Think of the human body as a complex network of mechanical systems that communicate with one another to process physical tasks. The digestive tract supports the entire network by receiving food, converting it to fuel and delivering it to cells and storage tissues. With the help of designated "workers" such as saliva, enzymes and stomach acid, the food molecules are divided into particles that are tiny enough to extract basic nutrients and pass through cell tissue.
Ingestion
When you ingest food, digestion begins. Your mouth responds to the sight, smell or taste of food by releasing liquid from the salivary glands. Food entering the mouth is mixed with saliva containing the digestive enzyme amylase, which catalyzes complex carbohydrates to produce simpler sugar molecules. Your teeth grind and mash the food as it is softened by saliva, while muscles in the tongue simultaneously knead the materials into a condensed format. When it is time to swallow, the tongue reflexively pushes food toward the throat. As carbohydrate metabolism begins almost immediately upon ingestion, carbs are digested faster than any other nutrient.
The Stomach
Muscles in the digestive tract guide food from one organ to the next in a rhythmic movement known as peristalsis. This process propels food along the throat and esophagus down to the acid-filled stomach chamber. The stomach consists of folded layers that can expand to hold up to one gallon of food, says Gretchen Hoffman, author of "Digestive System" in the "Amazing Human Body" series. The stomach churns incoming substances in gastric juices containing hydrochloric acid and protein-catalyzing enzymes. Proteins are broken down into individual amino acids, while the stomach acid attacks bacteria in the food. Different foods leave the stomach at their own rate. A hamburger full of protein and fatty acids will take longer to exit the stomach than bread or fruit juice rich in carbohydrates.
The Small Intestine
Food and nutrients are reduced to a liquid substance known as chyme before transitioning from the stomach to the small intestine, according to Kids Health. The digestive process is completed as enzyme-rich juices from the small intestine, pancreas and gallbladder mix with partially-digested food to produce a thin, watery substance. Each type of nutrient is reduced to its smallest molecular form by reacting with enzymes. Fats are broken down by lipases and proteins by proteases, while each type of carbohydrate reacts with a corresponding enzyme. For example, sucrose is broken down by sucrase and maltose by maltase. Bile also helps to break fat down to its basic components, fatty acids and glycerol. Bile is produced in the liver and stored in the gallbladder, traveling into the intestine by way of connective ducts during digestion.
Absorption
Absorption occurs as nutrients are diffused through the membrane of the intestinal wall. Tiny protrusions known as villi line the walls of the small intestine, increasing its surface area to enable high rates of absorption. After digestion, nutrients are small enough to penetrate the cells of the villi and pass into the bloodstream. Absorption of fat requires more time than other nutrients. After bile is released into the small intestine, it attaches to the catalyzed fat molecules and aids in passing through the intestinal wall into the blood vessels. Food that is not digested or stored travels into the large intestine, where the process of waste removal begins.
