What churns food and adds juices

Learning Outcomes After reading this chapter, you will be able to:

3.1 Describe the processes and organs involved in digestion.

3.2 Explain how food is propelled through the gas- trointestinal tract.

3.3 Identify the role of enzymes and other secre- tions in chemical digestion.

3.4 Describe how digested nutrients are absorbed.

3.5 Explain how hormones and the nervous sys- tem regulate digestion.

3.6 Explain how absorbed nutrients are trans- ported throughout the body.

3.7 Discuss the most common digestive disorders.

True or False? 1. Saliva can alter the taste of food. T/F 2. Without mucus, the stomach would digest itself. T/F 3. The major function of bile is to emulsify fats. T/F 4. Acid reflux is caused by gas in the stom-ach. T/F 5. The primary function of the large intes-tine is to absorb water. T/F 6. Feces contain a high amount of bacteria. T/F

7. The lymphatic system transports all nutrients through the body once they’ve been absorbed. T/F 8. Hormones play an important role in digestion. T/F 9. Diarrhea is always caused by bacterial infection. T/F

10. Irritable bowel syndrome is caused by an allergy to gluten. T/F See page 110 for the answers.

Digestion, Absorption, and Transport

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76 Chapter 3 | Digestion, Absorption, and Transport

The digestion of food begins even before you take that first bite. Just the sight and smell of homemade apple pie stimulates the release of saliva in the mouth. The secretion of saliva and other digestive juices starts a cascade of

events that prepares the body for digestion, the chemical and mechanical

processes by which the body breaks food down into individual nutrient

molecules ready for absorption. Food components that aren’t absorbed are

excreted as waste (feces) by elimination. Although these are complex

processes, they go largely unnoticed. You consciously chew and swallow the

pie, but you don’t feel the release of chemicals or the muscular contractions

that cause it to be digested or the absorption of nutrient molecules through

the intestinal lining cells. In fact, you may be unaware of the entire process

until about 48 hours after eating, when the body is ready to eliminate waste.

In this chapter, we explore the processes of digestion, absorption, and

elimination, the organs involved, and the other biological mechanisms that

regulate our bodies’ processing of food and nutrients. We also discuss the causes

and treatments of some common gastrointestinal conditions and disorders.

What Are the Processes and Organs Involved in Digestion? LO 3.1 Describe the processes and organs involved in digestion.

Digestion, absorption, and elimination occur in the gastrointestinal (GI) tract, a mus- cular tube approximately 20–24 feet long in an adult. Stretched vertically, the tube would be about as high as a two-story building. It provides a barrier between the food within the lumen (the hollow interior of the tract), which is technically external, and our body cells, which are internal.

Although the prefix gastro- means “stomach,” the GI tract actually extends from the mouth to the anus. Its six organs are the mouth, pharynx, esophagus, stomach, small intestine, and large intestine. Food moves from one organ to the next by propulsion. Various sphincters along the way allow food to pass. These muscular rings act like one-way doors, allowing the mixture of food and digestive juices to flow into the next organ but not to flow back. Focus Figure 3.1 illustrates the organs and processes of the digestive system.

Digestion Begins in the Mouth The body digests food chemically, by the actions of digestive secretions, and mechanically, by the actions of the teeth and the powerful muscles of the GI tract. Both chemical digestion and mechanical digestion begin in the mouth. During mastication, the teeth, powered by strong jaw muscles, mechanically cut and grind food into smaller pieces as the tongue mixes it with saliva. Saliva dissolves small food particles, which allows them to react with the taste buds so we can savor food. About 99 percent water, saliva moistens and binds food to lubricate it for comfortable swallowing and traveling down the esophagus. Saliva also contains enzymes, compounds that help accelerate the rate of chemical reactions. Enzymes are discussed in detail later in this chapter. The primary enzyme in saliva is sali- vary amylase, which begins to break down carbohydrates. (You can taste this enzyme working when you eat a starch-containing food such as a cracker; as the enzyme breaks down starch into sugars, the flavor becomes sweeter.)

digestion Process that breaks down food into individual molecules small enough to be absorbed through the intestinal wall.

absorption Process of moving nutrients from the GI tract into the circulatory system.

elimination Excretion of undigested and unabsorbed food through the feces.

gastrointestinal (GI) tract Tubular organ system including the mouth, pharynx, esopha- gus, stomach, and small and large intestines, by means of which food is digested, nutrients absorbed, and wastes expelled.

lumen Channel or inside space of a vessel such as the intestine or artery.

propulsion Process that moves food along the GI tract during digestion.

sphincters Circular rings of muscle that open and close in response to nerve input.

chemical digestion Breaking down food through enzymatic reactions.

mechanical digestion Breaking down food by chewing, grinding, squeezing, and mov- ing it through the GI tract by peristalsis and segmentation.

mastication Chewing food.

saliva Secretion from the salivary glands that softens and lubricates food and begins the chemical breakdown of starch.

Whole foods must first be broken down into individual nutrients that can be used by the body’s cells.

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What Are the Processes and Organs Involved in Digestion? 77

Head to Mastering Nutrition and watch a narrated video tour of this figure by author Joan Salge Blake.

Figure 3.1 The Digestive SystemFOCUS

PANCREAS

GALLBLADDER

LIVER Produces bile to digest fats.

Stores bile before release into the small intestine through the bile duct.

Produce saliva, a mixture of water, mucus, enzymes, and other chemicals

SALIVARY GLANDS