Write about digestive system in human …

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Digestion: Digestion is the process by which food is broken down into simple absorbable substances. Digestion of food takes place in the digestive system. Digestive system is made up of alimentary canal and associated glands.
  
Parts of alimentary canal:
The mouth, oesophagus, stomach, small intestine and large intestine form the alimentary canal. It is also known as the digestive tract.

Mouth is guarded by upper lip and lower lip. The process of taking in food through mouth is called as ingestion.

Buccal cavity is the inner region of the mouth. It encloses teeth and tongue.

Teeth are of different types namely, incisors, canines, premolars and molars.
Incisors are used for biting food.
Canines are sharp and pointed and are used to pierce or tear food.
Premolars and molars are the principal grinders helping to chew and grind the food.

Tongue is a muscular organ attached to the floor of the buccal cavity at the back. It helps in mixing of food with saliva and swallowing it. It also helps in tasting different types of food with the help of taste buds present on it. The buccal cavity leads into pharynx.

Pharynx is at the back of buccal cavity. The pharynx is the common channel for food and air. When you swallow food, a flap-like valve called the epiglottis closes the windpipe. It prevents the entry of food particles into respiratory tract.

Oesophagus also called as food pipe helps in conveying the food from buccal cavity to stomach. The oesophagus is also known as the gullet. It is about 25 centimetres long.   Food inside the oesophagus reaches the stomach by peristalsis movements.

Stomach is the widest part of the alimentary canal. It is a J-shaped muscular organ with stomach occurs with the help of digestive juice. Digestive juice comprises of hydrochloric acid, mucous and some enzymes.

Small intestine is made up of three regions namely duodenum, jejunum and ileum.   Partially digested food called the chyme from the stomach is received by the duodenum. Duodenum also receives bile form the liver, enzymes from pancreas and secretes some enzymes on its own. All these substances bring about digestion of food in the intestine. The inner walls of the small intestine have millions of small finger like projections called the villi. Villi increase the surface area for digestion as well as absorption of digested food by eight times.

Large intestine comprises of colon and rectum. Undigested food is sent into large intestine. Water from the food is reabsorbed to a great extent in the large intestine. Undigested waste is stored in the rectum for defecation.
**** is the opening of the alimentary canal to the exterior. This helps in the elimination of faeces by the process of egestion.

Associated glands:   
These glands include salivary glands, gastric glands, intestinal glands, liver and pancreas.

Salivary glands are present inside the buccal cavity. They secrete saliva. This saliva plays an important role in breaking down complex components like starch into simple sugars.

Gastric glands present inside the stomach secrete gastric juice which helps in the digestion of food.

Intestinal glands secrete various enzymes which aid in the process of digestion of food.

Liver is the largest gland in our body. The liver secretes a yellowish green watery fluid called bile. It is temporarily stored in a sac called the gall bladder. Bile plays an important role in the digestion of fats.   Bile is sent into duodenum through a narrow tube-like structure called the bile duct. Bile breaks the larger fat molecules into tiny droplets, thereby increasing their surface area, which helps in the digestion of fats easily.

Pancreas is the mixed gland. It acts as a both endocrine and exocrine gland. The pancreas secretes the pancreatic juice that helps to digest carbohydrates, proteins and fats. The pancreatic juice converts carbohydrates into simple sugars and glucose, proteins into amino acids, and the lipids into fatty acids and glycerol.