1. Digestion: How Food Becomes Nutrition
Concept 6

2. Pop Quiz True or False? The GI tract is essentially a long tube.
You absorb only 75 percent of the nutrients in your food.
Heartburn occurs when stomach acid leaks into the chest cavity and contacts heart muscle.
We absorb most of the nutrients in foods via our stomach.
Stool is mostly made up of food remnants and bacteria.
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3. What Is Digestion and Why Is It Important?
Digestion Occurs in the GI Tract
Digestion Allows You to Absorb Nutrients from Foods

4. What Is Digestion and Why Is It Important?
To use food for energy, it must be broken down into molecules small enough to be absorbed.
Digestion: the breakdown of foods into absorbable components in the gastrointestinal (GI) tract using mechanical and chemical means

5. Digestion Occurs in the GI Tract
The GI tract is a 23-foot-long tube consisting of:
Mouth
Esophagus
Stomach
Small and large intestines
Other organs
The main roles of the GI tract are to:
Break food down into its smallest components
Absorb the nutrients
Prevent microorganisms or other harmful compounds in foods from entering the tissues of the body

6. Digestion Occurs in the GI Tract
Digestion is mechanical and chemical.
Mechanical—breaking down food through chewing and grinding
Peristalsis: rhythmic muscular contractions that move food through the GI tract and mix it with enzymes
Chemical—breaking down food with enzymes or digestive juices

7. Digestion Allows You to Absorb Nutrients from Foods
Absorption is the process by which digested nutrients move into the tissues to be transported and used by the body’s cells.
The body has two transport systems for absorption:
The circulatory system
The lymphatic system
You digest and absorb 92 to 97 percent of the nutrients from your food.

8. Animation: Overview Digestion Absorption

9. How Does Digestion Happen?
You Begin Breaking Down Food in Your Mouth
The Esophagus Propels Food into the Stomach
The Stomach Stores, Mixes, and Prepares Food for Digestion
Most Digestion and Absorption Occurs in the Small Intestine
The Large Intestine Absorbs Water and Some Nutrients
The Liver, Gallbladder, and Pancreas Are Accessory Organs

10. How Does Digestion Happen?
Begins before we eat
Hunger and thirst
Physical needs for food and water that drive how much and how often we eat
Appetite
A powerful drive to eat, but less reliable
Influenced by environmental and psychological cues
May cause eating without being hungry or needing nourishment

11. Digestion and the Organs of the GI Tract
Figure 6.1

12. You Begin Breaking Down Food in Your Mouth
Saliva, a watery substance produced by glands in the mouth, helps moisten and soften food.
By chewing, your teeth grind the food into smaller pieces and mix it with saliva.
Your tongue also helps mix food and saliva and pushes the food mass (bolus) to the back of the mouth into the pharynx.
The epiglottis closes off the trachea during swallowing to prevent food from lodging in the windpipe.
Food is directed down the esophagus.

13. The Epiglottis Prevents Food from Entering the Larynx When You Swallow
Figure 6.2

14. The Esophagus Propels Food into the Stomach
Once swallowed, the food bolus is pushed down the esophagus by peristalsis.
The lower esophageal sphincter (LES)—the circular muscle at the bottom of the esophagus—relaxes and allows food into the stomach.
The LES closes to prevent backflow of hydrochloric acid (HCl) from the stomach.
Reflux of stomach acid causes heartburn, an irritation of the lining of the esophagus.

15. Peristalsis Helps Move Food Down the Esophagus
Figure 6.3

16. The Stomach Stores, Mixes, and Prepares Food for Digestion
The stomach churns and contracts to mix food with powerful digestive secretions, forming chyme.
The stomach can hold 2 to 4 liters of chyme.
Hydrochloric acid (HCl) has important digestive functions:
Activates the digestive enzyme pepsin
Enhances absorption of minerals
Breaks down connective tissue in meats
Destroys ingested microorganisms
Mucus protects the stomach lining from damage.

17. The Stomach Stores, Mixes, and Prepares Food for Digestion
Low-calorie, high-carbohydrate foods require minimal digestion and exit the stomach faster.
Foods high in fat, protein, and fiber exit slower, and keep you feeling full longer.
The pyloric sphincter—the circular muscle between the stomach and the small intestine—slowly releases chyme and prevents backflow of intestinal contents.
Allows about 1 tsp of chyme to enter small intestine every 30 seconds

18. Most Digestion and Absorption Occurs in the Small Intestine
Small intestine: long, narrow, coiled segment of the GI tract
Inner lining covered with villi and microvilli, which increase surface area and maximize absorption
Blood vessels and lymphatic vessels pick up absorbed nutrients and transport them throughout the body.
Circular folds further increase surface area and cause chyme to spiral forward.
Food takes 3 to 10 hours to pass through the small intestine.

19. Surface Area in the Small Intestine
Figure 6.4

20. The Large Intestine Absorbs Water and Some Nutrients
Most nutrients in chyme are absorbed before it enters the large intestine through the ileocecal sphincter.
The large intestine is wider than the small intestine and not as tightly coiled.
Helpful bacteria living in the large intestine play a role in chemical digestion.
Produce some vitamins
Break down fiber
Ferment some undigested dietary carbohydrates into methane gas, carbon dioxide, and hydrogen

21. The Large Intestine Absorbs Water and Some Nutrients
The cells of the large intestine absorb fluid, leaving a semi-solid mass of fecal matter (stool, or feces).
Transit time ranges from 12 to 70 hours, depending on a person’s age, health, diet, and fiber intake.
The rectum is the final portion of the large intestine where stool is stored.
Distention of the rectum stimulates the defecation reflex and the internal sphincter of the anus is relaxed to voluntarily eliminate stool.

22. The Liver, Gallbladder, and Pancreas Are Accessory Organs
The liver is the largest abdominal organ in the body.
Helps regulate metabolism of carbohydrates, fats, and protein
Stores some nutrients, including certain vitamins, minerals, and glycogen
Essential for processing and detoxifying alcohol
Makes bile, a greenish yellow liquid that is important for fat digestion

23. The Liver, Gallbladder, and Pancreas Are Accessory Organs
Bile has two main functions:
Breaks up large fat globules to increase surface area of the droplets and enhance absorption of fat
Acts as an emulsifier by dispersing fat throughout the chyme and helping enzymes make contact with it and digest it
Bile is collected, drained, and released into the gallbladder, a small organ responsible for storing bile and releasing it into the intestine in response to fat ingestion.

24. The Liver, Gallbladder, and Pancreas Are Accessory Organs
The pancreas produces important digestive enzymes that are released into the small intestine.
Enzymes made by the pancreas are responsible for the digestion of 90 percent of ingested fat and 50 percent of ingested protein and carbohydrate.
The pancreas also produces sodium bicarbonate to neutralize acidic chyme and protect the tissues of the small intestine.
It produces two substances important in regulating the level of sugar in your blood.

26. What Other Body Systems Affect Your Use of Nutrients?
The Nervous System Stimulates Your Appetite
The Circulatory and Lymphatic Systems Distribute Nutrients
The Excretory System Passes Urine Out of the Body

27. The Nervous System Stimulates Your Appetite
Your brain communicates and interprets the messages of hunger and thirst.
If you ignore the signals of hunger and thirst you may feel dizzy or weak, or develop a headache.
The nervous system helps us to decide what to eat, when to eat, where to eat, and when to stop eating.

28. The Circulatory and Lymphatic Systems Distribute Nutrients
The blood transports oxygen, nutrients, and other substances throughout the body and removes waste products from the cells to be excreted.
Blood picks up nutrients absorbed through the lining of the small intestine and transports them to the liver, then to the cells of the body.
The lymphatic system is a network of vessels, valves, lymph nodes, and ducts.
Picks up a fluid called lymph that has seeped into tissues and filters it before returning it to the bloodstream
Fats and fat-soluble vitamins travel in lymph from the GI tract to your blood

29. The Circulatory and Lymphatic Systems
Figure 6.5

30. The Excretory System Passes Urine Out of the Body
The excretory system eliminates wastes from the circulatory system.
The kidneys filter the blood and excrete the waste products into urine, which is stored in the bladder until it is released.

31. The Excretory System
Figure 6.6

32. What Are Some Common Digestive Disorders?
Disorders of the Mouth and Esophagus
Disorders of the Stomach
Gallbladder Disease
Disorders of the Intestines

33. Disorders of the Mouth and Esophagus
Properly nourishing yourself is difficult if you have extensive tooth decay, gingivitis, or periodontal disease.
Gingivitis: gum swelling, bleeding, and oral pain
Periodontal disease: more severe inflammation caused by infections or plaque on the teeth
These conditions are serious but treatable with various dental procedures, optimal food choices, and excellent oral hygiene.

34. Disorders of the Mouth and Esophagus
Heartburn affects 25 to 35 percent of adults occasionally and 7 percent of the population on a daily basis.
HCl flows from the stomach into the esophagus.
Symptoms include a sour taste in the mouth, nausea, bloating, belching, or a burning sensation.
Chronic heartburn can lead to gastroesophageal reflux disease (GERD).
It is often caused by a weak LES.
Certain foods or lifestyle factors can worsen the condition.
Treatment includes dietary changes, behavior modification, over-the-counter antacids or prescription drugs, or surgery in very severe cases.

35. Disorders of the Stomach
Stomachache may be caused by overeating, eating too fast, or eating foods that are spicy, acidic, or high in fat or fiber.
Gastroenteritis is an inflammation of the stomach or intestines caused by a virus or bacteria.
Rest, oral rehydration therapy, and a soft diet are the recommended treatment.
A peptic ulcer is an erosion of the stomach lining caused by drugs, alcohol, or a bacterium.
Symptoms include abdominal pain, vomiting, fatigue, bleeding, and general weakness.
It is treated with prescription drugs and dietary modification.

36. Peptic Ulcer
Figure 6.7

37. Gallbladder Disease
Gallstones: small, hard, crystalline structures formed in the gallbladder or bile duct due to abnormally thick bile
May be asymptomatic or cause pain, fever, nausea, vomiting, cramps, and obstruction of the bile duct
Treated with prescription medications, shock-wave therapy, or surgical removal of the gallbladder

38. Disorders of the Intestines
Serious intestinal problems are often characterized by malabsorption.
Malabsorption diseases are characterized by abdominal pain, nausea, vomiting, bloating, loss of appetite, diarrhea, anxiety, weight loss, and fatigue.
Celiac disease is a more common cause of malabsorption.

39. Celiac Disease: An Issue of Absorption
Celiac disease: immune system response to gluten, causing damage to the lining of the small intestine and malabsorption
Gluten is a protein found in wheat, rye, and barley.
More common in people of European descent
Affects 1 in 133 Americans
Symptoms include abdominal bloating, cramping, diarrhea, gas, fatty and foul-smelling stool, weight loss, anemia, fatigue, bone or joint pain, or a skin rash.

40. Celiac Disease: An Issue of Absorption
Complications of celiac disease include:
Osteoporosis
Diminished growth
Possible seizures
Increased risk of cancer of the small intestine
Gold standard for diagnosis is surgical detection of damaged villi
Treatment for celiac disease is a gluten-free diet.
Many processed foods contain hidden sources of gluten.
Even traces of gluten can set off an immune reaction.

41. Disorders of the Intestines
Constipation: difficulty in passing stool
Often due to insufficient fiber or water intake
Stress, inactivity, certain medications, and various illnesses can contribute
Diarrhea: the passage of frequent, watery, loose stools
Results in the loss of fluids, certain minerals, and nutrients
Many causes, including contaminated water, microorganisms, stress, or excessive fiber intake

42. Disorders of the Intestines
Irritable bowel syndrome (IBS): a disorder of bowel function caused by hypersensitivity to stimuli in the large intestine
Alternating patterns of diarrhea, constipation, and abdominal pain
Cause is unknown
Dietary modification, stress management, and occasional use of prescription drugs can help

43. The Top Five Points to Remember
Digestion is the process of breaking down food into absorbable nutrients through mechanical or chemical processes.
Digestion begins in the mouth when food is chewed and mixed with saliva. The stomach mixes foods with enzymes and stores it. Most digestion and absorption occurs in the small intestine. The villi in the small intestine greatly increase the surface area for absorption. The large intestine absorbs water and some nutrients before waste is eliminated through the anus. Sphincters control entry and exit of food and chyme through the organs of the GI tract.

44. The Top Five Points to Remember
The liver produces bile, metabolizes and stores certain nutrients, and detoxifies alcohol. The gallbladder stores and releases bile. The pancreas produces enzymes important for digestion.
Other body systems, including the nervous, circulatory, lymphatic, and excretory systems, help you use the nutrients you eat.
Common digestive disorders include heartburn, peptic ulcers, celiac disease, constipation, diarrhea, and irritable bowel syndrome.

45. Answers to the Pop Quiz
TRUE. The GI tract runs through the body and connects the mouth to the anus.
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46. Answers to the Pop Quiz
FALSE. Your body is very efficient and absorbs over 90 percent of the nutrients in your food.
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47. Answers to the Pop Quiz
FALSE. Heartburn is actually caused by a backflow of stomach acid into the lower esophagus.
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48. Answers to the Pop Quiz
FALSE. The small intestine is the primary organ responsible for nutrient absorption.
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49. Answers to the Pop Quiz
TRUE. Stools contain leftover food residue, nondigestible fibers, bacteria, gases, and sloughed-off intestinal cells.
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