Understanding the Human Digestive System
The Human Digestive System
Parts of the Digestive System
The digestive system is essentially a complex tube, primarily muscular, of endodermal and ectodermal origin. It begins at the mouth and ends at the anus, featuring associated glands that release necessary digestive products into the tube. This tube is lined with four layers of tissue, from innermost to outermost: the mucosa (epithelium), the submucosa (connective tissue containing major blood vessels), the muscularis (typically two layers, one longitudinal and one circular), and the fibroserosa (the visceral layer of the peritoneum).
The components of the digestive system include: the mouth (oral cavity), salivary glands, pharynx, esophagus, cardia, stomach, pylorus, accessory glands (liver, gallbladder, and pancreas), small intestine, ileocecal valve, large intestine, rectum, and anus.
The Mouth or Oral Cavity
Separated from the outside by the lips, the oral cavity contains the teeth, palatal vault, floor of the mouth, soft palate, uvula, and tongue.
Humans, like all mammals, have lips (fleshy folds separating the oral cavity from the outside) and teeth for shredding and breaking down food. The lips are important for modulating sounds and suckling.
Children have 20 deciduous (milk) teeth, which are replaced by 28 permanent teeth between the ages of 6 and 12. Four wisdom teeth may erupt later, bringing the total to 32. Teeth are anchored in the jawbone by roots. The types of teeth are:
- 4 incisors: sharp edges for cutting
- 2 canines: long, tapered for tearing
- 4 premolars (bicuspids): flat surfaces for grinding and crushing
- 6 molars (including wisdom teeth): for grinding and shredding
Each tooth has three parts: the crown (above the gumline), the neck (at the gumline), and the root (embedded in the jawbone by the periodontal ligament). The tooth’s structure includes the pulp chamber (containing blood vessels and nerves), the dentine (ivory), the enamel (hard outer layer of the crown), and the cementum (covering the root).
The roof of the mouth includes the palatal vault (hard palate, a bony structure) and the soft palate, which ends in the uvula. The tongue, a muscular organ on the floor of the mouth, aids in taste, chewing, swallowing, and speech.
Salivary Glands
Besides mucus-secreting cells in the oral mucosa, there are three pairs of salivary glands: the parotid glands, the submandibular glands, and the sublingual glands. These glands produce saliva, which contains ptyalin (a digestive enzyme) and mucus.
The Pharynx
This muscular cavity, common to the respiratory and digestive systems, is divided into three parts: the nasopharynx, the oropharynx, and the laryngopharynx. The pharynx plays a role in swallowing, voice tone, and warming inspired air. Tonsils, lymphoid tissue in the pharynx, provide defense against infection.
The Esophagus
This tube, about 30 cm long, connects the pharynx to the stomach. It moves food to the stomach via peristalsis.
The Cardia
This sphincter controls the passage of food into the stomach.
The Stomach
Located in the upper left abdomen, the stomach has three parts: the fundus, the body, and the antrum (pyloric region). Its muscular walls churn food and mix it with gastric juice, which is secreted by gastric glands and contains pepsin, hydrochloric acid, intrinsic factor, and mucus.
The Pylorus
This sphincter controls the release of food from the stomach into the small intestine.
Accessory Glands
The liver, gallbladder, and pancreas are essential for digestion. The liver produces bile, which is stored and concentrated in the gallbladder. The pancreas produces pancreatic juice and hormones (insulin and glucagon).
The Small Intestine
This 7-meter long tube has three parts: the duodenum, the jejunum, and the ileum. Intestinal glands secrete intestinal juice, and numerous villi absorb nutrients.
The Ileocecal Valve
This valve prevents backflow from the large intestine into the small intestine.
The Large Intestine
The large intestine, or colon, includes the cecum (with the vermiform appendix), the ascending colon, the transverse colon, the descending colon, and the sigmoid colon.
The Rectum
The final 17-20 cm of the digestive tract, the rectum, stores feces before elimination.
The Anus
The anus is the opening for the expulsion of feces, controlled by sphincter muscles.