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

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 –, PO 4 2–, HCO 3 – Salivary amylase and lingual lipase; mucin; Metabolic wastes—urea and uric acid; lysozyme, IgA, defensins, and a cyanide compound protect against microorganisms

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 oSympathetic impulses inhibit secretion and motility oParasympathetic impulses stimulate  Chemical and mechanical receptors are located in organ walls that trigger reflexes

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

Food Breakdown in the Stomach  Gastric juice is regulated by neural 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 from parietal cells makes the stomach contents very acidic, kills bacteria  Heartburn from Acid Reflux Acid reflux animation online Ulcer formation movie online Chief cell pepsinogen

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

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

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

Liver  Largest gland in the body  Connected to the gall bladder via the common hepatic duct  Bile Production in the Liver Composition oBile 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.

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

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

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

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

Chemical Digestion : Pancreas Figure 14.6  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

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 +

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

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)

Figure (1 of 4) Carbohydrate digestion Glucose and galactose are absorbed via cotransport with sodium ions. Fructose passes via facilitated diffusion. 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. Starch and disaccharides Oligosaccharides and disaccharides LactoseMaltoseSucrose Glucose Fructose Salivary amylase Mouth Pancreatic amylase Brush border enzymes in small intestine (dextrinase, gluco- amylase, lactase, maltase, and sucrase) Small intestine Small intestine Foodstuff Galactose Path of absorption Enzyme(s) and source Site of action Summary of Carbohydrate Digestion (monosaccharides)

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

Figure Epithelial cells of small intestine Fat droplets coated with bile salts Fat globule Lacteal Bile salts Micelles made up of fatty acids, monoglycerides, and bile salts 1 Large fat globules are emulsified (physically broken up into smaller fat droplets) by bile salts in the duodenum. 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. 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. Lipid Digestion and Fatty Acid/Glycerol Import

Figure (3 of 4) Fat digestion Small intestine Small intestine Foodstuff Unemulsified fats Emulsification by the detergent action of bile salts ducted in from the liver Pancreatic lipases Monoglycerides and fatty acids Glycerol and fatty acids Path of absorption Enzyme(s) and source Site of action Fatty acids and monoglycerides enter the intestinal cells via diffusion. 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. The chylomicrons enter the lacteals of the villi and are transported to the systemic circulation via the lymph in the thoracic duct. 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. Summary of Lipid Digestion

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

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

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 CH 4 H 2 S Vitamin K Vitamin B

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

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