The Digestive System

1. The Digestive System Guts, teeth and glands! Images from: http://www.whfreeman.com/life/update/

2. Why Guts? • Multicellular animals must have specialized structures for obtaining and breaking down their food. • There are two processes: feeding and digestion. • Animals are heterotrophs, they must absorb nutrients or ingest food sources.

3. How to dine... • Ingestive eaters (us). • Absorptive feeders (tapeworm) • Filter feeders (clam) • Substrate feeders (earthworms) • Fluid feeders (mosquito)

4. Vertebrate Digestion • The digestive system uses mechanical and chemical digestion to breakdown food.

5. Tube in a tube • Vertebrates have a tube-within-a-tube system. • digestion occurs in the lumen with the nutrient molecules being transferred to the blood.

6. Stages of digestion • Movement of food • Secretion of digestive juices • Digestion of food into molecules • Absorption of molecules • Elimination of undigested food and wastes

7. The Digestive System • Mouth, pharynx, • esophagus, stomach • small intestine • large intestine • anus • salivary glands • pancreas • liver and gall bladder

8. The Start • In the mouth, teeth, jaws and the tongue begin the mechanical breakdown of food. • Chemical breakdown of starch by amylase • Mucus moistens food and lubricates the esophagus. • The chewed food and saliva is then pushed into the pharynx and esophagus. • The esophagus uses peristalsis to send the food to the stomach.

9. Move the food • Food is chewed and passed to the stomach through the esophagus. The name of the movement is peristalsis. (see video)

10. The Stomach • Holds 1 to 2L (folds) • The stomach secretes mucus, hydrochloric acid and pepsin. HCl lowers pH of the stomach to activate pepsin. • Pepsin hydrolysis of proteins into peptides. • The stomach also mechanically churns the food. Chyme, leaves the stomach and enters the small intestine. • Ulcers occur when mucus lining is reduced.

11. The Small Intestine • 3 m long tube with coils and folding plus villi. Very large surface area! • Final digestion of all food and absorbtion. • Villi produce enzymes which complete the digestion of peptides and sugars. • The absorption process in the villi.

12. Villi • Sugars and amino acids go into the bloodstream via capillaries in each villus. • Glycerol and fatty acids go into the lymphatic system. Absorption is an active transport, requiring cellular energy.

13. Duodenum - busy place • Secretions from the liver and pancreas are used for digestion in the duodenum. • The pancreas secretes digestive enzymes and stomach acid-neutralizing bicarbonate. • The liver produces bile, which is stored in the gall bladder before entering the bile duct into the duodenum.

14. Small intestine - more • Digestion of carbohydrates, proteins, and fats continues in the small intestine. See table in text - page 166. • Bile emulsifies fats so that lipases can completely digested lipids. • Most absorption occurs in the ileum and jejeunum (second third of the small intestine).

15. Liver and Gall Bladder • The liver produces bile and helps to detoxify of blood • synthesis of blood proteins • destruction of old erythrocytes • storage of glucose as glycogen • De-amination amino groups and ammonia. (this produces urea, less toxic)

16. Glycogen-Glucose • Low glucose levels in the blood cause glucagon to stimulate breakdown of glycogen into glucose. • Insulin helps store glucose and glycogen in the liver (see page 929) • When no glucose or glycogen is available, amino acids are converted into glucose in the liver.

17. The Large Intestine what to do with left overs! • The large intestine produces an alkaline mucus that neutralizes acids produced by bacterial metabolism. • Water, salts, and vitamins are absorbed, the remaining contents in the lumen form feces (mostly cellulose, bacteria, bilirubin). • Bacteria in the large intestine, such as E. coli, produce vitamins (including vitamin K) that are absorbed.

18. Nutrition • See the basics of nutrition in your text. • Have a hand out!