Nutrition in Human Beings (Complex Multicellular Animal):

The various organs of the human digestive system in sequence are: Mouth, Oesophagus (Food pipe), Stomach, Small intestine and Large intestine.

The glands which are associated with the human digestive system are: Salivary glands, Liver and Pancreas.

The various steps of nutrition in human beings are as follows:

Ingestion:

In human beings, food is ingested through the mouth. The food is put into the mouth with the help of hands.


Digestion:

The digestion of food begins in the mouth itself.

The teeth cut the food into small pieces, chew and grind it. (Physical digestion)

The salivary glands in our mouth produce saliva (watery liquid) which contains an enzyme salivary amylase which digests the starch (carbohydrate) present in the food into sugar. (Chemical digestion)

Our tongue helps in mixing this saliva with food.

The digestion of food remains incomplete in mouth.


Oesophagus:

The slightly digested food in the mouth is swallowed by the tongue and goes down the food pipe called oesophagus.

When the slightly digested food enters the food pipe, the walls of food pipe start contraction and expansion movements called as peristaltic movement.

This peristaltic movement of food pipe pushes the slightly digested into the stomach.


Stomach:

The stomach is a J-shaped organ present on the left side of the abdomen.

The stomach walls contain s three tubular glands in it walls which secrete gastric juice.

The gastric juice contains three substances: Hydrochloric acid, the enzyme pepsin and mucus.

The hydrochloric creates an acidic medium which facilitates the action of the enzyme pepsin i.e. digestion of protein.

The mucus helps to protect the stomach wall from its own secretions of hydrochloric acid.

The partially digested food then goes from the stomach into the small intestine.


Small intestine:

From the stomach, the partially digested food enters the small intestine.

The small intestine is the largest part (about 6.5m) of the alimentary canal.

The small intestine is very narrow and arranged in the form of a coil in our belly.

The small intestine in human beings is the site of complete digestion of food (like carbohydrates, proteins and fats)

The small intestine receives the secretion of two glands: Liver and Pancreas.

Liver secretes bile (greenish yellow liquid made in the liver and stored in gall bladder).

Bile performs two functions:

Makes the acidic food coming from the stomach alkaline so that pancreatic enzymes can act on it.

Bile salts break the fats present in the food into small globules making it easy for the enzymes to act and digest them.

The pancreas secretes pancreatic juice which contains enzymes like pancreatic amylase for breaking down starch, trypsin for digesting proteins and lipase for breaking down emulsified fats.

The walls of the small intestine contain glands which secretes intestinal juice. The enzymes present in it finally convert the proteins into amino acids, complex carbohydrates into glucose and fats into fatty acids and glycerol.

In this way the process of digestion converts the large and insoluble food molecules into small water soluble molecules.


Absorption:

The small intestine is the main region for the absorption of digested food.

The inner surface of the small intestine has numerous finger-like projections called villi which increase the surface area for rapid absorption of digested food.

The digested food which is absorbed through the walls of the small intestine goes into our blood.


Assimilation:

The blood carries digested and dissolved food to all the parts of the body where it becomes assimilated as part of the cells and is utilised for obtaining energy, building up new tissues and the repair of old tissues.


Egestion:

The unabsorbed food is sent into the large intestine where more villi absorb water from this material.

The rest of the material is removed from the body via the anus.

The exit of this waste material is regulated by the anal sphincter.