Approximately one cup of chyme is released into the small intestine every 30 seconds.

75

3

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

MOUTH

PHARYNX AND ESOPHAGUS

STOMACH

SMALL INTESTINE

LARGE INTESTINE

RECTUM

The human digestive system consists of the organs of the gastrointestinal (GI) tract and associated accessory organs. The processing of food in the GI tract involves ingestion, mechanical digestion, chemical digestion, propulsion, absorption, and elimination.

Ingestion Food enters the GI tract via the mouth.

Mechanical digestion Mastication tears, shreds, and mixes food with saliva, forming a bolus.

Chemical digestion Salivary amylase begins carbohydrate breakdown.

Produces digestive enzymes and bicarbonate ions that are released into the small intestine via the pancreatic duct.

Mechanical digestion This process mixes and churns the bolus with acid, enzymes, and gastric fluid into a liquid called chyme.

Chemical digestion Pepsin begins digestion of proteins.

Absorption A few fat-soluble substances are absorbed through the stomach wall.

Mechanical digestion and Propulsion Segmentation mixes chyme with digestive juices; peristaltic waves move it along the tract.

Chemical digestion Digestive enzymes from the pancreas and brush border digest most classes of food.

Absorption Nutrients are absorbed into blood and lymph through enterocytes.

Chemical digestion Some remaining food residues are digested by bacteria.

Absorption Salts, water, and some vitamins are reabsorbed.

Propulsion Compacts waste into feces.

Elimination Feces are temporarily stored before voluntary release through the anus.

Propulsion Swallowing and peristalsis move the bolus from mouth to stomach.

ORGANS OF THE GI TRACT ACCESSORY ORGANS

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78 Chapter 3 | Digestion, Absorption, and Transport

Once food has been adequately chewed and moistened, the tongue rolls it into a bolus and thrusts it into the pharynx to be swallowed. The pharynx is the gateway to the esophagus, as well as to the trachea (or windpipe, the tube that connects to the lungs). Normally, a flap of cartilage called the epiglottis closes off the trachea during swallowing, so that food doesn’t accidentally “go down the wrong pipe” (see Figure 3.2). When the epiglottis doesn’t work properly, food can get lodged in the trachea and potentially cause choking.

The esophagus has only one function—to transport food and fluids from the mouth to the stomach. As food passes through the pharynx, the upper esophageal sphincter opens, allowing the bolus to enter the esophagus. After swallowing, rhythmic muscular contractions, with the help of gravity, move the bolus toward the stomach. The esophagus narrows at the bottom (just above the stomach) and ends at the lower esophageal sphincter (LES) (Figure 3.3). Under normal conditions, when the bolus reaches the stomach, the LES relaxes and allows food to pass into the stomach. The stomach also relaxes to comfortably receive the bolus. After food enters the stomach, the LES closes to prevent the stomach contents from regurgitating backward into the esophagus.

The Stomach Stores, Mixes, and Prepares Food for Digestion The primary function of the stomach is to mix food with various gastric juices to chemi- cally break it down into smaller and smaller pieces (Figure 3.4). The stomach lining includes four layers. The innermost layer contains goblet cells and gastric pits or ducts, which contain gastric glands that secrete a variety of critical digestive juices. Various other cells in the stomach lining, among them parietal (puh-RAHY-i-tl ) cells, chief cells, and mucous neck cells, secrete other gastric juices and mucus. The gastric juices are discussed later in this chapter.

Mechanical digestion in the stomach occurs as the longitudinal, circular, and diagonal muscles that surround the organ forcefully push, churn, and mix the contents of the stomach with the gastric juices. These powerful muscles can also stretch to accom- modate different volumes of food. An empty stomach can hold a little less than a cup, but the numerous folds of the stomach lining can stretch out after a large meal to hold up to 1 gallon (4 liters).1 For several hours, food is continuously churned and mixed in the stomach.

bolus Soft mass of chewed food.

pharynx Area of the GI tract between the mouth and the esophagus; also called the throat.

esophagus Tube that connects the mouth to the stomach.

epiglottis Cartilage at the back of the tongue that closes off the trachea during swallowing.

upper esophageal sphincter Muscular ring located at the top of the esophagus.

lower esophageal sphincter (LES) Mus- cular ring located between the base of the esophagus and the stomach.

stomach J-shaped muscular organ that mixes and churns food with digestive juices and acid to form chyme.

goblet cells Cells throughout the GI tract that secrete mucus.

gastric pits Indentations or small pits in the stomach lining where the gastric glands are located; gastric glands produce gastric juices.

parietal cells Specialized cells in the stomach that secrete the gastric juices hydrochloric acid and intrinsic factor.

chief cells Specialized cells in the stomach that secrete pepsinogen, an inactive form of the protein-digesting enzyme pepsin.

mucus Secretion produced throughout the GI tract that moistens and lubricates food and protects membranes.

▲ Figure 3.2 The Role of the Epiglottis The epiglottis prevents food from entering the trachea during swallowing.

Esophagus

Trachea (open)

Esophagus

Trachea (closed)

Bolus

Tongue

Epiglottis (up)

Bolus

Tongue

Epiglottis (down)

▲ Figure 3.3 Sphincters at Work Sphincters control the passage of food by contracting or relaxing.

The LES (lower esophageal sphincter) relaxes after swallowing to allow the bolus to enter the stomach.

1 The LES contracts to prevent stomach contents from returning to the esophagus.

2

Esophagus

Bolus LES relaxes

Stomach

Esophagus

LES contracts

Stomach

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What Are the Processes and Organs Involved in Digestion? 79

Once food has been adequately chewed and moistened, the tongue rolls it into a bolus and thrusts it into the pharynx to be swallowed. The pharynx is the gateway to the esophagus, as well as to the trachea (or windpipe, the tube that connects to the lungs). Normally, a flap of cartilage called the epiglottis closes off the trachea during swallowing, so that food doesn’t accidentally “go down the wrong pipe” (see Figure 3.2). When the epiglottis doesn’t work properly, food can get lodged in the trachea and potentially cause choking.

The esophagus has only one function—to transport food and fluids from the mouth to the stomach. As food passes through the pharynx, the upper esophageal sphincter opens, allowing the bolus to enter the esophagus. After swallowing, rhythmic muscular contractions, with the help of gravity, move the bolus toward the stomach. The esophagus narrows at the bottom (just above the stomach) and ends at the lower esophageal sphincter (LES) (Figure 3.3). Under normal conditions, when the bolus reaches the stomach, the LES relaxes and allows food to pass into the stomach. The stomach also relaxes to comfortably receive the bolus. After food enters the stomach, the LES closes to prevent the stomach contents from regurgitating backward into the esophagus.

The Stomach Stores, Mixes, and Prepares Food for Digestion The primary function of the stomach is to mix food with various gastric juices to chemi- cally break it down into smaller and smaller pieces (Figure 3.4). The stomach lining includes four layers. The innermost layer contains goblet cells and gastric pits or ducts, which contain gastric glands that secrete a variety of critical digestive juices. Various other cells in the stomach lining, among them parietal (puh-RAHY-i-tl ) cells, chief cells, and mucous neck cells, secrete other gastric juices and mucus. The gastric juices are discussed later in this chapter.

Mechanical digestion in the stomach occurs as the longitudinal, circular, and diagonal muscles that surround the organ forcefully push, churn, and mix the contents of the stomach with the gastric juices. These powerful muscles can also stretch to accom- modate different volumes of food. An empty stomach can hold a little less than a cup, but the numerous folds of the stomach lining can stretch out after a large meal to hold up to 1 gallon (4 liters).1 For several hours, food is continuously churned and mixed in the stomach.

bolus Soft mass of chewed food.

pharynx Area of the GI tract between the mouth and the esophagus; also called the throat.

esophagus Tube that connects the mouth to the stomach.

epiglottis Cartilage at the back of the tongue that closes off the trachea during swallowing.

upper esophageal sphincter Muscular ring located at the top of the esophagus.

lower esophageal sphincter (LES) Mus- cular ring located between the base of the esophagus and the stomach.

stomach J-shaped muscular organ that mixes and churns food with digestive juices and acid to form chyme.

goblet cells Cells throughout the GI tract that secrete mucus.

gastric pits Indentations or small pits in the stomach lining where the gastric glands are located; gastric glands produce gastric juices.

parietal cells Specialized cells in the stomach that secrete the gastric juices hydrochloric acid and intrinsic factor.

chief cells Specialized cells in the stomach that secrete pepsinogen, an inactive form of the protein-digesting enzyme pepsin.

mucus Secretion produced throughout the GI tract that moistens and lubricates food and protects membranes.

▲ Figure 3.4 Anatomy of the Stomach The cross section of the stomach illustrates the gastric cells that secrete digestive juices.

Longitudinal muscles

Stomach Cross section of inner stomach walls

Small intestine

Esophagus

Lower esophageal sphincter

Circular muscles

Diagonal muscles

Inner stomach walls

Chief cells

Gastric pits

Parietal cells

Submucosa

Gastric glands

Goblet cells

By the time the mixture reaches the lower portion of the stomach, it is a semiliquid mass called chyme, which contains digestive secretions plus the original food. As the chyme accumulates near the pyloric sphincter, between the stomach and the small intestine, the sphincter relaxes and the chyme gradually enters the small intestine. You eat much faster than you can digest and absorb food, so the stomach also acts as a holding tank for chyme until it can be released into the small intestine. Approximately 1–5 milliliters (1 teaspoon) of chyme is released into the small intestine every 30 seconds.2 The pyloric sphincter prevents chyme from exiting the stomach too soon and blocks the intestinal contents from returning to the stomach.

Most Digestion Occurs in the Small Intestine As chyme passes through the pyloric sphincter, it enters the long, coiled small intestine. This organ consists of three segments—the duodenum, jejunum, and ileum— and extends from the pyloric sphincter to the ileocecal valve at the beginning of the large intestine (Figure 3.5). The first segment, the duodenum, is approximately 10 inches long. The second, the jejunum, measures about 8 feet long, and the third, the ileum, is about 12 feet long. The “small” in “small intestine” refers to its diameter (about 1 inch), not its extended length.

As in the stomach, both mechanical and chemical diges- tion occur in the small intestine. Muscular contractions squeeze chyme forward while digestive secretions from the pancreas, gallbladder, and intestinal lining chemically break down the nutrients.

Numerous fingerlike projections, called villi, line the small intestine. They increase the surface area to maximize absorption and help mix the partially digested chyme with intestinal secretions (Focus Figure 3.6). Each villus contains capillaries and lymphatic vessels called lacteals that pick up digested nutrients during absorption. The villi extend about 1 millimeter into the lumen, creating a velvety appearance, and are arranged into hundreds of overlapping, circular folds. The circular folds cause chyme to spiral forward through the small intestine, further increasing its exposure to the villi.

chyme Semiliquid, partially digested food mass that leaves the stomach and enters the small intestine.

small intestine Long coiled chamber that is the major site of food digestion and nutrient absorption.

villi Small, fingerlike projections that line the lumen of the small intestine.

▲ Figure 3.5 Anatomy of the Small Intestine The small intestine is highly adapted for absorbing nutrients. Its length— about 20 feet—provides a huge surface area, and its wall has three structural features—circular folds, villi, and microvilli—that increase its surface area by a factor of more than 600.

Duodenum

Ileum

Jejunum

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80 Chapter 3 | Digestion, Absorption, and Transport

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

Figure 3.6 Structures of the Small Intestinal WallFOCUS

The lining of the small intestine is heavily folded, resulting in increased surface area for the absorption of nutrients.

CIRCULAR FOLDS

The folds are covered with villi, thousands of fingerlike projections that increase the surface area even further. Each villus contains capillaries and a lacteal for picking up nutrients absorbed through the enterocytes and transporting them throughout the body.

VILLI

The cells on the surface of the villi, enterocytes, end in hairlike projections called microvilli that together form the brush border through which nutrients are absorbed.

MICROVILLI

The small intestine is highly adapted for absorbing nutrients. Its length—about 20 feet—provides a huge surface area, and its wall has three structural features—circular folds, villi, and microvilli— that increase its surface area by a factor of more than 600.

Crypt

Goblet cell

Enterocyte

Microvilli (brush border)

Lacteal

Enterocyte

Capillaries

Small Intestine

Villi

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What Are the Processes and Organs Involved in Digestion? 81

Epithelial cells called enterocytes cover the villi. They have smaller projections called microvilli, referred to collectively as the brush border, which trap nutrients and absorb them into the enterocyte interior. There, the nutrients enter blood and lymphatic vessels that transport them throughout the body. Enterocytes also secrete several enzymes that help digest specific nutrients. We discuss these secretions in more detail later in the chapter.

Goblet cells scattered along the villi secrete lubricating mucus into the intestine. Between the villi lie glands called crypts that secrete intestinal juices. Within the crypts, stem cells continually divide, producing younger cells that travel up the villi to replace mature cells when they die. A constant source of nutrients is needed to replace these cells and maintain a healthy absorptive surface. Without the proper nutrients, the villi deteriorate and flatten, result- ing in malabsorption.

Depending on the amount and type of food eaten, the contact time in the small intes- tine is between 3 and 10 hours. Usually, by the time you sit down to dinner, your breakfast is just about reaching the end of the small intestine.

The Large Intestine Absorbs Water and Some Nutrients The large intestine is only about 5 feet long—much shorter than the small intestine— but its lumen is larger, about 2.5 inches in diameter. It has three segments: the cecum, colon, and rectum (Figure 3.7). The cecum (SEE-kum) marks the beginning of the large intestine. Chyme from the small intestine passes through the ileocecal valve into the cecum before entering the colon. The colon is the largest portion of the large intes- tine, and it is further subdivided into the ascending, transverse, descending, and sigmoid regions. These regions are relatively long and straight. Note that though the terms colon and large intestine are often used interchangeably, technically they’re not the same thing.

By the time chyme enters the large intestine, the majority of the nutrients, except water and the electrolytes sodium, potassium, and chloride, have been absorbed. The cells of the large intestine absorb water and these electrolytes much more efficiently than the cells of the small intestine. The large intestine also produces mucus that protects the cells and acts as a lubricant for fecal matter.

Helpful bacteria that colonize the colon are collectively known as the GI flora or microflora. These GI flora produce some vitamins, including vitamin K, thiamin, ribo- flavin, biotin, and vitamin B12. Only biotin and vitamin K can be absorbed in the colon, however. Bacteria also ferment some of the undigested and unabsorbed dietary carbohy- drates into simpler compounds, methane gas, carbon dioxide, and hydrogen. This fermen- tation process is the major source of intestinal gas. Similarly, some of the colon’s bacteria break down undigested fiber and produce various short-chain fatty acids. Amino acids that reach the colon are converted to hydrogen, sulfide, some fatty acids, and other chemi- cal compounds. Given the importance of bacteria in the healthy functioning of the colon, it isn’t surprising that foods containing such beneficial living microorganisms, called pro- biotics, have become popular in recent years. The Examining the Evidence feature evalu- ates the claims of health benefits associated with consumption of probiotic foods.

enterocytes Absorptive epithelial cells that line the lumen of the small intestine.

microvilli Tiny projections on the villi in the small intestine.

crypts Glands at the base of the villi; they contain stem cells that manufacture young cells to replace the cells of the villi when they die.

large intestine Lowest portion of the GI tract, where water and electrolytes are absorbed and waste is eliminated.

cecum Pouch at the beginning of the large intestine that receives waste from the small intestine.

ileocecal valve Sphincter that separates the small intestine from the large intestine.

colon Another name for the large intestine.

GI flora Microorganisms that live in the GI tract of humans and animals.

ferment To metabolize sugar into carbon dioxide and other gases.

▲ Figure 3.7 Anatomy of the Large Intestine By the time chyme reaches the large intestine, most of its nutrients have been absorbed. However, water and some electrolytes are absorbed in the colon. The final waste products of digestion pass out of the body through the anus.

Rectum

Anal sphincter

Cecum

Appendix

Ileocecal valve

Ileum

Ascending colon

Transverse colon

Descending colon

Sigmoid colon

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82 Chapter 3 | Digestion, Absorption, and Transport

EXAMINING THE

EVIDENCE

If you’re like most people, the thought of eating food that contains living microorgan- isms is not appealing. But did you know that consuming certain microbes, called probiotics, might improve your health? Pro- biotics (pro = for, bios = life) are “live microorganisms, which, when administered in adequate amounts, confer a health benefit on the host.”1 They are similar to the more than 10 trillion beneficial microbes, mostly strains of bacteria, that colonize your GI tract. Medications, stress, a poor diet, and illness can disrupt the balance of friendly GI flora, and consuming them in probiotic foods can help restore their numbers. In the United States, sales of probiotic supplements and foods reached $1.14 billion in 2014, and sales are expected to nearly double by 2019.2 Why?

Research Supports the Health Benefits of Probiotics Probiotics function in the same way that the native bacteria in your GI tract do. For example, they pro- duce organic acids that inhibit the growth of disease-causing micro- organisms. They also compete with these pathogens for nutrients and receptor sites, keeping the popula- tion of harmful microbes in check. Other proposed benefits of probiotics include:3–9

• May reduce constipation, in part by reducing transit time of chyme through the GI tract.

• May prevent or reduce the symp- toms of diarrhea and the cramping and bloating associated with lactose intolerance.

• May reduce inflammation. • May play a role in preventing food

allergies. • May shorten the duration and reduce

the severity of the symptoms of the common cold, especially in academi- cally stressed students.

• May reduce signs of colic in infants.

Prebiotics and Synbiotics Support the GI Flora Prebiotics, which are present in thou- sands of plant-based foods, are nondi- gestible resistant starches—a form of dietary fiber—that support the growth and health of your GI flora. Because the body can’t digest them, they reach the large bowel intact. There, the GI flora digest them. Eating foods rich in pre- biotics thus helps build and maintain a healthy population of GI flora. The two most common prebiotics are fructo- oligosaccharides (FOS) and inulin.

Products called synbiotics are processed foods and supplements that contain both probiotic bacteria and prebiotic starches. For example, yogurts to which inulin has been added qualify as synbiotics. Such products both supplement your native GI flora and support their growth and activity.

Where Can You Find Probiotics, Prebiotics, and Synbiotics? Probiotics are found primarily in fer- mented dairy and soy products and in dietary supplements (see the accom- panying table). For example, eating Activia yogurt is one way to increase probiotics through diet. The food label should identify the strain of the bac- teria used, when the product expires,

the suggested serving size, and how to store the product to make sure the bacteria are still alive when you eat it. A serving should contain at least one billion CFUs (a measure of live bacteria called colony-forming units) to provide the level of probiotics found to deliver health benefits.1 When choosing yogurt, look for brands low in saturated fat and added sugars. Plain Greek-style yogurt is a healthful and satisfying choice.

The food richest in prebiotics is bananas. Other foods containing prebi- otics include garlic, onions, asparagus, leeks, squash, jicama, beets, carrots, tur- nips, parsnips, sweet potatoes, and arti- chokes. Oats, barley, and certain other whole grains also provide prebiotics.

Some yogurt producers add inulin, producing a synbiotic. You can also create your own synbiotics by eating foods rich in probiotics and prebiotics together. Consider, for example, yogurt with oatmeal and bananas, or tempeh (fermented soy) in a stir-fry with aspara- gus, garlic, and onions.

There’s no downside to increasing your consumption of foods rich in probiotics, prebiotics, and synbiotics. Although there is a great deal we still don’t know about their impact on our health, we do know that the dairy and plant-based foods providing them are nutrient dense and delicious.

Do Probiotics, Prebiotics, and Synbiotics Improve Your Health?

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5. Spaiser, S. J., et al. 2015. Lactobacillus gas- seri KS-13, Bifidobacterium bifidum G9-1, and Bifidobacterium longum MM-2 Inges- tion Induces a Less Inflammatory Cytokine Profile and a Potentially Beneficial Shift in Gut Microbiota in Older Adults: A Random- ized, Double-Blind, Placebo-Controlled, Crossover Study. Journal of the American College of Nutrition 34(6):459–469. doi: 10.1080/07315724.2014.983249.

6. Savaiano, D. A., A. J. Ritter, et al. 2013. Improving Lactose Digestion and Symptoms of Lactose Intolerance with a Novel Galacto- Oligosaccharide (RP-G28): A Randomized, Double-Blind Clinical Trial. Nutrition Journal 12:160. doi: 10.1186/1475-2891-12-160.

7. Ritz, B. W. 2011. Probiotics for the Prevention of Childhood Eczema. Available at http://natu- ralmedicinejournal.com. Accessed January 2017.

8. Langkamp-Henken, B., et al. 2015. Bifidobac- terium bifidum R0071 Results in a Greater Proportion of Healthy Days and a Lower Percentage of Academically Stressed Students

Reporting a Day of Cold/Flu: A Ran- domised, Double-blind, Placebo-controlled Study. British Journal of Nutrition 113(3):426– 434. doi: 10.1017/S0007114514003997.

9. Anabrees, J., F. Indrio, et al. 2013. Probiotics for Infantile Colic: A Sys- tematic Review. BMC Pediatrics 13:186. doi:10.1186/1471-2431-13-186.

References 1. Kechagia, M. D., S. Basoulis, et al. 2013.

Health Benefits of Probiotics: A Review. ISRN Nutrition, Article ID 481651, doi:10.5402/ 2013/481651. Accessed January 2017.

2. Statista: The Statistics Portal. 2016. Dollar Sales of Probiotic Supplements in the United States in 2015, By Channel (in million U.S. dollars). Available at https://www.statista. com/statistics/493536/dollar-sales-of- probiotic-supplement-in-the-us-by-channel. Accessed January 2017.

3. Ritchie, M. L., and T. N. Romanuk. 2012. A Meta-Analysis of Probiotic Efficacy for Gastrointestinal Diseases. PLoS ONE 7(4):e34938. doi:10.1371/journal. pone.0034938.

4. Beserra, B. T., et al. 2014. A Systematic Review and Meta-analysis of the Prebiotics and Synbiotics Effects on Glycaemia, Insulin Concentrations, and Lipid Parameters in Adult Patients with Overweight or Obe- sity. Clinical Nutrition 34(5):845–858. doi: 10.1016/j.clnu.2014.10.004.

probiotics Live microorganisms that, when consumed in adequate amounts, confer a health benefit on the host.

Beneficial Bacteria

If You Have This Problem Try This Probiotic Found in These Foods and Dietary Supplements

Diarrhea ■■ Lactobacillus reuteri 55730 ■■ Saccharomyces boulardii yeast

■■ BioGai tablets, drops, and lozenges ■■ Florastor dietary supplement

Constipation, irritable bowel syndrome, and overall digestion problems

■■ Bifidobacterium animalis DN-173 010 ■■ Bifidobacterium infantis 35624

■■ Dannon Activia yogurt ■■ Align supplement

Poor immune system ■■ Bifidobacterium lactis Bb-12 ■■ Lactobacillus casei Shirota ■■ Lactobacillus casei DN-114 001

■■ Yo-Plus Yogurt ■■ Yakult fermented dairy drink ■■ Dannon’s DanActive dairy drink

Vaginal infections ■■ Lactobacillus rhamnosus GR-1 combined with Lactobacillus reuteri

■■ Fem Dophilus dietary supplement

About l liter of fluid material—consisting of water, undigested or unabsorbed food particles, indigestible residue, and electrolytes—passes into the colon each day. Gradually, the material is reduced to about 200 grams of brown fecal matter (also called stool, or feces). Stool consists of the undigested food residue, as well as sloughed-off cells from the GI tract and a large quantity of bacteria. The brown color is due to unabsorbed iron mix- ing with a yellowish-orange substance called bilirubin. The greater the iron content, the darker the feces. The intestinal matter passes through the colon within 12–70 hours, depending on a person’s age, health, diet, and fiber intake.

Stool is propelled through the large intestine until it reaches the final 8-inch portion called the rectum. The anus is connected to the rectum and controlled by an internal and an external sphincter. Under normal conditions, the anal sphincters are closed. When stool distends the rectum, the action stimulates stretch receptors that in turn stimulate the internal anal sphincters to relax, allowing the stool to enter the anal canal. This causes nerve impulses of the rectum to communicate with the rectum’s muscles, resulting in defecation. The final stage of defecation is under voluntary control and influenced by age, diet, prescription medicines, health, and abdominal muscle tone.

stool Waste produced in the large intestine; also called feces.

prebiotics Nondigestible starch found in plant foods that promotes the growth and health of your GI flora.

rectum Final 8-inch portion of the large intestine.

anus Opening of the rectum, or end of the GI tract.

What Are the Processes and Organs Involved in Digestion? 83

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84 Chapter 3 | Digestion, Absorption, and Transport

The Accessory Organs Secrete Digestive Juices The salivary glands, liver, gallbladder, and pancreas are considered accessory organs because food does not pass through them (Figure 3.8). These organs are still key to the digestive process, however. They contribute digestive secretions such as saliva, bile, and enzymes that help with breakdown and transport of nutrients.

The salivary glands, located beneath the jaw and under and behind the tongue, produce about 1 quart of saliva per day.3 In addition to water and electrolytes, saliva

contains several enzymes, including salivary amylase and lysozyme, an enzyme that destroys certain oral bacteria. It also contains mucus, which helps lubricate food,

helps it stick together, and protects the inside of the mouth. Weighing in at about 3 pounds, the liver is the largest organ in the

body. It is located just beneath the rib cage and functions as a major player in the digestion of food and the absorption and transport of nutrients. The liver plays an essential role in carbohydrate metabolism, produces

proteins, and manufactures bile salts, which contribute to the break- down of fats. The bile produced by the liver is secreted into the gall- bladder for storage. The liver also metabolizes alcohol and removes and degrades toxins and excess hormones from the circulation.

The gallbladder is located beneath the right side of the liver. This pear-shaped organ receives bile from the liver through the

common hepatic duct, concentrates it, and secretes bile into the small intestine through the common bile duct.

The pancreas is a flat organ about 10–15 centimeters long that rests behind the stomach in the bend of the duodenum. The function of the pan- creas is both endocrine (endo = inside) and exocrine (exo = outside). As an endocrine organ, the pancreas releases into the bloodstream hormones that help regulate blood glucose levels. As an exocrine organ, the pancreas produces and secretes digestive enzymes through the pancreatic duct into the small intestine.

salivary glands Cluster of glands located underneath and behind the tongue that release saliva in response to the sight, smell, and taste of food.

liver Accessory organ of digestion located in the upper abdomen and responsible for the synthesis of bile, the processing of nutrients, the metabolism of alcohol, and other functions.

gallbladder Pear-shaped organ that stores and concentrates bile produced by the liver and secretes it through the common bile duct into the small intestine.

pancreas Large gland located behind the stomach that releases digestive enzymes and bicarbonate after a meal. Also secretes the hormones insulin and glucagon, which control blood glucose.

Duodenum

Liver

Salivary glands

Gallbladder

Pancreas

Common bile duct

▲ Figure 3.8 The Accessory Organs The salivary glands, liver, gallbladder, and pancreas produce digestive secretions that flow into the GI tract through various ducts.

LO 3.1: THE TAKE-HOME MESSAGE Digestion takes place in the GI tract. Chemicals break the molecular bonds in food so that nutrients can be absorbed and transported throughout the body. Saliva mixes with and moist- ens food in the mouth, making it easier to swallow. Once a bolus of food mixes with gastric juices in the stomach, it becomes chyme. Maximum digestion and absorption occur in the small intestine. Undigested residue enters the large intestine, where water is removed from the chyme as it is prepared for elimina- tion. Eventually, the remnants of digestion reach the anus and exit the body in the feces. The salivary glands, liver, gallbladder, and pancreas are important accessory organs. The salivary glands produce saliva. The liver produces bile, which the gallbladder concentrates and stores. The pancreas produces diges- tive enzymes and hormones.

How Is Food Propelled through the GI Tract? LO 3.2 Explain how food is propelled through the gastrointestinal tract.

Wavelike movements of the muscles throughout the GI tract propel food and liquid for- ward. These contractions, which depend on coordination between the muscles, nerves, and hormones in the GI tract, help mechanically digest the food by mixing and pushing

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How Is Food Propelled through the GI Tract? 85

it at just the right pace through each organ. The two primary types of contractions are called peristalsis and segmentation (Figure 3.9).

Peristalsis begins in the esophagus, as contractions of circular muscles prevent food from moving backward. Repeated waves of contractions follow as the circular muscles relax and the longitudinal muscles constrict and push the food forward.

In the stomach, circular, longitudinal, and diagonal muscles move the food from the top of the stomach toward the pyloric sphincter at the base of the stomach. The waves of contractions in the stomach are slower than in other GI organs, as peristalsis mixes and churns the stomach contents with gastric juices until the food is liquefied.

As the partially digested food leaves the stomach, the second form of mechanical digestion, called segmentation, begins. Segmentation moves the food back and forth, helping to break it down into smaller pieces while mixing it with the chemical secretions of the intestine. Segmentation differs from peristalsis in that food is shifted (rather than squeezed) back and forth along the intestinal walls. This shifting action increases the time food is in contact with the intestinal lining, moving food through the small intestine at a rate of 1 centimeter per minute.4

Mass movement, also known as mass peristalsis, is a series of strong, slow contrac- tions in the large intestine that move the chyme through the colon. Three or four times a day, these slow but powerful muscular contractions force the waste products toward the rectum. Meanwhile, segmentation within the colon helps dry out the feces, allowing for the maximum amount of water to be absorbed. These contractions often occur shortly after eating and are stronger when the diet contains more fiber.

peristalsis Forward, rhythmic muscular con- tractions that move food through the GI tract.

segmentation Muscular contractions of the small intestine that move food back and forth, breaking the mixture into smaller and smaller pieces and combining it with digestive juices.

mass movement (mass peristalsis) Strong, slow peristaltic movements, occurring only three or four times a day within the colon, that force waste toward the rectum.

▲ Figure 3.9 Peristalsis and Segmentation

Longitudinal, circular, and diagonal muscles constrict in wavelike motions to propel food through the GI tract.

1

Longitudinal and circular muscles in the small intestine mix and squeeze food back and forth along the intestinal wall.

2

Stomach

Small intestine Segmentation

Peristalsis

Longitudinal

Circular

Longitudinal

Circular

Diagonal

LO 3.2: THE TAKE-HOME MESSAGE Food is propelled through the GI tract by strong muscular contractions. Peristalsis in the esophagus, stomach, and small intestine squeezes the food and propels it forward, while segmentation in the small and large intestines shifts food back and forth along the intestinal walls. Mass movement moves waste slowly and powerfully toward the rectum.

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86 Chapter 3 | Digestion, Absorption, and Transport

How Is Food Chemically Digested? LO 3.3 Identify the role of enzymes and other secretions in chemical digestion.

While food travels through the organs of the GI tract, digestive enzymes and other chemicals break it down into nutrients. How do these chemicals work?

Enzymes Drive the Process of Digestion Enzymes are compounds, most of which are proteins, that catalyze or speed up chemi- cal reactions. One of the most important roles of enzymes is to accelerate hydrolysis (hydro = water, lysis = break) reactions, in which water breaks the bonds of digestible carbohydrates, fats, proteins, and alcohol. Hydrolysis produces single molecules small enough to be absorbed by the intestines. During hydrolysis, the hydroxyl (OH) group from water joins one of the molecules, while the hydrogen ion (H) joins the other molecule, forming two new molecules. This is illustrated in the Chemistry Boost. Enzymes aren’t changed in the reaction and can thus be used over and over again.

enzymes Substances, mostly proteins, that increase the rate of chemical reactions; also called biological catalysts.

hydrolysis Chemical reaction that breaks the bond between two molecules with water. A hydroxyl group is added to one molecule and a hydrogen ion is added to the other molecule.

Once food enters the mouth, enzymes begin to chemically break the bonds that bind the nutrients.

Hydrolysis The process known as

hydrolysis digests most food molecules, in which the addition of water and a specific enzyme breaks down the corresponding molecule. In the figure below, a molecule of sucrose (sugar) is digested by the enzyme sucrase when it breaks the bond with the addition of water by adding a hydroxyl group (OH) to form glucose and hydrogen (H) to form fructose.

Chemistry Boost

H2O

H

H

OH

O

OH

H

HO

H OH

CH2OH

C

C C

C

Glucose (C6H12O6) Fructose (C6H12O6)

+

H

HO

OH

H

HO

H OH

CH2OH

C

C

CH

CH

C O

C

Sucrose (C12H22O11)

Sucrase

OH H

O

H HO

OH H

HOCH2 CH2OH

C

C C

C

OH H

O

H

OH H

HOCH2 CH2OH

C

C C

C

In order for enzymes to catalyze hydrolytic reactions, three conditions must be present.

1. The compatible enzyme and nutrient must both be present. Enzymes are compatible only with a specific compound or nutrient, referred to as a substrate. Each enzyme has a binding site that only fits certain substrates, much like a key fits a specific lock. When the substrate binds to the active site of the enzyme, the bond is hydrolyzed. This reaction is illustrated in Figure 3.10. Enzymes are often named according to the type of substrate they act upon, plus the suffix -ase. So, sucrase hydrolyzes the sugar sucrose, and maltase hydrolyzes maltose. Some enzymes, such as pepsin, were named before this new nomenclature was devel- oped and don’t follow these naming rules.

substrate Substance or compound that is altered by an enzyme.

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How Is Food Chemically Digested? 87

2. The pH of the environment must fall in the appropriate range. Enzymes are most effective when the fluid environment falls within a certain pH range, a range of acidity or alkalinity (see the Chemistry Boost). When the pH falls outside of that range, the activity of the enzyme is decreased or even halted. For example, saliva contains bicarbonate, which neutralizes acids in foods. It has a pH of about 6.4, which is optimal for the effective action of salivary amylase. When the bolus containing the salivary enzymes reaches the stomach, where the pH is closer to 1, salivary amylase activity is stopped. However, another enzyme, pepsin, becomes activated in this acidic environment. As chyme continues to travel through the GI tract, various organs secrete digestive juices that produce the optimal range of pH for the enzymes to function.

3. The temperature of the environment must fall within the appropriate range. As temperature falls below optimal levels, enzyme activity slows. If the temperature becomes too high, the enzyme is inactivated. In the body, the opti- mal temperature for enzymatic activity is 98.6°F (35.7°C), which is considered normal body temperature. Temperature also influences enzyme activity in foods. Cooling food in a refrigerator slows enzyme activity, and cooking food com- pletely inactivates any enzymes it contains.

Digestive enzymes are secreted all along the GI tract, but most are produced in the pancreas. The pancreas secretes digestive enzymes into the duodenum through the pancreatic duct. The brush border of the small intestine releases the last of the digestive enzymes. Table 3.1 summarizes the digestive enzymes, the organs that secrete them, and their actions.

pH Measure of the acidity or alkalinity of a solution.

▲ Figure 3.10 An Enzyme in Action Enzymes increase the rate of digestion without altering their shape.

Enzyme

Substrate

Bond

H2O

The substrate binds to the active site of the enzyme.

1 The individual products are released and the enzyme is ready to take action again.

3 The bond is broken between the two molecules by hydrolysis.

2

Organ or Gland Enzyme Action Nutrient

Salivary glands Salivary amylase Begins the digestion of starch Carbohydrates

Stomach Pepsinogen S Pepsin Begins the hydrolysis of polypeptides Protein Gastric lipase Begins digestion of lipids Lipids

TABLE 3.1 Digestive Enzymes and Their Actions

(continued)

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88 Chapter 3 | Digestion, Absorption, and Transport

pH Scale Acids and bases describe the chemical

properties of a substance. Acidity level is expressed using a pH scale that measures the hydrogen ion concentration. The range of a pH scale is 0–14, with 7—the pH of pure water—considered neutral. A solution that has a pH lower than 7 is considered acidic (0 is the most acidic); one higher than 7 is basic (14 is the most basic).

The lower the number on the pH scale, the greater the concentration of hydro- gen ions (H+) in a solution. The more H+ in a solution, the stronger the acid. For example, gastric juice, with a pH of 1, has a higher concentration of H+ and is much more acidic than pancreatic juice, which has a pH of 8.

Bases—alkaline compounds—contain more hydroxide ions (OH-) and have a low concentration of H+. The more OH- in a solution, the stronger the base. For example, sodium hydroxide, an acid, has a pH of 1, whereas bile, which is a base, has