To and Through the Stomach

In the mouth, a bite of chicken sandwich is processed by chemical digestion through the enzyme amylase, which breaks down starches. It is also processed by mechanical digestion as it is broken into small pieces during chewing. What happens to the bolus next?

The esophagus is colored red in the image.

The stomach.

The bolus enters the next part of the digestive tract, which is called the esophagus. The esophagus is the tube that leads from the pharynx (in the back of the throat) to the stomach. The esophagus contains a layer of muscle that helps push the chewed food through a special motion called peristalsis. Peristalsis looks a little like people doing "the wave" in a sports stadium. The muscles in the esophagus move in a wavelike fashion to push the food down into the stomach.

The stomach is a muscular, hollow pouch that comes after the esophagus in the digestive tract. Although the stomach is a relatively small organ, it plays many roles in digestion. After the bolus enters the stomach, both mechanical and chemical digestion occur to break down the food and to make its parts available to be absorbed into the blood.

First, mechanical digestion takes place as the muscles in the lining of the stomach contract in the same wavelike motion that brought the bolus through the esophagus, peristalsis. Second, chemical digestion takes place as the food comes in contact with gastric juice. Gastric juice contains mostly two chemicals:

• an enzyme called pepsin which breaks down proteins (like in the chicken meat) into smaller parts.
  
  
• hydrochloric acid, which helps pepsin to function and also kills any germs that may have been swallowed.

The stomach also makes a mucus that is similar to the mucus in your nose and throat to prevent the acid from harming the stomach itself. After an hour or two, the stomach contents are about the consistency of cake batter. This substance is now called chyme. Chyme enters the next part of the digestive tract.

Answer these questions to test your understanding of how the bolus moves to the stomach and what happens to it once it gets there.