Human Anatomy and Physiology

Secretory functions of the alimentary tract

Secretion from tract

Saliva Gastric secretion Pancreatic secretion Bile Small i ntestine secretion Large intestine secretion

TOTAL

Daily volume (liters) 1.0 1.5 1.0 1.0 2.0 0.2

6.7

pH 6 – 7 1.0 – 3.5 8.0 – 8.3 7.8 7.5 – 8.0 7.5 – 8.0

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 stomach HCl pepsin small intestine affect nullified HCO 3

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