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I am assuming this question refers to the human digestive system. If so, the alimentary canal is a tube within a tube arrangement within the body. The outer tube is the body wall and the inner tube is the alimentary canal or digestive system. Digestion begins in the mouth. Chewing helps with mechanical digestion by cutting the size of food particles down. Salivary glands secrete saliva, which contains the enzyme ptyalin. This begins to hydrolyze starch in your foods and begins to break them down to sugar. This begins the process of chemical digestion. The food is moistened and swallowed. In the esophagus, a muscular tube, peristaltic action begins and food is sent to the stomach. The low or acidic ph in the stomach is the perfect environment for the protease enzymes in gastric juice to begin the digestion of proteins. The mass of food is now called chyme. It is sent through the pyloric cecum to the upper small intestine called the duodenum. In the small intestine, peristalsis continues to push the food along. In here, enzymes from the intestinal juice and pancreatic juice can break down all the food groups--carbohydrates,proteins and lipids. Bile which helps emulisify fat, is produced by the liver and stored in the gall bladder. When needed, it travels via the bile duct to the small intestine and acts on fats, making them break up into smaller globules which are easier for the enzymes to act upon. The ph in the small intestine is alkalkine. At the conclusion of digestion, nutrients are absorbed into tiny projections of the small intestinal lining called villi. Endproducts like glucose and amino acids enter the circulating blood in tiny capillaries inside the villi. Fatty acids enter a lacteal, a small projection of the lymph system inside the villi and eventually, these too are returned to the circulating blood. Undigested materials enter the large intestine where they accumulate into feces and are eliminated by the anus. Water is reabsorbed through the large intestine lining which helps maintain homeostasis.
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