1. Copyright © 2009 Pearson Education, Inc. The Digestive System  The digestive system is arranged as a series of organs along a tube called the gastrointestinal (GI) tract, into which various accessory glands release their secretions  Each organ helps to break down food into molecules small enough to be absorbed into the bloodstream and utilized by the body

2. Copyright © 2009 Pearson Education, Inc. The Digestive System  Along most of its length, the wall of the digestive system has four basic layers 1.Mucosa- secretes mucous 2.Submucosa- under mucosa 3.Muscularis- contains smooth muscle 4.Serosa – outer protective layer

3. Copyright © 2009 Pearson Education, Inc. The Digestive System Has Specialized Compartments  Digestion begins in the mouth where food enters the digestive process both chemically and physically  The teeth bite, tear, and crush food into smaller pieces  Increases the amount of surface area available for the action of digestive enzymes  Different teeth for different functions

4. Copyright © 2009 Pearson Education, Inc. The Digestive System Has Specialized Compartments  Three pairs of salivary glands release secretions, called saliva, into the mouth  Saliva  Moistens food  Dissolves the chemicals in the food  Contains the enzyme, salivary amylase  Begins digestion of carbohydrates

5. Copyright © 2009 Pearson Education, Inc. The Digestive System Has Specialized Compartments  The tongue  A large skeletal muscle studded with taste buds  Helps form food into a bolus  A soft mass of food, suitable for swallowing  The pharynx (throat)  Where the nasal and oral cavities join  During swallowing the epiglottis covers the trachea, or windpipe, to prevent choking

6. Copyright © 2009 Pearson Education, Inc. The Digestive System Has Specialized Compartments  The esophagus  A tube that transports food from the mouth to the stomach  Food is pushed through our digestive system by a series of muscular wave-like contractions called peristalsis

7. Copyright © 2009 Pearson Education, Inc. The Digestive System Has Specialized Compartments  The stomach  Responsible for the storage of food  Turns food into a soupy mixture called chyme  Adds digestive enzymes and acids (from gastric glands) that begin chemical digestion of proteins  Segmentation or mixing of food happens here

8. Copyright © 2009 Pearson Education, Inc. The Digestive System Has Specialized Compartments  Sphincter muscles  One-way “gates”  Allow food to enter the stomach from the esophagus  Release food from the stomach into the small intestine

9. Copyright © 2009 Pearson Education, Inc. The Digestive System Has Specialized Compartments  In the stomach, hydrochloric acid (HCl)  Kills bacteria  Begins the breakdown of protein  Activates the digestive enzyme pepsin  The stomach itself is protected from being digested by this caustic combination of chemicals

10. Copyright © 2009 Pearson Education, Inc. The Digestive System Has Specialized Compartments  The small intestine  Food digested into its smallest chemical components  Primary site of absorption  3 regions- duodenum, jejunum, and illeum

11. Copyright © 2009 Pearson Education, Inc. The Digestive System Has Specialized Compartments  The complex chemicals of carbohydrates, proteins, fats, and nucleic acids are broken down into their simplest forms in the small intestine by enzymes of the pancreas and the small intestine aided by bile from the gall bladder

12. Copyright © 2009 Pearson Education, Inc. The Digestive System Has Specialized Compartments  Bile  Produced in the liver  Concentrated in the gallbladder  Aids in the digestion and absorption of fats  Pancreatic juice contains  Enzymes, Water and Ions  Includes bicarbonate ions that are important in neutralizing the acid in chyme when it emerges from the stomach

13. Copyright © 2009 Pearson Education, Inc. The Digestive System Has Specialized Compartments  The lining of the small intestine  Pleated  Has numerous finger-like projections called villi to increase surface area so the maximal absorption of nutrients can occur  Each villus is covered with microvilli  Gives the small intestine a velvety appearance  Called the brush border

14. Copyright © 2009 Pearson Education, Inc. The Digestive System Has Specialized Compartments  Gallbladder  Stores bile produced by the liver  Occasionally, gallstones form from cholesterol and other substances  The liver  Monitors blood glucose levels  Packages fat molecules with proteins to aid transport in the blood  Removes poisonous substances

15. Copyright © 2009 Pearson Education, Inc. The Digestive System Has Specialized Compartments  Hepatitis  An inflammation of the liver caused by a virus  Six viruses  Designated as A, B, C, D, E, and G  The liver stops filtering bilirubin from the blood  Results in jaundice

16. Copyright © 2009 Pearson Education, Inc. The Digestive System Has Specialized Compartments  Undigested and indigestible materials that have not been absorbed by the small intestine move to the large intestine  The large intestine absorbs the water, ions, and vitamins found in these materials  Eventually the feces, now in a consistency suitable for elimination, pass from the colon into the rectum and leave the body through the anal canal

17. Copyright © 2009 Pearson Education, Inc. Nerves and Hormones Control Digestive Activities  Processes of the digestive system that are quick or anticipatory are controlled by the nervous system  Processes that take more time and are able to move at a slower pace are controlled by hormones

18. Copyright © 2009 Pearson Education, Inc. Nerves and Hormones Control Digestive Activities  Gastrin  Released by the stomach lining  Vasoactive intestinal peptide (VIP), secretin, and cholecystokinin  Released by the lining of the small intestine  All cause the release of digestive enzymes