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Digestive System

Anatomy is the study of structure and function of the body.

Physiology is the branch of science that deals with various functions of living organisms and the processes which regulate them.


 The purpose of digestion is to change the foodstuffs by mechanical and chemical action to simple forms, which can be easily absorbed into blood and utilized by various tissues in the body.

 It is also involved in maintaining the water and electrolyte balance of the body.

The process of the digestion involves breaking up large particles of foodstuff like polysaccharides, fats and proteins into the smaller particles like monosaccharides, fatty acids and amino acids which can be easily absorbed. These absorbed products are metabolised for the production of energy for various activities of the body system.


Gastrointestinal tract is formed by two types of organs: –

1)Primary digestive organs.

(Mouth, Pharynx, Esophagus, Stomach, Small intestine, Large intestine)

2)Accessory digestive organs.

(Teeth, Tongue, Salivary glands, Exocrine part of pancreas, Liver, Gallbladder)


                                                     The process of digestion takes place in the alimentary canal and is assisted by some accessory organs like salivary glands, liver and pancreas.

Food is processed within the body in 4 steps: –

  • Ingestion. (Taking in food and chewing are functions performed by mouth and teeth, aided by tongue. Pharynx and esophagus are concerned with swallowing)
  • Digestion. (it occurs in the stomach and upper part of the small intestine.)
  • Absorption. (Can occur from any part of alimentary canal: – small intestine)
  • Excretion. (A large intestine absorbs a major quantity of water and the residue is excreted in the form of fauces.)

Mouth—-> Pharynx—-> Esophagus—>Stomach—->Duodenum—-> Jejunum—>Ileum—->Caecum and appendix—> Ascending colon—>Transverse colon—>Descending colon—>Sigmoid colon—->Rectum—->anal canal.

Mouth: –

        The mouth, or oral cavity, is the first part of the digestive tract. It is adapted to receive food by ingestion, break it into small particles by chewing, and mix it with saliva. The lips, cheeks, and palate form the boundaries. The oral cavity contains the teeth and tongue and receives the secretions from salivary glands.


                            The lips and cheeks help hold food in the mouth and keep it in place for chewing. They are also used in the formation of words for speech. The lips contain numerous sensory receptors that are useful for judging the temperature and texture of foods.


          The palate is the roof of the oral cavity. It separates the oral cavity from the nasal cavity. The anterior portion, the hard palate, is supported by bone. The posterior portion, the soft palate, is skeletal muscle and connective tissue. Posteriorly, the soft palate ends in a projection called the uvula. During swallowing, the soft palate and uvula move upward to direct food away from the nasal cavity and into the oropharynx.


            The tongue manipulates food in the mouth and is used in speech. The surface is covered with papillae that provide friction and contain the taste buds.


         A complete set of deciduous (primary) teeth contains 20 teeth. There are 32 teeth in a permanent (secondary)set. The shape of each tooth type corresponds to the way it handles food.


             Food is forced into the pharynx by the tongue. When food reaches the opening, sensory receptors around the fauces respond and initiate an involuntary swallowing reflex. This reflex action has several parts. The uvula is elevated to prevent food from entering the larynx and trachea in order to direct the food into the esophagus.

Peristaltic movements propel the food from the pharynx into the esophagus.


                   The esophagus is a collapsible muscular tube that serves as a passageway between the pharynx and stomach. As it descends, it is posterior to the trachea and anterior to the vertebral column . It passes through an opening in the diaphragm, called the esophageal hiatus, and then empties into the stomach. The mucosa has glands that secrete mucus to keep the lining moist and well lubricated to ease the passage of the food. Upper and lower esophageal sphincters control the movement of food into and out of the esophagus.The lower esophageal sphincter is sometimes called the cardiac sphincter and resides at the esophagogastric junction.


               The stomach, which receives food from the esophagus , is located in the upper left quadrant of the abdomen. The stomach is divided into the fundic, cardiac, body and pyloric regions. The lesser and greater curvatures are on the right and left sided respectively, of the stomach.


                      The regulation of gastric secretion is accomplished through neural and hormonal mechanisms. Gastric juice is produced all the time but the amount varies subject to the regulatory factors. Regulation of gastric secretions may be divided into cephalic, gastric, and intestinal phases. Thoughts and smells of food start the cephalic phase of gastric secretion ;the presence of acid chyme in the small intestine begins the intestinal phase.


                          Relaxation of the pyloric sphincter allows chyme to pass from the stomach into the small intestine. The rate of which this occurs depends on the nature of the chyme and the receptivity of the small intestine.


                      The small intestine extends from the pyloric sphincter to the ileocecal valve, where it empties into the large intestine. The small intestine finishes the process of digestion, absorbs the nutrients, and passes the residue on to the large intestine. The liver, gallbladder, and pancreas are accessory organs of the digestive system that are closely associated with the small intestine. Small intestine is divided into the duodenum, jejunum ileum. The small intestine follows the general structure of the tract in that the wall has a mucosa with simple columnar epithelium, submucosa, smooth muscle with inner circular and outer longitudinal layers, and serosa. The absorptive surface area of the small intestine is increased by plicae circulares ,villi and microvilli.

The most important factor for regulating secretions in the small intestine is the presence of chyme. This is largely a local reflex action in response to chemical and mechanical irritation from chyme and in response to distention of the intestinal wall.


                               The large intestine is larger in diameter than the small intestine. It begins at the ileocecal junction, where the ileum enters the large intestine and ends at the anus. The large intestine consists of the colon,rectum and anal canal.

The last 2-3 cm of the digestive tract is the anal canal, which continues from the rectum and opens to the outside at the anus. The mucosa of the rectum is folded to form longitudinal anal columns. The smooth muscle layer is thick and forms the internal and sphincter at the superior end of the anal canal. This sphincter is under involuntary control. There is an external anal sphincter composed of skeletal muscle and is under voluntary control.


                                    The salivary glands, liver, gallbladder and pancreas are not part of the digestive tract, but they have a role in digestive activities and are considered accessory organs.

                SALIVARY GLANDS: –

                                   3 pairs of major salivary glands(parotid, submandibular and sublingual glands)and numerous smaller ones secrete saliva into the oral cavity, where it is mixed with food during chewing. Saliva contains water, mucus and enzyme amylase. Functions of saliva include the following: –

  • It has a cleansing action on the teeth.
  • It moistens and lubricates food during chewing and swallowing.
  • It dissolves certain molecules so that food can be tasted.

It begins the chemical digestion of starches through the action of amylase, which breaks down polysaccharides into disaccharides.


        The liver is located primarily in the right hypochondriac and epigastric regions of the abdomen, just beneath the diaphragm. It is the largest gland in the body. On the surface, the liver is divided into 2 major lobes and 2 smaller lobes. The functional units of the liver are lobules with sinusoids that carry blood from the periphery to the central vein of the lobule.

Liver functions include the following: –

  • Secretion
  • Synthesis of bile salts
  • Synthesis of plasma protein 
  • Storage
  • Detoxification
  • Excretion
  • Carbohydrate metabolism
  • Lipid metabolism
  • Protein metabolism
  • filtering.


                     The gallbladder is a pear-shaped sac that is attached to the visceral surface of the liver by the cystic duct. The principal function of the gallbladder is to serve as a storage reservoir for bile. Bile is a yellowish-green fluid produced by liver cells. The main components of bile are water, bile salts, bile pigments, and cholesterol.

 Bile salts act as emulsifying agents in the digestion and absorption of fats. Cholesterol and bile pigments from the breakdown of haemoglobin are excreted from the body in the bile.


                    The pancreas has both endocrine and exocrine functions. The endocrine portion consists of the scattered islets of Langerhans, which secrete the hormones insulin and glucagon into the blood. The exocrine portion is the major part of the gland. It  consists of pancreatic acinar cells that secrete digestive enzymes into tiny ducts interwoven between the cells. Pancreatic enzymes include amylase, trypsin, peptidase and lipase. Pancreatic secretions are controlled by the hormones secretin and cholecystokinin.