Gastrointestinal tract

The gastrointestinal tract, often called the GI tract, digestive tract, or alimentary canal, is the continuous passageway of the digestive system that runs from the mouth to the anus. It ranks among the largest systems in the body and houses all the primary digestive organs—the esophagus, stomach, and intestines. Food enters through the mouth, where digestion breaks it down to extract nutrients and absorb energy, with leftover waste exiting the anus as feces. The term "gastrointestinal" specifically refers to the stomach and intestines.

Other animals

While most animals possess a complete, through-gut digestive tract, some simpler creatures differ. Sponges have tiny pores (ostia) for digestion and a larger dorsal pore (osculum) for excretion; comb jellies feature a ventral mouth and separate dorsal anal pores; and cnidarians and acoels use a single pore for both digestion and excretion. In humans, the GI tract includes the esophagus, stomach, and intestines, and is split into upper and lower sections. It encompasses all structures from the mouth to the anus, forming a continuous pathway that includes the main digestive organs: the stomach, small intestine, and large intestine.

The full human digestive system comprises the GI tract plus accessory organs—the tongue, salivary glands, pancreas, liver, and gallbladder. The tract can also be divided by embryonic origin into foregut, midgut, and hindgut. At autopsy, the entire human GI tract measures about nine meters (30 feet) long. In a living body, it is considerably shorter because the intestines, composed of smooth muscle, maintain constant tone in a semi-tense state but can relax locally to allow for distension and peristalsis.

Structure

The human gut microbiota consists of roughly 4,000 different strains of bacteria, archaea, viruses, and eukaryotes, playing diverse roles in immune health and metabolism. Enteroendocrine cells in the GI tract release hormones—such as gastrin, secretin, cholecystokinin, and ghrelin—to regulate digestion. These hormones act through intracrine or autocrine mechanisms, and the cells that release them are evolutionarily conserved structures. The GI tract’s structure and function are described by both gross anatomy and microscopic anatomy (histology).

It is divided into upper and lower tracts, and the intestines into small and large intestines. The upper GI tract includes the mouth, pharynx, esophagus, stomach, and duodenum. The exact boundary between upper and lower tracts is the suspensory muscle of the duodenum, which marks the embryonic border between foregut and midgut and is used clinically to classify gastrointestinal bleeding as upper or lower.

Although the duodenum appears as a single organ, it has four segments based on function, location, and internal anatomy: bulb, descending, horizontal, and ascending (moving from the stomach toward the jejunum). The suspensory muscle of the duodenum, a thin muscle derived from embryonic mesoderm, suspends the superior border of the ascending duodenum from the diaphragm and serves as a key landmark dividing the duodenum from the jejunum (the first and second parts of the small intestine). The lower GI tract includes most of the small intestine and all of the large intestine. In human anatomy, the intestine (also called bowel or gut) extends from the pyloric sphincter of the stomach to the anus and, as in other mammals, consists of two segments: the small intestine and the large intestine.

The small intestine is further subdivided into the duodenum, jejunum, and ileum. The large intestine is subdivided into the cecum; the ascending, transverse, descending, and sigmoid colons; the rectum; and the anal canal. The small intestine is a tubular structure about 6 to 7 meters long, starting at the duodenum and ending at the ileum. Its mucosal area in an adult human is roughly 30 m² (320 sq ft).

Circular folds, villi, and microvilli increase the absorptive surface area about 600-fold, giving the entire small intestine a total area of about 250 m² (2,700 sq ft). Its main function is to absorb digestion products—carbohydrates, proteins, lipids, and vitamins—into the bloodstream. It has three major divisions: - A short section (about 20–25 cm long) that receives chyme from the stomach, along with pancreatic juice containing digestive enzymes and bile from the gallbladder. Digestive enzymes break down proteins, and bile emulsifies fats into micelles.

The duodenum contains Brunner’s glands, which produce a mucus-rich, alkaline secretion containing bicarbonate. These secretions, combined with bicarbonate from the pancreas, neutralize stomach acids in the chyme. - The midsection of the small intestine, connecting the duodenum to the ileum. It is about 2.5 m (8.2 ft) long and features circular folds (plicae circulares) and villi that increase its surface area. Here, sugars, amino acids, and fatty acids are absorbed into the bloodstream. - The final section of the small intestine, about 3 m long, with villi similar to the jejunum.