Digestive System II: Digestive Activities

1. Digestive System II: Digestive Activities • Digestive Activities in the Mouth, Swallowing • Digestive Activities in the Stomach • Gastric juice • Hormones that stimulate stomach acid and enzyme secretion • Contributions of the Liver in Digestion • Digestion in the Small Intestine • Pancreatic juice • Hormonal stimulation of pancreas • Methods employed to digest and absorb carbohydrates, proteins, lipids, and nucleic acids • Absorption in the Large Intestine and Feces Formation

2. Salivary Glands • Extrinsic salivary glands (parotid, submandibular, and sublingual) • Intrinsic (buccal) salivary glands are scattered in the oral mucosa • Secretion (saliva) • Cleanses the mouth • Moistens and dissolves food chemicals • Aids in bolus formation • Contains enzymes that begin the breakdown of starch • Composition • 97–99.5% water, slightly acidic solution containing: • Electrolytes—Na+, K+, Cl–, PO4 2–, HCO3– • Salivary amylase and lingual lipase; mucin; • Metabolic wastes—urea and uric acid; lysozyme, IgA, defensins, and a cyanide compound protect against microorganisms

3. Control of Digestive Activity and Mouth • Mostly controlled by reflexes via the parasympathetic division • Enteric Nervous System • (ENS) Intrinsic nerve supply of the alimentary canal • Sympathetic impulses inhibit secretion and motility • Parasympathetic impulses stimulate • Chemical and mechanical receptors are located in organ walls that trigger reflexes

4. Digestive System II: Digestive Activities • Digestive Activities in the Mouth, Swallowing • Digestive Activities in the Stomach • Gastric juice • Hormones that stimulate stomach acid and enzyme secretion • Contributions of the Liver in Digestion • Digestion in the Small Intestine • Pancreatic juice • Hormonal stimulation of pancreas • Methods employed to digest and absorb carbohydrates, proteins, lipids, and nucleic acids • Absorption in the Large Intestine and Feces Formation

5. Food Breakdown in the Stomach • Gastric juice is regulated byneural and hormonal factors • Presence of food or falling pH causes the release of gastrin • Gastrin causes stomach glands (chief cells) to produce protein-digesting enzymes (pepsinogen) • Hydrocholoric acid fromparietal cells makes the stomach contents very acidic, kills bacteria • Heartburn from Acid Reflux pepsinogen Chief cell Acid reflux animation online Ulcer formation movie online

6. Digestion and Absorption in the Stomach • Protein digestion enzymes (proteases) • Stomach proteins are exported into the stomach as zymogens (extra long, inactive polypeptide chains) • Pepsin – an active protein digesting enzyme (converted from pepsinogen through action of HCl and active pepsin) • Rennin (chymosin)– works on digesting milk casein protein in infants • The only absorption that occurs in the stomach is of alcohol and aspirin Conversion of pepsinogen to pepsin The action of chymosin on casein

7. Regulation of Stomach Activity Stimulatory events Inhibitory events 1 1 Sight and thought of food Loss of appetite, depression Cerebral cortex Lack of stimulatory impulses to parasym- pathetic center Cerebral cortex Cephalic phase Conditioned reflex 2 Stimulation of taste and smell receptors Hypothalamus and medulla oblongata Vagus nerve 1 1 Stomach distension activates stretch receptors Excessive acidity (pH <2) in stomach Vagovagal reflexes Medulla Vagus nerve Gastrin secretion declines G cells Gastric phase 2 Emotional upset Local reflexes Overrides parasym- pathetic controls Sympathetic nervous system activation 2 Food chemicals (especially peptides and caffeine) and rising pH activate chemoreceptors G cells Gastrin release to blood Stomach secretory activity 1 Distension of duodenum; presence of fatty, acidic, hypertonic chyme, and/or irritants in the duodenum Entero- gastric reflex Local reflexes 1 Presence of low pH, partially digested foods, fats, or hypertonic solution in duodenum when stomach begins to empty Intestinal (enteric) gastrin release to blood Vagal nuclei in medulla Brief effect Intestinal phase Pyloric sphincter 2 Distension; presence of fatty, acidic, partially digested food in the duodenum Release of intestinal hormones (secretin, cholecystokinin, vasoactive intestinal peptide) Stimulate Inhibit Figure 23.17

8. Digestive System II: Digestive Activities • Digestive Activities in the Mouth, Swallowing • Digestive Activities in the Stomach • Gastric juice • Hormones that stimulate stomach acid and enzyme secretion • Contributions of the Liver in Digestion • Digestion in the Small Intestine • Pancreatic juice • Hormonal stimulation of pancreas • Methods employed to digest and absorb carbohydrates, proteins, lipids, and nucleic acids • Absorption in the Large Intestine and Feces Formation

9. Liver • Largest gland in the body • Connected to the gall bladder via the common hepatic duct • Bile Production in the Liver • Composition • Bile salts, bile pigment (broken down hemoglobin, cholesterol, phospholipids • Bile is introduced into the duodenum in the presence of fatty food to cause emulsification • Emulsification solubilizes lipids so that they can come in contact with water-soluble lipases and be broken down into fatty acids and glycerol.

10. Role of the Liver in Metabolism Produces blood proteins (albumin, clotting proteins) and lipoproteins Degrades hormones

11. The Liver Processes/Detoxifies Absorbed Food in Lobules (a) (b) Lobule Central vein Connective tissue septum Figure 23.25a, b

12. Digestive System II: Digestive Activities • Digestive Activities in the Mouth, Swallowing • Digestive Activities in the Stomach • Gastric juice • Hormones that stimulate stomach acid and enzyme secretion • Contributions of the Liver in Digestion • Digestion in the Small Intestine • Pancreatic juice • Hormonal stimulation of pancreas • Methods employed to digest and absorb carbohydrates, proteins, lipids, and nucleic acids • Absorption in the Large Intestine and Feces Formation

13. Digestion in the Small Intestine • Enzymes from the brush border • Break double sugars into simple sugars (carbohydrases) • Complete some protein digestion (proteases) • Pancreatic enzymes play the major digestive function; usually zymogens • Help complete digestion of starch (pancreatic amylase) • Carry out about half of all protein digestion (trypsin, etc.) • Responsible for fat digestion (lipase) • Digest nucleic acids (nucleases) • Bicarbonate ion neutralizes acidic chyme, raises pH to 8

14. Chemical Digestion : Pancreas • Produces a wide spectrum of digestive enzymes that break down all categories of food • Acinar cells produce enzymes that are secreted into the duodenum • Alkaline fluid introduced with enzymes neutralizes acidic chyme Figure 14.6

15. Stimulation of the Release of Pancreatic Juice • Vagus nerve:  secretion of enzymes, weak release of bile • Local hormones from duodenal mucosa • Secretin(enzyme release,  bile production, gastrin &  stomach activities) • Cholecystokinin(enzyme and bicarbonate release,  bile release) Liver +

16. Digestive System II: Digestive Activities • Digestive Activities in the Mouth, Swallowing • Digestive Activities in the Stomach • Gastric juice • Hormones that stimulate stomach acid and enzyme secretion • Contributions of the Liver in Digestion • Digestion in the Small Intestine • Pancreatic juice • Hormonal stimulation of pancreas • Methods employed to digest and absorb carbohydrates, proteins, lipids, and nucleic acids • Absorption in the Large Intestine and Feces Formation

17. Other Activities in the Small Intestine • Water absorption • Nutrient absorption by mucosal cells via active or passive transport • Lipids are absorbed by diffusion into lacteals • Substances are transported to the liver by the hepatic portal vein or lymph (lacteal)

18. Summary of Carbohydrate Digestion Carbohydrate digestion Enzyme(s) and source Site of action Foodstuff Path of absorption • Glucose and galactose are absorbed via cotransport with sodium ions. Starch and disaccharides Salivary amylase Mouth • Fructose passes via facilitated diffusion. Pancreatic amylase Small intestine Oligosaccharides and disaccharides • All monosaccharides leave the epithelial cells via facilitated diffusion, enter the capillary blood in the villi, and are transported to the liver via the hepatic portal vein. Brush border enzymes in small intestine (dextrinase, gluco- amylase, lactase, maltase, and sucrase) Small intestine Lactose Maltose Sucrose Galactose Fructose Glucose (monosaccharides) Figure 23.32 (1 of 4)

19. Summary of Protein Digestion Protein digestion Enzyme(s) and source Site of action Foodstuff Path of absorption • Amino acids are absorbed by cotransport with sodium ions. Protein Pepsin (stomach glands) in presence of HCl Stomach • Some dipeptides and tripeptides are absorbed via cotransport with H+ and hydrolyzed to amino acids within the cells. Large polypeptides Pancreatic enzymes (trypsin, chymotrypsin, carboxypeptidase) + Small intestine Small polypeptides, small peptides • Amino acids leave the epithelial cells by facilitated diffusion, enter the capillary blood in the villi, and are transported to the liver via the hepatic portal vein. Brush border enzymes (aminopeptidase, carboxypeptidase, and dipeptidase) Small intestine Amino acids (some dipeptides and tripeptides) Figure 23.32 (2 of 4)

20. Lipid Digestion and Fatty Acid/Glycerol Import Fat globule 1 Large fat globules are emulsified (physically broken up into smaller fat droplets) by bile salts in the duodenum. Bile salts 2 Digestion of fat by the pancreatic enzyme lipase yields free fatty acids and monoglycerides. These then associate with bile salts to form micelles which “ferry” them to the intestinal mucosa. Fat droplets coated with bile salts Micelles made up of fatty acids, monoglycerides, and bile salts 3 Fatty acids and monoglycerides leave micelles and diffuse into epithelial cells. There they are recombined and packaged with other lipoid substances and proteins to form chylomicrons. 4 Chylomicrons are extruded from the epithelial cells by exocytosis. The chylomicrons enter lacteals. They are carried away from the intestine by lymph. Epithelial cells of small intestine Lacteal Figure 23.34

21. Summary of Lipid Digestion Fat digestion Enzyme(s) and source Site of action Foodstuff Path of absorption Unemulsified fats • Fatty acids and monoglycerides enter the intestinal cells via diffusion. Emulsification by the detergent action of bile salts ducted in from the liver Small intestine • Fatty acids and monoglycerides are recombined to form triglycerides and then combined with other lipids and proteins within the cells, and the resulting chylomicrons are extruded by exocytosis. Pancreatic lipases Small intestine • The chylomicrons enter the lacteals of the villi and are transported to the systemic circulation via the lymph in the thoracic duct. Monoglycerides and fatty acids Glycerol and fatty acids • Some short-chain fatty acids are absorbed, move into the capillary blood in the villi by diffusion, and are transported to the liver via the hepatic portal vein. Figure 23.32 (3 of 4)

22. Summary of Nucleic Acid Digestion Nucleic acid digestion Enzyme(s) and source Site of action Foodstuff Path of absorption • Units enter intestinal cells by active transport via membrane carriers. Nucleic acids Pancreatic ribo- nuclease and deoxyribonuclease Small intestine • Units are absorbed into capillary blood in the villi and transported to the liver via the hepatic portal vein. Brush border enzymes (nucleosidases and phosphatases) Small intestine Nucleotides Figure 23.32 (4 of 4)

23. Digestive System II: Digestive Activities • Digestive Activities in the Mouth, Swallowing • Digestive Activities in the Stomach • Gastric juice • Hormones that stimulate stomach acid and enzyme secretion • Contributions of the Liver in Digestion • Digestion in the Small Intestine • Pancreatic juice • Hormonal stimulation of pancreas • Methods employed to digest and absorb carbohydrates, proteins, lipids, and nucleic acids • Absorption in the Large Intestine and Feces Formation

24. Processes in the Large Intestine • No digestive enzymes are produced • Resident bacteria digest remaining nutrients • Produce some vitamin K and B • Release methane and hydrogen sulfide gases • Water and vitamins K and B are absorbed • Remaining materials are eliminated via feces CH4 H2S Vitamin K Vitamin B

25. Propulsion in the Large Intestine (Colon) • Sluggish peristalsis • Mass movements • Slow, powerful movements • (Three to four times per day) • Lack of fiber and weakening of colon walls cause diverticula • Presence of feces in the rectum causes defecation reflex • Internal anal sphincter is relaxed • Defecation occurs with relaxation of the voluntary external anal sphincter Neural control of the rectum and anal sphicters

26. Digestive System II: Digestive Activities • Digestive Activities in the Mouth, Swallowing • Digestive Activities in the Stomach • Gastric juice • Hormones that stimulate stomach acid and enzyme secretion • Contributions of the Liver in Digestion • Digestion in the Small Intestine • Pancreatic juice • Hormonal stimulation of pancreas • Methods employed to digest and absorb carbohydrates, proteins, lipids, and nucleic acids • Absorption in the Large Intestine and Feces Formation