Digestive System In Humans - Process Of Digestion

What is the Digestive System?

The human digestive system consists of organs that help in converting food molecules into energy and other basic nutrients which are essential for life. In humans, the process is quite simple due to our monogastric nature. This means that we have a one-chambered stomach, unlike other animals such as cows, which have four chambers.

Digestion Process

The process of digestion begins from the mouth and ends in the small intestine – the large intestines’ main function is to absorb the remaining water from the undigested food and enable bacterial fermentation of materials that can no longer be digested.

The alimentary canal or the gastrointestinal tract is a series of hollow organs and tubes that begins from the mouth cavity and continues into the pharynx, through the stomach, small intestines, large intestines, and finally ending at the anus.

Food particles get digested gradually as they travel through various compartments of the gastrointestinal tract. It has six steps, which are as follows:

  1. Ingestion

The very first step involves mastication (chewing). The salivary glands, along with the tongue helps to moisten and lubricate food, before being pushed down into the food pipe.

  1. Mixing and movement

It involves the process of lubricating and manipulating food and pushing it down the food through the food pipe (using peristalsis), and into the stomach.

  1. Secretion

The stomach, small intestine, liver, and pancreas secrete enzymes and acids to aid the process of digestion. It functions by breaking down food particles into simple components and easily absorbable components.

  1. Digestion

The process of converting complex food particles into simpler substances in the presence of enzymes and acids secreted by different digestive organs.

  1. Absorption

This process begins in the small intestine where most of the nutrients and minerals are absorbed. The excess water in the indigestible matter is absorbed by the large intestines.

  1. Excretion

The process of removing indigestible substances and waste by-products from the body through the process of defecation.

Digestive system

Parts of the Digestive System

The digestive system has various components that play specific roles during the process of digestion. They are:

  1. Mouth & Buccal Cavity

Food begins its journey at the buccal cavity or the oral cavity. The mouth has many accessory organs which include teeth, salivary glands, and tongue that help in the digestion of food. Teeth function by masticating (chewing) food into small pieces. Salivary glands secrete saliva and they moisten and soften the food. The tongue, along with the help of other muscles pushes the bolus (small, round mass of food) into the pharynx.

  1. Pharynx

Next part of the digestive system is the pharynx which is also called the throat. It is a part of both the digestive and respiratory systems. It is a triangular or a tube-shaped chamber covered by the mucous membrane and connected to the mouth. During the digestion process, pharynx functions by moving the chewed food into the oesophagus. In the respiration process, it plays an important role in allowing air to the larynx. Pharynx comprises a flap of tissue known as the epiglottis that acts as a switch in transporting food to the oesophagus and air to the larynx during digestion and respiration respectively.

  1. Oesophagus (or) Esophagus

The oesophagus is also known as the food pipe. It is a part of the alimentary canal connecting the pharynx to the stomach. It functions by carrying the swallowed portions of chewed food along its length and gradually pushes it down into the stomach for further process. At the lower end of the oesophagus lies a muscular ring called the lower oesophagal sphincter which closes the end of the food pipe to trap food in the stomach.

  1. Stomach

The stomach is a thick-walled, muscular organ present on the left side of the abdominal cavity. It plays a major role in digestion and is the largest part of the alimentary canal. Stomach functions by storing the food received from the pharynx until the complete digestion process is completed. It receives food from the food pipe at one end and opens into the small intestine at the other end. The interior walls of the stomach secrete digestive enzymes, mucus, and hydrochloric acid, which helps in continuing the process of digestion.

    • Mucous: It is an aqueous secretion produced by the mucous membranes. It functions by protecting the stomach lining and gastric pits from the acid which is produced by the glands to destroy the bacteria that entered along with the food particles.
    • Digestive enzymes: They are the group of enzymes which functions by breaking down polymeric macromolecules like biopolymers into their smaller and simpler substances.
    • Hydrochloric acid: It is the digestive fluid formed by the stomach during the process of digestion. It functions by destroying harmful microorganisms present in the food particles.
  1. Small Intestine

The small intestine is a highly coiled long, thin tube of about 7.5 meters. It receives secretions from the liver and the pancreas. The interior walls of the small intestine also secrete juices for digesting food. The complete small intestine is twisted or rolled like a hose with its inside surface being full of ridges and folds. The folds are used to increase the digestion of food and absorption of nutrients.

Once the food is broken down into its molecular constituents, it is ready to be absorbed by the body. The inner walls of the small intestine have numerous finger-like outgrowths called villi, which increase the surface area for the absorption of nutrients. Each villus (singular for villi) has a network of thin blood vessels. When villi absorb the nutrients, it is transported to different parts of the body through the blood vessels, where, they are again used to build complex substances like proteins which are required by the body. This process of building complex substances from simpler forms is called assimilation.

  1. Large Intestine

The last part of the digestive system is the large intestine. It is a long, thick tube, about 1.5 meters long. It is located just below the stomach and coils around the external border of the small intestine. The large intestine absorbs a very minimal amount of nutrients and water from the undigested food molecules with the help of several symbiotic bacteria residing within it.

  1. Rectum

The unused wastes particles move into the end of the large intestine called the rectum. It is stored here as semi-solid faeces which later exits from the body through the anal canal through the process of defecation.

  1. Accessory Organs

These are not part of the digestive system, but they aid in the process of digestion:

      • The liver is a roughly triangular, reddish brown accessory organ of the digestive system located to the right of the stomach. It produces bile, which helps in the digestion of fat in the small intestine. The bile is stored and recycled in the gallbladder. It is a small, pear-shaped organ which is located just next to the liver.
      • The pancreas is a large gland located just below the stomach. It secretes digestive enzymes into the small intestine to complete the chemical digestion of foods. The liquid secreted by the pancreas gland is referred to as pancreatic juice which breaks down proteins, carbohydrates, and fats into simpler substances.
      • The tongue enables us to perceive taste due to the presence of various taste buds. Furthermore, they also help in manipulating and swallowing our food.

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Practise This Question

A conversation between two people is given below:

Person 1:  I can’t have this food.

Person 2: But why?

Person 1:  Because this food will add 10,000 calories to my body. I will become fatter.

Choose the option which correctly explains what 10,000 calories is.