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Human beings are heterotrophic omnivorous organisms. They obtain food from plants and animals by holozoic mode of nutrition.

The human diet includes

· Carbohydrates

· Fats

· Proteins

· Vitamins

· Minerals

· Water

The human digestive system consists of alimentary canal and digestive glands.


Alimentary canal: [Digestive tract]

The alimentary canal is a long tube of about 9 metres long starting from mouth to anus. It is made up of epithelial lining in which the secretions of digestive glands are secreted.

Different organs of alimentary canals are:

(a) Mouth

(b) Buccal cavity (Oral cavity)

(c) Pharynx

(d) Oesophagus

(e) Stomach of small intestine

(f) Large intestine

(g) Anus


It is the opening of the digestive tract through which food is ingested. It consists of two soft movable lips and opens into oral cavity.

Oral cavity:

It is the large space that consists teeth, tongue and salivary glands. The food ingested is chewed by teeth. The human dental formula is Incisor – 2/2, Canine – 1/1, Pre molar – 2/2, Molar – 3/3.


Tongue mixes the chewed food with the salivary juice secreted by salivary glands. Salivary juice consists of salivary amylase (ptyalin) that digests starch into maltose. Enzymes are absent for the digestion of proteins and fats. About 30% of starch digestion occurs in oral cavity.



It is a long tubular structure, which shows muscular peristaltic movements by which food is pushed down into the stomach. In fact these movements occur throughout the digestive tract.


Stomach is the widest part of the alimentary canal. Partially digested food reaches to the stomach through pharynx and oesophagus. The muscular walls of the stomach helps in mixing the food thoroughly with digestive juices secreted by gastric glands present in the stomach wall.

Gastric juice comprises of – HCl, Mucous, Gastric amylase (minute amount), Gastric lipase (minute amount), Pepsin.

  • HCl (Hydrochloric Acid):

(a) Kills the pathogens entered with the food.

(b) Required for the activity of pepsin.

  • Mucous:

Protects inner lining of the stomach from the acidic condition.

  • Pepsin:

Digests the proteins into peptones.

Gastric amylase, salivary amylase (mixed in the starch food) are inactivated by the action of HCl. So carbohydrate digestion does not take place in the stomach.

  • Gastric lipase: Breakdown of fats.

Now the acidic food (chyme) enters the small intestine.


Small intestine:

Small intestine is the longest part of the digestive tract. It ranges about 6 to 7 metres in length. The length of small intestine is longer in the herbivores for the digestion of cellulose. It is shorter in case of carnivores as meat is digested easily compared to cellulose.

The food arrived from the stomach is acidic in nature, it mixes with the liver and pancreatic juices. Bile juice secreted form liver and bicarbonates of pancreatic juice neutralises the acidic medium, this helps for the activity of pancreatic enzymes.

· Bile juice (Bile salts) acts on the fat molecules and break them into small globules by the process called emulsification.

· Pancreatic amylase causes breakdown of starch.

· Trypsin causes breakdown of proteins.

Finally this food mixes with the intestinal juice which is secreted by small intestine. The enzymes of small intestine juice causes conversion of proteins into amino acids, starch into glucose molecules and lipids into fatty acids and glycerol.


The simple molecules glucose, amino acid and fatty acids are absorbed into the blood stream from the small intestine through lumen. Small intestine consists of finger like structures called villi, they increase the surface area of the small intestine for the absorption of these simple molecules.

Complete digestion of food and absorption takes place in the small intestine.

Large intestine:

Small intestine is followed by large intestine. It consists of 3 parts coecum, colon and rectum. The undigested food is converted as faeces and absorption of water takes place. Mucus separated by large intestine mixes with the faeces and excreted out through anal opening of the activity of anal sphincter.


Entry of absorbed simple molecules into the cell and their conversion into complex molecules called as assimilation.