Human Anatomy and Physiology

Secretory functions of the alimentary tract

Secretion from tract

Daily volume (liters)

pH

Saliva 1.0 6 – 7

Gastric secretion 1.5 1.0 – 3.5

Pancreatic secretion 1.0 8.0 – 8.3

Bile 1.0 7.8

Small intestine secretion 2.0 7.5 – 8.0

Large intestine secretion 0.2 7.5 – 8.0

TOTAL 6.7

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

pepsinogen pepsin

HCl HCO3

affectnullified

stomach small intestine

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)

Ductcells

Acinarcells

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

Ductcells

Acinarcells

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