The digestive system

1. The digestive system By: Jocelyn Pearce

4. The Mouth • Inside the mouth, food is mixed with saliva. Humans have three pairs of salivary glands that do this. The parotid, submandibular, and sublingual glands. • The nervous system controls the secretions • Although our mouth is constantly secreting saliva, it increases when food enters the mouth • When food is ready to be swallowed, the tongue pushes it to the back of the mouth

5. The mouth cont. • Swallowing occurs when the soft palate moves up and pushes against the pharynx • Pressure on the pharynx stimulates a reflex, where the larynx is raised, pushing the glottis against the epiglottis. • All of this directs the food into the esophagus, making sure it does not enter the nasal passage or respiratory tract

6. The esophagus • After being swallowed, food enters the esophagus, a muscular tube connecting the pharynx to the stomach • Food is moved down the esophagus by rhythmic waves of muscle contractions called peristalsis • Sphincters in the stomach let food in but prevents it from exiting

7. The stomach • It is like a sack that can fold up when empty and expand when filled • Contains an extra layer of smooth muscle that mixes food with gastric juices (an acidic substance secreted by the walls of the stomach) • Two kinds of secretory glands, parietal and chief cells • Parietal cells secrete hydrochloric acid • Chief cells secrete pepsinogen

8. The Stomach cont. • The pepsinogen a weak protein-digesting enzyme that is activated by the HCL’s low pH • When activated they cleave each other producing a more active protein- digesting enzyme called pepsin • This prevents the chief cells from eating themselves • Pepsin makes food proteins into shorter polypeptides that are digested in the small intestines • The parietal cells also secrete intrinsic factor, a polypeptide needed for the intestines to absorb vitamin B12

9. The Stomach Cont. • The mixture of food and gastric juice is called chyme • The acidic solution on the stomach kill most of the bacteria that we ingest in food • The bacteria not killed grows and multiplies in the intestines • Chyme leaves through the pyloric sphincter to travel to the small intestine

10. The Small Intestine • Neural and hormonal signals are used as communications between the stomach and the small intestines • Limited room in the intestines and the digestion takes a long time • Approx. 4.5 meters long in living person • The first 25 cm is the duodenum • The rest is divided into the jejunum and the ileum

11. The Small Intestine cont. • The duodenum receives chyme from the stomach • It also takes in digestive enzymes and bicarbonate from the pancreas, and bile from the liver and gallbladder • These juices help to digest food into smaller molecules • This happens primarily in the duodenum and jejunum

12. The small intestine cont. • The inner wall of the small intestine is covered with villi • The villi are covered in microvilli • The villi and microvilli increase the surface area of the intestine • They also contain digestive enzymes which help with digestion of disaccharides like lactose and sucrose

13. Absorption in the small intestines • When we digest proteins and carbohydrates, amino acids and monosaccharides are left • They are transported across the microvilli and then across the membrane, into the blood capillaries within the villi • These products are then taken into the liver by way of the hepatic portal vein • Fats are absorbed in a different way • Fats are broken down into fatty acids which are absorbed into the epithelial cells and then are reassembled into fats. The fats then combine with proteins called chylomicrons and then absorbed into the lymphatic capillaries

14. The Pancreas • A large gland near the connection of the stomach and small intestine • Pancreatic fluid is secreted into the duodenum through the pancreatic duct • This fluid contains many enzymes that digest proteins, starches, etc. • Enter duodenum as inactive zymogens and activate when they come in contact with the microvilli

15. The Pancreas cont. • Pancreatic fluid contains bicarbonate neutralizing the HCL from the stomach. Produced from acini • The pancreas also secretes hormones into the the blood that control the levels of glucose and other nutrients

16. The Liver • Secretes bile made up of bile pigments and bile salts that are delivered to the duodenum during digestion • Bile pigments do not help in digestion! They are just waste products and in the end are eliminated with feces • Bile salts help digest fat by making fat into smaller droplets • After bile is produced in the liver it is stored in the gallbladder

17. The Gallbladder • When fatty foods arrive, in the duodenum, they trigger trigger neural reflexes • The reflexes cause the gallbladder to contract, injecting bile into the duodenum

18. The large intestine • Called large because of its larger diameter, not length (which is shorter than the small intestine) • Makes up the last meter of the digestive tract • No digestion occurs here • Primary function is to concentrate waste materials, although it does absorb some water, sodium, and vitamin K

19. The large intestine cont. • Undigested material is compacted and stored here • Many bacteria live and reproduce here and are incorporated into feces • Compacted feces pass from the large intestines into the rectum, driven by peristaltic contractions • From the rectum, the feces then exit through the anus

20. Why is it necessary? • This system is necessary because it is our way of breaking down foods and getting the nutrients out of them. If we didn’t have the digestive system then we would be in trouble. There would be no way to really eat, therefore we would starve and also die from the lack of nutrients. We would have to inject ourselves with nutrients or something like that.

21. Interdependency • Many parts of this system are controlled by neural and hormonal signals, for example the movement of fluids from the stomach to the small intestine • Also, the secretions by the salivary glands are controlled by the nervous system • Works with the endocrine and nervous systems

22. Evolutionary development • Single-celled organisms digest their food intracellularly while other organisms digest their food extracellularly in digestive cavities • The most primitive digestive tract is in nematodes, a tubular gut lined with epithelial membrane. • Earthworms have specialized systems for the ingestion, storage, fragmentation, digestion, and absorption of foods • Vertebrates have similar specialization, more complex

23. Diseases/Disorders • Gastric Ulcers- overproduction of gastric acid. The acid eats a hole through the lining of the stomach • Gallstones- formed from a hardened precipitate of cholesterol. These block the bile duct. • Herpes esophagitis is a viral infection of the esophagus, caused by the herpes simplex virus