The Esophagus and Swallowing
The esophagus is a simple transport tube - no digestion or absorption happens here. Its only job is to move the food bolus from the pharynx to the stomach. But that job involves an elegant reflex that keeps food out of your lungs.
The Swallowing Reflex (Deglutition)
Swallowing has three phases:
1. Voluntary (oral) phase - Your tongue pushes the bolus to the back of the mouth toward the pharynx. This is the only voluntary part of swallowing.
2. Pharyngeal phase (involuntary) - Once the bolus touches the pharynx, a reflex takes over:
- The soft palate rises to close off the nasal passages (prevents food from going up your nose)
- The epiglottis folds down to cover the trachea (prevents food from entering the airway)
- The upper esophageal sphincter relaxes to let the bolus enter the esophagus
- Breathing is briefly inhibited
3. Esophageal phase (involuntary) - Peristaltic waves push the bolus down the esophagus toward the stomach. This takes about 8-10 seconds. Gravity helps but is not required - you can swallow while upside down because peristalsis does the work.
Esophageal Anatomy
The esophagus is about 25 cm long and runs behind the trachea. Key features:
- Upper third - contains skeletal muscle (voluntary-ish, used in the initial swallowing push)
- Middle third - mix of skeletal and smooth muscle
- Lower third - smooth muscle only (completely involuntary)
- No serosa - the esophagus has adventitia instead (unlike most of the GI tract, which has serosa)
The Lower Esophageal Sphincter (LES)
At the junction between the esophagus and stomach sits the lower esophageal sphincter (also called the cardiac sphincter). It stays contracted to prevent stomach acid from splashing back into the esophagus, and relaxes only when a peristaltic wave delivers a bolus.
When the LES fails to close properly, stomach acid enters the esophagus. This causes the burning sensation of acid reflux.