The pharynx is divided into three parts. The upper portion, or nasopharynx, is the portion from the base of the skull to the inferior extent of the soft palate, which establishes the boundary between the nasopharynx and the oropharynx. The anterolateral walls of the oropharynx comprise the tonsils and their corresponding pillars. The posterior and lateral walls of the oropharynx house the inferior constrictors and overlying mucosa. Anteriorly, the oropharynx includes the base of the tongue, which is covered by lymphoid tissue or lingual tonsils. Just inferior to the base of the tongue are the valleculae, which may include small- to medium-sized inclusion cysts. The hypopharynx is the remainder of the pharynx that is not visible transorally and includes the pyriform sinuses and its posterior and lateral walls. The pyriform sinuses end inferiorly at the cricopharyngeus muscle, which is the most inferior structure of the pharynx and serves as the valve at the top of the esophagus. The posterior and lateral walls house the inferior pharyngeal constrictors and an overlying mucosa. Common variants that may be seen on endoscopy in the hypopharynx include the pulsations of the internal carotid arteries, lymphoid follicles, and the thyroid and hyoid greater cornu.
The esophagus originates at the level of the sixth cervical vertebra, posterior to the cricoid cartilage. In the thorax, the esophagus passes behind the aortic arch and left mainstem bronchus; it descends in the posterior mediastinum along the right side of the descending thoracic aorta and then courses in front of and a bit to the left of the aorta as it enters the diaphragm at the level of the tenth thoracic vertebrae. About 2 to 4 cm of esophagus are normally below the diaphragm. There are three anatomic areas of narrowing in the esophagus: (1) at the level of the cricoid cartilage (pharyngoesophageal or upper esophageal sphincter); (2) in the mid thorax from compression by the aortic arch and the left main stem bronchus; and (3) at the level of the esophageal hiatus of the diaphragm (gastroesophageal or lower esophageal sphincter). The upper esophageal sphincter has a greater resting pressure (100 mm Hg) than the lower esophageal sphincter (15 to 24 mm Hg), with the upper sphincter relaxing first in response to a food bolus, followed 5 to 10 seconds later with lower sphincter relaxation. The distance from the upper central incisors to the cricopharyngeus muscle is 15 to 20 cm, to the aortic arch 20 to 25cm, to the inferior pulmonary vein 30 to 35 cm, and to the gastroesophageal junction approximately 40 to 45 cm. The musculature of the upper third of the esophagus is skeletal (striated muscle) and the remainder is smooth muscle. The mucosal lining is stratified squamous epithelium with mucous glands. The esophagus does not have a serosal layer and as a result does not heal as readily when injured as other segments of the gastrointestinal tract. (Note: The anatomic differentiation of the upper, middle, and lower thirds of the esophagus does not correspond with the echocardiographic delineation of mid- or upper-esophageal landmarks.)
The stomach is composed of multiple segments. The cardia is located at the gastroesophageal (GE) junction. The fundus is the portion that lies cephalad to the GE junction, while the corpus is the large central body of the stomach. The pyloric antrum is its terminal segment with the pylorus as the boundary between the stomach and the duodenum. The mucosal folds, rugae, contain mucus-secreting cells: parietal cells and chief cells. The smooth muscle comprises an inner circular layer and outer longitudinal layer, with outer serosal covering.