Chapter 18 The Digestive System 18-1

18-4

Functions of GI Tract 18-5

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, CCK, GIP, GLP-1, guanylin, VIP, and somatostatin 18-7

Absorption Is passage of digested end products into blood or lymph 18-8

Digestive System continued Organs include oral cavity, pharynx, esophagus, stomach, and small and large intestine Accessory organs include teeth, tongue, salivary glands, liver, gallbladder, and pancreas 18-12

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 movements--the enteric nervous system GI motility is influenced by paracrine and hormonal signals 18-18

Stomach Is most distensible part of GI tract Empties into the duodenum Functions in: storage of food; initial digestion of proteins; killing bacteria with high acidity; moving soupy food mixture (chyme) into intestine 18-25

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

HCl in Stomach continued Is secreted in response to the hormone gastrin; and ACh from vagus These are indirect effects since both stimulate release of histamine which causes parietal cells to secrete HCl 18-32

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

Protection of Stomach Against HCL and Pepsin Both HCL and pepsin can damage lining and produce a peptic ulcer 1st line of defense is the adherent layer of mucus = a stable gel of mucus coating the gastric epithelium Contains bicarbonate for neutralizing HCL Is a barrier to actions of pepsin Gastric epithelial cells contain tight junctions to prevent HCL and pepsin from penetrating the surface Gastric epithelial cells are replaced every 3 days 18-34

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

Gastric and Peptic Ulcers Peptic ulcers are erosions of mucous membranes of stomach or duodenum caused by action of HCl In Zollinger-Ellison syndrome, duodenal ulcers result from excessive gastric acid in response to high levels of gastrin Helicobacter pylori infection is associated with ulcers Antibiotics are useful in treating ulcers And also proton pump inhibitors such as Prilosec Acute gastritis is an inflammation that results in acid damage due to histamine released by inflammation Is why histamine receptor blockers such as Tagamet and Zantac can treat gastritis 18-36

Small Intestine (SI) continued Absorption of digested food occurs in SI Facilitated by long length and tremendous surface area 18-39

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

Small Intestine (SI) continued Each villus is covered with columnar epithelial cells interspersed with goblet cells Epithelial cells at tips of villi are exfoliated and replaced by mitosis in crypts of Lieberkuhn Inside each villus are lymphocytes, capillaries, and central lacteal 18-41

Intestinal Enzymes Attached to microvilli are brush border enzymes that are not secreted into lumen Enzyme active sites are exposed to chyme 18-43

Large Intestine (LI) or Colon Has no digestive function but absorbs H2O, electrolytes, B and K vitamins, and folic acid Internal surface has no villi or crypts and is not very elaborate Contains large population of microflora = 400 different species of commensal bacteria which produce folic acid and vitamin K and ferment indigestible food to produce fatty acids And reduce ability of pathogenic bacteria to infect LI antibiotics can negatively affect commensals 18-49

Fluid and Electrolyte Absorption in LI SI absorbs most water but LI 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 LI can also secrete H2O via AT of NaCl into intestinal lumen 18-51

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

Secretion of Glucose, Triglycerides and Ketones Liver helps regulate blood glucose by removing it from blood or releasing it to blood Removes it via glycogenesis and lipogenesis Or produces it via glycogenolysis and gluconeogenesis Can convert free fatty acids into ketone bodies (ketogenesis) that can be used for energy during fasting 18-66

Neural and Endocrine Regulation Neural and endocrine mechanisms modify activity of GI system Vagus nerve is heavily involved in regulating and coordinating digestive activities GI tract is both an endocrine gland and target for action of hormones Hormones include secretin, gastrin, CCK, and GIP 18-77

Enteric Nervous System Submucosal and myenteric plexuses contain as many neurons as spinal cord Includes preganglionic Parasymp axons, ganglion cell bodies, postganglionic Symp axons; and afferent intrinsic and extrinsic sensory neurons; interneurons, and glia Peristalsis is controlled by enteric NS 18-84

Digestion and Absorption of Protein Begins in stomach when pepsin digests proteins to form polypeptides In SI, endopeptidases (trypsin, chymotrypsin, elastase) cleave peptide bonds in interior of polypeptides SI exopeptidases (carboxypeptidase, aminopeptidase) cleave peptide bonds from ends of polypeptides 18-91

Digestion and Absorption of Lipids Occurs in SI Arrival of lipids in duodenum causes secretion of bile Fat is emulsified by bile salt micelles Forms tiny droplets of fat dissolved in bile salt micelles Greatly increases surface area for fat digestion 18-93

Digestion and Absorption of Lipids continued Pancreatic lipase hydrolyzes triglycerides to free fatty acids and monglycerides Phospholipase A breaks down phospholipids into fatty acids and lysolecithin 18-94

Transport of Lipids continued Cholesterol and triglycerides from liver form VLDLs which are secreted and take triglycerides to cells Once triglycerides are removed, VLDLs become LDLs LDLs transport cholesterol to organs and blood vessels HDLs transport excess cholesterol back to liver High ratio of HDL-cholesterol to total cholesterol is believed to confer protection against atherosclerosis 18-99