Pharynx and Esophagus – Alimentary Canal
Pharynx and Esophagus
The pharynx is a cavity posterior to the mouth from which the tubular esophagus leads to the stomach. The pharynx and the esophagus do not digest food, but both are important passageways whose muscular walls function in swallowing.
Structure of the Pharynx
The pharynx (far′inks) connects the nasal and oral cavities with the larynx and esophagus. It has three parts:
1. The nasopharynx (na′′zo-far′inks) communicates with the nasal cavity and provides a passageway for air during breathing. The auditory tubes, which connect the pharynx with the middle ears, open through the walls of the nasopharynx.
2. The oropharynx (o′′ro-far′inks) is posterior to the soft palate and inferior to the nasopharynx. It is a passageway for food moving downward from the mouth and for air moving to and from the nasal cavity.
3. The laryngopharynx (lah-ring′′go-far′inks), just inferior to the oropharynx, is a passageway to the esophagus.
Swallowing has three stages. In the first stage, which is voluntary, food is chewed and mixed with saliva. Then the tongue rolls this mixture into a mass, or bolus, and forces it into the oropharynx. The second stage of swallowing begins as food reaches the oropharynx and stimulates sensory receptors around the pharyngeal opening. This riggers the swallowing reflex, which includes the following actions:
1. The soft palate (including the uvula) raises, preventing food from entering the nasal cavity.
2. The hyoid bone and the larynx are elevated. A flaplike structure attached to the larynx, called the epiglottis (ep′′ı˘-glot′is), closes off the top of the larynx so that food is less likely to enter the trachea.
3. The tongue is pressed against the soft palate, sealing off the oral cavity from the nasopharynx.
4. The longitudinal muscles in the pharyngeal wall contract, pulling the pharynx upward toward the food.
5. Muscles in the laryngopharynx relax, opening the esophagus.
6. A peristaltic wave begins in the pharyngeal muscles and forces food into the esophagus.
The swallowing reflex momentarily inhibits breathing. Then, during the third stage of swallowing, peristalsis transports the food in the esophagus to the stomach.
The esophagus (e˘-sof′ah-gus), a straight, collapsible tube about 25 centimeters long, is a food passageway from the pharynx to the stomach. The esophagus begins at the base of the laryngopharynx and descends posterior to the trachea, passing through the mediastinum. It penetrates the diaphragm through an opening, the esophageal hiatus (e˘-sof′′ah-je′al hi-a′tus), and is continuous with the stomach on the abdominal side of the diaphragm.
Mucous glands are scattered throughout the submucosa of the esophagus. Their secretions moisten and lubricate the tube’s inner lining. Just superior to the point where the esophagus joins the stomach, some of the circular smooth muscle fibers have increased muscle tone, forming the lower esophageal sphincter, or cardiac sphincter. These fibers usually remain contracted. They close the entrance to the stomach, preventing the stomach contents from regurgitating into the esophagus. When peristaltic waves reach the stomach, these muscle fibers temporarily relax and allow the swallowed food to enter.