Digestive Physiology

Overview Inside gastrointestinal (GI) tract, food is broken down by hydrolysis reactions into molecular monomers Most digestion of nutrients and absorption of monomers occurs in small intestine (90%) 18-3

Functions of the Digestive System Ingestion--taking food into mouth Mastication--chewing food and mixing it with saliva Deglutition--swallowing food Peristalsis--rhythmic wave-like contractions that move food through GI tract Digestion– mechanical and chemical breakdown of food Absorption--Is passage of digested end products into blood or lymph Storage and Elimination--Includes temporary storage and subsequent elimination of indigestible components of food 18-6

Functions of Digestive System Secretion: Includes release of exocrine and endocrine products into GI tract Exocrine secretions include: HCl, H2O, HCO3-, bile, lipase, pepsin, amylase, trypsin, elastase, and histamine Endocrine includes hormones secreted into stomach and small intestine to help regulate GI system e.g. gastrin, secretin, cholescytokinin, gastric inhibitory peptide, and somatostatin 18-7

Digestive System Is composed of GI tract (alimentary canal) and accessory digestive organs Organs include oral cavity, pharynx, esophagus, stomach, small and large intestine Accessory organs include teeth, tongue, salivary glands, liver, gallbladder, and pancreas

Regulation of GI Tract Parasympathetic effects, arising from vagus and spinal nerves, stimulate motility and secretions of GI tract Sympathetic activity reduces peristalsis and secretory activity GI tract contains an intrinsic system that controls its peristaltic movements--the enteric nervous system GI motility is also influenced by paracrine and hormonal signals 18-18

From Mouth to Stomach continued Peristalsis propels food thru esophagus and GI tract = wave-like muscular contractions After food passes into stomach, the lower esophageal sphincter constricts, preventing reflux 18-23

Stomach Is most distensible part of GI tract Empties into the duodenum Functions in: storage of food; initial chemical digestion of proteins some mechanical digestion of all nutrients kills bacteria with high acidity (HCl) moves soupy food mixture (chyme) into intestine 18-25

Stomach Gastric mucosa has gastric pits in its folds Cells that line folds deeper in the mucosa, are exocrine gastric glands 18-28

Stomach continued Gastric glands contain cells that secrete different products that form gastric juice 1. Goblet cells secrete mucus 2. Parietal cells secrete HCl and intrinsic factor (necessary for B12 absorption in intestine) 3. Chief cells secrete pepsinogen (precursor to pepsin) 18-29

Stomach continued 4. Enterochromaffin-like cells secrete histamine and serotonin 5. G cells secrete gastrin 6. D cells secrete somatostatin 18-30

HCl in Stomach Is secreted into stomach lumen by proton pumps of epithelial parietal cells in response to the histamine secreted by ECL cells ; and ACh from vagus (parasympathetic stimulation) These are indirect effects since release of histamine is due to gastrin release from G cells Proton pump inhibitors (medicines) are common and work to reduce stomach acids (treat ulcers) 18-32

HCl in Stomach continued Makes gastric juice very acidic which denatures proteins to make them more digestible Also converts pepsinogen into pepsin Pepsin is more active at low pHs 18-33

Digestion and Absorption in Stomach Proteins are partially digested by pepsin Carbohydrate digestion by salivary amylase is soon inactivated by acidity Water, alcohol and aspirin are the only commonly ingested substances that are absorbed here 18-35

Small Intestine (SI) continued Surface area increased by foldings and projections Large folds are plicae circulares Microscopic finger-like projections are villi Cell apical hair-like membrane projections are microvilli 18-40

Intestinal Enzymes Attached to microvilli are brush border enzymes Enzyme active sites are exposed to chyme 18-43

Large Intestine or Colon Has no digestive function of its own but absorbs H2O, electrolytes, B and K vitamins, and folic acid Internal surface has no villi (relatively smooth) Intestinal Flora: Contains large population of microflora 400 different species of commensal bacteria Which produce folic acid and vitamin K and ferment indigestible foods to produce fatty acids And reduce ability of pathogenic bacteria to infect colon Antibiotics can kill commensals 18-49

Fluid and Electrolyte Absorption in Colon SI absorbs most water but colon absorbs 90% of water it receives Begins with osmotic gradient set up by Na+/K+ pumps Water follows by osmosis Salt and water reabsorption stimulated by aldosterone Colon can also excrete H2O via active transport of NaCl into intestinal lumen (an ANP influence) 18-51

Accessory Organs:The Liver Day 28 complete

Functions of the Liver Overall it over 500 functions! Lipid metabolism: lipolysis, lipogenesis, synthesis of cholesterol Protein metabolism: synthesizes the plasma proteins (albumin, fibrinogen, alpha and beta globulins, and prothrombin); breaks down proteins and converts the to carbohydrates or lipid for storage. Carbohydrate metabolism: helps to maintain normal blood glucose levels by; breaking down glycogen into glucose and then secreting it into the blood converting serum glucose into glycogen and triglycerides for storage

Functions of the Liver Detoxification: processes drugs and hormones; detoxifies substances such as alcohol or excretes drugs such as the antibiotics into bile. Synthesis of bile salts: bile salts are used in the small intestine for the emulsification and absorption of lipids, cholesterol, phospholipids, and lipoproteins. Storage: stores glycogen, vitamins and minerals. Phagocytosis: Kupffer cells phagocytize RBCs, WBCs, bacteria, and toxins.

Detoxification of Blood Liver can remove hormones, drugs, and other biologically active molecules from blood by: Excretion in bile Phagocytosis by Kupffer cells Chemical alteration of molecules e.g. ammonia is produced by deamination of amino acids in liver Liver then converts it to urea which is excreted in urine 18-64

The Gallbladder Stores and concentrates bile continuously produced by liver When SI is empty, sphincter of Oddi in common bile duct closes and bile is forced up into gallbladder Expands as it fills with bile When food is in SI, sphincter of Oddi opens, gall bladder contracts, and bile is ejected thru ducts into duodenum 18-69

The Pancreas Has both endocrine and exocrine functions Endocrine function performed by Islets of Langerhans cells Secrete insulin and glucagon Exocrine secretions include bicarbonate (HCO3-) solution and digestive enzymes These pass in pancreatic duct to small intestine Exocrine secretory units are acini 18-70

The Physiology of Digestion Regulation of Gastric Function Extrinsic control of gastric function is divided into cephalic, gastric, and intestinal phases 18-78

Cephalic Phase Refers to control by brain via vagus nerve Stimulated by sight, smell, thought, and taste of food Activation of vagus nerve stimulates: Salivary glands to secrete saliva Chief cells to secrete pepsinogen G cells to secrete gastrin ECL cells to secrete histamine Parietal cells to secrete HCl 18-79

Gastric Phase The presence of proteins, polypeptides and amino acids in the stomach raises the pH. This change in chemical nature, along with stomach distension, activates the gastric (stomach) phase. 18-81

Gastric Phase Proteins, polypeptides and amino acids present in the stomach stimulate G cells to secrete gastrin and chief cells to secrete pepsinogen Gastrin then stimulates ECL cells to secrete histamine which stimulates parietal cell secretion of HCl This is a positive feedback mechanism: as more HCl and pepsinogen are secreted, more polypeptides and amino acids are liberated, and more digestive processes are stimulated. 18-81

Gastric Phase As polypeptides leave the stomach and move into the duodenum, the pH begins to drop again and the gastric phase slows. 18-81

Summary of the Interactions Among Gastric Gland Cells and Secretions 1. Presence of proteins in stomach, an increased pH, and stomach distension (ie: stretch) all stimulate G cells to secrete gastrin and chief cells to secrete pepsinogen. 2. Gastrin stimulates ECL cells to secrete histamine. 3. Histamine stimulates parietal cells to secrete HCL. 4. HCl denatures proteins and activates pepsin from pepsinogen. 5. Pepsin digests proteins into polypeptides. 6. Polypeptides also stimulate G cells to secrete gastrin (a positive feedback effect).

Intestinal Phase Begins when chyme enters the small intestine Arrival of chyme in duodenum causes a neural reflex that inhibits gastric motility and secretion Fat in chyme stimulates SI to secrete enterogasterones--hormones that inhibit gastric motility and secretion Include Somatostatin, Cholecystokinin, Secretin, and Gastric Inhibitory Peptide 18-83

Intestinal Phase Enterogasterones secreted by intestines whem chyme arrives: Somatostatin: inhibits stomach acid secretion. Cholecystokinin: stimulates secretion of pancreatic enzymes; stimulates contraction of gall bladder; brings about a feeling of fullness after eating; slows gastric motility and acid secretion. 18-83

Intestinal Phase Enterogasterones secreted by intestines whem chyme arrives: Secretin: stimulates secretion of pancreatic HCO3- and bile from the liver; inhibits acid production and gastric motility. Gastric Inhibitory Peptide: Inhibits gastric motility and secretion; stimulates secretion of insulin from pancreas. 18-83

Digestion and Absorption of Carbohydrates Most carbohydrates are ingested as starch-- structured of glucose Salivary amylase begins starch digestion in the mouth and continues for awhile in the stomach (until it is denatured by the low pH) 18-90

Digestion and Absorption of Carbohydrates Pancreatic amylase secreted into duodenum converts starch to oligosaccharides Oligosaccharides are hydrolyzed by small intestine brush border enzymes into monosaccharides Monosaccharides are absorbed directly into the bloodstream 18-90

Digestion and Absorption of Proteins Chemical digestion begins in stomach when pepsin digests proteins to form polypeptides In small intestine, endopeptidases (trypsin, chymotrypsin, elastase) from pancreas cleave peptide bonds from interior of polypeptides Also in small intestine exopeptidases (carboxypeptidase, aminopeptidase) cleave peptide bonds from ends of polypeptides. Carboxypeptidase is a pancreatic enzyme while aminopeptidase is a brush border enzyme. 18-91

Digestion and Absorption of Protein Protein digestion in small intestine results in free amino acids, dipeptides, and tripeptides Which are absorbed into small intestine cells where they are broken down into amino acids Which are then secreted directly into the bloodstream 18-92

Digestion and Absorption of Lipids Small amount of lipid chemical digestion begins in mouth via lingual lipase No chemical digestion in stomach (except in infants who can digest milk fats) Arrival of lipids in duodenum causes secretion of bile from gall bladder Fat is emulsified by bile salt micelles Form tiny droplets of fat Greatly increases surface area for digestion by pancreatic lipase 18-93

Digestion and Absorption of Lipids continued Pancreatic lipase then hydrolyzes exposed triglycerides to free fatty acids and monoglycerides which are then absorbed into epithelial cells 18-94

Digestion and Absorption of Lipids continued Products of fat digestion are dissolved in micelles which move to the brush border 18-95

Digestion and Absorption of Lipids continued Free fatty acids and monoglycerides leave micelles and are absorbed into epithelial cells Inside epithelial cells, they are resynthesized into triglycerides and phospholipids, and then packaged into protein transport structures termed chylomicrons. 18-96