Digestion and functions of digestive system

Digestion is defined as the process by which food is broken down into simple chemical substances that can be absorbed and used as nutrients by the body. Most of the substances in the diet cannot be utilized as such. These substances must be broken into smaller particles, so that they can be absorbed into blood and distributed to various parts of the body for utilization. Digestive system is responsible for these functions. Digestive process is accomplished by mechanical and enzymatic breakdown of food into simpler chemical compounds. A normal young healthy adult consumes about 1 kg of solid diet and about 1 to 2 liter of liquid diet every day. All these food materials are subjected to digestive process, before being absorbed into blood and distributed to the tissues of the body. Digestive system plays the major role in the digestion and absorption of food substances.

Thus, the functions of digestive system include:

1. Ingestion or consumption of food substances

2. Breaking them into small particles

3. Transport of small particles to different areas of the digestive tract

4. Secretion of necessary enzymes and other substances

for digestion 5. Digestion of the food particles

6. Absorption of the digestive products (nutrients)

7. Removal of unwanted substances from the body.


Digestive system is made up of gastrointestinal tract (GI tract) or alimentary canal and accessory organs,

which help in the process of digestion and absorption. GI tract is a tubular structure extending from

the mouth up to anus, with a length of about 30 feet. It opens to the external environment on both ends.

GI tract is formed by two types of organs:

1. Primary digestive organs.

2. Accessory digestive organs.

1. Primary Digestive Organs

Primary digestive organs are the organs where actual digestion takes place.

Primary digestive organs are:

i. Mouth

ii. Pharynx

iii. Esophagus

iv. Stomach v. Small intestine

vi. Large intestine.

2. Accessory Digestive Organs

Accessory digestive organs are those which help primary digestive organs in the process of digestion.

Accessory digestive organs are:

i. Teeth

ii. Tongue

iii. Salivary glands

iv. Exocrine part of pancreas

v. Liver

vi. Gallbladder.


In general, wall of the GI tract is formed by four layers which are from inside out:

1. Mucus layer

2. Submucus layer

3. Muscular layer

4. Serous or fibrous layer.


Mucus layer is the innermost layer of the wall of GI tract. It is also called gastrointestinal mucosa or mucus membrane. It faces the cavity of GI tract. Mucosa has three layer of structures:

i. Epithelial lining

ii. Lamina propria

iii. Muscularis mucosa.

Epithelial Lining

Epithelial lining is in contact with the contents of GI tract. The type of cells in this layer varies in different parts of GI tract. The inner surface of mouth, surface of tongue, inner surface of pharynx and esophagus have stratified squamous epithelial cells. However, mucus membrane lining the other parts such as stomach, small intestine and large intestine has columnar epithelial cells.

Lamina Propria

Lamina propria is formed by connective tissues, which contain fibro blasts, macrophages, lymphocytes and eosinophils.

Muscularis Mucosa

Muscularis mucosa layer consists of a thin layer of smooth muscle fibers. It is absent in mouth and pharynx. It is present from esophagus onwards.


Submucus layer is also present in all parts of GI tract, except the mouth and pharynx. It contains loose collagen fibers, elastic fibers, reticular fibers and few cells of connective tissue. Blood vessels, lymphatic vessels and nerve plexus are present in this layer.


Muscular layer in lips, cheeks and wall of pharynx contains skeletal muscle fibers. The esophagus has both skeletal and smooth muscle fibers. Wall of the stomach and intestine is formed by smooth muscle fibers. Smooth muscle fibers in stomach are arranged in three layers:

i. Inner oblique layer

ii. Middle circular layer

iii. Outer longitudinal layer.

Smooth muscle fibers in the intestine are arranged

in two layers:

i. Inner circular layer

ii. Outer longitudinal layer.

Auerbach nerve plexus is present in between the circular and longitudinal muscle fibers. The smooth

muscle fibers present in inner circular layer of anal canal constitute internal anal sphincter. The external

anal sphincter is formed by skeletal muscle fibers.


Outermost layer of the wall of GI tract is either serous or fibrous in nature. The serous layer is also called serosa or serous membrane and it is formed by connective tissue and mesoepithelial cells. It covers stomach, small intestine and large intestine. The fibrous layer is otherwise called fibrosa and it

is formed by connective tissue. It covers pharynx and esophagus.

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