The Organs of Digestion
The upper gastrointestinal tract—that part of the digestive system (alimentary canal) from mouth to stomach—is primarily responsible for the breakdown of food. While the different regions of the alimentary canal may vary in their specific structure and function, each area has the same four-layer foundation.
Mucosa
The inner layer of the alimentary canal is the mucosal layer and is composed of the luminal epithelium, which comes in contact with the processed food. A variety of cells can be found in this layer, depending on the function of the particular region. Beneath the epithelium is the lamina propria, which has loose connective tissue and an abundance of lymphocytes. The boundary of the mucosa is a thin layer of smooth muscle called the muscularis mucosae, which functions in the mechanical processing of material.
Submucosa
This region of connective tissue underlies the mucosa and contains blood vessels, lymphatics, and nervous system plexuses that control the muscular contractions (peristalsis) of the canal. Meissner’s plexus (submucosal plexus) is located in this layer and provides parasympathetic control of the various secretions in a particular region. This is part of the autonomic nervous system, particularly the enteric nervous system (the part of the intrinsic nervous system that oversees the function of the gastrointestinal system.
Anatomy of a Word
plexus
A plexus is a network of interconnecting nerves (or vessels). Plexuses (sometimes “plexi”) is the plural form of plexus and refers to a group of such networks.
Muscularis
Made up of thick layers of smooth muscle, this third layer of the alimentary canal facilitates the peristaltic movement of each region. The general structure of the muscularis is an inner layer of smooth muscle and an outer layer of longitudinal muscle. Rhythmic contractions of both circular and longitudinal muscle layers moves material progressively down the alimentary canal.
These contractions are controlled by another component of the enteric nervous system, Auerbach’s plexus (myenteric plexus), located between the circular and longitudinal smooth muscle layers.
Adventitia/Serosa
The outermost connective tissue of the alimentary canal is the tunica adventitia. This connective tissue allows the alimentary canal to be secured to the connective tissue of the body wall in certain areas. In other locations, the canal is not attached to the body, but is covered by a thin layer of mesothelium (squamous cell membrane) called the visceral peritoneum. In this case, the outer layer is called the serosa.
Pharynx and Esophagus
As food becomes mixed with saliva, it is processed by the teeth and tongue, and turned into a spherical mass called a bolus. This bolus is moved to the back of the mouth in preparation for swallowing. From the oral cavity, the bolus is moved into the oropharynx, which is commonly called the throat.
If the pharynx were considered a pipe, then in plumbing it would be a T intersecting pipe. The oropharynx would be the stem of the T, and would connect to the upper portion descending from the nasal cavity (nasopharynx) and the lower portion connecting with the larynx and esophagus (laryngopharynx).
As the food is swallowed (deglutition), a cartilaginous flap, the epiglottis, reflexively covers the glottis (opening of the trachea) and prevents material from being aspirated into the airway. Additionally, the epiglottis creates a ramp to help direct the bolus into the esophagus.
Composed of an epithelium that is identical to that of the oral cavity, the esophagus uses its muscularis to propel food downward and into the stomach. The upper portion of the esophagus contains a high proportion of skeletal muscle in the muscularis layer, and this can be under either voluntary or involuntary control. As the esophagus descends, the muscle transitions from skeletal to smooth. By the time the esophagus connects to the stomach, the muscle is 100 percent smooth muscle, a portion of which becomes the lower esophageal sphincter, which prevents regurgitation of stomach contents, including acid, upward into the esophagus. The esophagus passes through an opening in the diaphragm (hiatus) to connect with the stomach in the upper abdominal cavity.
Stomach
In the stomach, mechanical and chemical digestion are increased. When empty, the interior lining of the stomach contains folds called gastric rugae. These folds accommodate the expansion of the organ.
Cells
At the cardia of the stomach (the location of attachment of the esophagus to the stomach), the mucosal lining cells change from the epithelium of the esophagus, which resists friction, to an epithelium that lines the stomach and resists the harsh chemical environment. Surface lining cells (SLCs) line the luminal surface of the stomach. These cells have tight junctional complexes between adjacent cells, creating a watertight barrier to keep stomach contents from leaking into the underlying tissue. The stomach epithelium is pitted in the mucosa. These gastric pits are invaginations of the epithelium downward through the lamina propria. They populate the entire mucosal layer, vastly increasing the surface area of the epithelium and creating a protected environment in the pits for the secretory cells.
The cells in the upper or neck area of the gastric pits produce a visible mucus, rich in bicarbonate, which coats the SLCs and protects them from the harmful HCl in the stomach. Farther in these pits, parietal cells become more numerous. These are the HCl-producing (oxyntic) cells of the stomach. Populating the base of the pits are chief cells, which produce and secrete an enzyme-rich mixture and are classified as zymogenic cells. The final category of cells found in the stomach is the enteroendocrine cells, which produce a number of hormones in response to stomach activity including glucagon (to mobilize liver glycogen), gastrin (to signal HCl production), and serotonin (to stimulate stomach peristalsis).
Divisions
In the cardia of the stomach, gastric glands rich in mucus-producing cells protect against the harmful acid in the stomach. In the fundus of the stomach, the dome-shaped superior portion, and the body of the stomach, the gastric glands (fundic glands) are densely populated with parietal cells. The most inferior region, just before joining the small intestine, is the pylorus, which, like the cardiac region, is rich in mucus-producing cells that neutralize the stomach acid before transferring stomach contents to the intestine. Finally, at the junction of the stomach and intestine, the inner circular layer of the stomach is expanded into the pyloric sphincter, which regulates the passage of material from the stomach into the first portion of the small intestine (duodenum).