1. The Basics of Digestion
Chapter 3
The Basics of Digestion

2. Objectives for Chapter 3
Define digestion, absorption, and nutrient transport.
Describe the organs involved in digestion and their primary functions.
Explain the roles of the gallbladder, liver, and pancreas in digestion.
Explain the role of enzymes in digestion.
List the main carbohydrate-, protein-, and fat-digesting enzymes and the tissues that secrete them.
Identify the hormones involved in digestion, including their primary action and their source of origin.
Explain the role of the small intestine, villi, and microvilli in digestion.
Explain how the circulatory and lymphatic systems transport absorbed nutrients throughout the body.
Describe the symptoms and causes of the most common digestive disorders.

3. What Makes Eating So Enjoyable?
Hunger and thirst: physical needs which drive how much and how often we eat
Appetite: another powerful drive, but unreliable
Influenced by food preferences and psychological stimulation
May eat without being hungry or needing nourishment

4. We Develop a Taste for Certain Foods
Taste: Five basic categories
Detected by at least 10,000 taste buds mostly on tongue, but also throat and elsewhere in mouth
Sweet: tip of tongue
Salty: sides of tongue
Sour: sides of tongue
Bitter: back of tongue
Savory (umami): throughout mouth; not all people are sensitive to this taste sensation

5. We Develop a Taste for Certain Foods
Humans share an innate preference for sweet (pleasure), salty (electrolytes), and fatty (rich textures and aromas) foods
Sometimes food preferences and nutritional needs conflict and make changing food choices challenging
Brain recognizes taste: food dissolves in saliva, contacts the tongue surface, taste cells send nerve impulses to brain, translated into taste sensations

6. Aromas and Flavors Enhance the Pleasure of Eating
Both mouth and nose contribute to tasting of foods
Aromas detected by olfactory cells in nasal cavity as food odors enter both nose and mouth (move to back of throat and into nasal cavity)
Flavor refers to both taste and aroma
Example: Chocolate tastes sweet but its flavor is “chocolate”

7. What Is Digestion and Why Is It Important?
Digestion: a multi-step process of breaking down foods into absorbable components using mechanical and chemical means in the gastrointestinal (GI) tract
Gastrointestinal tract consists of:
Mouth
Esophagus
Stomach
Small and large intestines
Other organs

8. What Is Digestion and Why Is It Important?
Main roles of the GI tract are to:
Break down food into smallest components
Absorb nutrients
Prevent microorganism or other harmful compounds in food from entering tissues of the body
GI tract is about 23 feet long with extensive surface area for nutrient absorption
Cells lining GI tract are replaced every three to five days

9. Digestion is the process by which
insoluble food, consisting of large molecules is broken down into soluble compounds

10. What are starch, proteins & fats broken down into?
enzyme
STARCH
Glucose
enzyme
PROTEIN
Amino acids
enzyme
FAT
Fatty acids &
glycerol

11. Why must food be digested?1
Food in
intestine 2
Food in
intestine
Blood
Blood
enzyme
Wall of
intestine
Food in
intestine 3
To be small enough to be absorbed through the wall of intestine.
Blood

12. Digestion starts at the mouth and ends at the ……………….
anus

13. Fig. 1 The human digestive system.
Lower Esophageal Sphincter

14. Food is broken down by two actions:
PHYSICAL /MECHANICAL
2. CHEMICAL

15. Why is it important to break down food physically?
To increase the surface area for enzyme action.

16. Peristalsis
Figure 3.1

17. Digestion Allows Us to Absorb Nutrients from Foods
After digestion, nutrients are absorbed through the
walls of the intestines into the body’s two transport
systems:
Circulatory system (blood)
Lymph system
Sent to the liver for processing before delivery to the body’s cells
GI tract is highly efficient: 92 to 97 percent of nutrients from food are digested and absorbed

18. What Are the Organs of the GI Tract and Why Are They Important?
Digestion begins in the mouth
Saliva released: contains water, electrolytes, mucus and a few enzymes
Softens, lubricates, dissolves food particles
Bolus (food mass) moves into pharynx, is swallowed, and enters the esophagus
Epiglottis closes off trachea during swallowing to prevent food from lodging in the windpipe

19. The Stomach Stores, Mixes, and Prepares Food for Digestion
Esophagus propels food into the stomach
Bolus is pushed down esophagus by peristalsis
Lower Esophageal Sphincter (LES): at bottom of esophagus relaxes and allows food into stomach
LES then closes to prevent backflow of hydrochloric acid (HCl) from stomach
“Reflux” of stomach acid causes “heartburn” (irritation of esophagus lining)

20. Food reaches the stomach

21. The stomach stores food for some time & releases it slowly
Food stays for 2-6 hours in the stomach
Chyme:
creamy liquid leaving
the stomach

22. Food remains inside stomach due to contraction of two sphincters
Oesophagus
Cardiac sphincter
Lining of
mucus
Pyloric sphincter
Churning due to:
Duodenum
Layers of muscle

23. Which sphincter is contracted?
Cardiac sphincter:
Pyloric sphincter:
relaxes to let chyme into the duodenum
contracted

24. Gastric glands secrete gastric juice

25. What happens to food inside the stomach?

26. Gastric juice contains:
HYDROCHLORIC ACID
PEPSIN
RENNIN
What is the pH in the stomach?
1-2

27. Functions of hydrochloric acid:
creates the optimum pH for pepsin
HCl
Pepsinogen
(inactive form)
Pepsin
(active enzyme) 2.
3. kills bacteria taken in with the food

28. Excessive production of gastric juice: burning sensation

29. Gastric juice enters oesophagus: not enough mucus to protect wall

30. Remedy for burning sensation in throat and chest:
Drink milk
Take antacids

31. The Stomach Stores, Mixes and Prepares Food for Digestion
Gastrin: stimulates digestive activities and secretion of HCl; increases gastric motility and emptying
Liquids, carbohydrates, low-fiber, and low-calorie foods exit stomach faster
High fiber, fat, and protein foods exit slower, keep you feeling full longer
Pyloric sphincter: located between the stomach and small intestine; allows about 1 tsp of chyme to enter the small intestine every 30 seconds
Prevents backflow of intestinal contents

32. Pepsin is a protease:
breaks:
Proteins
Polypeptides

33. Which conditions are needed for a piece of meat to be digested?
Pepsin in acidic conditions

34. Some substances that are absorbed by the stomach:
ethanol
vitamins
certain salts and drugs
some water
glucose
How did I get drunk so quickly?

35. Rennin is an enzyme found in the stomach of young children Function:
Pepsin then acts on the insoluble protein
Soluble protein in milk
Changes into insoluble protein

36. In cheese-making, rennet [contains rennin] is added to milk

37. Anatomy of the Stomach
Figure 3.4

38. Surface Area in the Small Intestine
Figure 3.5

39. Where is bile: i) made? ii)stored?

40. Made in liver
Stored in gall bladder

41. Bile: has no enzymes consists of: water sodium chloride bile salts
bile pigments
 pH = 8

42. Functions of Bile: dilutes contents from stomach
creates optimum pH for pancreatic enzymes to work in duodenum
contains bile salts which emulsify fats
Bile salts

43. Most Digestion and Absorption Occur in the Small Intestine
Small intestine: long, narrow, coiled
Three segments:
Duodenum (10 inches)
Jejunum (8 feet)
Ileum (12 feet)
Interior surface area tremendously increased by villi, microvilli, circular folds.

44. Model of the ileum showing numerous villi
Villus: a finger-like projection

45. Most Digestion and Absorption Occur in the Small Intestine
Mechanical and chemical digestion in small intestine:
Peristalsis moves chyme through intestine
Segmentation mixes chyme with chemical secretions
Pendular movement enhances nutrient absorption
Chyme moves though at rate of 1 centimeter per minute
Total contact time 3 to 10 hours, depending on amount and type of food

46. proteins & polypeptides
Small Intestine
amylase
starch molecule
maltose
trypsin
proteins & polypeptides
peptides +
lipase
fatty acids
glycerol
fat molecule

47. Large Intestine Absorbs Water and Some Nutrients
Ileocecal sphincter: prevents backflow of fecal matter into ileum
Most of nutrients in chyme have been absorbed when it reaches large intestine
Large intestine has three sections: cecum, colon, rectum
About 5 feet long, 2.5 inches in diameter
Absorbs water and electrolytes
No digestive enzymes; chemical digestion done by bacteria

48. Large Intestine Absorbs Water and Some Nutrients
Intestinal matter passes through colon in 12 to 70 hours depending on age, health, diet, fiber intake
Bacteria in colon produce vitamin K and biotin and break down fiber and undigested carbohydrates, producing methane, carbon dioxide, hydrogen gas, and other compounds
Stool stored in rectum
Anus controlled by two sphincters: internal and external
Final stage of defecation is under voluntary control

49. Anatomy of the Large Intestine
Figure 3.6

50. Enzymes, Hormones, and Bile Aid Digestion
Complete digestion of chyme requires:
Enzymes: break down food into absorbable nutrient components
Secreted by stomach, small intestine, pancreas
Amylases, lipases, proteases
Hormones: gastrin, insulin, glucagon, secretin, cholecystokinin
Control digestive secretions and regulate enzymes

51. Process of Digestion and Organs of the GI Tract
Figure 3.7

52. The Liver, Gallbladder, and Pancreas Are Accessory Organs
Liver: largest gland in body
Produces bile needed for fat digestion
Metabolism of carbohydrates, fats, and protein
Stores nutrients: vitamins A, D, B12, E, copper, iron, glycogen (glucose storage form)
Detoxifies alcohol
Emulsify -
Gallbladder: concentrates and stores bile
Released into GI tract when fat is ingested

53. The Liver, Gallbladder, and Pancreas Are Accessory Organs
Produces hormones: insulin and glucagon regulate blood glucose
Sodium bicarbonate neutralizes acidic chyme, protects enzymes from inactivation by acid
Digestive enzymes:
Amylase: digests carbohydrate
Lipase: digests fats
Trypsin, chymotrypsin, and carboxypeptidase: digest protein

54. Ingestion
(2 L)
Salivary gland secretions
(1 L)
Gastric secretions
(2 L)
Pancreatic secretions
(1.2 L)
Bile
(0.7 L)
Although 8.9 L of water are ingested or secreted daily, the faeces are dry. Explain.
Small intestine
secretions
(2 L)
1% in
faeces

55. Absorbed in the small intestine & colon.
Ingestion
(2 L)
Salivary gland secretions
(1 L)
Gastric secretions
(2 L)
Pancreatic secretions
(1.2 L)
Bile
(0.7 L)
Absorbed in the small intestine & colon.
Small intestine
secretions
(2 L)
1% in
faeces

56. Table 3.2

57. Diet Dream Drug: Hope or Hype: Pros and Cons of Alli
Discussion Questions
What does Alli claim to do in the body to help you lose weight?
What are some of the unpleasant side effects of using Alli?
What is the typical behavior of people using diet drugs?

58. What Other Body Systems Affect Your Use of Nutrients?
Nervous system stimulates your appetite
Hormone ghrelin signals brain to eat when stomach is empty
Circulatory system transports nutrients, oxygen, waste products through your blood
Lymphatic system distributes fat and fat-soluble nutrients through your lymph
Excretory system eliminates wastes from circulatory system via the urine

59. What Are Some Common Digestive Disorders?
Disorders of the mouth and esophagus:
Gingivitis and periodontal disease
Swallowing problems: dysphagia
Esophageal problems
Heartburn (acid reflux) may be caused by weak LES
Certain foods, smoking, drinking alcohol, being overweight or obese, tight-fitting clothes, reclining after eating, large evening meals may worsen condition

60. What Are Some Common Digestive Disorders?
Disorders of the stomach:
Gastroenteritis
Peptic ulcers
Gallbladder disease:
Gallstones

61. What Are Some Common Digestive Disorders?
More serious intestinal disorders:
Irritable Bowel Syndrome (IBS)
Celiac disease
Crohn’s disease
Colon cancer

62. THE END
I HAVE SO MUCH TO DIGEST!!