The digestive tract (also known as the alimentary canal) is the system of organs within multicellular animals that takes in food, digests it to extract energy and nutrients, and expels the remaining waste. The major functions of the GI tract are ingestion, digestion, absorption, and defecation. The GI tract differs substantially from animal to animal. Some animals have multi-chambered stomachs, while some animals' stomachs contain a single chamber. In a normal human adult male, the GI tract is approximately 6.5 meters (20 feet) long and consists of the upper and lower GI tracts. The tract may also be divided into foregut, midgut, and hindgut, reflecting the embryological origin of each segment of the tract.
Upper gastrointestinal tract
The upper GI tract consists of the mouth, pharynx, esophagus, and stomach.
- The mouth contains the buccal mucosa, which contains the openings of the salivary glands; the tongue; and the teeth.
- Behind the mouth lies the pharynx, which leads to a hollow muscular tube, the esophagus.
- Peristalsis takes place, which is the contraction of muscles to propel the food down the esophagus which extends through the chest and pierces the diaphragm to reach the stomach..
Lower gastrointestinal tract
The lower GI tract comprises the intestines and anus.
- Bowel or intestine
- Small intestine, which has three parts:
- Large intestine, which has three parts:
- Cecum (the vermiform appendix is attached to the cecum).
- Colon (ascending colon, transverse colon, descending colon and sigmoid flexure)
- Rectum
- Anus
Accessory organs
Accessory organs to the alimentary canal include the liver, gallbladder, and pancreas. The liver secretes bile into the small intestine via the biliary system, employing the gallbladder as a reservoir. Apart from storing and concentrating bile, the gallbladder has no other specific function. The pancreas secretes an isosmotic fluid containing bicarbonate and several enzymes, including trypsin, chymotrypsin, lipase, and pancreatic amylase, as well as nucleolytic enzymes (deoxyribonuclease and ribonuclease), into the small intestine. Both of these secretory organs aid in digestion.
Embryology
The gut is an endoderm-derived structure. At approximately the 16th day of human development, the embryo begins to fold ventrally (with the embryo's ventral surface becoming concave) in two directions: the sides of the embryo fold in on each other and the head and tail fold towards one another. 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 yolk sac remains connected to the gut tube via the vitelline duct. Usually this structure regresses during development; in cases where it does not, it is known as Meckel's diverticulum.
During fetal life, the primitive gut can be divided into three segments: foregut, midgut, and hindgut. Although these terms are often used in reference to segments of the primitive gut, they are nevertheless used regularly to describe components of the definitive gut as well.
Each segment of the primitive gut gives rise to specific gut and gut-related structures in the adult. Components derived from the gut proper, including the stomach and colon, develop as swellings or dilatations of the primitive gut. In contrast, gut-related derivatives—that is, those structures that derive from the primitive gut but are not part of the gut proper—in general develop as outpouchings of the primitive gut. The blood vessels supplying these structures remain constant throughout development.
