Digestive PhysiologyDigestive Physiology

Physiological functions of the stomach

By: M.H.Dashti

Lecture 4

Anatomy of StomachAnatomy of Stomach

Size when empty?– large sausage– stretches due to rugae

Parts of stomach

– cardia

– fundus---air in x-ray

– body

– pylorus---starts to narrow as approaches pyloric sphincter

Physiological functions of the Stomach - secretions

Physiological functions of the Stomach - secretions By Oxyntic and Pyloric

glandular cells exocrine

Gastric juice-2-3 L/day– Water & electrolytes – enzymes

Mucus Endocrine: Gastrin Paracrine

– Histamine– Somatostatin

Gastric glandular cells & their secretionsGastric glandular cells & their secretions

surface mucous cells & mucous neck cells Alkaline Mucin

Parietal cell– Hydrochloric acid and

Intrinsic factor Chief ,peptic or

zymogene cell– Gastric lipase,

pepsinogen and rennin G cell

– Gastrin hormone

,rennin

Entrochromafin cells produce paracrine secretions of Histamine & D cells

Somatostatin

Mechanisms of acid secretion Mechanisms of acid secretion

ATPNa+

K+

control of acid secretioncontrol of acid secretion

M3

CckB

Histamine

Ach

Gastrin

Somatostatin

Prostaglandins

EGF

PGE2

cAMP-

IP3IP3

Ionic concentrations of gastric juice as a function of secretion rate

Ionic concentrations of gastric juice as a function of secretion rate .At low flow rates the

juice is hypotonic At high flow rates it

approaches isotonicity and contains predominantly H+ and Cl-

Gastric alkaline mucosal barrierGastric alkaline mucosal barrier

Factors affecting Gastric alkaline mucosal barrierFactors affecting Gastric alkaline mucosal barrier

stimulation– Prostaglandins– parasympathetic

inhibition– Sympathetic– Bile salts– Alcohol– Citric acid

pH~7 at Cell surface

Helicobacter pylori & GastritisHelicobacter pylori & Gastritis

cytokines

Control of the chief cellsControl of the chief cells Secretin , VIP &

β-adrenergic agonists act via CAMP

Ach Gastrin & cck act via IP3 & Ca2+

Mechanism for Potentiation effect of H+ is not known

Pepsinogen &Gastric

lipase

Gastric motility : Gastric motility :

Gentle mixing waves (type I)– every 20 seconds

(BER)– mixes bolus with

2 .5 L/day of gastric juice to turn it into chyme

More vigorous waves (type II) – travel from body of

stomach to pyloric region

Intense waves Intense waves (type III )(type III )

open theopen the pylorus and pylorus and squirt out 3ml of chyme squirt out 3ml of chyme

with each wavewith each wave type II & III are stimulated by Ach & type II & III are stimulated by Ach &

Gastrin and inhibited by NEGastrin and inhibited by NE

type Itype I

type IItype II typeIIItypeIII

Gastric motility: Mechanical DigestionGastric motility: Mechanical Digestion

Pyloric sphincter & Pyloric pump Pyloric sphincter & Pyloric pump

antrumduodenum

Myenteric plex

Sub mucosal plexus

Circular muscle layer Longitudinal muscle

layerPyloric sphincter

Control of Gastric functions Three PhasesControl of Gastric functions Three Phases

Cephalic Phase = “Stomach Getting Ready”

Cephalic Phase = “Stomach Getting Ready”

Sight, smell, taste & thought – stimulate

parasympathetic nervous system

– Vagus nerve– increases

stomach muscle and glandular activity

Receptive relaxation

Gastric Phase = “Stomach Working”

Gastric Phase = “Stomach Working” Nervous control via

stretch receptors & chemoreceptor keeps stomach active– vigorous peristalsis

and glandular secretions continue

– chyme is released into the duodenum

Endocrine gastrinEndocrine gastrin hormone increases hormone increases stomach churning and pyloric sphincter stomach churning and pyloric sphincter relaxationrelaxation

Intestinal Phase = “Stomach Emptying”

Intestinal Phase = “Stomach Emptying” Stretch receptors in

duodenum slow stomach activity & increase intestinal activity

Distension, fatty acids or sugar signals medulla – sympathetic nerves slow

stomach activity Hormonal influences (from

SI) – secretin hormone

decreases stomach secretions

– cholecystokinin(CCK) decreases stomach emptying

Takes 2-4 hrs to completely empty the stomach

Hormones ,short&

Inhibit gastric

function

Regulation of Gastric Emptying

Chemical Digestion in the StomachChemical Digestion in the Stomach

Protein digestion begins– HCl denatures (unfolds) protein molecules– HCl transforms pepsinogen into pepsin that

breaks peptides bonds between certain amino acids (aromatics)

– Rennin in infants Fat digestion continues

– gastric lipase splits the triglycerides in milk fat most effective at pH 5 to 6 (infant stomach)

Absorption of Nutrients by the StomachAbsorption of Nutrients by the Stomach

Water especially if it is cold Electrolytes Some drugs (especially aspirin) & alcohol