Human Anatomy and Physiology Secretory functions of the alimentary tract

Secretion from tract

Phases of digestion Cephalic: before food enters the stomach Gastric: events in the stomach Intestinal: events in the intestine

Saliva Parotid, submandibular, sublingual Secretions initiated in cephalic phase by parasympathetic influence Composition 99.5% water, 0.5% protein and electrolytes Protein: amylase, mucus, lysozyme Functions Water: softens food Amylase: polysaccharide breakdown Mucus: lubrication Lysozyme: kills bacteria

Esophageal secretion Mucus Provides lubrication for swallowing Transit time is about 10 seconds

Gastric secretions 1. HCl By parietal cells A. Cephalic phase HCl secreted B. Gastric phase Breakdown proteins to peptides More distention = more HCl release Protein + HCl = gastrin release Gastrin increase HCl release Beneficial positive feedback

Gastric secretions 2. Pepsinogen - gastric phase Stored in chief cells as zymogen granules

Gastric secretions C. Intestinal phase Inhibits gastric secretions - 3 mechanisms Decrease parasympathetic stimulation Local reflex action Release of secretin, GIP and cholecystokinin

Gastric secretions 3. Mucus Goblets cells Functions Prevents mechanical injury Self digestion against pepsin Neutralizes HCl

Inhibitors of gastric motility 1. Gastric inhibitory peptide (GIP) Released from duodenal mucosa Stimulus is fat and sugars in duodenum Decreases gastric motility and secretions

Inhibitors of gastric motility 2. Secretin Released from duodenal mucosa Stimulus is high acidity in duodenum Decreases gastric motility Increases pancreas secretions

Inhibitors of gastric motility 3. Cholecystokinin Released from duodenal mucosa Stimulus is presence of lipid and protein Decreases gastric motility Increases pancreas secretions

Pancreas Location and anatomy (acinar and duct cells) Duct cells Acinar cells

Acinar cell secretions Proteolytic enzymes (trypsin, chymotrypsin carboxypeptidase) Peptides Lipase Fatty acids and monoglycerides Amylase Disaccharides: Lactose and maltose

Duct epithelial cell secretion Bicarbonate ions Function: neutralizes stomach acidity Duct cells Acinar cells

Hormones effecting pancreatic secretion 1. Secretin Secreted by duodenal mucosa In response to high [H + ] Increases bicarbonate secretion 2. Cholecystokinin Secreted by duodenal mucosa In response to fat and protein Increases total amount of enzymes secreted 3. Acetylcholine Released from parasympathetic influence

Liver Bile Bile salts, bicarbonate, lecithin, water Functions Bile salt and water: emulsifies lipid Lecithin: prevent reaggregration of emulsification Bicarbonate: neutralizes gastric HCl

Hormone effecting bile release Cholecystokinin Released into blood by presence of fats (and protein) in duodenum Causes gall bladder release of bile Causes Sphincter of Oddi to relax

Small intestine secretions Crypts of Lieberkühn Mucus (Brunner’s glands) and aqueous salt Functions Lubrication and protection from stomach acid Aid in enzyme breakdown of chyme

Large intestine secretions Crypts of Lieberkühn without villi secretes mucus Functions Protection against abrasion Holds feces together Protection against bacterial activity