The Mouth, Pharynx, and Esophagus

The mouth: where digestion starts. In the mouth (oral cavity), both types of digestion begin:

• Mechanical: the teeth chew (mastication), and the tongue mixes food, breaking it into a soft mass called a bolus — increasing surface area for enzymes.
• Chemical: saliva (from the salivary glands) contains salivary amylase, an enzyme that begins breaking down starch into smaller sugars. Saliva also moistens food, contains lysozyme (mild antibacterial action), and dissolves food chemicals so you can taste them.

So even before you swallow, starch digestion has begun and the food is prepared for the trip down.

Swallowing (deglutition). Swallowing safely moves the bolus from mouth to stomach and has two main phases:

1. Voluntary phase: the tongue pushes the bolus toward the back of the mouth into the pharynx.
2. Involuntary (pharyngeal–esophageal) phase: once in the pharynx, swallowing becomes an automatic reflex. Critically, the epiglottis covers the larynx (airway) so food goes down the esophagus, not the trachea (recall respiratory anatomy). If this protection fails, food can enter the airway (aspiration), risking choking or pneumonia.

The esophagus. The esophagus is a muscular tube that carries the bolus from the pharynx to the stomach. It moves food by peristalsis (waves of smooth-muscle contraction), so swallowing works even lying down or against gravity. At each end is a sphincter (a muscular ring):

• The upper esophageal sphincter controls entry from the pharynx.
• The lower esophageal sphincter (LES), where the esophagus meets the stomach, opens to let food in and closes to prevent stomach acid from backing up. When the LES doesn't close properly, acidic stomach contents reflux into the esophagus, causing heartburn/GERD (gastroesophageal reflux disease) — a very common condition.

How it works

From bite to stomach:

Mouth: teeth chew + tongue mix → bolus; saliva (amylase) starts starch digestion
   → tongue pushes bolus into pharynx (voluntary)
   → swallow reflex: epiglottis covers airway → bolus enters esophagus (involuntary)
   → peristalsis down esophagus → lower esophageal sphincter opens → stomach

Comparisons

Common confusions

• Chemical digestion starts in the mouth (salivary amylase on starch), not just the stomach.
• Epiglottis protects the airway — it isn't part of digestion itself.
• Peristalsis, not gravity, moves food — you can swallow upside down.
• The LES prevents reflux — its weakness causes GERD.

Memory aids

• "Amylase = Attacks starch (both start with a soft 'a' sound; A for starch's first digester)."
• Epiglottis = "the airway's lid during swallowing."
• LES = "the anti-reflux gate."

Quick review

• The mouth begins mechanical digestion (teeth/tongue → bolus) and chemical digestion (salivary amylase on starch).
• Swallowing has a voluntary phase (tongue pushes bolus) and an involuntary reflex where the epiglottis protects the airway.
• The esophagus moves food by peristalsis; the lower esophageal sphincter prevents acid reflux (its weakness causes GERD).
• Dysphagia and aspiration are key clinical safety concerns.