Parts of the Human Digestive System Alimentary canal: Long tube like structure. u Mouth u Tongue u Pharynx (throat) u Esophagus u Stomach u Small intestine u Large intestine u Rectum u Anus Digestive glands: Produce enzymes, bile, and other substances important for digestion. u Salivary glands u Pancreas u Liver and gallbladder

Human Digestive System

Mouth: u Ingest and mechanically break down food. u Digestion: Saliva lubricates and starts to digest food. F Starch is digested by salivary amylase u Other enzymes in saliva kill bacteria.

Part of the Human Digestive System Pharynx (Throat): u Throat opens into both the trachea (respiratory system) and esophagus (digestive system). u As food enters pharynx, swallowing reflex is triggered: F Esophageal sphincter relaxes F Epiglottis blocks tracheal opening F Food is directed towards esophagus Esophagus: u Muscular tube that conveys food to stomach. u Peristalsis: Wavelike involuntary muscle contractions squeeze food through alimentary canal (towards the stomach).

Swallowing Reflex and Esophageal Peristalsis

Part of the Human Digestive System Stomach: u Stores food (can stretch to accommodate up to 2 liters of food and water), and breaks it down with acids and enzymes. u Gastric juice is secreted by stomach. Contains hydrochloric acid (HCl), enzymes (pepsin), and mucus. u Protein digestion starts in stomach with pepsin.

Stomach: Food Storage and Digestion

Part of the Human Digestive System Small Intestine: u Huge surface area, about 300 square meters. u Most digestion and absorption occurs here. u Pancreas and liver empty digestive enzymes and bile into the small intestine. F Protease: Break down proteins F Lipases: Break down fats F Bile: Helps fat digestion by emulsifying fats. u Very large surface area for absorption due to: F Large circular folds (villi) F Tiny cell surface projections (microvilli). u Capillaries drain nutrients from small intestine and then sends them to first to liver and then rest of body.

Small Intestine: Site of Digestion and Absorption

Human Digestive System Large Intestine (Colon): u 1.5 m long and 5 cm wide (diameter) u Most water absorption occurs here (up to 90%). u Undigested remainder of food is converted into feces. u Site of bacterial synthesis F Vitamin K F Several B vitamins u Appendix: Small fingerlike projection. Involved in immunity. Rectum: u Stores feces until ready to eliminate

Large Intestine: Water Absorption and Formation of Feces

Human Digestive System Digestive Glands: Liver: u Produces bile which is stored in gallbladder. u Bile is released into the small intestine after a meal. u Bile contains no enzymes, but helps solubilize fat particles. u Has many other functions. Pancreas: u Produces several digestive enzymes which are emptied into small intestine. u Enzymes digest starch, protein, fats, and nucleic acids.

Digestive Glands: Liver and Pancreas Empty Contents into Small Intestine

Nutritional Needs of Animals The diet provides: 1. Fuel (chemical energy) to power body activities. 2. Organic raw materials to make animal’s own macromolecules. 3. Essential nutrients or substances that the animal cannot make for itself and must obtain prefabricated from food: u Essential amino acids u Vitamins u Minerals

Essential Amino Acids Are Not Synthesized by Humans Most Vegetables are Incomplete Protein Sources