The five major organs that secrete digestive juices are the salivary glands, stomach, pancreas, liver and small intestine. Each of these organs synthesizes its own enzyme-rich mixture of digestive juices that breaks down food into smaller pieces that can be absorbed into the body.
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 carbohydrates. Carbohydrates are found in foods like bread and rice. 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.
Role of the Stomach
The stomach, an important organ for digestion, is distinctive for its ability to produce hydrochloric acid. This acid is so powerful that the stomach must produce large amounts of mucus to protect itself from the acid. Hydrochloric acid works with an 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 is also responsible for the secretion of a substance called intrinsic factor, which helps the small intestine absorb vitamin B12.
Pancreas and Fat Digestion
The pancreas is a leaf-shaped organ that lies below the stomach. It secretes juices containing enzymes that are capable of digesting all major types of food -- carbohydrates, fats and proteins. The pancreas is the first organ to begin digesting fats. It secretes 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. Pancreatic juice also contains large amounts of bicarbonate, which neutralizes the acid from the stomach.
Liver and Gall Bladder
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 to be absorbed by the body. Bile acts like soap, breaking the bonds that hold the spheres together and turning them into tiny globules that are easily taken up by the body.
The small intestine is covered with tiny fingerlike extensions from its walls, called villi. The villi are where the nutrients from food are absorbed into the body. The tips of villi have many enzymes, such as peptidases, disaccharidases and intestinal lipases. They perform the final steps of digestion for all major food types, dissolving them into their basic building blocks to enable absorption. 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