As you read the chapter for this week, summarize the functions by classifying them into a chemical breakdown or mechanical breakdown using the followi

Because learning changes everything. ®

HUMAN BIOLOGY Seventeenth Edition

Sylvia S. Mader Michael Windelspecht

Chapter 9 Digestive System

and Nutrition

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9.1 Overview of Digestion 1

Learning Outcomes: • State the function of each organ of the

gastrointestinal tract. • List the accessory organs and name a function for

each. • Describe the structure of the gastrointestinal tract

wall.

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9.1 Overview of Digestion 2

Overview of the digestive system. The organs are located within the gastrointestinal (GI) tract.

Function—to hydrolyze, or break down, the macromolecules found in food. • The subunit molecules (monosaccharides, amino acids,

fatty acids, and glycerol) can cross plasma membranes using facilitated and active transport.

The nutrients made available are transported by the blood to our cells.

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Organs of the GI Tract and Accessory Structures of Digestion (Figure 9.1)

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Processes of Digestion

Digestion processes: Ingestion—intake of food via the mouth.

Digestion—mechanically or chemically breaking down foods into their subunits. • Mechanical digestion—chewing in the mouth and

contractions of smooth muscles in the stomach.

• Chemical digestion—digestive enzymes hydrolyze macromolecules into subunits.

• Begins in the mouth, continues in the stomach, and is completed in the small intestine.

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Stages of Digestion

Digestion processes, continued.

Movement—food is passed from one organ to the next, normally by contractions of smooth muscle called peristalsis; indigestible must be expelled.

Absorption—movement of nutrients across the GI tract wall into the blood; they are then delivered to cells.

Elimination—removal of indigestible wastes. • Defecation.

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Wall of the Digestive Tract 1

Wall of the digestive tract. Lumen—open area of a hollow organ or vessel; in the GI tract, it contains food or feces.

Layers: • Mucosa—innermost layer; produces mucus for protection; also

produces digestive enzymes.

• Submucosa—loose connective tissue; contains blood vessels, lymphatic vessels, and nerves.

• Muscularis—made of two layers of smooth muscle (inner, circular layer, and an outer, longitudinal layer) that move food along the GI tract.

• Serosa—outer lining; is part of the peritoneum.

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The Layers of the Gastrointestinal Tract Wall (Figure 9.2)

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Wall of the Digestive Tract 2

Diverticulosis. A condition in which the mucosa of any part of the GI tract (usually the large intestine) pushes through the other layers and forms pouches where food collects.

Diverticulitis—when the pouches become infected. • This happens in 10–25% of people with diverticulosis.

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Bowel Disease

Inflammatory bowel disease (IBD)—chronic diarrhea, abdominal pain, fever, and weight loss. Irritable bowel syndrome (IBS)—contractions of the muscularis cause abdominal pain, constipation, and diarrhea. • The underlying cause is not known.

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Check Your Progress 9.1

List the digestive organs and accessory structures of the digestive system.

Describe the processes that occur during the digestive process.

Identify the four layers of the GI tract from the lumen outward.

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9.2 The Mouth, Pharynx, and Esophagus

Learning Outcomes: • Identify the structures of the mouth, pharynx, and

esophagus and provide a function for each. • Explain the series of events involved in swallowing. • Summarize the diseases and conditions associated

with the mouth, pharynx, and esophagus.

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The Mouth 1

Mouth (oral cavity). Receives food and begins mechanical and chemical digestion.

The roof of the mouth separates the nasal cavity from the oral cavity. • Two parts: a bony hard palate and a soft palate.

• The soft palate is made of muscle; ends in the uvula.

Tonsils—in the back of the mouth on either side of the tongue. • Lymphatic tissue; help protect from disease.

• There is a single pharyngeal tonsil in the nasopharynx, commonly called the adenoids.

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Structures of the Mouth (Figure 9.3a) 1

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The Mouth 2

Mouth (oral cavity), continued. Three pairs of salivary glands secrete saliva, which contains: • Salivary amylase—begins carbohydrate digestion.

• Lysozyme—antibacterial enzyme.

Tongue. • Covered in taste buds.

• Assists in mechanical breakdown, movement of food.

• Forms a bolus (mass of chewed food) and moves it toward the pharynx.

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The Mouth 3

Teeth. Mechanically digest food.

20 smaller deciduous (baby) teeth, 32 adult teeth.

Two main divisions: • Crown—the part of the tooth above the gum line.

• Root—the portion below.

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The Mouth 4

Teeth, continued. • Enamel—extremely hard outer layer. • Dentin—a thick layer of bonelike material. • Pulp—nerves and blood vessels. • Periodontal membranes—anchor the tooth to the

jawbone.

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Structures of the Mouth (Figure 9.3b) 2

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The Mouth 5

Teeth, continued. Dental caries—tooth decay, cavities. • Occur when bacteria metabolize sugar and produce acids,

which erode the teeth.

• Can be painful when it reaches the nerves of the pulp.

Gingivitis—inflammation of the gums; can spread to the periodontal membrane, causing loosening of the teeth.

Periodontitis—loss of bone and loosening of the teeth.

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The Pharynx and Esophagus

The mouth and nasal passages lead to the pharynx. In turn, the pharynx opens into both the food passage (esophagus) and air passage (trachea, or windpipe). • These two tubes are parallel to each other; the trachea is

in front of the esophagus.

• The esophagus is a muscular tube that leads to the stomach.

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Swallowing

Swallowing. Starts off voluntary, but once food or drink is pushed back into the pharynx, it becomes an involuntary reflex.

Food normally enters the esophagus because other possible avenues are blocked. • The soft palate moves back to close off the nasal passage,

and the trachea moves up under the epiglottis to cover the glottis, the opening to the larynx.

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The Process of Swallowing (Figure 9.4)

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Peristalsis 1

Peristalsis—contractions that push food through the digestive tract. Sphincter—a ring of muscle that acts as a valve. When it contracts, it stops food from moving through; when it relaxes, it allows food through.

That is, the lower esophageal sphincter is between the esophagus and the stomach. • Heartburn—failure of this sphincter; stomach contents move

from the stomach into the esophagus.

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Peristalsis 2

Vomiting—when strong contractions of the abdominal muscles and the diaphragm (the muscle separating the thoracic and abdominal cavities) force the contents of the stomach into the esophagus and oral cavity.

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Check Your Progress 9.2

Describe the role of the mouth, pharynx, and esophagus in digestion. Detail how mechanical digestion and chemical digestion occur in the mouth. Explain what ordinarily prevents food from entering the nose or trachea when you swallow.

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9.3 The Stomach and Small Intestine

Learning Outcomes: • Describe the structure of the stomach and explain

its role in digestion. • Describe the structure of the small intestine and

explain its role in digestion. • Explain how carbohydrates, lipids, and proteins are

processed by the small intestine.

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The Stomach 1

Stomach. Stores food, starts digestion of proteins, and controls movement of food into the small intestine.

Does not absorb nutrients. • However, it does absorb alcohol, because alcohol is fat-

soluble and can pass through membranes easily.

There are three layers of muscle in the muscularis layer (instead of two) to help in mechanical digestion. • Circular, longitudinal, and third oblique layer.

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Structure of the Stomach (Figure 9.5)

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(c): Steve Gschmeissner/Brand X Pictures/SPL/Getty Images 28

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The Stomach 2

Heartburn (GERD). • The lining of the esophagus is thinner than the lining

of the stomach; if chyme enters the esophagus, it produces a burning sensation.

• Can be caused by pregnancy and excess fat pushing on the stomach.

• Gastroesophageal reflux disease (GERD)—chronic heartburn.

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The Stomach 3

Stomach, continued. The mucosa has deep folds called rugae.

The mucosa also has gastric pits, which contain gastric glands. • The gastric glands produce gastric juice, which contains:

• Pepsin—digests proteins.

• Hydrochloric acid (HCl)—pH of 2.

• Kills bacteria and activates pepsin.

• Mucus.

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The Stomach 4

Stomach, continued. Normally, the stomach empties in 2–6 hours.

Chyme—a mixture of food and gastric juice. • Pyloric sphincter—allows only a small amount of chyme

to enter the small intestine at a time.

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The Small Intestine 1

The small intestine—named for its small diameter compared with the large intestine’s. The small intestine is very long, longer than the large intestine. • Averaging about 6 m (18 ft) in length, whereas the large

intestine is about 1.5 m (4.5 ft).

It consists of three regions: • Duodenum.

• Jejunum.

• Ileum. 32

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The Small Intestine 2

The Small Intestine Is the Main Digestive Organ. Pancreas—secretes digestive enzymes through a duct into the duodenum, the first portion of the small intestine.

Another duct brings bile from the liver and gallbladder into the duodenum.

Bile emulsifies fat. • Mechanical digestion; causes fat droplets to disperse in water to

expose more of it to lipase.

• Produced by the pancreas, it hydrolyzes fats into glycerol and fatty acids.

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The Small Intestine 3

The Small Intestine Is the Main Digestive Organ, continued. Pancreatic amylase—produced by the pancreas, secreted into the duodenum. • Digests carbohydrates.

Trypsin—produced by the pancreas, secreted into the duodenum. • Digests proteins, so is a protease—a class of enzymes that digest

proteins.

Pancreatic juice contains sodium bicarbonate, which neutralizes acidic chime.

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Major Digestive Enzymes (Table 9.1) 1

Table 9.1a Major Digestive Enzymes: Carbohydrate Digestion Enzyme Produced By Site of Action Optimum pH Function

Salivary amylase Salivary glands Mouth Neutral Begins the breakdown of starch to maltose

Pancreatic amylase Pancreas Small intestine Basic Breakdown of starch to maltose Maltase Small intestine Small intestine Basic Breakdown of maltose to glucose Lactase Small intestine Small intestine Basic Breakdown of lactose to glucose

and galactose

Table 9.1b Major Digestive Enzymes: Protein Digestion Enzyme Produced By Site of Action Optimum pH Function

Pepsin Gastric glands Stomach Acidic Breakdown of proteins to peptides and amino acids

Trypsin Pancreas Small intestine Basic Breakdown of proteins to peptides and amino acids

Peptidases Small intestine Small intestine Basic Breakdown of peptides to amino acids

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Major Digestive Enzymes (Table 9.1) 2

Table 9.1c Major Digestive Enzymes: Nucleic Acid Digestion Enzyme Produced By Site of Action Optimum pH Function

Nuclease Pancreas Small intestine Basic Breakdown of nucleic acids to nucleotides

Nucleosidases Small intestine Small intestine Basic Breakdown of nucleotides to phosphates, bases, and sugars

Table 9.1d Major Digestive Enzymes: Fat Digestion Enzyme Produced By Site of Action Optimum pH Function

Salivary lipase

Salivary glands Mouth Neutral Begins the breakdown of triglycerides into fatty acids

Lipase Pancreas Small intestine Basic Breakdown of lipids to glycerol and fatty acids

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The Small Intestine 4

Nutrients Are Absorbed in the Small Intestine. The mucosa contains fingerlike projections called villi (sing., villus). The cells that cover the villi have microvilli. • Give the villi a fuzzy “brush border.”

• Contain brush border enzymes that complete digestion.

These two structures greatly increase the surface area of the small intestine for absorption of nutrients.

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Absorption in the Small Intestine (Figure 9.6)

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(photos) (b): Al Telser/McGraw Hill; (c): Steve Gschmeissner/Science Photo Library/Getty Images 38

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The Small Intestine 5

Nutrients Are Absorbed in the Small Intestine, continued. Nutrients are absorbed into the villi, which contain blood capillaries and a small lymphatic capillary called a lacteal. • Monosaccharides and amino acids enter the blood capillaries of a

villus.

• Glycerol and fatty acids form lipoprotein droplets called chylomicrons, which then enter a lacteal.

After nutrients are absorbed, they are eventually carried to all the cells of the body by the blood.

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Digestion and Absorption of Organic Nutrients (Figure 9.7)

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Lactose Intolerance

Lactose Intolerance. Lactose is the primary sugar in milk.

Lactase—the brush border enzyme that digests lactose.

Characterized by diarrhea, gas, bloating, and abdominal cramps after ingesting milk and other dairy products. • Diarrhea occurs because the undigested lactose causes fluid retention

in the small intestine.

• Gas, bloating, and cramps occur when bacteria break down the lactose anaerobically.

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Celiac Disease

Celiac Disease. • Autoimmune response against a protein called

gluten, which is naturally found in grains such as wheat, barley, and rye.

• The presence of gluten in the small intestine results in an inflammatory response, which damages the villi and microvilli of the small intestine.

• Can lead to abdominal pain, diarrhea, and malnutrition.

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Check Your Progress 9.3

Describe the functions of the stomach and how the wall of the stomach is modified to perform these functions. Detail the functions of the small intestine and how the wall of the small intestine is modified to perform these functions. Summarize the differences in how carbohydrates, proteins, and fats are digested and absorbed by the small intestine.

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9.4 The Accessory Organs and Regulation of Secretions

Learning Outcomes: • Explain the functions of the pancreas, liver, and

gallbladder during digestion. • List the secretions of the pancreas, liver, and

gallbladder. • Summarize how secretions of the accessory organs

are regulated.

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The Accessory Organs 1

Pancreas.

Located behind the stomach.

Most pancreatic cells produce pancreatic juice, which enters the duodenum via the pancreatic duct. • Contains sodium bicarbonate and digestive enzymes.

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Accessory Organs of the Digestive System (Figure 9.8)

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The Accessory Organs 2

Pancreas, continued. Also an endocrine gland; secretes the hormone insulin when the blood glucose levels rise.

Type 1 diabetes—not enough insulin. • Normally diagnosed in childhood.

Type 2 diabetes—the body’s cells are insulin- resistant. • Normally occurs in adulthood.

• Risk factors: obesity, inactivity, family history.

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The Accessory Organs 3

Liver. The largest gland in the body.

Lies mainly in the upper right abdominal cavity, under the diaphragm.

Lobules—structural and functional units.

The hepatic portal vein brings blood to the liver from the GI tract. • The lobules filter this blood, removing poisonous

substances.

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Functions of the Liver (Table 9.2)

Table 9.2 Functions of the Liver 1. Destroys old red blood cells; excretes bilirubin, a breakdown product of

hemoglobin in bile, a liver product 2. Detoxifies blood by removing and metabolizing poisonous substances 3. Stores iron (Fe raised to the 2 plus power), the water-soluble vitamin B12, and the fat-soluble

vitamins A, D, E, and K 4. Makes plasma proteins, such as albumins and fibrinogen, from amino

acids 5. Stores glucose as glycogen after a meal; breaks down glycogen to glucose

to maintain the glucose concentration of blood between eating periods 6. Produces urea after breaking down amino acids 7. Helps regulate blood cholesterol level, converting some to bile salts

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The Accessory Organs 4

Liver, continued. Stores vitamins.

Involved in blood glucose homeostasis. • Stores glucose as glycogen; when blood glucose is low, it

releases glucose by breaking down glycogen.

• Converts glycerol and amino acids to glucose.

• As amino acids are converted to glucose, urea is formed.

Produces plasma proteins.

Regulates blood cholesterol levels.

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The Accessory Organs 5

Liver, concluded. Produces bile, which contains bile salts, water, cholesterol, and bicarbonate. • Contains bilirubin, a green pigment formed from the

breakdown of hemoglobin.

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The Accessory Organs 6

Gallbladder. • Pear-shaped organ just below the liver. • Stores bile. • Gallstones—made of a stone-like material.

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Liver Disorders

Liver disorders: hepatitis and cirrhosis. Liver disease can cause bile pigments to leak into the blood, causing jaundice. • Yellowish tint to the whites of the eyes and the skin.

Hepatitis. • Inflammation of the liver.

• Has different forms.

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Hepatitis

Hepatitis, continued. Hepatitis A—usually acquired from sewage- contaminated drinking water and food.

Hepatitis B—usually from sexual contact, but also from blood transfusions or contaminated needles. • More contagious than the AIDS virus and is spread in the

same way.

• Vaccines are available for hepatitis A and B.

Hepatitis C—usually acquired by infected blood. • Can lead to chronic hepatitis, liver cancer, and death.

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Cirrhosis

Cirrhosis. Chronic disease; first, liver becomes fatty, and then filled with fibrous scar tissue.

Often seen in people with obesity and alcohol use disorder, due to malnutrition and the excessive alcohol (a toxin) the liver is forced to break down.

The liver can regenerate and recover if the rate of regeneration exceeds the rate of damage. • During liver failure there may not be enough time to let

the liver heal so will need a liver transplantation.

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Regulation of Digestive Secretions 1

Secretion of digestive juices is controlled by the nervous system and digestive hormones. When you look at or smell food, the parasympathetic nervous system stimulates gastric secretion.

A meal rich in protein causes the stomach to produce the hormone gastrin. • Increases the secretory activity of gastric glands.

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Hormonal Control and Regulation of Digestion (Figure 9.9)

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Regulation of Digestive Secretions 2

Nervous system and hormonal control, continued. Secretin—secreted by the duodenum. • Release is stimulated by HCl (present in chyme).

Cholecystokinin (CCK)—released by the duodenum when proteins and fat are present. • Causes the liver to increase production of bile and causes

the gallbladder to contract and release stored

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