Located behind the trachea, the esophagus is a muscular tube connecting the pharynx and the stomach and lined with stratified squamous epithelium. The only substance secreted by the esophagus is mucus. The protective mucus provides lubrication for the passage of food and helps to prevent damage to the esophageal wall by coarse food materials. The esophagus is sealed off by two sphincters, one at either end of the tube: the upper esophageal sphincter (UES) and the lower esophageal sphincter (LES). Each is normally closed except during the process of swallowing. The normal function of the respiratory system creates a subatmospheric pressure in the thoracic cavity. If, indeed, the esophagus were open to the atmosphere, this pressure gradient would pull air into the esophagus and stomach during each inspiration. Therefore, the closure of these sphincters prevents large volumes of air from entering the digestive tract.

The esophageal stage of the swallowing reflex involves a primary peristaltic wave of contraction initiated by the swallowing center and mediated by the vagus nerve. This wave begins at the UES and moves slowly down the esophagus at a rate of 2 to 6 cm/sec until it reaches the LES. Some particularly large or sticky food particles may remain in the esophagus after the primary peristaltic wave. The distension of the esophagus by the presence of these particles elicits secondary peristaltic waves that do not involve the swallowing center. The smooth muscle of the LES relaxes immediately prior to the arrival of the peristaltic contraction to allow for movement of food into the stomach.

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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