Week 21 Physiology

G-I hormones Site of production Actions Control of release
Gastrin Gastric antrum, duodenum and jejunum to lesser extent (G cells) + gastric acid & pepsin secretion

+ growth of the mucosa of

the stomach and small and large intestines

+ glucagon secretion

+ gastric motility

Stimuli – luminal distension, peptides, amino acids, increased vagal tone, blood borne Ca or adrenaline


Inhibitors – luminal acid or somatostatin, blood borne secretin, GIP, VIP, glucagon, calcitonin

CCK Duodenum and Jejunum, ileum to lesser extent (I cells) + pancreatic enzyme secretion

+ Gall bladder contraction

Relaxation of sphincter of Oddi

Augments action of secretin

+ glucagon secretion

Stimulated by luminal peptides and amino acids, fatty acids–> positive feed back but this is terminated when the products move down the GI tract.
Secretin Duodenum and Jejunum, ileum to lesser extent (S cells) + bicarbonate secretion by pancreas

Augments CCK

Inhibits gastric acid secretion

? Cause contraction of pyloric sphincter

Stimulated by protein products and acid
VIP Nerves of GIT + intestinal secretion of electrolytes & water.

Relaxation of intestinal

smooth muscle, including sphincters; dilation of peripheral

blood vessels; and inhibition of gastric acid secretion.

GIP(gastric inhibitory peptide) Duodenum and Jejunum, (K cells) +Insulin secretion

Inhibit gastric acid secretion and motility

Stim by luminal glucose and fat
Somatostatin Hypothalamus, D cells in pancreatic islets Inhibit GH

Inhibit secretion of gastrin, secretin, VIP, GIP

Inhibit pancreatic exocrine secretion

Inhibit gall bladder contraction

Inhibit gastric acid secretion

Inhibit absorption of glucose, AA and TGs.

Stim by acidic contents in lumen,


  • Amylase – initiates starch digestion
  • Lingual lipase – start fat digestion
  • Mucus – lubricant
  • IgA
  • Proteins and enzymes eg lysozyme
  • Electrolyte solution to moisten food – Hypotonic and alkaline
  • NaCl extracted and K and bicarb added to saliva
  • 1000-1500mL of saliva per day
  • Control of secretion almost all neural – parasympathetic stimulation of the glands (otic and submandibular ganglions)
  • Other stimuli – chewing, thinking about/smelling food, nausea
  • Inhibited by fear and during sleep


  • Food made into bolus and pushed to back of mouth actively
  • Swallowing is an unconscious reflex


Regulation of gastric acid secretion and motility

  • Gastric acid secretion
    • Promoter – gastrin, Ach, Histamine
    • Inhibitor – secretin, VIP, GIP, somatostatin
  • Gastric motility
    • Promoter – gastrin
    • Inhibitor – VIP, GIP


Functions of the stomach.

  • Gastric acid production
  • Control of secretion and motility
  • Mixing of food
  • Digestion of starches
  • Absorption of water, nutrients, B12
  • EtOH break down


Exocrine pancreas.

  • Retroperitoneal organ
  • Secretes 2-2.5L per day
  • Proenzymes
    • Tryprsinogen
    • Chymotrypsinogen
    • Procarboxypeptidase
    • Proelastase
    • Kallikreinogen
    • Phospholipase A
    • Conversion of proenzymes requires conversion of trypsinogen to trypsin by duodenal enteropeptidase.
  • Enzymes released in active form
    • Lipase
    • amylase


Functions of the liver.

  • Nutrient uptake from portal circulation –> glycogen synthesis, protein synthesis, fat synthesis, cholesterol synthesis
  • Gluconeogenesis, glycogenolysis, lipolysis in starvation
  • Metabolism – phase 1 and phase 2 to modify toxins, proteins, drugs
  • Synthesis of coagulation factors
  • Synthesis of bile
  • Synthesis of IGF-1
  • Synthesis angiotensinogen
  • Fetal RBC production
  • Break down of insulin, hormones, ammonia to urea.
  • Storage of ADEK vitamins, copper, iron,  blood reservoir
  • Immune role – Kuppfer cells


Composition of bile

  • Water 97%
  • Bile acid 0.7%
  • Pigment 0.2%
  • Fatty acid 0.15%
  • Cholesterol 0.06%
  • Phosphatidylcholine 0.2%
  • Alkaline electrolyte solution


Bilirubin metabolism / excretion

  • RBC break down –> haem–> unconjugated bilirubin –> bound to albumin in plasma –> hepatic uptake –> glucuronidation/conjugation –> water soluble conjugated bilirubin –> excreted in bile –> colon and out or metabolisedby gut bacteria –> urobilinogen–> reabsorbed in bowel –> excreted renally.



  • Cholesterol stones
  • Black pigment stones
  • Mixed stones


Viva questions:

  1. Tell me about gastric acid secretion.
  2. Tell me about the control of gastric secretion.
  3. What are the functions of the stomach ?
  4. What are the functions of the liver ?
  5. Tell me about bilirubin metabolism.
  6. Tell me about the function and composition of bile