Describe the structure of the GIT (in order, including all accessory organs)
- Mouth (tongue, salivary glands - parotid, sublingual, submandibular)
- oesophagus
- Stomach
- Small intestines (gall-bladder, liver, pancreas)
- Large intestine
- Rectum
- Anus
What is a sphincter?
- an opening/entryway that opens/closes a tube when required
Discuss the properties of the basic architectural components of the GIT (mucosa and submucosa)
Mucosa: - epithelial cells - lamina propria - muscularis mucosa Submucosa: - circular muscle - longitudinal muscle - nerve plexus: intrinsic (submucosal, myenteric) and extrinsic (parasympathetic - vagus and pelvic nerves - and sympathetic - T8-L2)
Discuss the borders of the fore-, mid- and hind-gut
Foregut: oral cavity to mid-duodenum
Mid-gut: duodenum to initial 2/3rds of transverse colon
Hind-gut: later 1/3rd transverse colon to the upper portion of anus
What is the function of the oral cavity in the GIT?
Physical digestion
Chemical digestion:
- saliva (mucin, buffers (carbonic acid, antibacterial)
- amylase (action: hydrolyses starch and glycogen to smaller poly-saccharides and maltose)
Tongue:
- taste buds and propulsion of food bolus
How does the bolus move through the oesophagus into the stomach?
Peristalsis - waves of muscular contraction
Discuss the functions of the stomach
- gastric juice and smooth muscles help digestion further (gastric juice = acid (pH 2) and pepsin)
- pepsin - hydrolyses proteins
- mucus
(regions of stomach, fundus, corpus, antrum-pylorus)
Discuss the functions of the liver
- bile production
- storage
- nutrient interconversion
- detoxification and first-pass
- phagocytosis
- synthesis
What is the function of the gall-bladder (biliary system)?
- concentrates bile
- secretory/resorptive function (particularly large ducts)
- highly resistant to concentrated bile acids
What causes the urge to defecate?
When the descending colon becomes full and stool passes into the rectum
What glands produce what secretions?
Parotid: (25% - serous, amylase)
Submandibular: (70% - mixed, lysozyme and lactoperoxidase)
Sublingual: (5% - mucus, lingual lipase)
What are the functions of the following enzymes: amylase, lipase, lysozyme, lactoperoxidase?
Amylase: poly-> di and trisaccharide
Lipase: TG -> glycerides and FA
Lysozyme: antibacterial
Lactoperoxidase: bactericidal
Which cranial nerves and parts of the brain are involved in taste?
- CNVII, CNIX, CNX
- thalamus
- medulla
- gustatory center in cerebral cortex
What are the functions of the following gastric mucosa cell types: parietal, chief, APUD, G-cells, Mucus secreting cells, ECL?
Parietal - HCl and intrinsic factors
Chief - pepsin
APUD - somatostatin (inhibits acid secretion)
G-cells - gastrin (stimulates acid secretion)
Mucus-secreting cells - alkaline mucus
ECL - histamine to activate parietal cells
How is gastric secretion controlled?
Primary activators: - 10th nerve, gastrin and histamine Primary inhibitors: - secretin and somatostatin Regulation: - cephalic - gastric - intestinal
Discuss gastric acid secretion
- uptake of CO2, Cl-, Na+ and water
- active secrete HCl into lumen of stomach (ACH, Gastrin, Histamine)
- carbonic acid into blood
What are the three components of the small intestine, and what are the primary purposes?
Duodenum - chyme mixes with bile and pancreatic juice
Jejunum - absorption (amino acids, lipids, CHOs, Fe and Ca2+)
Ileum - B12 and bile salts absorption
What are the different duodenal mucosal cells and their functions?
Columnar - absorption
Goblet - mucus
Brunner’s glands - alkaline, mucus
What are the different routes of cellular absorption?
- ATP energy source
- transcellular (in cell)
- paracellular (between cells)
- water absorption against an osmotic gradient
What are some similarities between bile and pancreatic secretion?
- simulation via CCK (cholecystokinin - a hormone)
- 10th nerve stimulation
Describe the process of carbohydrate absorption
Dietary polysaccharide conversion
- Disaccharides in brush border
- Na+ and energy-dependent secondary active transport
- Monosaccharide fructose enters cell by passive facilitated diffusion
- Glucose, galactose and fructose exit the cell at the basal membrane by passive facilitated diffusion
- Simple diffusion moves monosaccharides
Describe the process of protein absorption
Na+ and energy-dependent secondary active transport via a symporter
- Na+ and amino acids enter cell through lumen
- Hydrogen and small peptides also brought into cell
- Intracellular peptidases converts small peptides into amino acids
- Amino acids exit the cell at the basal membrane via various passive carriers
- Amino acids enter the blood via simple diffusion
Describe the process of fat absorption
- Bile salts undergo lipid emulsion, become pancreatic lipase
- Pancreatic lipase breaks down into monoglycerides and free fatty acids
- Become bile salts again - micellar diffusion?
- Fatty acids and monoglycerides are then passively absorbed through the microvillus and into the cell
- Monoglycerides + fatty acids = triglycerides
- Triglycerides are aggregated and coated with lipoprotein - become chylomicrons
- Then exit basal membrane by exocytosis
Describe the process of iron absorption
(Absorbable iron = heme and ferrous)
(Iron in feces = iron not absorbed by cells, iron lost as cell is sloughed)
1. Energy dependent transport of heme and ferrous iron into cell
2. Heme carrier protein 1 and divalent metal transported 1 -> iron absorbed into blood
3. Ferroportin exits basal membrane
4. Iron absorbed into blood, bound to transferrin
5. Ferritin - pool of iron not absorbed into blood
6. Iron lost as cell is sloughed
7. Dietary iron
etc. etc.
