GI System

• Mouth
• Oropharynx
• Laryngopharynx
• Oesophagus
• Stomach
• Small Intestine
• Large intestine
• Rectum
• Anal canal

Food is broken down and mixed with fluids to become chyme

• Duodenum 25cm
• Jejunum 2.5m
• Ileum 3.5m

An organ associated with the GI tract but not part of the tube itself.

e.g. pancreas, liver, salivary glands, gallbladder

Synthesise and deliver material to the lumen of the GI tract via a duct system

Parotid glands secrete watery serous fluid and are located just below the ear

Submandibular glands secrete mucous fluid as well as serous fluid and are located below the mandible (jaw bone)

Sublingual glands secrete mucous fluid and are located below the tongue

The fauces

Oral cavity

Stratified squamous epithelium. Multiple layers prevent damage from food

Gastroesophageal junction

• Epithelium lines the lumen
• Lamina propria supports the epithelium
• Muscularis mucosa underlies the lamina propria
• * Epithelium and lamina propria are folded to allow the tract to distend *

• Lies beneath the mucosa
• Connective tissue
• Contains glands and ducts as well as larger blood vessels, nerves and lymphatics

• Two layers of smooth muscle.
• - Inner cicular layer. Contraction leads to deacrease in diameter of the lumen
• - Outer longitudinal layer. Contraction leads to a decrease in length.

• Surrounds the outside of the GI tract
• Fibrous connective tissue
• Adventitia + peritoneum = serosa

• Two layers of serous membrane
• - Outer parietal layer lines the abdominal wall
• - Inner visceral layer wraps around most organs
• Cavity = peritoneal - contains serous fluid

• Double layer of visceral peritoneum.
• Attaches organs loosely to the posterior abdominal wall

• Lesser omentum: two layers of peritoneum
• Greater omentum: four layers of peritoneum - protective layer, prevent spread of infection

• Temporary storage of food and fluids
• Mechanical digestionL requires muscle
• Chemical digestion (enzymes and chemicals are secreted from stomach epithelium)
• Mixing of food: requires muscle

• - Made up of the fundus, body and pylorus
• - Increases in size to accomodate large volume (1-1.5L)
• - Three layers of muscle
• - Right border = lesser curvature
• - Left border = greater curvature
• - Folded surface (rugae) increase surface area

Inferior to diaphragm and liver

• Serosa: supportive CT lined by SSE
• Muscularis: outer longitudinal, middle circular, inner oblique
• Submucosa: CT contains blood vessels and nerves
• Muscosa: muscalaris mucosae next to the submucosa, gastric glands open into the lumen via gastric pits, on top of the muscalaris mucosae is the lamina propria (carries blood vessels), then a final layer of simple columnar epithelial cells.

Jejunum

Simple columnar epithelial cells, specialised for absorption

Ileocaecal valve. It is loose and allows free passage of material into the LI without preventing backflow into the SI

• Ascending: retroperitoneal
• Transverse: intraperitoneal
• Descending: retroperitoneal

• Outer longitudinal layer is divided into 3 bands: taeniae coli
• Inner circular layer forms semilunar folds that divide the large intestine into pouches called haustra

• Mucosa: contains mucous secreting glands. Lamina propria of the mucosa contains collagen and immune cells
• Muscularis layer is discontinuous
• Peritoneum forms fat-filled pouches called apendices epiploicae

• Internal: thickened smooth muscle, relaxes involuntarily
• External: thickened skeletal muscle, relaxes voluntarily

• Upper portion is lined with columnar epithelium
• Lower portion is lined with stratified squamous epithelium

• Half of the bile flows into the duodenum via the common bile duct
• The other half is diverted to the gallbladder via the cystic duct for storage (which then flows through the common bile duct when required)

Ampulla of Vater at the greater duodenal papilla

Muscular valve which controls the flow of secretions from the liver, gallbladder and pancreas from the ampulla into the duodenum

It drains into the portal venous system which carries it to the liver sinusoids for processing and detoxification.

Propelling of food along the GI tract. Occurs due to sequential contraction of adjacent segments of smooth muscle

Occurs only in the intestines. Involves simultaneous contraction of certain regions of smooth muscle which results in mixing of food and exposure to absorptive surfaces

• Spontaneous variations in membrane potential.
• When the slow wave reaches threshold an AP will fire and muscle contraction will result.
• The frequency varies in different parts of the GI tract...
• - Stomach contracts 3 times/min
• - Duodenum contracts 12 times/min
• - Ileum contracts 9 times/min

• Occurs 4-5 hours after a meal and is repeated every 2 hours after that until the next meal
• - Waves of motility from stomach to SI clear the GI tract.
• 3 distinct phases
• - Inactivity ~hour
• - Intermittent activity ~50mins
• - Intense activity 5-10mins

Every 30 minutes. Only a small amount of extraction of electrolytes and water occurs in the large intestine.

• In response to stomach distension, partially digested proteins, vagal stimulation
• Actions: increase HCl and pepsin secretion, increase stomach motility

• In response to the presence of fatty acids and glucose
• Actions: Stimulate insulin secretion, affect fatty acid metabolism

• In response to presence of acid and partially digestion proteins and fatty acids
• Actions: Inhibit acid secretion, increase HCO3- secretion, bile ejection from gallbladder