| 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, |
Saliva
- 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
Swallowing.
- 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.
Gallstones.
- Cholesterol stones
- Black pigment stones
- Mixed stones
Viva questions:
- Tell me about gastric acid secretion.
- Tell me about the control of gastric secretion.
- What are the functions of the stomach ?
- What are the functions of the liver ?
- Tell me about bilirubin metabolism.
- Tell me about the function and composition of bile