peristalsis activity will make the bolus to pass downwards and the relaxing lower esophageal sphincter will allow the food to enter the stomach.