Explanation:

Digestion begins when food enters the MOUTH (oral cavity). Both mechanical and chemical digestion occur in the mouth. Teeth grind and break up food (mechanical), while an enzyme in saliva called amylase begins to break down CARBOHYDRATES (chemical).

After it is swallowed, the chewed food (now called a bolus) moves down the esophagus. The esophagus acts as a connection between the mouth and the stomach, but no digestion occurs here.

The bolus then reaches the stomach, where more mechanical and chemical digestion take place. The muscles in the stomach walls churn the bolus (mechanical), allowing it to mix with DIGESTIVE enzymes and gastric acids (chemical). This process converts the bolus into a liquid called chyme.

Digestion continues in the stomach for several hours. During this time, an enzyme called pepsin breaks down most of the protein in the food.

Image detailing the structures of the digestive system

Image detailing the structures of the digestive system

Image from OpenStax, CC BY 4.0

The chyme is slowly TRANSPORTED into the small intestine, where most chemical digestion takes place. Bile, which is made in the liver, is released from the gallbladder to help digest fats. In addition, enzymes from the pancreas and intestinal walls combine with the chyme to start the final part of digestion.

Most 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 undigestible substances are left behind.

The chyme then enters the large intestine. Here, water is removed and bacteria break down some undigestible 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.

Accessory organs

Accessory organs help with digestion but are not part of the digestive tract. These include:

Salivary glands: moisten food and begin chemical digestion of starches.

Liver: creates bile for FAT digestion, detoxifies blood, processes absorbed vitamins

Gallbladder: stores bile produced by the liver

Pancreas: secretes pancreatic juices to help digestion of proteins and carbohydrates