Introduction to the Gastrointestinal System and Oral Cavity

Core Concepts

The alimentary canal and its accessory organs

The alimentary canal consists of the mouth (oral cavity), pharynx, esophagus, stomach, small intestine, and large intestine. The accessory organs — salivary glands, liver, gallbladder, and pancreas — deliver secretions into the tube through ducts. The liver produces bile (stored and concentrated by the gallbladder), the pancreas produces digestive enzymes and bicarbonate, and the salivary glands produce saliva. This division matters for pharmacology because drugs can act on the tube itself (e.g., laxatives, antiemetics) or be metabolized by accessory organs (e.g., the liver's first-pass metabolism of many oral drugs).

The four layers of the GI wall

From the lumen outward, the wall of the GI tract has four layers: the mucosa (the inner lining that secretes mucus and enzymes and absorbs nutrients), the submucosa (connective tissue carrying blood vessels, lymphatics, and nerves), the muscularis externa (smooth muscle layers that produce motility), and the serosa (the outer covering). This layering is clinically relevant: ulcers, inflammation, and tumors begin in the mucosa and can penetrate deeper, and the muscularis is what peristalsis — and drugs that speed or slow motility — acts upon.

Motility: how the tube moves material

Two main patterns of motility move food and waste. Peristalsis is a wave of contraction and relaxation that propels material forward. Segmentation is a rhythmic chopping motion in the small intestine that mixes chyme with digestive juices and keeps it in contact with the absorptive surface. Motility is controlled by the enteric nervous system — sometimes called the "second brain" — a network of neurons in the gut wall that can operate independently, with input from the autonomic nervous system (parasympathetic activity generally promotes motility and secretion; sympathetic activity generally slows them). This is why anticholinergic drugs (which block parasympathetic effects) are used to slow the gut, and why prokinetic drugs exist to speed it up.

The oral cavity: where digestion begins

Digestion actually starts in the mouth. The teeth mechanically break food into smaller pieces (mastication), increasing surface area for enzymes. The tongue manipulates the food and contains taste buds. The salivary glands produce saliva, which contains salivary amylase, an enzyme that begins breaking down starch into smaller sugars, and mucus that lubricates the food into a soft mass called a bolus. Saliva also contains lysozyme, which has mild antibacterial activity, and its continuous flow helps keep the mouth clean and teeth protected. The mouth's mucosa is thin and richly vascularized — the reason sublingual (under the tongue) and buccal (against the cheek) routes deliver drugs rapidly, and why nitroglycerin tablets, for example, are given sublingually to bypass the digestive tract and liver.

Swallowing: the three phases

Swallowing (deglutition) moves the bolus from the mouth to the stomach in three phases: the oral (voluntary) phase, in which the tongue pushes the bolus to the back of the mouth; the pharyngeal phase, a reflex that closes the airway (the epiglottis and larynx move to protect the trachea) and propels the bolus through the pharynx; and the esophageal phase, peristalsis that carries the bolus down the esophagus. The pharyngeal phase is the dangerous one: if the airway is not protected, food or liquid enters the lungs — aspiration — a major nursing concern in people with dysphagia (difficulty swallowing), stroke, or altered consciousness. This is why nurses assess swallowing before oral intake and why some patients receive thickened liquids or tube feedings.

The oral cavity in nursing assessment and medication administration

The nurse's mouth assessment covers the lips, teeth, gums, tongue, palate, and mucosa: color, moisture, lesions, and dentition. Dry mouth (xerostomia) can result from many medications (anticholinergics, some antidepressants) and makes chewing, swallowing, and dissolving sublingual tablets difficult. Aspiration risk is assessed by watching for coughing or choking during meals, and specific swallowing evaluations are done by trained clinicians where policy requires. Scope note: assessment, medication administration per orders, and basic mouth care are core nursing activities; swallowing studies, dietary texture changes, and enteral feeding decisions are ordered and supervised per institutional policy and scope of practice.

Common Confusions