1. Digestion Physiology Chapter 26
Test (Chapters 25 & 26) – Tuesday, 3/30 (Tentative Date )

2. Primary Mechanisms of Digestive System (Table 26-1)
Ingestion – take food in
Digestion – break down complex nutrients into simpler nutrients
Motility – movement of GI tract; aids in digestion
Secretion – enzymes are required for digestion to take place
Absorption – movement across GI mucosa into the internal environment
Elimination – process of eliminating unabsorbed material

3. Mechanical Digestion
Includes all motility of the digestive tract that cause the following changes:
Change in physical state of the food from large particles to smaller particles
Churning of the GI lumen to mix particles with digestive juices
Propelling food forward ending with elimination

4. Mastication Mastication = chewing Requires tongue, cheek and lips
Reduces particle size
Mix food with saliva

5. Deglutition Deglutition = swallowing Oral stage Pharyngeal stage
Bolus is formed voluntarily
Tongue pushes bolus against the palate and into the oropharynx
Soft palate acts as a valve to prevent food from entering the nasopharynx
Pharyngeal stage
Involuntary reflexes push bolus toward esophagus
Epiglottis prevents food from entering trachea
Esophageal stage
Involuntary reflexes move bolus towards stomach

7. Motility
Smooth muscle contractions take over in the lower portion of the esophagus
Peristalsis – progressive wavelike ripple of the muscle layer of a hollow organ
Bolus stretches the GI tract wall  triggers contraction of circular smooth muscle  bolus moves forward

8. Motility
Segmentation – mixing movement; back and forward movement within a single region
Mechanically breakdown food particles
Mix food and digestive juices together
Facilitate absorption
Peristalsis and segmentation can occur in alternating sequence to churn/mix and progress food

9. Peristalsis vs. Segmentation

10. Regulation of Motility
Gastric Motility
Emptying the stomach takes approx 2-6 hours after a meal
Food is churned with digestive juices to form chyme
Ejection every 20 seconds into the duodenum
Controlled by hormonal and nervous mechanisms

11. Regulation of Motility
Hormonal Control
Fats and nutrients in duodenum cause secretion of gastric inhibitory peptide (GIP) from the intestinal mucosa into the bloodstream
Slows peristalsis in stomach; decreasing passage of food into the duodenum

12. Regulation of Motility
Nervous Control
Nerve receptors in duodenum are sensitive to acid and distention
Vagus nerve (CN X) causes inhibition (stops) gastric peristalsis

13. Regulation of Motility
Intestinal Motility
Takes approx 5 hours for food to pass through the small intestine
Segmentation
Mixes chyme and digestive juices from liver, pancreas and intestinal mucosa
causes contact with intestinal mucosa to increase absorption
Peristalsis
Continues in the jejunum to move food into the large intestine
Stimulated by the hormone cholecystokinin-pancreozymin (CCK)
Secreted by intestinal endocrine cells in the presence of chyme

14. Mechanical Digestion
Summarized in Table 26-2; page 775

15. Chemical Digestion
Consists of all the changes in the chemical composition of food
Result of hydrolysis
Compounds combine with water then split into simpler compounds
Enzymes catalyze the hydrolysis of foods

16. Chemical Digestion Six main types of chemical substances:
Carbohydrates, proteins, fats, vitamins, mineral salts, water
Only carbohydrates, proteins and fats must undergo chemical digestion to be absorbed

17. Properties of Digestive Enzymes
Extracellular enzymes
Classified as hydrolases
Function optimally at a specific pH
Ex: amylase vs pepsin
Continually destroyed or eliminated

18. Carbohydrate Digestion
Carbohydrates are saccharide compounds
Contain one or more saccharide groups
Polysaccharides – starches & glycogen
Disaccharides – sucrose, lactose and maltose
Monosaccharides – glucose, fructose & galactose

19. Carbohydrate Digestion
Polysaccharides are hydrolyzed by amylases
Present in saliva and pancreatic juice
Sucrose, lactose and maltose are hydrolyzed by sucrase, lactase, and maltase
Located on epithelial cells lining villi in small intestine
End product (usually glucose) is absorbed in the small intestine

21. Protein Digestion
Proteins are large molecules composed of twisted chains of amino acids
Proteases catalyze the hydrolysis of proteins into smaller compounds
Proteins  proteoses or peptides  amino acids
The peptides bonds holding amino acids together can vary = increased need for varying proteases

22. Protein Digestion - Proteases
Pepsin – gastric juice (stomach)
Trypsin and chymotrypsin – pancreatic juice (pancreas)
Peptidases – intestinal brush border (small intestine)

23. Proteoses

24. Fat Digestion
Fats are insoluble in water so they must be emulsified prior to digestion
Emulsify = dispersed as small droplets
Lecithin and bile salts emulsify oils and fats in the small intestine by forming micelles (fig 26-8, page 778)
Micelles are further broken down by lipase

26. Residuals of Digestion
Certain compounds cannot be digested in humans b/c we lack the enzyme required for hydrolysis
These compounds are excreted in the feces
Cellulose (dietary fiber), connective tissue from meat (collagen), undigested fats combined with calcium and magnesium, bacteria, pigments, water, mucous

27. Chemical Digestion
Summarized in Table 26-3, page 779

28. Secretion Release of substances from exocrine glands in the GI tract
Examples: Saliva, gastric juice, bile, pancreatic juice, intestinal juice

29. Saliva Secreted from salivary glands
Water component helps liquefies food  chyme
Allows enzymes to mix with food particles
Mucus lubricates food to protect mucosa lining
Amylase – chemically digest (breakdown) complex carbohydrates
Lipase (small amounts) – digest lipids
Decreased function when fat are not emulsified
Sodium bicarbonate (NaHCO3)
Dissociated in water
Bicarbonate ions bind with H+ to increase pH

30. Control of Salivary Secretion
Controlled by reflex mechanisms:
Olfactory & visual stimuli send afferent impulses to centers in the brainstem  efferent impulses to salivary glands
Chemical and mechanical stimuli come from the presence of food in the mouth

31. Gastric Juice Gastric juice = water, mucous, enzymes & acid
Secreted by gastric glands in the stomach
Chief cells – secrete enzymes of digestive juices
Pepsin
Parietal cells
Secrete HCl
Decreases stomach pH

32. Control of Gastric Secretion
Gastric secretion is controlled by 3 phases:
Cephalic phase (“psychic phase”)
Sight, smell, taste, thought of food activate control centers in medulla oblongata
Parasympathetic fibers of the vagus nerve conduct impulses to gastric glands
Vagal impulses stimulate production of gastrin
Gastrin stimulates gastric secretion

33. Control of Gastric Secretion
2. Gastric phase:
Gastrin secretion is further stimulated by the presence of products of protein digestion & distention
Gastrin continues to stimulate the secretion of gastric juices (pepsin and HCl)

34. Control of Gastric Secretion
3. Intestinal phase:
Gastric inhibitory peptide (GIP) in secreted in the small intestine in the presence of fats and carbohydrates
Decrease gastric motility and secretion
Secretin secreted in the small intestine in the presence of acid, digested proteins and fats
Inhibit gastric secretion
Simulate secretion of pancreatic enzymes
Stimulate ejection of bile into small intestine
CCK
Secreted in the small intestine in the presence of chyme
Stimulates ejection of bile from gallbladder
Stimulates secretion of pancreatic juices
Opposes action of gastrin; raises pH of gastric juice

36. Pancreatic Juice Secreted by exocrine portion of the pancreas
Mostly water
Enzymes:
Trypsin and chymotrypsin (proteases)
Lipases
Amylase (starch digesting enzyme)
Secrete bicarbonate into the GI lumen and H+ into the blood to buffer the effects parietal cell secretion (fig and fig 26-11)

38. Control of Pancreatic Secretion
Secretin & CCK
Stimulates the secretion of pancreatic fluid high in bicarbonate to neutralize acidity of chyme in the small intestine
See notes under “Control of Gastric Section – Intestinal phase”

39. Bile Secreted by liver and stored in gallbladder Bile contains:
Lecithin and bile salts
Emulsify fats by creating a hydrophilic “shell” around tiny fats droplets
Sodium Bicarbonate – increase pH of chyme in small intestine
Excretions:
Cholesterol, products of detoxification, bilirubin (product of hemolysis)

40. Control of Bile Secretion
Controlled by CCK and secretin
See Table 26-5, page 782

41. Intestinal Juice Mucus – provides lubrication
Sodium bicarbonate – increases pH to allow intestinal enzymes to function at optimal level
Water – carries mucus and NaHCO3

42. **Study These Tables**
Table 26-4: Digestive Secretions
Table 26-5: Actions of Digestive Hormones
Both on page 782

43. Absorption
Passage of substances (digested foods, vitamins, salts, water) across the mucosa into the blood
Majority of absorption takes place in small intestine where surface area is increased

44. Elimination
Expulsion of digestive residuals from the digestive tract in the form of feces
Defecation
Normally rectum is empty
Massive peristalsis of feces into the rectum stimulate receptors and relax the external anal sphincter
Voluntary control

45. Elimination Constipation Diarrhea
Contents move through large intestine at a slower rate
Increased water absorption occurs resulting in hardened feces
Diarrhea
Result of increased motility of the small intestine
Water absorption does not occur

46. GI Tract Disorders Common S/S Gastroenteritis
Gastritis – stomach inflammation
Enteritis – intestinal inflammation
Anorexia: chronic loss of appetite
Nausea: feeling of needing to vomit; may progress to vomiting
Emesis: vomiting
Diarrhea: elimination of liquid feces; abdominal cramps may also be present
Constipation: decreased motility of colon; difficulty in defecating

47. GI Tract Disorders Ulcers In stomach or duodenum
Cause pain and may lead to perforation of the wall of the GI tract
Bleeding  anemia
Causes:
Hyperacidity
H. phylori bacterium

48. GI Tract Disorders Stomach cancer
Linked to excessive alcohol use, chewing tobacco, eating heavily preserved foods
Early signs:
Heartburn, belching, nausea
Later signs:
Chronic indigestion, vomiting, anorexia, stomach pain, blood in feces

49. GI Tract Disorders
Diverticulosis – presence of abnormal sac-like projections on the large intestine (diverticula)
When inflamed causes diverticulitis
S/S: pain, tenderness, fever

50. GI Tract Disorders Colitis – inflammation of the large intestine
s/s: diarrhea, abdominal cramps, constipation, bleeding, intestinal ulcers
Crohn’s Disease: autoimmune colitis
Treatment: surgical removal of affected portions of the intestine

51. GI Tract Disorders Irritable bowel syndrome (IBS) “spastic colon”
Noninflammatory condition usually caused by stress
Diarrhea or constipation

52. GI Tract Disorders Colorectal cancer Occurs after 50
Associated with low-fiber, high-fat diet
Early signs: change in bowel habits, fecal blood, rectal bleeding, abdominal pain, unexplained anemia, weight loss, fatigue

53. Liver Disorders Hepatitis – inflammation of the liver
S/S: jaundice, liver enlargement, anorexia, abdominal discomfort, gray-white feces, dark urine
Causes: alcohol or drug abuse; bacterial or viral infection
Cirrhosis – degenerative liver condition
Tissue can no longer regenerate