The human digestive tract functions to break down food particles into smaller molecules the body can absorb through the lining of the small intestines. The body performs two main types of digestion; mechanical digestion involves the physical breakdown of food such as the action of chewing, and chemical digestion involves digestive acid, digestive enzymes and bile salts, also commonly called bile acids or just bile. Bile salts and digestive enzymes perform different functions in different sections of the digestive tract.
Several different glands produce digestive enzymes. The salivary glands produce and secrete saliva, which contains the digestive enzyme known as salivary amylase. As food progresses to the stomach, two additional enzymes aid in the process of digestion: pepsin and gastric amylase. The pancreas, classified as a solid digestive organ because food does not pass through it, secretes the three digestive enzymes known as pancreatic amylase, pancreatic protease and pancreatic lipase. In contrast to these enzymes, the liver produces bile salts and releases the substance to the gall bladder, which stores it until needed in the small intestine.
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Digestive enzymes are proteins that enhance biochemical reactions that occur in the digestive tract. Although stomach acid produced by the cells lining the stomach performs a major part of the digestive process, digestive enzymes speed up the process by breaking down different types of food molecules. Bile describes the fluid produced in the liver that contains water, electrolytes and organic molecules including the bile acids, cholesterol, fats and bilirubin – the waste product produced by the breakdown of old red blood cells. The liver uses cholesterol to produce two main bile acids: cholic acid and chenodeoxycholic acid. When the bile acids combine with the amino acids glycine or taurine, they form the bile salts.
The different types of digestive enzymes break down different types of macronutrients – those nutrients the body needs in large amounts. The amylase enzymes break down carbohydrate molecules. Salivary amylase and gastric amylase cleave the long chains of sugar molecules into smaller molecules consisting of only two sugars, while pancreatic amylase breaks those down further into the simple sugar known as glucose. Pepsin, classified as a protease enzyme, and pancreatic protease break down complex proteins into the individual amino acids the body can absorb. Pancreatic lipase breaks down fat molecules. In order for pancreatic lipase to work, the fat must mix with digestive fluids. Because bile salts contain a fat-soluble portion and a water-soluble portion, they act as a detergent, causing the fat globules to break down into tiny droplets that allow the lipase enzyme to access and break down the fat.
Digestive enzymes work throughout the digestive tract, from the first contact with food in the mouth to the small intestine, where the body absorbs the majority of nutrients. Bile salts come into contact with the food particles only in the small intestine.
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