The esophagus is a member of the digestive tract whose main responsibility is to transfer food from the pharynx to the stomach. It measures about 10 inches in length and is tubular and collapsible. The esophagus can be traced from the originating point of the larynx running behind the trachea. It traces down the mediastinum of the thorax until it runs into the diaphragm, ending just above the stomach’s opening. The esophageal hiatus refers to the opening in the diaphragm.
Nonkeratinized stratified squamous epithelium lines the esophagus. The walls themselves are either created by skeletal muscles or by smooth muscle tissue. Skeletal muscle lines the upper 1/3 of the esophagus, while the middle 1/3 is a combination of the two muscle tissues, leaving the remaining 1/3 as smooth muscle tissue. Where the stomach and the esophagus meet, there is a thickening of the circular muscle fibers, creating the gastroesophageal sphincter, or the lower esophageal sphincter. This sphincter is responsible for maintaining the food and fluids inside the stomach, preventing regurgitation up into the esophagus. Air in the lungs creates an additional pressure in the lower thoracic region, encouraging regurgitation of foods and fluids.
The Mechanisms of Swallowing
Deglutition is the term given to the process of swallowing, a highly mechanical and functionally complex process that allows the initiation of digestion to occur. Deglutition is typically described in three basic phases, allowing for clarity.
The initial stage of deglutition is preceded by mastication if there is a solid in the process. This is a voluntary stage, meaning that a human must begin this stage consciously. The oral cavity closes and the process of breathing is temporarily suspended. The bolus has been formed through the process of mastication, and the tongue then lifts the bolus firm against the transverse palatine folds of the hard palate. The mylohyoid muscle and the styloglossus muscle contract to produce this experience.
The secondary stage takes over from this point, passing the bolus through the pharynx. The second stage of deglutition is involuntary. Sensory receptors that are positioned at the opening of the oropharynx stimulate the secondary action. The tongue is pressing the bolus against the transverse palatine folds of the hard palate, which creates a seal against the nasopharynx. This not only prevents the bolus from entering the airway, but it also stimulates the pressure senses of the oropharynx and forces the bolus into the opening. The soft palate along with the uvula close off the rest of the nasopharynx while the bolus passes into the gullet, and the hyoid bone and the larynx elevate protectively. The constrictor muscles of the pharynx then contract in a rhythmic and intentional sequence in order to force the bolus through the pharynx, where it will then enter the esophagus. This entire stage can be completed in less then a second.
The final stage of deglutition is also involuntary, and it deals specifically with the bolus entering and passing through the esophagus. Peristalsis kicks in naturally and the bolus in pressured down the esophagus. Fluids are rushed through this entire process, all three stages, in under a second, while an average sized bolus takes approximately 5 to 8 seconds to complete the three stages.