Discuss the major steps involved in …

i. Ingestion – The process of taking food into the body is called ingestion.

ii. Digestion – Dissolving of solid food is called digestion. During digestion, the food containing large insoluble substances is broken down into small, water soluble substances. Digestion involves both physical and chemical methods for digesting the large substances present in food.

The human digestive system consists of the alimentary canal and its associated glands. Food is taken into the body through the mouth.

Digestion in human beings starts in the mouth. The mouth contains teeth, tongue and salivary glands. The teeth cut the food into small pieces, chew and grind it. The tongue mixes the food with saliva. The salivary glands secrete a watery liquid called saliva.

The slightly digested food goes into the food pipe or oesophagus by peristalsis. No digestion of food takes place in the oesophagus.

The slightly digested food reaches the stomach through oesophagus for further digestion where it is broken down into smaller pieces. The inner lining of the stomach secretes gastric juice which includes mucus, hydrochloric acid and digestive juices. Mucus protects the lining of the stomach from hydrochloric acid. Hydrochloric acid makes the medium in the stomach acidic to facilitate the digestion of proteins. The digestive juices break down the protein into simpler substances. Then the partially digested food goes into the small intestine.

The small intestine is a very long and narrow tube where complete digestion of food takes place. It receives secretions of liver, pancreas and its own walls. Liver secretes liquid called bile which helps in the digestion of fats and converts them into small droplets. Pancreas secretes pancreatic juice which breaks down fats into fatty acids and glycerol. Pancreatic juice also breaks down carbohydrates and proteins into simpler forms. The intestinal juice secreted by the walls of small intestine breaks down the carbohydrates into glucose, proteins into amino acids. Thus, the food gets completely digested in small intestine.

iii. Absorption: The digested food material passes into the blood vessels in the walls of small intestine. This process is called absorption. The inner walls of small intestine have millions of tiny, finger-like outgrowths called villi which increase the surface area for rapid absorption of digested food. Each villus has a network of blood vessels which absorbs the digested food materials into the blood flowing through them.

iv. Assimilation: The blood carries the absorbed food materials to the cells in all the parts of the body where it is used for growth, energy and repair. This is called assimilation. Glucose breaks down utilizing oxygen to form carbon dioxide and water, and releases energy.

The undigested and unabsorbed food passes from the small intestine into large intestine. Most of the water is absorbed from the undigested food materials in the large intestine.

v. Egestion: The undigested food is temporarily stored in the last part of the large intestine called rectum and finally the semi-solid waste is passed out from the body through **** in the form of faeces. This is called egestion.