Gastrointestinal tract

The gastrointestinal tract, also called the GI tract, digestive tract, and the alimentary canal, is the passageway of the digestive system that leads from the mouth to the anus. It is one of the largest systems in the body and contains all the major organs of digestion, including the esophagus, stomach, and intestines. Food enters through the mouth, gets digested to extract nutrients, and the body absorbs energy. What is left exits the body as feces through the anus.

Most animals have a complete digestive tract, but some simpler creatures have different ways of handling digestion and waste. The human gastrointestinal tract is divided into the upper and lower tracts and includes the esophagus, stomach, small intestine, and large intestine. The whole tract is about nine meters (30 feet) long but is shorter in living bodies because the intestines can relax and tighten.

The human gut also hosts many tiny living things, such as bacteria, archaea, viruses, and other cells. These help keep the body healthy and manage how it uses food. Special cells in the GI tract release hormones that control digestion, helping the process work smoothly throughout life.

Human gastrointestinal tract

Structure

The gastrointestinal tract, or GI tract, is the passageway for digestion in the body. It includes major organs like the esophagus, stomach, and intestines. The tract is divided into an upper and lower part, with the intestines split into small and large sections.

Upper gastrointestinal tract

Main articles: Mouth, Pharynx, Esophagus, Stomach, and duodenum

The upper GI tract includes the mouth, pharynx, esophagus, stomach, and duodenum. The suspensory muscle of the duodenum marks the division between the upper and lower tracts. The duodenum has four segments: bulb, descending, horizontal, and ascending.

Lower gastrointestinal tract

The lower GI tract includes most of the small intestine and all of the large intestine. The small intestine has three parts: duodenum, jejunum, and ileum. The large intestine includes the cecum, colon, rectum, and anal canal.

Small intestine

Main articles: Small intestine, Duodenum, Jejunum, and Ileum

The small intestine is about 6 to 7 meters long. It absorbs digestion products into the bloodstream. It has three parts: duodenum, jejunum, and ileum.

Large intestine

Main article: Large intestine

The large intestine absorbs water and salts. It includes the cecum, colon, rectum, and anal canal.

Development

Main article: Development of the digestive system

The gut develops from the embryo. It forms from a piece of the yolk sac and becomes the primitive gut. This develops into three segments: foregut, midgut, and hindgut.

Histology

Main article: Gastrointestinal wall

The GI tract has four layers: mucosa, submucosa, muscular layer, and adventitia or serosa.

Mucosa

See also: Intestinal mucosa, Oral mucosa, and Gastric mucosa

The mucosa is the innermost layer. It contacts digested food and has layers like epithelium, lamina propria, and muscularis mucosae.

Submucosa

Main article: Submucosa

The submucosa has connective tissue, blood vessels, and nerves.

Muscular layer

The muscular layer has circular and longitudinal layers that help move food through the tract.

Adventitia and serosa

Main articles: Serous membrane and Adventitia

The outer layer of the GI tract is made of connective tissue.

Gene and protein expression

About 75% of human genes are active in the digestive system. Specific proteins help with digestion and nutrient uptake.

Transit time

Food takes about 2.5 to 5 hours to leave the stomach and 30 to 50 hours to pass through the colon.

Immune function

The GI tract is part of the immune system. It has barriers and immune components to protect against harmful microbes.

Immune barrier

The digestive tract has a large surface area and includes barriers like low stomach pH and mucus to protect against pathogens.

Immune system homeostasis

Beneficial bacteria help keep the immune system balanced. A high-fiber diet can support this balance.

Gastrointestinal microbiota

Main article: Gut microbiota

The large intestine has beneficial bacteria that help break down molecules and produce vitamins. These bacteria also help with water absorption and nutrient reabsorption.

Other animals

See also: Equine anatomy § Digestive system, and Ruminant § Digestive system of ruminants

Many animals, including amphibians, birds, reptiles, egg-laying mammals, and some fish, have a special opening called a cloaca where the urinary and other systems meet, instead of a separate anus. Mammals, including humans, have separate openings for these systems.

Some animals, like ruminants, have extra stomach parts to help break down tough plants and can bring up food to chew again. Birds and other animals often have a special part in their stomach called a gizzard to grind up food. Many birds also have a pouch called a crop near their esophagus.

Scientists discovered a very old fossil of a digestive tract from a wormlike creature that lived about 550 million years ago during the late Ediacaran period.

Clinical significance

Main articles: Gastrointestinal disease and Gastroenterology

Further information: Clinical significance

Diseases

Many diseases can affect the gastrointestinal system. These include infections, inflammation, and cancer. Bacteria that cause foodborne illnesses can lead to gastroenteritis, which is inflammation of the stomach and small intestine. Gastroenteritis is the most common disease of the GI tract.

Gastrointestinal cancer can occur anywhere in the tract, including the mouth, tongue, esophagus, stomach, and colon. Other conditions include inflammation of the ileum (ileitis) or large intestine (colitis), and appendicitis, which is inflammation of the appendix.

Diverticular disease is common in older people and affects the large intestine. It involves pouches forming on the intestinal wall, which can become inflamed. Inflammatory bowel disease includes Crohn's disease and ulcerative colitis, both of which cause inflammation in the bowel.

Symptoms

Several symptoms can indicate problems with the gastrointestinal tract, such as vomiting, diarrhea, constipation, and blood in stool.

Treatment

Gastrointestinal surgery can often be done as an outpatient procedure. In the United States, operations on the digestive system are among the most common outpatient surgeries.

Imaging

Various imaging methods can be used to examine the gastrointestinal tract, including swallowing special dyes for X-rays, endoscopy, colonoscopy, and capsule endoscopy.

Other related diseases

Other diseases related to the gastrointestinal tract include cholera, giardiasis, pancreatitis, peptic ulcer disease, and yellow fever. Helicobacter pylori is a bacterium that can infect the stomach and cause gastritis and other symptoms. Coeliac disease is triggered by eating gluten and requires a lifelong gluten-free diet. Other conditions include ileus (a blockage of the intestines), endometriosis affecting the intestines, and Hirschsprung's disease.

Uses of animal guts

Intestines from animals are used in many interesting ways. For example, intestines from young calves are used to make a special substance called rennet, which helps turn milk into cheese. Pig and calf intestines can be eaten, and pig intestines are often used to hold sausage.

Animal guts are also used to make strings for musical instruments. Long ago, musicians used strings made from lamb gut, but today most strings are made from silk or strong materials like nylon or steel. Some musicians still prefer gut strings for their special sound. Gut strings were also used for tennis rackets in the past, and the best ones today come from cow gut. Gut cord was used for the strings of snare drums, though most drums now use metal wire. Traditional drums from North Africa still use gut cord.

Animal gut is also used to make natural sausage casings. Foods like kokoretsi and haggis are wrapped in or served with animal gut. Chitterlings are a food made from cleaned pig’s gut. In the past, animal gut was used to make cords for clocks, but now metal wire is used instead. The oldest known condoms were made from animal intestine long ago.