GI Physiology - Stomach & Intestines

Question 1

What is the function of the stomach?

Answer 1

Storage of food
Up to 4L, mediated by NO & VIP
Lower oesophageal sphincter closes around 20mmHg above intragastric pressure
Regulation of food passage into duodenum via pyloric sphincter
Digestion
Mechanical (syncytium formed via gap junctions, contraction intensity determined by the frequency of spike potentials. Increased by parasympathetic stimulation
Chemical (enzymes requiring low pH to function)
Secretion of hormones
Intrinsic factor

Question 2

What factors affect lower oesophageal sphincter tone?

Answer 2

Decrease tone
Atropine & glycopyrrolate
Swallowing
Dopamine
Thiopentone
Alcohol
Oestrogen & progesterone
Hiatus hernia (loss of acute angle)

Increase tone
Acetylcholine
Gastrin
Metoclopramide
Cylcizine
Suxamethonium

Question 3

What glands can be found in the stomach?

Answer 3

Oxyntic glands
Fundus & body of the stomach
Chief cells (Pepsinogen
Mucous cells
Parietal cells (HCl and Intrinsic factor)

Pyloric glands
Antrum
Mucous cells
G-cells (Gastrin

Question 4

Explain the secretory functions of the stomach

Answer 4

Stomach acid
Immune function (Destroys pathogens)
Activates enzymes (Such as pepsin)
Improves ion solubility (such as calcium/iron)

Gastrin
Hormone produced by G cells, stimulates release of mucus, enzymes, increases motility
Produced in response to vagal input, protein in stomach, and gastric stimulation
Inhibited by acid & secretin

Intrinsic factor
Glycoprotein that binds & protects vitamin B12, allowing it to reach terminal ileum where it is absorbed
Produced by Parietal cells, deficiency causes megaloblastic anaemia

Therefore gastrectomy can result in iron/B12 deficiency and associated anaemia

Other hormones include somatostatin (D cells), HCl (Parietal cells( and pepsinogen (Chief cells)

Question 5

How is gastric secretion regulated?

Answer 5

Phases of secretion
Cephalic phase
Sight & smell of food triggers vagus nerve and increases gastric secretion
Gastic phase
Food enters stomach, stimulates vagus nerve & local reflexes (particularly gastrin)
Intestinal phase
As gastric content (particularly protein) enters duodenum, further stimulates gastric secretion

Factors that increase gastric secretion
ACh (Vagus)
Histamine
Gastrin
Amino acids
Caffeine
Alcohol

Factors that decrease gastric secretion
Prostaglandins
Enterogastric reflex
Secretin (released in response to stomach distension, increase in pH of gastric contents, protein, fat)

Question 6

Describe the structure, function, and innervation of the GI tract

Answer 6

5 layers
Mucosa, submucosa, circular muscle, longitudinal muscle, outer serosa layer

Two muscle movements
Peristalsis
Smooth muscle fibres joined longitudinally allow action potentials to travel along tract
Stimulated by chyme entering duodenum, CCK and gastrin
Inhibited by glucagon & secretin
Prolonged fasting triggers migrating motor complex to clear excess secretions

Mixing
Muscularis externa of the stomach contains three layers of muscle, outer longitudinal, middle circular, and inner oblique, work together to churn stomach contents

Controlled by enteric nervous system -
two plexuses integrated into the autonomic nervous system - Auberach’s myenteric plexus and Meissner’s submucosal plexus

Also influenced by the ANS itself
Parasympathetic
Vagal input to proximal tract
Sacral parasympathetic to distal

Sympathetic
T5-L1 - reduced motility, increased sphincter tone

Question 7

What factors affect gastric emptying time, and how can it be measured?

Answer 7

Delay
Atropine & Glycopyrrolate
High fat & protein content entering duodenum
Distension of duodenum
Stress, pain, trauma
Opioids
Alcohol

Accelerate
Gastric distension
Acetylcholine & anticholinergics
Prokinetics

Measured though ultrasound, applied potential tomography, or systemic absorption of paracetamol