The Human Digestive System

1. The Human Digestive System

2. The Human Digestive System • The human digestive tract is similar to that of the earthworm, in terms of both the anatomy and physiology. • The digestive tract is actually a tube, with a number of specialized organs along its path.

3. Digestive System continued • If the tube was stretched out, it would be about 9 m in length.

4. Path of Ingested Food • Oral Cavity (mouth) • Pharynx (throat) • Esophagus (gullet) • Stomach • Small intestine • Large intestine • Rectum • Anus

5. Digestive Glands • Along the digestive path, there are several digestive glands that include the salivary glands, the liver and the pancreas. • These glands produce secretions that enter the tube via small tubes called ducts.

6. The Mouth and Pharynx • When food enters the mouth there are two important processes that occur: • 1. Mechanical breakdown- grinding action of teeth. • 2. Chemical digestion-action of saliva. Refer to Handout-Saliva and Salivation. Bolus Bolus

7. Mouth and Pharynx continued. • When the food has been chewed, moistened, and turned into the bolus, it is pushed to the back of the pharynx by the tongue. • When the bolus arrives at the back of the throat, the swallowing reflex is activated. • Food is forced into the esophagus. Bolus Bolus

8. The Esophagus • Food is pushed along the esophagus to the stomach, by means of wave-like contractions called peristalsis. • In peristalsis, muscles in front of the bolus relax, and those behind the bolus contract. This sends food ahead to the stomach. See handout called Peristalsis and Segmentation. Esophagus

9. The Stomach • When the bolus reaches the stomach it encounters a ring of muscle tissue called a sphincter. • Because of peristalsis, the sphincter relaxes and food enters the stomach. • The stomach is a thick-walled muscular sac and can hold more than 2 L of material. • Refer to handout- Stomach: Structure and Functions.

10. Chemical Digestion of Food • Chemical breakdown of food begins in the stomach. • Two types of glands are involved here: • 1. Pyloric Glands - These glands secrete mucus that protects the stomach from enzymes and acids i.e. prevents “self-digestion”.

11. Chemical Digestion of Food continued • 2. Gastric Glands - produce gastric juice which contains • (a) Hydrochloric Acid - pH 1.5-2.5 • (b) Pepsin- works best in an acid environment. Breaks down large proteins into smaller chains of amino acids called polypeptides.

12. Release of Gastric Juice • 1. Sight, thought, smell - stimulation of the brain causes gastric glands to release gastric juice (GJ). • 2. Food touching stomach lining results in GJ release. • 3.Food stretching stomach or certain substances (protein, caffeine, alcohol), results in release of the hormone gastrin, from the stomach lining. Gastrin in turn stimulates release of GJ.

13. From Stomach to Intestine ! • In the stomach, food is turned into a soupy material called chyme. • Chyme passes through the pyloric sphincter and into the duodenum (the first part of the small intestine). • This takes about 2-6 hours. Duodenum

14. The Small Intestine (S.I.) • The S.I. Is about 6.5 m long and 2.5 cm in diameter. • S.I environment is alkaline. • Most chemical digestion occurs in the S.I. • Nearly all absorption occurs in the S.I. • Peristalsis and segmentation move food and digestive juices through the S.I.(see handout).

15. The Small Intestine (S.I.) con’t. • In the S.I., chyme is mixed with the following: • 1. Bile from the liver • 2. Pancreatic juice from the pancreas. • 3. Intestinal juice from the cells in the intestinal wall. • The above secretions contain enzymes that complete digestion.

16. Digestive Juices in the S.I. Chyme from stomach Pancreatic juice and enzymes Secretin and cholecystokinin

17. Pancreas Secretions • Two products: • 1. Pancreatic Juice- contains sodium bicarbonate, that neutralizes the acid chyme. • 2. Pancreatic Enzymes- Three enzymes. Pancreas

18. Pancreas Enzymes • 1. Amylase -breaks down starch into maltose. • 2. Proteases - two types; trypsin and chymotrypsin. Both act on large protein molecules. • 3. Lipases - break down fats. Pancreas

19. Bile • Produced in liver and stored in the gall bladder. • Bile release is stimulated by cholecystokinin. • Bile travels to S.I. via bile duct, where it breaks up fats and oils into small droplets. This process is called emulsification. Gall bladder

20. Intestinal Juice • Intestinal glands produce this juice. • Contains several enzymes: • Fats- fatty acids and glycerol. • Carbohydrates-simple sugars. • Protein-amino acids. Intestinal glands

21. Absorption in the S.I. • S.I. is the site of absorption. • Simple sugars, amino acids, vitamins and minerals are absorbed through the wall of the S.I. • Fatty acids and glycerol are absorbed by small lymphatic vessels called lacteals. S.I.

22. Absorption in the S.I. Con’t. • S.I. is an excellent absorption site: • 1. Very long(6.5m)-allows time for absorption to occur. • 2. Highly folded lining. • 3. Lining is covered in villi. • 4. Epithelial cells of the intestinal lining have microvilli.. Villus

23. The Villus

24. The Large Intestine • 1.5 m in length, 6 cm in diameter. • No digestion here ! • Reabsorption of water. • Absorption of vitamins produced by bacteria (Vit. K & Vit. B). • Gets rid of indigestible material. • Storage of feces. Large Intestine