The human digestive system is a series of interconnected organs consisting of the digestive tract and the accessory digestive organs. The digestive tract is made by the mouth, pharynx, esophagus, stomach, small intestine and large intestine. The small intestine is subdivided into duodenum, jejunum and ileum, and the large intestine is subdivided into caecum, appendix, ascending colon, transverse colon, descending colon, sigmoid colon, rectum and anal canal. The external opening of the anal canal is called anus. Therefore, the digestive tract starts at mouth and ends at the anus, and the entire tract are approximately 9 meters in length in adult. The accessory organs associated with the human digestive system are the tongue, salivary glands, liver, gallbladder and pancreas. The digestive system provides the human body with a continual supply of nutrients, water and electrolytes.

Functions

To achieve a continual supply of nutrients, water and electrolytes, the human digestive system performs the following functions:

Ingestion: Taking food into the mouth is called ingestion.

Propulsion: Propulsion is the transport of food through the digestive tract. It occurs by the processes of swallowing and peristalsis. Swallowing is a reflex response that propels the food from the mouth into the pharynx and then from the pharynx into the esophagus. Peristalsis is a composite wave consisting of a wave of contraction followed by a wave of relaxation that occurs from the esophagus to the rectum. Peristalsis propels the food from the esophagus to the downward direction.

 Secretion: The mucous layer of the digestive tract contains different types of glands which secrete digestive enzymes that help in digestion, and also secrete mucus that lubricate and protect the digestive tract. The accessory digestive organs including salivary glands, liver and pancreas lie completely outside the wall of the digestive tract which also secrete digestive juices that reach in the digestive tract via their duct and helps in digestion.

Digestion: It is a process by which the food particles are broken down into small absorbable units within the digestive tract with the help of digestive juices. Learn more.

Absorption:  Absorption is the movement of nutrients, water and electrolytes from the digestive tract to the blood or lymph. Learn more.

Defecation: Defecation is the elimination of indigestible materials through the anus.

Parts

1. Digestive tract 2. Accessory digestive organs
(i) Mouth

(ii) Pharynx

(iii) Esophagus

(iv) Stomach

(v) Small intestine

  1. Duodenum
  2. Jejunum
  3. Ileum

(vi) Large intestine

  1. Caecum
  2. Appendix
  3. Colon – Ascending colon, Transverse colon, Descending colon and Sigmoid colon
  4. Rectum
  5. Anal canal
(i) Tongue

(ii) Salivary glands

  1. Parotid gland
  2. Submandibular gland
  3. Sublingual gland

(iii) Liver

(iv) Gallbladder

(v) Pancreas

Diagram

Human digestive system

Figure: Human digestive system

Mouth

Mouth is the first portion of the digestive tract and is bounded by the lips, cheeks, palate, and tongue. It is continuous with the oropharynx posteriorly. Mouth receives food and starts chewing, mixes the food with saliva and forms the solid food into a soft rounded mass called bolus with the help of teeth and tongue.

Tongue

Tongue is a mass of skeletal muscle covered with mucous membrane. Its anterior two thirds reside in the oral cavity and posterior third resides in the oropharynx.  Tongue helps in the act of chewing, receives taste, mixes food with saliva and forms the bolus, initiates swallowing and helps in speech.

Salivary glands

Three pairs of salivary glands present in the human digestive system those are parotid, submandibular or submaxillary and sublingual glands. One of each pair remains on one side of the mouth and opens into oral cavity through their ducts. All of these glands are exocrine in nature and produce saliva that is conveyed through their ducts into oral cavity, which perform the digestive, lubricating and immunologic function.

Pharynx

It is a fibromuscular tube of about 10 cm in length, divided into three parts; nasopharynx, oropharynx and hypopharynx or laryngopharynx. Nasopharynx communicates anteriorly (infront) with nasal cavity and inferiorly (below) with oropharynx. Oropharynx communicates anteriorly with mouth cavity and inferiorly with hypopharynx. The hypopharynx is continuous with esophagus and communicates antero-inferiorly (infront and below) with larynx. Nasopharynx is a part of respiratory system. But the oropharynx and hypopharynx are the common passage of both digestive and respiratory system. The oropharynx and hypopharynx, as a part of the digestive system, propels the food bolus from the mouth into the esophagus by the process of  swallowing.

Esophagus

It is a muscular tube, 25 cm in length, divided into three parts; cervical part (located behind the throat), thoracic part (located behind the chest) and abdominal part ((located within the abdomen). It receives food from the pharynx and passes it into the stomach by a series of peristaltic contraction. Esophagus also prevents the retrograde flow of stomach contents. Learn more.

Stomach

Stomach is a J shaped muscular tube and the most dilated part of the human digestive tract. It acts as a reservoir of food, converts food into a chyme of uniform consistency due to repeated churning, kill the swallowed organisms with the help of secreted HCl and allows digestion of proteins with the help of enzyme pepsin. Learn more.

Small intestine

It a long muscular tube of about 6.5 metres in length. It is also called small bowel and consists of the proximal fixed part, duodenum (25 cm), and the remaining mobile part that includes jejunum (2.5 metres) and ileum (3.6 metres). Most of the digestion and absorption of food takes place in the small intestine.

Large intestine

Large intestine also called large bowel. It is 1.5 metres in length. It includes caecum (6 cm), appendix (2-20 cm), colon (1.3 metres that includes ascending, transverse, descending and sigmoid colon), rectum (12 cm) and anal canal (3.8 cm). The unabsorbed residues of foodstuffs are delivered from small intestine into large intestine in a fluid form. By the time the contents reach the lower portion of colon and the semisolid consistency of feces is formed. Large intestine allows absorption of about 500 ml of water per day. The digestion of cellulose takes place in the large intestine with the help of putrefactive bacteria. The feces are stored in the descending and sigmoid colon, and are voided via the rectum and anal canal only during defaecation.

Liver

It is the largest gland in the human digestive system and also in the human body, and consists of both exocrine and endocrine portions. The average weight of the liver in adult is 1500 gram. It is situated in the upper and right side of the abdominal cavity. The exocrine portion of the liver plays an important role in digestive system by secreting bile, an important fluid that helps in fats digestion within the small intestine.

Gall bladder

It is not a gland, just a pear shaped hollow organ attached to the lower surface of the liver. The major function of the gall bladder is to store bile, concentrate bile ten times more than the liver bile by absorbing its water and release it when necessary into the digestive tract.

Pancreas

Pancreas is a soft lobulated gland, consisting of both exocrine and endocrine portions. It is situated in the upper part of the posterior abdominal wall. The exocrine portion of the pancreas produces pancreatic juice contains digestive enzymes that help in digestion of foodstuffs within the small intestine.