Describe the alimentary canal of man.

The alimentary canal is the main organ of the digestive system. It starts from the mouth and ends in the anus.

The main organs of the alimentary canal are:

The mouth (or buccal cavity), pharynx, esophagus (food pipe), stomach, small intestine, large intestine, rectum, and anus.

The major glands associated with the alimentary canal of man are salivary glands, gastric glands, intestinal glands, liver, and pancreas.

Their functions are:

1. Salivary glands: The main function of the salivary gland is to produce saliva through a system of ducts. It contains salivary amylase which is responsible for the digestion of starch into simple sugars.

2. Gastric glands: The stomach wall or gastric glands secretes gastric juice containing three substances hydrochloric acid, mucus, and pepsin.

The functions of hydrochloric acid are to kill bacteria that may enter the stomach with food and the activation of inactive pepsinogen into active pepsin. The mucus protects the inside layer of the stomach from the damaging effect of the substance hydrochloric acid whereas the substance pepsin is an enzyme for digestion.  It helps in the digestion of proteins. The partially digested food then enters the small intestine for further digestion. 

3. Intestinal glands: These glands produce intestinal juices responsible for the complete digestion of carbohydrates, proteins, and lipids into an absorbable form.

4. Liver: The liver produces bile juice which is stored in the gall bladder. It helps to digest fats.

5. Pancreas: It secretes pancreatic juice and hormones. Pancreatic juice contains enzymes that combine with the juices of intestinal walls to form chyme. Pancreatic amylase helps in breaking down the starch. Pancreatic lipase breaks down fats. While pancreatic proteases break down proteins.

Digestion of Food in Humans:
1. Digestion begins when food enters the mouth (oral cavity). Teeth are used to grind and break up food. This is called physical or mechanical digestion. 
2. An enzyme in saliva called amylase begins to break down into maltose sugar. This is called chemical digestion
3. After it is swallowed, the chewed food moves down the esophagus. The esophagus acts as a connection between the mouth and the stomach.
4. The bolus (the chewed food coming from the mouth) then reaches the stomach, where mechanical and chemical digestion takes place further. The muscles in the stomach walls churn the bolus allowing it to mix with digestive enzymes and gastric acids like HCl. This process converts the bolus into a liquid called chyme.
The digestion in the stomach continues for several hours. During this process, an enzyme called pepsin breaks down most of the protein in the food. The chyme is slowly transported into the small intestine, where most chemical digestion takes place.
5. Liver produces bile juice which is stored in the gall bladder.
6. Bile juice is released from the gallbladder to help digest fats.
7. Enzymes from the pancreas and intestinal walls combine with the chyme. 
8. Most of the nutrient absorption occurs in the small intestine. Nutrients are absorbed through its walls into the circulatory system and by the time the chyme exits the small intestine, only water, and indigestible substances are left behind.
9. The chyme then enters the large intestine, where water is removed and bacterias break down some indigestible materials, producing important compounds (such as vitamin K). The concentrated waste material that remains is called feces, which is passed into the rectum and eliminated from the body through the anus.

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