The Stomach
- The partially digested food is now in the stomach.
- The stomach is a muscular sac with thick, expandable walls.
- The stomach walls are made of layers of muscles that contract in opposite directions.
- Mechanical digestion occurs when the stomach walls contract strongly, mixing and churning the food. These contractions are responsible for the “growling” noises our stomach makes, they are the loudest when we have an empty stomach.
- Chemical digestion in the stomach begins with the actions of hydrochloric acid and an enzyme called pepsin. Glands in the stomach secrete both substances.
- Pepsin breaks down protein, and works best in an acidic environment, which is provided by the hydrochloric acid.
- Another fluid secreted by glands in the stomach is mucus. Mucus lubricates food so that it can travel through the digestive tract more easily.
- Mucus also coats the walls of the stomach, protecting the muscle tissue from being broken down by other digestive fluids.
- The lives of stomach wall cells are short; they are replaced about every three days.
- After about three hours (2-3 hours) of mechanical and chemical treatment in the stomach, food is reduced to a soft pulp called chyme (kym).
- Chyme is a thick liquid made up of partially digested proteins, starches, and acids, and undigested sugars and fats.
- At this point, the pyloric valve between the stomach and small intestine opens, allowing small amounts of chyme to pass into the small intestine.
- By the time chyme has left the stomach, most proteins have been broken down into smaller polypeptides. Sugars and fats have not yet been chemically altered. Some starch molecules have been broken down into disaccharides.