1. The Digestive System
Section 9.3

2. The Digestive System
Learning Goal: To explore the digestive system of humans.
Success Criteria:
I know I am succeeding when I can…
explain that physical and chemical digestion begin in the mouth
explain how food moves through the esophagus
explain that food is both chemically digested and stored in the stomach
explain the roles of gastric juice, mucus, gastrin, and pepsin in digestion
explain how acid reflex and ulcers are caused

3. The Digestive System
food contains the nutrients we need but not in a form that our bodies can use directly
digestion: process in which food is broken down into nutrients that can then be absorbed into the blood and carried to the cells of the body
the organs that work together to carry out digestion is called the digestive system

4. The Digestive System Cont’d…
the digestive system is made up of the digestive tract and accessory organs
digestive tract is essentially one long tube that includes, the mouth, esophagus and, stomach, intestines, rectum and anus
accessory organs are the tongue, salivary glands, liver, gall bladder, pancreas

5. Steps in Digestion four main steps in digestion:
ingestion  taking in of nutrients
digestion  mechanical (physical breakdown of food into smaller parts) and chemical digestion(enzymes break down food molecules)
absorption  transfer of digested nutrients from the digestive system to the bloodstream
egestion  removal of waste food materials from the body

6. Mouth digestion begins in the mouth
food is broken down into smaller pieces by the teeth (physical digestion)
teeth in the front of your mouth, called incisors and canines, grab and cut food
premolars and molars are broad flattened teeth that grind and crush food

7. Mouth Cont’d… salivary glands secrete a watery fluid called saliva
saliva contains enzymes— chemicals that increase the rate of chemical reactions in living things
amylase breaks down starch into smaller disaccharides (chemical digestion)
saliva also dissolves food particles so we can taste food
saliva contains mucus moistens the food into a ball, or bolus, so that it can be swallowed

8. Pharynx
tongue pushes food to the back of the mouth where it is swallowed
palate raises to prevent food from entering the nasal passages
the larynx is raised against a flap of soft tissue called the epiglottis which covers the entrance to the trachea to prevent food from entering the lungs

9. Esophagus
food enters the esophagus which is a long, muscular tube that carries food from the pharynx to the stomach
food moves through the esophagus by peristalsis (not gravity!)
peristalsis: rhythmic, wave-like contractions of smooth muscles of the GI tract

10. Stomach
mechanical and chemical digestion continues (proteins are partly digested in the stomach) in the stomach

11. Stomach Cont’d… the stomach is a J-shaped, muscular organ
movement of food into and out of the stomach is controlled by circular muscles called sphincters

12. Stomach Cont’d… The stomach has four layers
mucosa: innermost, extensively folded layer, that secretes gastric juice(i.e. a mixture of digestive enzymes, acid, and mucus)
submucosa: layer of connective tissue that contains networks of nerves and blood vessels
muscularis: consists of smooth muscles which contract frequently, churning and mixing the food with gastric juices to produce a semi-liquid material called chyme
serosa: smooth, outermost layer, that holds the stomach in place and secretes a lubricating fluid that eliminates friction between organs

13. Stomach Cont’d…
the nerves in the submucosa detect when food is present and initiate the release of a hormone called gastrin which stimulates the release of gastric juice
gastric juice is made up of mucus, acid and digestive enzymes
muscular contractions of the stomach mixes the food with gastric juice (physical digestion)

14. inactive zymogen: pepsinogen
Stomach Cont’d…
the mucus coats and protects the lining of the stomach from acid and digestive enzymes
the acid is very strong and kills many harmful micro-organisms that are ingested with food
also provides the pH for the activation of other digestive enzymes
hydrochloric acid converts pepsinogen to its active form, pepsin, which begins the breakdown of proteins into amino acids
low pH
masking sequence
active site
active enzyme: pepsin
inactive zymogen: pepsinogen

15. Acid Reflex
if the gastroesophageal sphincter does not close completely, acid from the stomach can enter the esophagus
this causes a burning sensation in the lower throat called acid reflux (a.k.a. heartburn)

16. Ulcers
bacteria(H. pylori) secretes acid-neutralizing enzymes and burrows through the mucosa
prevents mucus-producing cells from producing enough mucus to protect the stomach lining
when the lining is exposed to the strong stomach acid, an open sore, called an ulcer
usually successfully eliminated with antibiotics

17. The Digestive System
Learning Goal: To explore the digestive system of humans.
Success Criteria:
I know I am succeeding when I can…
explain that physical and chemical digestion begin in the mouth
explain how food moves through the esophagus
explain that food is both chemically digested and stored in the stomach
explain the roles of gastric juice, mucus, gastrin, and pepsin in digestion
explain how acid reflex and ulcers are caused

18. Homework
Answer 9.4 questions #1-6