Physiology + Anatomy of GI Tract Flashcards
The role of the GI Tract is absoorbing nutrients + excretion of waste products.
What does this require?
breakdown of large molecules
secretion of enzymes
transport mechanisms
secretion of compounds to aid absorption
How is the motility of the GI tract controlled?
Cyclical contraction and relaxation of sheets of smooth muscle that line tubular tissue (peristalsis)
- lumen cavity constricts + dilates alternately
- synchronised and wavelike
- unidirectional movement of lumen contents
Nerve plexus (network of nerves) between 2 layers of muscle
- e.g. myenteric plexus
- nerve network
- co-ordinates contraction-relaxation cycle at local level
What is the Interstitial cells of Cajal?
Specialised pacemaker cells
Create electrical potential which helps coordinate contraction of GI smooth muscle
Integrate signals from enteric nerves to smooth muscle
What receptors are involved in Motility regulation
Controlled by ENS
via local reflexes triggered by local stimuli involving ACh, NA, 5-HT, NO, + ATP ENS is controlled by parasympathetic + sympathetic NS
ANS: acts on enteric nerves and direct on smooth muscle, opening Ca2+ ion channels
PNS: release of ACh, acts on muscarinic receptors, stimulates contraction
What muscarinic receptors are involved with GI motility?
Mostly M3 - salivary glands, smooth muscle (GI airways)
M2 - heart-rate of contraction, GI smooth muscle contraction, CNS
GI motility can be controlled therapeutically.
This is by stimulation or antagonism of the _____ receptors + _____ receptors
Stimulation/antagonism of muscarinic receptors (e.g. Buscopan-IBS)
Stimulation or antagonism of pre-synaptic receptors
- D2 receptor antagonists (bloating)
- Opioid receptor agonists (diarrhoea)
- 5-HT receptor agonists (constipation)
What is the function of the stomach?
Storage of food Initiation of digestion of proteins + CHO
Kill digested bactria (via acid)
Formation of Chyme, before transfer to small intestines (Stomach absorbs alcohol, not food)
What is the function of the small intestine?
Mechanical + chemical digestion of nutrients
Absorption of nutrients
What receives chyme from stomach?
Duodenum
Ducts empty into duodenum to deliver enzymes from acinar cells in pancreas
—lipase —amylase —proteases —nucleases —HCO3
Where does nutrient absorption take place?
Duodenum + Jejunum
At the terminal ileum, vitamin 12 + bile is absorbed
Inner surface of SI is folded - from folds to villi to microvilli
Discuss the structure of villi
Villi projecting from the folds increase SA another 10x
Surface of each villi lined with epithelial cells interspersed with mucous cells
Microvilli project from villi (brush border)
Tight junctions between cells limits transport of contents between cells
What are Peyer’s patches?
Lymphoid nodules in ileum
Colletions of sub-epithelial lymphoid follicles
Sample contents of lumen
Act as immune sensors - pathway important in oral vaccination
How are fats absorbed?
When fats reach the SI, they meet bile.
Bile
- made from cholesterol and in the liver
- stored in gallbladder
- secreted into duodenum
- Bile emulsifies fats and helps forms micelles to aid fat absorption
- enterophatic recycling = reuptake of bile back into liver
How is bile formed?
1-2L of bile produced daily Required for Fat uptake
Contains bilirubin, bile salts, phospholipids + cholesterol
how are the following vitamins + minerals absorbed?
Fat-soluble vitamins
Vitamin C
Vitamin B1
Vitamin B6
Vitamin B12
Folic acid
Fat soluble vitamines = micelles
Vit C = Na+ dependent active transport
Vit B1 = active transport in jejunum
Vit B6 = simple diffusion
Vit B12 = binds to intrinsic factor released by parietal cells in stomach
Folic acid = facilitated transport
