Digestion Physiology Chapter 26

1. Digestion PhysiologyChapter 26 Test (Chapters 25 & 26) April 6th and 7th (next block day)

2. March 28, 2011 Announcements: • Turn in last week’s warm ups to the box • Pick up new warm up sheet • Practical grades in gradebook • Practical make up: • Today after school @ 3:15 • Tuesday morning 6:45 Warm Ups: • In the large intestine longitudinal muscles are grouped into strips called _______; circular muscles are grouped into rings are form _______. • What is produced in the appendix? • The two lobes of the liver are connected by the: • What are gallstones? • Through what two pathways does blood enter the hepatic lobules?

3. 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

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

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

6. Deglutition • Deglutition = swallowing • Oral 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

8. 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

9. 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

10. Peristalsis & Segmentation

11. 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

12. 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

13. Regulation of Motility • Nervous Control • Nerve receptors in duodenum are sensitive to acid and distention • Sensory and motor fibers within the vagus nerve (CN X) cause a reflex inhibition of gastric peristalsis (enterogastric reflex)

14. 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

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

16. 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

17. 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

18. Digestive Enzymes Properties • Extracellular enzymes • Classified as hydrolases • Function optimally at a specific pH • Ex: amylase vs pepsin • Continually destroyed or eliminated • Most digestive enzymes are synthesized and secreted as proenzymes • Kinases convert proenzymes to active enzymes

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

21. 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 result (usually glucose) is located at site of absorption (“contact digestion”)

23. Protein Digestion • Proteins are large molecules composed of twisted chains of amino acids • Proteases catalyze the hydrolysis of proteins into smaller compounds • Proteoses  peptides  amino acids • Proteins have varying peptide bonds holding amino acids together = increased need for varying proteases

24. Protein Digestion - Proteases • Pepsin – gastric juice • Trypsin and chymotrypsin – pancreatic juice • Peptidases – intestinal brush border

25. Proteoses

26. Fat Digestion • Fats are insoluble in water – 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) • Lecithin mixes with fat to form micelles • Fats broken down by mechanical digestion are further broken down by lipase • Action of lipase is enhanced by colipase (released from the pancreas)

28. 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

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

30. Secretion • Release of substances from exocrine glands in the GI tract • Saliva, gastric juice, bile, pancreatic juice, intestinal juice

31. 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 starch and glycogen • 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

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

33. Gastric Juice • Secreted by gastric glands surrounds by gastric pits • Chief cells – secrete enzymes of digestive juices • Pepsin (proenzyme = pepsinogen) • Pepsinogen is activated by hydrochloric acid (HCl) • Parietal cells • Secrete HCl • Decreases stomach pH; increases blood pH • Secrete intrinsic factor • Binds to molecules of vitamin B12 to facilitate absorption in the small intestine

34. 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

35. Control of Gastric Secretion 2. Gastric phase: • Gastrin secretion is further stimulated by the presence of products of protein digestion & distention • Gastrin continue to stimulate the secretion of gastric juices (high pepsinogen and HCl content)

36. 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

38. Pancreatic Juice • Secreted by exocrine acinar cells of the pancreas • Mostly water • Enzymes: • Trypsin and chymotrypsin (proteases) • Lipases • Nucelases (RNA and DNA digesting enzymes) • Amylase (starch digesting enzyme) • Secrete bicarbonate into the GI lumen and H+ into the blood to buffer the effects parietal cell secretion (fig 26-10 and fig 26-11)

40. Control of Pancreatic Secretion • Secretin • 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” • CCK – see above

41. Bile • Secreted by liver and stored in gallbladder • 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)

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

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

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

45. Wednesday/Thursday 3/30-3/31 Warm Up: • Name the 5 components of saliva. • What is the proenzyme of pepsin? What is needed to activate this proenzyme? • Explain the role of bicarbonate in the GI tract. • The control of gastric secretion can be broken up into 3 phases. Name these phase and briefly describe each one. • Name and give the function of the 5 enzymes found in pancreatic juice.

46. 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

47. Mechanisms of Absorption • Some substances (water) are absorbed via diffusion • Secondary Active Transport (ex: Sodium) • Na+ is actively transported from the basal (backside) of epithelial cells lining the lumen of the small intestine into blood capillaries • Creates a low intracellular sodium concentration • Na+ ions diffuse passively from the lumen into epithelial cells • Fig 26-14, page 785

48. Mechanisms of Absorption • Sodium cotransport • Glucose is very large and hydrophilic • Requires carrier to cross intestinal mucosa • Carriers bind sodium and glucose together to passively transport out of lumen • Amino Acid absorption • Transported by passive carriers on luminal and basal surfaces of absorptive cells • Brush border enzymes can also act as carriers • Polypeptides can diffuse into absorptive cells, hydrolyze into amino acids, diffuse into blood

50. Mechanisms of Absorption • Fatty acid/monoglyceride absorption • Bile salts and lecithin form micelles • Fat digestion takes place within these tiny spheres • At the intestinal brush border, micelle contents can diffuse into absorptive cells • Inside triglycerides reform within chylomicrons • Water soluble chylomicrons allow fats to be transported through lymph and into bloodstream