structure and function of digestive organs; Mouth, oesophagus, stomach, intestines, rectum, anus-3

Pharynx

The pharynx is extended muscular tube lying behind the nose and mouth, commonly referred to as the throat. The oral part of the pharynx is visible in an open mouth when the tongue is depressed. The palatine tonsils may be seen at either side. The pharynx also extends upward to the nasal cavity and downward to the level of the larynx. The pharynx is composed of skeletal muscle, lined by mucus membrane, and is divided into three parts:

• The nasophyranx,
• The orophyranx,
• The laryngophyranx.

The laryngophyranx and orophyranx both have digestive functions, but the nasophyranx exclusively has respiratory functions. Following ingestion, ingested food travels from the mouth through the orophyranx and laryngophyranx before exiting the body. Food is propelled into the oesophagus and ultimately into the stomach thanks to the muscular contraction of these regions. A bolus, or little amount of chewed food and saliva forced into the pharynx by the tongue during swallowing. The tongue is also lifted to seal the rear of the oral cavity while the soft palate and uvula are raised to prevent food and drink from entering the nasal cavity. The epiglottis, which covers the aperture of the larynx, protects the trachea's entry during swallowing.

Esophagus

The esophagus, which is posterior to the trachea and is a muscular tube that can collapse, is roughly 25 cm (10 inches) long and 2 cm in diameter. Beginning at the inferior end of the laryngopharynx, the esophagus travels anteriorly through the mediastinum to the level of the tenth thoracic vertebra. It travels via the esophageal hiatus, a hole in the diaphragm, before joining the stomach. The major jobs of the esophagus are to carry food from the pharynx to the stomach and to release mucus.

Esophagus can be divided into:

• Cervical esophagus: 2 to 3 cm,
• Thoracic esophagus: 18 to 20 cm,
• Abdominal esophagus: 2 to 3 cm.

Constrictions of Esophagus

• Cervical Constriction (upper esophageal sphincter): At its beginning at the pharyngo- esophageal junction, approximately 15cm from the incisor teeth; caused by the cricopharyngeus muscle.
• Thoracic (broncho-aortic) Constriction: A compound constriction where it is first crossed by the arch of the aorta, 22.5 cm from the incisor teeth, and then where it is crossed by the left main bronchus, 27.5 cm from the incisor teeth; the former is seen in anteroposterior views, the latter in lateral views.
• Diaphragmatic Constriction: Where it passes through the esophageal hiatus of the diaphragm, approximately 40cm from the incisor teeth.

Awareness of these constrictions is important when passing instruments through the esophagus into the stomach, and when viewing radiographs of patients who are experiencing dysphagia (difficulty in swallowing).

Structure of the Esophagus

The esophagus consists of four layers:

The Mucosa: The mucosa is made up of stratified squamous epithelium containing numerous mucous glands

The Submucosa: The submucosa is a thick, loose fibrous layer connecting the mucosa to the muscularis. This layer contains blood vessels and mucus glands.

The Muscular layer: The muscularis is composed of an inner layer, in which the fibres are circular, and an outer layer of longitudinal fibres. The muscle of the esophagus is skeletal, the middle third is mixed (skeletal and visceral) and the lower third contains visceral muscle fibers.

Tunica Adventitia: The outer (superficial) layer of the esophagus, the tunica adventitia, is composed of loose fibrous tissue that connects the esophagus with neighboring structures. Except during the act of swallowing, the esophagus is normally empty, and its lumen, or channel, is essentially closed by the longitudinal folds of the mucosal and submucosal layers. Just superior to the point where the esophagus joins the stomach, some of the circular muscle fibers have increased sympathetic muscle tone, forming the lower esophageal sphincter or cardiac sphincter. These fibers usually remain contracted, and they close the entrance to the stomach. In this way, sphincter helps for prevention of regurgitation of the stomach contents into the esophagus.

Blood Supply of Esophagus

The arterial blood supply to the thoracic region is by the paired esophageal arteries (branches of thoracic aorta). The abdominal region is supplied by the branches of the inferior phrenic arteries and the left gastric branch of the celiac arteries. Venous blood is drained from the thoracic region into the azygos and hemiazygos veins and from abdominal region into left gastric vein.

Nerve Supply of Esophagus

The parasympathetic and sympathetic nerves supply the esophagus.

Swallowing (Deglutition)

Swallowing or deglutition is a mechanism that moves food from the mouth into the stomach. The process of swallowing is divided into three stages.

First Stage: It is the voluntary stage in which food is chewed and mixed with saliva. Then, tongue rolls this mixture into a mass, or bolus and forces it into pharynx.

Second Stage: It is also called pharyngeal stage that begins as food reaches pharynx which stimulates sensory receptors around pharyngeal opening. This triggers swallowing reflex includes following actions:

• Food cannot enter the nasal cavity because the soft palate (and uvula) rise.
• The larynx and hyoid bone are high. The epiglottis, a flap-like structure linked to the larynx, seals off the top of the trachea to prevent food from entering.
• The uvula and soft palate are forced together by the tongue, separating the nasal cavity from the oral cavity.
• In order to draw the pharynx upward toward food, longitudinal muscles in the pharyngeal wall contract. Lower inferior constrictor muscles relax, allowing the esophagus to expand.
• When the superior constrictor muscles contract, the pharyngeal muscles start to experience a peristaltic wave. This wave shovels food down the throat. Breathing is briefly impeded by the swallowing reflex.

Third Stage: It is also called esophageal stage, the involuntary passage of the bolus through the esophagus to the stomach.