Every morsel of food we eat has to be broken down into nutrients that can be absorbed by the body, which is why it takes hours to fully digest food. In humans, protein must be broken down into amino acids, starches into simple sugars, and fats into fatty acids and glycerol. The water in our food and drink is also absorbed into the bloodstream to provide the body with the fluid it needs.
The digestive system is made up of the alimentary canal and the other abdominal organs that play a part in digestion, such as the liver and pancreas. The alimentary canal (also called the digestive tract) is the long tube of organs — including the esophagus, the stomach, and the intestines — that runs from the mouth to the anus. An adult's digestive tract is about 30 feet long.
Digestion begins in the mouth, well before food reaches the stomach. When we see, smell, taste, or even imagine a tasty snack, our salivary glands, which are located under the tongue and near the lower jaw, begin producing saliva. This flow of saliva is set in motion by a brain reflex that's triggered when we sense food or even think about eating. In response to this sensory stimulation, the brain sends impulses through the nerves that control the salivary glands, telling them to prepare for a meal.
As the teeth tear and chop the food, saliva moistens it for easy swallowing. A digestive enzyme called amylase (pronounced: ah-meh-lace), which is found in saliva, starts to break down some of the carbohydrates (starches and sugars) in the food even before it leaves the mouth.
Swallowing, which is accomplished by muscle movements in the tongue and mouth, moves the food into the throat, or pharynx. The pharynx (pronounced: fair-inks), a passageway for food and air, is about 5 inches long. A flexible flap of tissue called the epiglottis (pronounced: ep-ih-glah-tus) reflexively closes over the windpipe when we swallow to prevent choking.
From the throat, food travels down a muscular tube in the chest called the esophagus (pronounced: ih-sah-fuh-gus). Waves of muscle contractions called peristalsis (pronounced: per-uh-stall-sus) force food down through the esophagus to the stomach. A person normally isn't aware of the movements of the esophagus, stomach, and intestine that take place as food passes through the digestive tract.
At the end of the esophagus, a muscular ring called a sphincter (pronounced: sfink-ter) allows food to enter the stomach and then squeezes shut to keep food or fluid from flowing back up into the esophagus. The stomach muscles churn and mix the food with acids and enzymes, breaking it into much smaller, more digestible pieces. An acidic environment is needed for the digestion that takes place in the stomach. Glands in the stomach lining produce about 3 quarts of these digestive juices each day.
Most substances in the food we eat need further digestion and must travel into the intestine before being absorbed. When it's empty, an adult's stomach has a volume of one fifth of a cup, but it can expand to hold more than 8 cups of food after a large meal.
By the time food is ready to leave the stomach, it has been processed into a thick liquid called chyme (pronounced: kime). A walnut-sized muscular tube at the outlet of the stomach called the pylorus (pronounced: py-lore-us) keeps chyme in the stomach until it reaches the right consistency to pass into the small intestine. Chyme is then squirted down into the small intestine, where digestion of food continues so the body can absorb the nutrients into the bloodstream.
