Figure 17.9 The Six Activities of Digestion Animation Video
To help us understand how digestion works, it is best to divide the digestion process into its six different activities. The first activity of digestion is ingestion—the process of getting food into the body. Ingestion involves the mouth, including the teeth, lips, and tongue. After food has been ingested, the second step, propulsion, begins. Propulsion is the movement of food through the GI tract by muscular contraction. Propulsion begins with swallowing at the pharynx and continues with peristalsis: symmetrical, rhythmic contraction of smooth muscle in the wall of the GI tract that moves food through the esophagus, stomach, and small and large intestines. Once the food reaches the stomach, mechanical breakdown increases. Chewing has already begun this process, but churning in the stomach and small intestines helps reduce food into small pieces. Mechanical breakdown increases the surface area of the food particles. The fourth step, chemical breakdown, begins in the stomach and continues in the small intestine. Enzymes in the lumen and on the walls of the GI tract break down large food molecules. Absorption, the fifth step in the process, occurs mainly in the small intestines. Nutrients absorbed from the food molecules move into (the) blood capillaries in the walls of the intestines. The blood then transports these nutrients throughout the body. The large intestine also absorbs some water, electrolytes, and vitamins. The sixth and final activity of digestion is defecation. Defecation is the expulsion of food molecules that were not absorbed. The waste matter produced, called feces, exits the body via the anus.
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