Digestive system
Revised 2025-08-28 12:55:17 PM EDT
The digestive system is a collection of organs that processes food, absorbs nutrients, and removes solid waste. It performs four main tasks:
The system includes the gastrointestinal (GI) tract – a long tube extending from the mouth to the anus – as well as several accessory organs that support digestion. The interior space of the GI tract, or lumen, remains continuous with the external environment even as it passes through the body. Organs within the GI tract include the mouth, pharynx, esophagus, stomach, small intestine, large intestine, and anus. The accessory organs – salivary glands, teeth, tongue, liver, gallbladder, and pancreas – are not part of the tract itself but contribute to digestive processes.
Anatomical location
Digestive organs are located in multiple body cavities.
Clinically, the abdominopelvic region is divided into four quadrants using vertical and horizontal lines intersecting at the umbilicus (belly button, or naval). These quadrants – right upper, left upper, right lower, and left lower – are used to localize pain and identify organ positions.
Headline organs for each abdominopelvic quadrant
GI tract wall structure
The wall of the GI tract is organized into four main layers:
Certain peritoneal extensions have specific names. For example, the greater omentum is a large sheet extending from the stomach and transverse colon that covers the small intestine. It contains fat and lymphatic tissue and can shift position to respond to inflammation or injury.
Mouth, pharynx, and esophagus
Digestion begins in the mouth. Chewing (mastication) breaks down food mechanically, and saliva begins chemical digestion, particularly of carbohydrates. Saliva comes from three pairs of salivary glands and contains both water and enzymes.
The tongue forms the chewed food into a bolus and pushes it into the pharynx. During swallowing (deglutition), the uvula lifts to block the nasal cavity, and the epiglottis folds to prevent food from entering the respiratory tract.
The swallowed bolus then travels down the esophagus, a muscular tube about 8–10 inches long. Peristaltic contractions move the bolus toward the stomach. A muscular ring called the lower esophageal sphincter (LES) controls entry into the stomach and prevents stomach contents from flowing backward.
Stomach
The stomach lies in the left upper quadrant of the abdominal cavity. When empty, its inner surface forms folds called rugae. As food enters, the wall relaxes and the rugae flatten.
The stomach’s mucosa contains glands that secrete enzymes (e.g., pepsin), hydrochloric acid (HCl), and mucus. These secretions help break down proteins, kill microbes, and protect the stomach lining. The muscularis layer of the stomach includes an additional oblique muscle layer that assists in mixing. The partially digested mixture, called chyme, exits the stomach through a muscular ring called the pyloric sphincter and into the small intestine.
Pancreas, liver, and gallbladder
As chyme enters the small intestine from the stomach it mixes with pancreatic juice and bile. The pancreas lies behind the stomach and secretes enzymes that continue digestion of carbohydrates, proteins, and fats. However, the acidic chyme can deactivate these enzymes. To prevent that an alkaline liquid called bile is produced by the liver. It both neutralizes the acidity of the chyme and emulsifies fats (i.e., breaks fats and oils into smaller globules for easier digestion). Bile can flow directly into the small intestine’s duodenum to join the chyme, or the bile can be stored in the gallbladder, a small organ that concentrates bile for later use.
Small intestine
The small intestine, approximately 10 feet in length, is the main site of digestion and absorption. It includes three segments, in this order:
- Duodenum: Receives chyme, bile, and pancreatic enzymes.
- Jejunum: The body’s major site of nutrient absorption.
- Ileum: Transfers remaining material to the large intestine.
The mucosa of the small intestine features villi and microvilli, which maximize surface area for greater absorption. Beneath the epithelial surface lies a dense network of blood capillaries and lymph lacteals. Blood capillaries absorb most nutrients; lymphatic lacteals transport large fats to the lymphatic system, eventually landing back in the deoxygenated blood supply.
The ileocecal valve at the end of the ileum regulates flow into the large intestine and prevents backflow.
Large intestine
The large intestine, about 5 feet long and wider in diameter, includes the cecum, colon, rectum, and anal canal. Unlike the small intestine, it lacks villi and circular folds but features haustra – pouches formed by the muscularis.
Its main functions are water and ion absorption and the formation of feces. Bacteria in the colon assist in breaking down undigested material, producing gas and vitamins such as vitamin K.
After passing the ileocecal valve, material enters the cecum. Attached to the cecum is the appendix, a small tube with lymphoid tissue that supports immune function.
From there, contents move through the colon in this order: ascending, transverse, descending, and sigmoid segments. The colon gradually absorbs water and compacts waste. Haustral contractions and peristalsis slowly compact fecal matter and move it toward the rectum.
The rectum leads to the anal canal. Two sphincters control defecation:
Feces are expelled through the anus.