The five major organs that secrete digestive juices are the salivary glands, stomach, pancreas, liver and small intestine. Each of these organs synthesizes its mixture of digestive juices that breaks down food into smaller pieces that can be absorbed into the body.
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The main salivary glands are found in the cheeks, under the tongue and around the jaw. They secrete about 1 quart of saliva each day. Amylase, also called ptyalin, is an enzyme in saliva that breaks down starches or complex carbohydrates -- such as bread, rice and potatoes. Lysozyme is another salivary enzyme, which helps to keep the mouth free from germs. Saliva also contains mucus, which coats the food and enables each bite to travel smoothly through the digestive tract.
The stomach, an important organ for digestion, produces gastric juice which is comprised of hydrochloric acid, water and enzymes. Hydrochloric acid works with the main gastric enzyme called pepsin to aid the digestion of protein-rich foods like eggs, meat and tofu. The production of acid is increased by a hormone known as gastrin, which is made by specific cells lining the stomach. The stomach also produces gastric lipase, which assists in digesting fats. Intrinsic factor, an enzyme-like compound which helps the small intestine absorb vitamin B12, is also produced in the stomach.
The pancreas is a leaf-shaped organ that lies below the stomach. It secretes juices rich in enzymes capable of digesting the 3 main energy nutrients -- carbohydrates, fats and proteins. Pancreatic juice also contains large amounts of sodium bicarbonate, which neutralizes the acid from the stomach and optimizes the environment for these enzymes to work. Pancreatic enzymes do most of the fat digestion, secreting pancreatic lipase, esterase and phospholipase, which break down chemically complex fats into simple, easy-to-absorb fats. Similarly, trypsin and carboxypolypeptidase break down proteins, and pancreatic amylase breaks down carbohydrates.
The liver produces a greenish juice called bile, which is stored and concentrated by the gall bladder. After a high-fat meal, such as one containing cheese, cream or bacon, the fats from the food tend to stick together to form large fat spheres. These are too big for the enzymes to work on, so the fat can be absorbed by the body. Bile acts like soap, breaking the bonds that hold these spheres together and turning them into tiny globules that are easily taken up by the body. Bile is not an enzyme but is is essential for the fat-digesting enzymes to work.
While the digestive process begins in the mouth and stomach, digestion gains momentum when food enters the small intestines. This is where secretions from the pancreas, liver and small intestines do most of the digestive work. The lining of the small intestine is covered with tiny finger-like extensions called villi -- which is where nutrients get absorbed into the blood. The tips of villi have many enzymes that digest protein, carbohydrates and fat, such as peptidases, disaccharidases and intestinal lipases. Enzymes that digest simple sugars are also secreted here, such as lactase and sucrase. The deep spaces between the villi are called crypts, which secrete mucus, bicarbonate and water. In addition to these secretions, the cells of the small intestine also produce hormones, like secretin and cholecystokinin, which stimulate the other organs to release their digestive juices.
- Guyton and Hall Textbook of Medical Physiology, 12th Edition; John E. Hall and Arthur C. Guyton
- Ganong’s Review of Medical Physiology, 23rd Edition; Kim E. Barrett, et al.
- Current Opinion in Gastroenterology: Gastric Secretion
- Nutrition and Diet Therapy; Linda K. Debruyne, et al.