The Human Digestive System: Biology Notes for UPSC Aspirants

The human digestive system is a complete type of digestive system, according to the evolutionary pattern. It is designed such that it converts the food into nutrients that the body utilizes for energy, growth and cell repair. The system is composed of organs arranged in a line forming the gastrointestinal tract.

the_Digestive_System image
      The Human Digestive System

The organs in the series are mouth, esophagus, stomach, small intestine, large intestine, and anus. Every part has a specific function that aids in the process of digestion.

Parts of the gastrointestinal tract and their functions

MOUTH: The process of digestion starts from here, the chewing beaks the food and it gets mixed with saliva. This converts the food into a bolus and from here it passes into the esophagus.

ESOPHAGUS:  The esophagus is a muscular tube that connects the mouth and stomach. By regular contractions, it delivers the food to the stomach. A sphincter present at the lower end prevents the backflow of food.

STOMACH: A sac-like muscular organ that holds the food for some time. It has both mechanical and chemical functions. It secretes acid and digestive enzymes that break down the food particles. The upper part of the stomach relaxes to allow the large volume to be stored; while the lower part contracts and churns the food. Then it transfers the food into the small intestine.

SMALL INTESTINE: It is the long tube that is loosely coiled in the abdomen. It is further divided into three segments: duodenum, jejunum, and ileum. The process of digestion continues and is aided by secretions from the pancreas and liver. Once the food is completely broken down, it is absorbed through the intestinal walls.

LARGE INTESTINE: It is the last part of the GI tract. Further subdivided into segments: ascending colon, transverse colon, descending colon and rectum. The last absorption occurs here and the waste is excreted out through the rectum.

Glands involved indigestion

The process of digestion is a complex one. Along with the muscular contractions of the tract, the various glands aid the breakdown of food by secreting enzymes and other substances.


The main function of the salivary glands is to secrete saliva. They are the main exocrine glands.

  • Saliva has important functions such as:

>lubrication of mouth

>swallowing of food

>aids in digestion

>protection of teeth from bacterial infections

The salivary glands consist of three pairs:

  1. Parotid glands
  2. Submandibular glands
  3. Sublingual glands

The saliva contains salivary amylase, which helps in the digestion of carbohydrates.


The pancreas has both exocrine and endocrine functions. It is located between the duodenum limbs.

The endocrine part secretes hormones: insulin and glucagon. These help to balance the glucose metabolism.

The exocrine part secretes the digestive enzymes, which together constitute the pancreatic juice.

The pancreatic ducts secrete bicarbonate ions and water to neutralize the pH of chime as it enters the duodenum.

The pancreatic juice s alkaline in nature and consists of enzymes like trypsinogen, chymotrypsinogen, elastase, carboxypeptidase, pancreatic lipase, nucleases, and amylases.


It is the largest gland of the human body. It is located in the abdominal cavity. The liver is composed of hepatic lobules which are the functional units of the liver. The hepatic cells secrete bile and pass it into the gallbladder. The hepatic duct from the liver and the cystic duct from the gallbladder from the common bile duct, which opens in the small intestine and releases bile.

Bile plays an important role in the emulsification of fats.

The hepatic vein and portal vein are connected to the liver. The hepatic vein carries the blood rich in oxygen while the portal vein carries the blood which is rich in digested nutrients.


These are the glands found the mucosa of the stomach, containing parietal cells that secrete hydrochloric acid and zymogenic cells that secrete pepsin.

There are three types of gastric glands depending on their location and type of secretion. The pyloric and cardiac gastric glands secrete mucus and protect the stomach from self-digestion.


The intestinal glands are found in the small intestine and are called crypts of lieberkuhn that secrete digestive enzymes such as maltase, peptidase that from the end products. The secretion is called as the succus entericus.

Physiology of digestion

The physiology can be divided under the following headings:

INGESTION: It is the process of intake of food,

DIGESTION: The breakdown of food into its simpler components so that it could be easily absorbed is called digestion. It starts from the mouth and ends in the small intestine.

ABSORPTION: The transfer of digested food from the alimentary canal into the blood is absorption. Maximum absorption occurs in the small intestine.

ASSIMILATION: The process of utilization of absorbed food by the different cells of the body.

EGESTION: The removal of undigested food from the alimentary canal through the anus is called egestion

The food we intake is composed of various nutrients. The digestive system breaks it down into simpler forms and provides it to the cells of the body for generating energy and growth of the body.

The various organs and enzymes play an important role in sustaining the life of the individual.

VIDEO CREDITS: Dandelion Team

Share on: