The GI tract contains all the major organs of the digestive system, in humans and other animals, including the esophagus, stomach, and intestines.

Food taken in through the mouth is digested to extract nutrients and absorb energy, and the waste expelled at the anus as feces.

[6][7] The gastrointestinal tract contains the gut microbiota, with some 1,000 different strains of bacteria having diverse roles in the maintenance of immune health and metabolism, and many other microorganisms.

This differentiates the embryonic borders between the foregut and midgut, and is also the division commonly used by clinicians to describe gastrointestinal bleeding as being of either "upper" or "lower" origin.

Upon dissection, the duodenum may appear to be a unified organ, but it is divided into four segments based on function, location, and internal anatomy.

The suspensory muscle is an important anatomical landmark that shows the formal division between the duodenum and the jejunum, the first and second parts of the small intestine, respectively.

[20] Its main function is to absorb the products of digestion (including carbohydrates, proteins, lipids, and vitamins) into the bloodstream.

There are three major divisions: The large intestine forms an arch starting at the cecum and ending at the rectum and anal canal.

The result is that a piece of the yolk sac, an endoderm-lined structure in contact with the ventral aspect of the embryo, begins to be pinched off to become the primitive gut.

The submucosa consists of a dense irregular layer of connective tissue with large blood vessels, lymphatics, and nerves branching into the mucosa and muscularis externa.

[24] Whilst the muscularis externa is similar throughout the entire gastrointestinal tract, an exception is the stomach which has an additional inner oblique muscular layer to aid with grinding and mixing of food.

The gut has intrinsic peristaltic activity (basal electrical rhythm) due to its self-contained enteric nervous system.

[25][26] Over 600 of these genes are more specifically expressed in one or more parts of the GI tract and the corresponding proteins have functions related to digestion of food and uptake of nutrients.

[27] The time taken for food to transit through the gastrointestinal tract varies on multiple factors, including age, ethnicity, and gender.

[35] The surface area of the digestive tract is estimated to be about 32 square meters, or about half a badminton court.

[19] With such a large exposure (more than three times larger than the exposed surface of the skin), these immune components function to prevent pathogens from entering the blood and lymph circulatory systems.

[37] Microorganisms also are kept at bay by an extensive immune system comprising the gut-associated lymphoid tissue (GALT) There are additional factors contributing to protection from pathogen invasion.

[39] Other factors in the GI tract contribution to immune function include enzymes secreted in the saliva and bile.

For example, Clostridia, one of the most predominant bacterial groups in the GI tract, play an important role in influencing the dynamics of the gut's immune system.

The large intestine contains multiple types of bacteria that can break down molecules the human body cannot process alone,[41] demonstrating a symbiotic relationship.

For example, certain strains in the large intestine produce vitamin B12;[42] an essential compound in humans for things like DNA synthesis and red blood cell production.

[43] However, the primary function of the large intestine is water absorption from digested material (regulated by the hypothalamus) and the reabsorption of sodium and nutrients.

Enzymes such as CYP3A4, along with the antiporter activities, are also instrumental in the intestine's role of drug metabolism in the detoxification of antigens and xenobiotics.

[45] In most vertebrates, including amphibians, birds, reptiles, egg-laying mammals, and some fish, the gastrointestinal tract ends in a cloaca and not an anus.

Therians (all mammals that do not lay eggs, including humans) possess separate anal and uro-genital openings.

Ruminants show many specializations for digesting and fermenting tough plant material, consisting of additional stomach compartments.

Many birds and other animals have a specialised stomach in the digestive tract called a gizzard used for grinding up food.

In 2020, the oldest known fossil digestive tract, of an extinct wormlike organism in the Cloudinidae was discovered; it lived during the late Ediacaran period about 550 million years ago.

Various pathogens, such as bacteria that cause foodborne illnesses, can induce gastroenteritis which results from inflammation of the stomach and small intestine.

Antibiotics to treat such bacterial infections can decrease the microbiome diversity of the gastrointestinal tract, and further enable inflammatory mediators.