Digestive system of human

Digestive system of human consists of the digestive tract (alimentary canal) which forms a continuous passage from the mouth to the anus. The food is taken by the mouth, and sent downwards. As the food moves downwards into the alimentary canal, it comes in contact with a series of juices, called digestive juices, which contain various enzymes. In the digestive tract, the food is broken down so that the carbohydrates, fats, proteins, minerals, vitamins etc. that it contains may be absorbed into the blood stream to fuel the body. The unabsorbed food remnants are passed out from the alimentary canal by a process called egestion.






Digestive system of humans


1) Mouth and Buccal cavity

The mouth is a transverse slit like aperture, bounded by two soft movable lips. Inside the Buccal cavity there is a muscular tongue and teeth.

a) Tongue

It is a thick, musculo-sensory organ present on the floor of the Buccal cavity. The posterior part of tongue is attached with a soft ligamentous fold said to be frenulum. The upper and lateral surface of the tongue remain covered with various types of Papillae.



Filiform Papillae are conical projection distribution in parallel rows over the anterior two-thirds of the tongue. They are whitesh and contain no taste buds.
Fungiform Papillae are mushroom like elevation distributed among the filiform papillae and are more numerous near the tip of the tongue, and most of them contain taste buds.
Ballard Papillae are aranged in the form of an inverted 'V' on the posterior lateral surface of the tongue and the contain taste buds.


b) Teeth

An adult human has 16 teeth on each jaw, embedded in jaw sockets (thecodont) . These teeths ate of four types in their structure and their functions (heterodont) and they appear twice in the life time (diphyodont).

In adult human being the dental formula is 2123/2123, in each half of the jaw, from middle to backwards, these are two incisors, one canine, two premolars and three molars.


The primary function of teeth is to grasp and hold the food in the mouth cavity. They are also modified to serve as a grinding mill for chewing the food. With the help of teeth, tongue and jaw movements, food is chewed and mixed with saliva in the mouth.





c) Structure of tooth

In general, a tooth consists of an upper exposed part, the crown, and one or more roots which are embedded in sockets in the jaw bone. The junction of crown and root is called neck. The greater part of tooth is formed by a bone like material called dentine. In the region of the crown, the dentine is covered by a much harder white material known as enamel. The dentine is covered by a thin layer of cement over the root. The cement is united to the wall of the bony socket in the jaw by a layer of fibrous tissue said to be periodontal ligament. There is a pulp cavity with in the dentine which contains a mass of cells, blood vessels, and nerves. These together constitute the pulp.




2) Pharynx

The pharynx, or throat, is the passageway leading from the mouth and nose to the esophagus and larynx. The pharynx permits the passage of swallowed solids and liquids into the esophagus, or gullet, and conducts air to and from the trachea, or windpipe, during respiration. The pharynx also connects on either side with the cavity of the middle ear by way of the Eustachian tube and provides for equalization of air pressure on the eardrum membrane, which separates the cavity of the middle ear from the external ear canal.

 The pharynx has roughly the form of a flattened funnel. It is attached to the surrounding structures but is loose enough to permit gliding of the pharyngeal wall against them in the movements of swallowing. The principal muscles of the pharynx, involved in the mechanics of swallowing, are the three pharyngeal constrictors, which overlap each other slightly and form the primary musculature of the side and rear pharyngeal walls.


3) Oesophagus

The Oesophagus is a 22-25 cm long, narrow, muscular and tubular structure. It runs downward through the neck behind the trachea, and opens in the stomach in the abdomen. The swallooed food is propelled towards the stomach through the oesophagus by the movement of its muscular wall.

A muscular sphincter (gastro-oesophageal) regulates the opening of Oesophagus into the stomach.





4) Stomach

The stomach is a large muscular and somewhat J-shaped sac. It occupies the left side of the upper part of the abdominal cavity. The upper part of the stomach is called the cardiac (fundus) the middle dome shaped part, the body, and the distal part, the pyloric (antrum). The stomach thus has two ends : the cardiac end where it receives Oesophagus and the pyloric end where it opens into the duodenum. The opening of the stomach into the duodenum is guarded by the pyloric sphincter. The stomach has two curvature : the greater curvature at the left and the lesser curvature at the right side.



The mucous membrane of the stomach is thick. It is thrown into numerous longitudinal folds, called rugae, when stomach is empty. In distended stomach these folds are not seen. There are numerous glands in the stomach which are called gastric glands. 

The muscles on the stomach walls churn and mix the food with the gastric juice. They also help in propelling the food to the small intestine through the pyloric end.



5) Small Intestine

It is the longest part of the alimentary canal, about 6 metre in length. It is narrow and tubular and occupies the central and lower parts of the abdominal cavity. It is divided into the following three regions.


a) Duodenum

The first part of the small intestine is called duodenum. It looks somewhat like the alphabet 'c' . The ducts conveying pancreatic juice as well as the duct carrying bile open in the duodenum.

b) Jejunum

The duodenum opens into jejunum, which is about 2.5 meters long.

c) Ileum

The Ileum is the longest part of the small intestine and it opens in the caecum in the lower part of the abdominal cavity. Both ileum and Jejunum are highly coiled structures. Inner mucosa of small intestine is raised into millions of minute finger like projections called villi.

All the three parts of the small intestine have tube like gland in there walls. These glands secrete intestine juice into the intestinal lumen. The muscles on the intestinal wall churn, knead and crush the food mix it with intestinal juice and propel it towards the large intestine by contractions. The digested products are absorbed mostly by the small intestine.





6) Large Intestine

The Ileum opens into the large intestine. It is much shorter and wider then the small intestine. It does not posses villi or brush bordered cells. It is distinguished into the following three regions.


a) Caecum

It is a small, pouch like structure which ends into a tubular part called vermiform appendix. Both, caecum and vermiform appendix are vestigial in function and are not involved in cellulose digestion.

b) Colon

It is a long sacculated structure which is differentiated into ascending colon extending up to liver on the right side transverse Colon which crosses the abdominal cavity below the pancreas, descending colon running downwards on the left side and pelvic Colon which is S-shaped continues into the rectum.

c) Rectum

It is slightly dilated and about 13 cm long. It opens outside by anus and is guarded by two anal sphincter muscles.

The large intestine does not secrete enzymes and plays only a minor role in the absorption of nutrients. It stores unabsorbed food remnants temporarily and also concentrates the contents of absorbing water to form faeces. The movement of coloncolonChelp to void faeces through anus.


OrganMovement
MouthChewing
EsophagusPeristalsis
StomachUpper muscle in stomach relaxes to let food enter, and lower muscle mixes food with digestive juice
Small intestinePeristalsis
PancreasNone
LiverNone
Large intestinePeristalsis




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