Chapter 14 – Part 5 The Digestive System

1. Chapter 14 – Part 5The Digestive System

2. Digestive Activities of the Mouth • Mechanical Breakdown • Food is physically broken down by chewing • Chemical Digestion – Begins carbohydrate digestion • Food is mixed with saliva • Breaking of starch into maltose by salivary amylase

3. Activities of the Pharynx and Esophagus • These organs have no digestive function • Serve as passageways to the stomach • Food propulsion only

4. Deglutition (Swallowing) • Deglutition – Swallowing • Complicated process that includes the coordinated efforts of the tongue, soft palate, pharynx, and esophagus • It has two phases: • Buccal Phase • Pharyngeal-Esophageal Phase

5. Deglutition (Swallowing) • Buccal Phase • Voluntary • Occurs in the mouth • Food is formed into a bolus (food mass) • The bolus is forced into the pharynx by the tongue

6. Deglutition (Swallowing) • Pharyngeal-esophageal phase • Involuntary transport of the bolus through the pharynx and esophagus • All passageways except to the stomach are blocked • Tongue blocks off the mouth • Soft palate (uvula) blocks the nasopharynx • Epiglottis blocks the larynx

7. Deglutition (Swallowing) • Pharyngeal-esophogeal phase (cont.) • Peristalsis moves the bolus toward the stomach • When food reaches the end of the esophagus, it presses against the cardioesophageal sphincter, causing it to open, and the food enters the stomach

8. Deglutition (Swallowing)

9. Food Breakdown in the Stomach • Secretion of gastric juice • Regulated by neural and hormonal factors • Under normal conditions, 2-3 liters are produced each day • Sight, smell, and taste of food increase the secretion of gastric juice • Presence of food or falling pH causes the release of the hormone gastrin

10. Food Breakdown in the Stomach • Gastrin causes the stomach glands to produce: • Protein-digesting enzymes (pepsinogens) • Mucus • Hydrochloric acid

11. Acid Environment in the Stomach • Hydrocholoric acid makes the stomach contents very acidic • The extremely acidic environment is necessary because it: • Activates pepsinogen to pepsin for protein digestion • Provides a hostile environment for microorganisms

12. Heartburn • Heartburn – Pain caused when gastric juice backs up into the esophagus when the cardioesophageal sphincter fails to close tightly • The esophagus has little mucus protection (unlike the stomach) • If uncorrected, it can cause inflammation of the esophagus (esophagitis) and even an ulceration to the esophagus

13. What are Peptic Ulcers? • Peptic Ulcers – Craterlike erosion in the mucosa of any part of the GI tract that is exposed to the secretions of the stomach • Affects 1 out of every 8 Americans • Most occur in the pyloric part of the stomach (gastric ulcers) or the first part of the duodenum (duodenal ulcers) • Develop most frequently between the ages of 50-70 years

14. Peptic Ulcers: Symptoms • After developing, they tend to recur -healing, then flaring up periodically - for the rest of one’s life if not treated • Produce a gnawing or burning pain in the abdomen that appears 1-3 hours after a meal and is relieved by eating • Other symptoms: loss of appetite, burping, nausea, and vomiting

15. Peptic Ulcers: Cause • Cause remains incompletely understood • A stressful lifestyle does seem to aggravate existing ulcers • Recent studies indicate that many ulcers are actually caused by a strain of acid-resistant bacteria (Helicobacter pylori)

16. Peptic Ulcers: Treatment First step is to avoid smoking, alcohol, ibuprofen, and aspirin (all aggravate ulcers) Antacid drugs to neutralize the stomach acids Triple drug therapy to promote healing and prevent reccurence

17. Digestion and Absorption in the Stomach • Other than the beginning of protein digestion, little chemical digestion occurs in the stomach • Protein digestion enzymes • Pepsin – an active protein digesting enzyme • Rennin – works on digesting milk protein • Virtually no absorption occurs in the stomach (Exception: alcohol and aspirin)

18. Food Propulsion in the Stomach • Food must first be well mixed • Rippling peristalsis occurs in the lower stomach

19. Propulsion in the Stomach • The pylorus meters out chyme into the small intestine (30 ml at a time) • The stomach empties in four to six hours

20. Vomiting • Vomiting – Essentially a reverse peristalsis occurring in the stomach • Accompanied by contraction of the abdominal muscles and the diaphragm, which increases the pressure on the abdominal organs • Caused by: • Local irritation of the stomach such as food poisoning • Disturbances of the equilibrium apparatus of the inner ear

21. Digestion in the Small Intestine • Food entering the small intestine is only partially digested • Carbohydrate and protein digestion have been started • Virtually no fats have been digested up to this point • By the time food reaches the end of the small intestine, chemical digestion is complete and nearly all food absorption has occurred

22. Enzymes of the Small Intestine • Brush border enzymes • Break double sugars into simple sugars • Complete some protein digestion • Enzyme-rich pancreatic juice (play the major digestive function)

23. Small Intestine: Pancreatic Enzymes • Pancreatic enzymes play the major digestive function: • Help complete the digestion of starch • Carry out about half of all protein digestion • Totally responsible for fat digestion • Digest nucleic acids • Alkaline content neutralizes acidic chyme coming in from the stomach

24. Absorption in the Small Intestine • Water is absorbed along the length of the small intestine • Absorption of the end products of digestion along the length of the small intestine • Most substances are absorbed by active transport through cell membranes • Lipids are absorbed by diffusion • Substances are then transported in the blood to the liver by the hepatic portal vein

25. Absorption in the Small Intestine • At the end of the small intestine (ileum), all that remains is: • Some water • Indigestible food materials • Large amounts of bacteria

26. Propulsion in the Small Intestine • Peristalsis is the major means of moving food • Segmental movements • Mix chyme with digestive juices • Aid in propelling food

27. Food Breakdown and Absorption in the Large Intestine • No digestive enzymes are produced • Resident bacteria digest remaining nutrients • Make some vitamins (Vitamin K and B) • Release gases (methane and hydrogen sulfide) that contribute to the odor of feces • The only absorption: Remaining water, vitamin K, and vitamin B • Remaining materials are eliminated via feces

28. Food Breakdown and Absorption in the Large Intestine • Feces – The more or less solid product delivered to the rectum • Contains: • Undigested food residues • Mucus • Millions of bacteria • Just enough water to allow their smooth passage

29. Food Breakdown and Absorption in the Large Intestine • About 500 ml of gas (flatus) is produced each day • Much more gas is produced when certain carbohydrate-rich foods (such as beans) are eaten

30. Propulsion in the Large Intestine • Two major types of propulsive movements occurring in the large intestine: • Sluggish peristalsis • Mass movements – Long, slow-moving, but powerful contractile waves that move over large areas of the colon • Occurs three to four times per day • Forces the contents towards the rectum

31. Propulsion in the Large Intestine • When feces are forced into the rectum by mass movements and its wall is stretched, a defecation reflex is initiated • Internal anal sphincter is relaxed • Defecation occurs with relaxation of the voluntary (external) anal sphincter • As feces are forced through the anal canal, you have time to make a decision as to whether the external voluntary sphincter should remain open or be constricted to stop the passage of feces

32. Diarrhea • Diarrhea – Watery stools • Results from any condition that rushes food residue through the large intestine before that organ has had time to absorb the water • Some Causes: Irritation of the colon by bacteria • Prolonged diarrhea may result in dehydration (lose fluids and ions)

33. Constipation • Constipation - Hard stool • Too much water is absorbed (food remains in the large intestine for too long) • Stool becomes difficult to pass • May result from: • Lack of fiber in the diet • Poor bowel habits (“failing to heed the call”) • Laxative abuse