Mechanical Digestion

The walls of the stomach have several layers of smooth muscle. As you can see in Figure 48-10, there are three layers of muscle—a circular layer, a longitudinal layer, and a diagonal layer. When food is present, these muscles work together to churn the contents of the stomach. This churning helps the stomach carry out mechanical digestion.

The inner lining of the stomach is a thick, wrinkled mucous membrane composed of epithelial cells. This membrane is dotted with small openings called gastric pits. Gastric pits, which are shown in Figure 48-10, are the open ends of gastric glands that release secretions into the stomach. Some of the cells in gastric glands secrete mucus, some secrete digestive enzymes, and still others secrete hydrochloric acid. The mixture of these secretions forms the acidic digestive fluid.

figure 48-9

Peristalsis is so efficient at moving materials down the esophagus that you can drink while standing on your head. The smooth muscles move the water "up" the esophagus, against the force of gravity.

figure 48-9

Peristalsis is so efficient at moving materials down the esophagus that you can drink while standing on your head. The smooth muscles move the water "up" the esophagus, against the force of gravity.

figure 48-10

Each of the muscle layers of the stomach is oriented in a different direction. The pH of the stomach is normally between 1.5 and 2.5, making it the most acidic environment in the body. Mucous cells lining the stomach wall protect the organ from damage.

www.scilinks.org Topic: Chemical Digestion Keyword: HM60267

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Maintained by the National Science Teachers Association figure 48-11

The liver is the body's largest internal organ, weighing about 1.5 kg (3 lb). If a small portion is surgically removed because of disease or injury, the liver regenerates the missing section.

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