DIGESTIVE ORGANS AND STRUCTURE | Medicoze

Digestive system is made up of gastrointestinal 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:

•  Primary digestive organs.
•  Accessory digestive organs.

Primary Digestive Organs

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

•  Mouth
•  Pharynx
•  Esophagus
•  Stomach
•  Small intestine
•  Large intestine.

Accessory Digestive Organs

Accessory digestive organs are those which help primary

digestive organs in the process of digestion.

Accessory digestive organs are:

•  Teeth
•  Tongue
•  Salivary glands
•  Exocrine part of pancreas
•  Liver
•  Gallbladder.

WALL OF GASTROINTESTINAL TRACT

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

•  Mucus layer
•  Submucus layer
•  Muscular layer
•  Serous or fibrous layer.

MUCUS 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:

•  Epithelial lining
•  Lamina propria
•  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

              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

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:

• Inner oblique layer
•  Middle circular layer
•  Outer longitudinal layer.

Smooth muscle fibers in the intestine are arranged in two layers:

• Inner circular layer
• 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.

 SEROUS OR FIBROUS LAYER

          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.