Theory:

The human digestive system is made up of the alimentary canal or the gastrointestinal tract and the various glands associated with it. The function of this system is to convert the complex food substances into simple forms and absorb the digested food.
The alimentary canal:
The alimentary canal is a tubular organ of about \(9\)meters long. It consists of a series of hollow organs from the mouth to the anus. It includes the,
  • Mouth or Buccal cavity
  • Pharynx
  • Oesophagus or food pipe
  • Stomach
  • Small intestine
  • Large intestine
  • Anus
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Alimentary canal or Gastrointestinal tract - Digestive system
 
Mouth:
It is a cavity that consists of the lips, tongue and teeth. It is the place where the mechanical and chemical digestion of the food begins. The digestion process occurs with the help of teeth, tongue and saliva.
 
Pharynx:
It is also known as the throat. It is the part of the alimentary canal that receives the food from the mouth.
 
Oesophagus:
It is a muscular tube that connects the mouth and stomach. As it is a muscular tube it can contract or expand to allow the passage of food.
 
Stomach:
It is a sac-like or pouch-like organ that stores, mixes and digests food with the help of the gastric juice it produces. The gastric juice consists of mucus, enzymes and hydrochloric acid.
 
Small intestine:
It is a long tube that is of about six meters long and is the place in which most of the chemical digestion occurs. All the nutrients present in the food are absorbed into the bloodstream when the food is present in the small intestine.
 
Large intestine:
The food that is properly not digested in the small intestine reaches the large intestine and the water in this food is absorbed here. The large intestine also absorbs certain important vitamins produced by the bacteria which harbours the large intestine.
 
Digestive system
Digestive glands:
These are the glands that are associated with the alimentary canal. These glands secrete various enzymes that help in the process of digestion of food in the digestive tract or alimentary canal. Some of the digestive glands are:
  • Salivary glands
  • Gastric glands
  • Liver
  • Pancreas
  • Intestinal glands
Salivary glands:
  
These are the glands that produce saliva which is a liquid that lubricates food so that it becomes easy for swallowing. Saliva contains certain antibacterial agents and the enzyme salivary amylase which breaks down starch.
 
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Salivary glands
 
Gastric glands:
  
It is a gland found in the linings of stomach and secretes the gastric juice that helps in the digestion of proteins. As this juice contains certain acids it kills the bacteria.
 
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Acid in the stomach
 
Liver:
  
It is an organ in which the blood carrying nutrients from the small intestine pass through. Here, the blood is filtered and the complex nutrients are broken down. Liver also plays a role in the synthesis of proteins, conversion of glucose to glycogen and also produces bile that helps in the digestion of fats and some vitamins.
 
Pancreas:
  
These are organs that play an essential role in the conversion of food into fuel. It secretes a digestive juice that contains an enzyme which is involved in the break down of carbohydrates, fats, and proteins.
 
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Digestive glands
 
Intestinal glands:
 
It is a gland that is found in between the small intestine and large intestine. They are involved in the secretion of digestive enzymes.
  
The overview of the digestive system:
 
Reference:
 
https://www.flickr.com/photos/mitopencourseware/3693598243
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