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Salivary glands Parotid gland Submandibular gland Sublingual gland Pharynx Tongue Esophagus Pancreas Pancreatic duct Stomach Ileum Anus Rectum Vermiform appendix Cecum Descending colon Ascending colon Transverse colon Bile duct Duodenum Gallbladder Liver Oral cavity
Upper and Lower human gastrointestinal tract

The digestive system is the system by which ingested food is acted upon by physical and chemical means to provide the body with nutrients it can absorb and to excrete waste products; in mammals the system includes the alimentary canal extending from the mouth to the anus, and the hormones and enzymes assisting in digestion.

In an adult male human, the gastrointestinal (GI) tract is approximately 5 metres (20 ft) long in a live subject, or up to 9 metres (30 ft) without the effect of muscle tone, 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.[1]

The GI tract releases hormones as to help regulate the digestion process. These hormones, including gastrin, secretin, cholecystokinin, and grehlin, are mediated through either intracrine or autocrine mechanisms, indicating that the cells releasing these hormones are conserved structures throughout evolution.[2]


Upper gastrointestinal tract

The upper gastrointestinal tract consists of the mouth cavity, pharynx, esophagus, stomach,and duodenum.

  • Behind the mouth lies the epiglottis which prevents food from entering the voice box and leads to a hollow muscular tube, the esophagus.

Lower gastrointestinal tract

The lower gastrointestinal tract comprises the most of the intestines and the anus.

Component Organs

The main organs of the digestive system are:

  • Mouth
  • Esophagus
  • Stomach
  • Small and Large intestines
  • Rectum
  • Anus

Other organs consist of the:

  • Salivary glands
  • Gallbladder
  • Liver
  • Pancreas

Accessory organs

Accessory organs to the alimentary canal include the liver, gallbladder, and pancreas. The liver secretes bile into the small intestine via the bile duct 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, which helps neutralize the acidic chyme, 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.



During fetal life, the primitive gut can be divided into three segments: foregut, midgut, and hindgut. Although these terms often are used in reference to segments of the primitive gut, they nevertheless are used regularly to describe components of the definitive gut as well.


part part in adult Gives rise to Arterial supply

Specialization of organs

Four organs are subject to specialization in the kingdom Animalia:[citation needed]

  • The first organ is the tongue, which is only present in the phylum Chordata.
  • The second organ is the esophagus. In birds, insects, and other invertebrates, the crop is an enlargement of the esophagus that is used to store food temporarily.
  • The third organ is the stomach. In addition to a glandular stomach (proventriculus), birds have a muscular "stomach" called the ventriculus or "gizzard". The gizzard is used to grind up food mechanically.
  • The fourth organ is the large intestine. Non-ruminant herbivores, such as rabbits, have an outpouching of the large intestine called the cecum, which aids in digestion of plant material such as cellulose.

Transit time

The time taken for food or other ingested objects to transit through the gastrointestinal tract varies depending on many factors, but roughly, it takes 2.5 to 3 hours after meal for 50% of stomach contents to empty into the intestines and total emptying of the stomach takes 4 to 5 hours. Subsequently, 50% emptying of the small intestine takes 2.5 to 3 hours. Finally, transit through the colon takes 30 to 40 hours.[4]



Immune function

The gastrointestinal tract also is a prominent part of the immune system.[5] The surface area of the digestive tract is estimated to be the surface area of a football field. With such a large exposure, the immune system must work hard to prevent pathogens from entering into blood and lymph.[6]




General structure of the gut wall

The gastrointestinal tract has a uniform general histology with some differences that reflect the specialization in functional anatomy.[7] The GI tract can be divided into four concentric layers:


The mucosa is the innermost layer of the gastrointestinal tract that is surrounding the lumen, or space within the tube. This layer comes in direct contact with food (or bolus), and is responsible for absorption and secretion, important processes in digestion.

The mucosa can be divided into:




Muscularis externa

The muscularis externa consists of an inner circular layer and a longitudinal outer muscular layer. The circular muscle layer prevents food from traveling backward and the longitudinal layer shortens the tract. The coordinated contractions of these layers is called peristalsis and propels the bolus, or balled-up food, through the GI tract.

Between the two muscle layers are the myenteric or Auerbach's plexus.


The adventitia consists of several layers of epithelia.


See also


  1. ^ lungs; i hate ms hawkiuns Hopkins, Charles William McLaughlin, Susan Johnson, Maryanna Quon Warner, David LaHart, Jill D. Wright. Human Biology and Health. Prentice Hall. ISBN 0-13-981176-1. 
  4. ^ Colorado State University > Gastrointestinal Transit: How Long Does It Take? Last updated on May 27, 2006. Author: R. Bowen.
  6. ^ Animal Physiology textbook


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