The digestive system in humans consists of an alimentary canal and associated digestive glands. The human alimentary canal (aliment: nourish) is a continuous muscular digestive tube about 8 to 10 m long that runs through the body. It is open at two ends with the openings, which are mouth at the anterior end and anus at the posterior end. It performs the function of the digestion of the food. It breaks the food down into smaller substances and absorbs the digested food. The main parts of the alimentary canal are

Alimentary Canal

Mouth and Buccal cavity:

Mouth:

The mouth is the entry point for food. This is the uppermost transverse slit-like opening of the alimentary canal. it is bound by the upper lip and the lower lip. The mouth is used to ingest the food.

Salivary Glands:

Saliva is released by the salivary glands into the oral (buccal) cavity when we smell food. Once the food enters the mouth, chewing (mastication) breaks food into smaller particles it helps enzymes in saliva to attack the broken down food.

Teeth: 

There are total 32 teeth in the buccal cavity of an adult human being. Our teeth can perform a cutting as well as grinding function to accomplish the task of the breaking of the food.

Tongue:

The tongue is a muscular fleshy, triangular-shaped organ which lies along the floor of the buccal cavity. The tongue helps in mixing the food with the saliva and then the tongue and roof of the mouth (soft palate) help to move the food into the pharynx and esophagus. The upper surface of the tongue bears numerous projections called papillae. These papillae contain sensory receptors called taste buds. Taste buds are used to detect tastes of different foods.

Pharynx (or throat):

It is a cavity at the back of the mouth. It is the transition area from the mouth to the esophagus. It is a common passage for the inhaled air and swallowed food. The opening of the respiratory tract in the pharynx is called glottis. It is guarded by a cartilaginous flap called epiglottis. During the act of swallowing the windpipe gets closed by the epiglottis. Thus epiglottis prevents the entry of food particles in the respiratory tract.

Oesophagus:

It is a narrow muscular tube arising from the pharynx, continuing through the thorax and ending in the stomach. It is about 25 cm long. Oesophagus (food pipe) contracts in a synchronized fashion (peristalsis) to move food down towards the stomach. While the muscles behind the food product contract, the muscles ahead of the food relax, causing the forward propulsion of the food. Peristalsis is the main mechanism by which food moves through our digestive system. Mucous secreted by the epithelial cells in the inner lining helps in the smooth passage of food. Once the food approaches the stomach, a muscular valve (esophageal sphincter or cardiac sphincter) relaxes and lets the food pass into the stomach.

Stomach:

The stomach is an elastic bag with highly muscular walls, located below the diaphragm. It can be divided into three parts namely a) Cardiac stomach, b) Fundus stomach, and c) Pyloric stomach. The cardiac stomach is connected with oesophagus and the opening of oesophagus into the cardiac stomach is guarded by the cardiac sphincter. This sphincter prevents the food or acid from the stomach from rentering in the oesophagus. The fundus is the middle part of the stomach. The pyloric stomach is connected with duodenum (small intestine) and the opening of the pyloric stomach into the duodenum is guarded by the pyloric sphincter. This sphincter prevents the food from the duodenum from rentering in the stomach.

The stomach has both a mechanical and a chemical function of digestion. In mechanical function, the upper part of the smooth (involuntary) stomach muscle relaxes to allow a large volume of food to be stored. The lower muscle then contracts in a rhythmical manner in order to churn the food inside and mix it together with the gastric juices. In chemical function, gastric acid (mainly hydrochloric acid) and digestive enzymes Pepsin, Gelatinase and Gastric Amylase and Lipase which break the food further.

At the end of this process, the food is transformed into a thick creamy fluid called chyme. Then the food is pushed into the small intestine.

Small intestine:

It is the longest part of the alimentary canal, a tube about 7 meters long and about 2.5 cm wide. Much coiled and folded, it is contained in the abdomen. It is divided into three parts (i) Duodenum (Short upper part, next to the stomach), (ii) Jejunum (Slightly longer part, about 2 meters long), and (iii) Ileum (Longest, about 4 meters long coiled and twisted).

The chyme from the stomach is then pushed into the duodenum. With the help of enzymes from the pancreas and bile from the liver. A further breakdown of the food occurs in the small intestine. The next two parts of the small intestine (jejunum and ileum) are mostly responsible for the absorption of nutrients from the digested food into the bloodstream through intestinal walls. Then the undigested food is pushed into the large intestine.

Large Intestine:

It is about 1.5 meters long, tube-like organ that is connected to the small intestine at one end and the anus at the other. The large intestine has four parts: cecum, colon, rectum, and anal canal.

• Caecum: It is a small blind pouch at the junction of the small and large intestine. A narrow worm-shaped tube (vermiform appendix) projects from the caecum. The vermiform appendix is a vestigial (functionless) organ in humans but is large and functional in herbivorous mammals for digestion of cellulose.
• Colon: It is a little over a 1-meter long tube, it has three parts termed ascending, transverse and descending limbs of the colon. The colon is lined internally by mucosal cells secreting mucous that makes the passage of undigested material easy. The colon removes water and some nutrients and electrolytes from partially digested food. The remaining material, solid waste called stool, moves through the colon to the rectum.
• Rectum and Anal Canal: It is the last part of the large intestine, about 15 cm. long. It has two parts, the rectum proper and the anal canal. The undigested material called faecal matter is stored in it temporarily.

Anus: 

The anus is the opening at the end of the digestive tract where bowel contents leave the body.  The process is called defaecation or egestion. The anus is surrounded by circular muscles (sphincters). The anal sphincter provides control over releasing stool or holding it. Once stool arrives in the rectum, feedback to the brain makes the person aware of the need for a bowel movement. Voluntary control over the anal sphincter lets us hold the stool until we go to the toilet.

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