Digestive System

The digestive system breaks down food, absorbs nutrients, and removes waste. It includes two major parts: the gastrointestinal tract and the accessory digestive organs. Together, these structures carry food from the mouth to the anus and support each step of digestion.

The gastrointestinal tract begins with the mouth, where food is chewed and mixed with saliva. Saliva contains enzymes that start the digestion of carbohydrates. The pharynx, or throat, then moves the chewed food from the mouth...

The peritoneum is a vital membrane in the abdominal cavity. It lines the cavity and covers most of the organs inside it. It helps protect the organs and gives them a smooth surface for movement.

The peritoneum has two layers: the parietal peritoneum and the visceral peritoneum. The parietal peritoneum lines the inner surface of the abdominal wall. It also includes the diaphragmatic peritoneum, pelvic peritoneum, and posterior abdominal wall peritoneum. The visceral peritoneum covers the...

The gastrointestinal (GI) tract is built from four continuous tissue layers. These layers are the mucosa, submucosa, muscularis, and serosa. Each layer changes to match its job along the digestive tract.

The mucosa is the inner layer and is often called a mucous membrane because it helps with mucus secretion. It includes the epithelium, which directly contacts swallowed material, and the lamina propria, a layer of connective tissue. A thin smooth muscle layer called the muscularis mucosae is...

The enteric nervous system helps control digestion in the gastrointestinal, or GI, tract. It is a network of nerves inside the gut and is often called the body’s “second brain.” This system helps manage digestion, nutrient absorption, and the movement of food.

The enteric nervous system has two main plexuses, or nerve networks. These are the myenteric plexus, also called Auerbach’s plexus, and the submucosal plexus, also called Meissner’s plexus. Each plexus sits in a different layer of the GI...

Digestive activity is controlled by reflexes, hormones, and local signals. Receptors in the walls of the stomach and intestines detect mechanical and chemical changes. They respond to stretching, pH, osmolarity, and the presence of food and its breakdown products.

These signals activate glands and smooth muscles. Smooth muscle reflexes help mix food and move it along the digestive tract. Glands can also be stimulated or inhibited. This changes the release of digestive enzymes into the lumen,...

Digestive blood flow depends on the splanchnic circulation, the network of blood vessels that supplies and drains the abdominal organs involved in digestion. These organs include the stomach, liver, pancreas, intestines, and spleen. The circulation brings oxygen and nutrients to these organs and carries away waste products.

Three main arteries make up this blood supply. The celiac artery, also called the celiac trunk, and the superior and inferior mesenteric arteries all arise from the...

The mouth, or oral cavity, is a complex part of the human body. It helps with chewing and swallowing food. It also supports speech and facial expressions.

Teeth are the hardest structures in the body. Humans have two sets of teeth during life: deciduous, or baby, teeth and permanent teeth. Each tooth has a crown, which is the visible part, and a root, which sits in the jaw. The tooth also includes enamel, dentin, pulp, and cementum. Enamel is the hard outer layer. Dentin lies under the enamel.

The tongue is a muscular organ in the oral cavity that helps with swallowing, speech, taste, and disease detection. It sits in the mouth and is held in place by the lingual frenulum, a fold of mucous membrane that attaches it to the floor of the mouth. The base of the tongue connects with the hyoid bone, the mandible, the styloid processes, and the pharynx.

The tongue contains eight muscles that work together to move and shape it. The extrinsic muscles begin outside the tongue and create its...

Salivary glands make saliva and help start digestion in the mouth. The three pairs of salivary glands are the parotid, submandibular, and sublingual glands. They also help keep the mouth moist and support oral health.

The parotid glands sit near the ears and below the masseter muscle. They release saliva into the mouth through the parotid duct of Stensen. The submandibular glands are on the floor of the mouth and send saliva through submandibular ducts. The sublingual glands lie under the...

Teeth develop before birth and continue to change from childhood through adulthood. This process is called odontogenesis. It begins around the sixth week of embryonic development and follows three main stages: the bud stage, the cap stage, and the bell stage.

During the bud stage, a tooth germ starts to form in the developing jawbone. In the cap stage, the tooth germ separates into the enamel organ, dental papilla, and dental sac. These structures later become the enamel, dentin and pulp, and...

A human tooth has several parts that work together for eating, speaking, and shaping the face. The visible part is the crown. It is covered by enamel, the hardest substance in the human body. The crown is shaped differently in each type of tooth, which helps teeth cut, tear, or grind food. The neck, also called the cervix, is the narrow area that connects the crown to the root. It is usually found at the gum line. The root holds the tooth in the jawbone and keeps it stable.

The number of roots...

The esophagus is a muscular tube that moves food from the pharynx to the stomach. It is about 10 inches, or 25.4 cm, long and lies behind the trachea. When a person is not swallowing, the tube stays collapsed. It also runs mostly straight through the thoracic mediastinum and passes into the abdominal cavity through the esophageal hiatus in the diaphragm.

Food enters the esophagus through the upper esophageal sphincter, which is formed mainly by the cricopharyngeus muscle. The wall of the...

Swallowing is the process that moves food from the mouth to the stomach. It uses the tongue, the muscles of the throat, and the esophagus. Saliva and mucus help food move smoothly. For semi-solid or solid food, swallowing takes about 4 to 8 seconds. Liquids or very soft food usually take about 1 second.

Swallowing has three stages: the voluntary phase, the pharyngeal phase, and the esophageal phase. The voluntary phase is under conscious control. During this stage, chewing ends and the tongue...

The human stomach is a key organ in the digestive system. It sits within the peritoneum, the serous membrane that lines the abdominal cavity. It helps process food and works with other digestive organs through coordinated digestive steps.

The stomach has a J-shape and four main regions. The cardia is the first part, where the esophagus connects to the stomach and food enters. Above and to the left of the cardia is the fundus, the rounded upper part of the stomach.

The body is the central...

The stomach wall has several layers that work together in digestion and protection. The outermost layer is the serosa, which gives support and helps protect the stomach. The muscularis externa contracts to mix food and move it through the stomach.

Beneath the muscularis externa is the submucosa. This layer contains connective tissue, blood vessels, nerves, and glands. These glands secrete mucus and other substances that support digestion.

The innermost layer is the mucosa. It secretes gastric...

Stomach hormones help control digestion by signaling when to make acid, move food, and adjust appetite. These hormones come from enteroendocrine cells in the stomach lining. Even though these cells make up only about 1% of stomach epithelial cells, they have a major effect on digestive function.

G cells secrete gastrin. Gastrin stimulates the production of gastric acid and enzymes. It also increases contraction of stomach muscles, which helps move food through the digestive tract.

D cells...

Digestion begins before food reaches the stomach. The cephalic phase is the first stage, and it is controlled by the central nervous system. It starts when food-related sensory cues, such as the sight and smell of food, send signals to the brain.

As eating begins, taste receptors strengthen these signals. The information travels to the cerebral cortex, then to the hypothalamus and medulla, where it is processed. These brain responses help prepare the stomach for digestion.

The output from the...

The gastric phase of digestion starts when food enters the stomach and stretches its walls. This stretch activates stretch receptors and local reflexes in the enteric nervous system, especially through the myenteric plexus. These neural signals help the stomach begin secretory activity and prepare food for further digestion and absorption.

During this phase, gastric cells release gastric juice. This fluid contains hydrochloric acid, pepsinogen, and mucus. Chemical signals also matter. Changes...

The intestinal phase of digestion begins when chyme enters the small intestine. Chyme is the partially digested mix of food and digestive enzymes that leaves the stomach. This final stage follows the cephalic and gastric phases and helps the body absorb nutrients efficiently.

As chyme enters the duodenum, the first part of the small intestine, it stretches the intestinal wall. This distension triggers the enterogastric reflex, a neural response that slows stomach activity. The reflex gives the...

The stomach uses parietal cells in the gastric glands to make hydrochloric acid, or HCl. This acid helps digestion because it is highly corrosive and can break down food. Parietal cells release hydrogen ions and chloride ions into the gland lumen, and these ions combine to form HCl.

Inside the parietal cell, water and carbon dioxide react to form carbonic acid. Carbonic acid then breaks apart into bicarbonate and hydrogen ions. The bicarbonate ions are exchanged for chloride ions from the...

Gastric motility is the movement of stomach muscles that turns food into chyme, a semi-liquid mixture ready for digestion in the intestines. It depends on coordinated contraction and relaxation of the stomach wall. Neural signals and smooth muscle action work together in this process.

The process begins when the vagus nerve causes the smooth muscles in the fundus and body of the stomach to relax. This lets these upper regions expand and hold food and liquid, up to about 1.5 liters. The fundus...

Gastric emptying is the process that slowly releases chyme from the stomach into the duodenum. Chyme is the partly digested food mixture that leaves the stomach. When the stomach stretches, it triggers gastrin release. Gastrin is a hormone that helps increase gastric acid secretion, which supports digestion.

Stomach distension also helps drive peristaltic waves. Peristalsis is the wave-like muscle movement that pushes food forward. These waves move stomach contents toward the pyloric region.

The liver is the largest gland in the human body. It is a firm, reddish-brown, wedge-shaped organ. In adults, it weighs about 1.5 kg and lies mainly in the right hypochondriac and epigastric regions. It sits under the diaphragm and extends farther to the right of the body’s midline than to the left.

The liver is almost completely protected by the rib cage. Its surface is covered by a strong fibrous capsule and a layer of visceral peritoneum. The only exception is the superior bare area, which...

The liver is built from tiny hexagonal lobules that form its microscopic structure. Each lobule is about the size of a sesame seed. Inside each one, plates of hepatocytes, or liver cells, radiate outward from a central vein.

Hepatocytes are highly active cells with many organelles, including rough and smooth endoplasmic reticulum, Golgi apparatus, peroxisomes, and mitochondria. These cells carry out many key jobs in the body. They secrete about 900 ml of bile each day, store glucose as...

Bile helps the body digest fats in the small intestine. It is a yellow-green, alkaline fluid made by the liver. From there, it moves through the common hepatic duct. It may enter the cystic duct and reach the gallbladder, or it may flow into the common bile duct.

The sphincter of Oddi controls bile flow at the entrance to the duodenum. When this sphincter is closed, bile is sent to the gallbladder for storage and concentration. When dietary fats enter the duodenum, the gallbladder contracts.

The gallbladder is a small, pear-shaped organ in the digestive system. It sits in a hollow area on the lower surface of the liver. About 10 cm long, it is roughly the size of a kiwi fruit.

Its main job is to store and concentrate bile. Bile is a fluid made by the liver that helps with digestion. When the gallbladder is full, it holds about 40 to 70 mL of bile.

The gallbladder has three regions: the fundus, body, and neck. A tube called the cystic duct extends from the neck. It joins the...

The liver helps keep blood chemistry stable and supports many body systems. It performs metabolic, hematological, endocrine, and bile-producing functions that are essential for homeostasis.

In metabolism, the liver helps regulate blood glucose levels in the normal range of 70 to 110 mg/dL. When glucose levels drop, it breaks down glycogen and releases glucose into the blood. It can also make glucose from other carbohydrates or amino acids through gluconeogenesis, which means making new glucose.

Liver and gallbladder diseases can affect how the body processes bile and bilirubin. Common conditions include cirrhosis, hepatitis, non-alcoholic fatty liver disease (NAFLD), and gallstones. Jaundice is also a frequent sign of liver and biliary disease.

Cirrhosis is a serious liver condition in which the hepatic lobules become scarred and are replaced by fibrous tissue. This change makes it harder for the liver to do its normal work. NAFLD happens when too much fat builds up in the liver and...

The pancreas is a pinkish-gray organ behind the stomach. It stretches from the duodenum toward the spleen and is tightly attached to the back wall of the abdominal cavity. Its surface is lumpy and lobular, and its shape includes a broad head, a slender body, and a short, blunt tail.

The pancreas has two jobs. Its exocrine part makes pancreatic juice for digestion, while its endocrine part releases hormones into the body. The exocrine tissue is built from acini and ducts. Acini are clusters of...

Pancreatic juice supports digestion in the small intestine. It is a clear fluid made by the pancreas and contains water, salts, sodium bicarbonate, and enzymes. These ingredients help break down large food molecules so nutrients can be absorbed.

Hormones control when pancreatic juice is released. Acidic chyme from the stomach entering the duodenum triggers secretin, a hormone that signals pancreatic juice secretion. After a fatty meal, cholecystokinin stimulates gallbladder contraction and...

The small intestine is the main site for digestion and nutrient absorption. It runs from the pyloric sphincter of the stomach to the ileocecal valve, where it joins the large intestine. Its long, folded structure helps food move through the digestive system and allows nutrients to be absorbed.

The small intestine has three sections: the duodenum, jejunum, and ileum. The duodenum is about 25 cm long and sits closest to the stomach. It acts like a mixing bowl, where chyme, or partially digested...

The small intestine has folds and cells that help it absorb nutrients from food. Its lining includes circular folds, villi, and specialized cells that work together during digestion. These structures increase surface area and support the movement of digested material through the gut.

Circular folds are transverse ridges in the intestinal wall. Each fold contains fingerlike villi, which are covered by simple columnar epithelium. These absorptive cells, also called enterocytes, have microvilli...

The small intestine uses mixing movements and enzymes to finish digestion. It performs both mechanical digestion and chemical digestion. These steps help break food into forms the body can absorb.

Mechanical digestion in the small intestine includes segmentations and migrating motility complexes, or MMCs. The myenteric plexus mainly controls these movements. Segmentations are local contractions in areas stretched by chyme, the partly digested food mixture. They mix chyme with digestive juices...

The large intestine follows a clear pathway from the ileocecal valve to the anus. It surrounds the small intestine on three sides and is divided into the cecum, colon, and rectum. The ileocecal sphincter, a fold of mucous membrane, controls the opening from the ileum into the large intestine.

The large intestine has four main layers: mucosa, submucosa, muscularis, and serosa. These layers work together to support water absorption, movement, and protection in the colon.

The mucosa is the innermost layer. It includes simple columnar epithelium, the lamina propria, and the muscularis mucosae. This layer contains absorptive cells that take up water and goblet cells that release mucus. The cells sit in tubular intestinal glands called crypts of Lieberkühn. Solitary lymphatic nodules are...

Gut bacteria in the colon help the body in several important ways. This community is part of the gut microbiome, the collection of microbes living in the digestive tract. It includes bacteria that begin living in the gut at birth and continue to change over time.

The large intestine also gains bacteria from food, and some enter the colon through the anus. Diet, lifestyle, and stress can all influence how this bacterial community develops. As the normal gut flora grows, it helps support health...

The large intestine helps finish digestion by absorbing water, vitamins, and electrolytes. It also moves leftover material toward the rectum. In the cecum, chyme arrives with undigested carbohydrates that are ready for fermentation.

Gut bacteria ferment these carbohydrates and make short-chain fatty acids. These fatty acids can provide some energy. They also help synthesize essential vitamins.

As chyme moves through the colon, it triggers two slow types of muscle movement: haustral churning...

Small intestine nutrient uptake moves digested nutrients from the intestinal lumen into the body. After digestion, carbohydrates, proteins, and fats are broken into simpler forms. These macronutrients, along with vitamins, minerals, and water, are then ready for absorption into the bloodstream.

Enterocytes are the specialized polar epithelial cells that line the mucosa of the small intestinal wall. Their apical ends have microvilli, which are tiny finger-like projections that increase surface...

Feces move through the large intestine as the final stage of digestion. After chyme stays in the large intestine for about 3 to 10 hours, it loses much of its water and becomes feces. Feces contain undigested dietary fiber such as cellulose, mucus, sloughed-off epithelial cells, and microbes.

The descending colon and sigmoid colon store feces before elimination. Haustral contractions help dry the material, but enough water remains to keep it semi-solid. Mass peristalsis then pushes feces into...

Digestive disorders can affect the esophagus, stomach, small intestine, and bowel. The gastrointestinal tract is vulnerable to several problems that can disrupt normal digestion and cause discomfort. Many of these conditions have distinct causes and symptoms.

When the lower esophageal sphincter is damaged, stomach acid can move back into the esophagus. This leads to gastroesophageal reflux disease, or GERD, which is commonly called heartburn. GERD can cause irritation of the esophageal lining,...