GI Tract Flashcards
Secretion of: Mucous neck cell
mucus (protects lining) & bicarbonate
Secretion of: Parietal cells
Gastric acid (HCl)
Secretion of: Chief cell
Pepsinogen (converted into Pepsin by HCL) & gastric lipase
Secretion of: D Cells
Somatostatin (inhibits HCL production)
Secretion of: G Cells
Gastrin (stimulates acid production)
What do the Crypts of Lieberkühn contain?
Paneth cells: defensins, antimicrobial, lysozyme, phagocytic.
Stem cells: Regeneration.
Goblet cells: Mucous.
Functions of: Salivary Glands
-Mucus acts as lubricant
-Produces thiocyanate & lysosomes for anti-microbial defences
-Digestion (amylase & lipase)
Function of Pancreas:
Makes enzymes to digest proteins, fats, & carbohydrates.
Produces the hormones e.g. insulin & glucagon.
And produces HCO3- which neutralises the acid from the stomach.
Swallowing Reflex
Stimulated when a food bolus stimulates pressure receptors in the back of the throat & pharynx, signalling the swallowing centre in the medulla, triggering nerve impulses, innervating skeletal muscles, causing involuntary contraction pushing food into the oesophagus.
Gastric Motility in the Stomach
Mixing wave initiated towards the pyloric sphincter. The more fluid chyme is pushed towards the pyloric sphincter, with the more solid region being pushed back by peristaltic waves, which then reverse to mix more thoroughly. The movement of liquid & solid is repeated, with the most liquid chyme passing through the pyloric sphincter, as mixing continues.
Defaecation
Faeces stretches rectum & stimulates stretch receptors, signalling the spinal chord, which causes a reflex in the rectum, relaxing the internal anal sphincter. Impulses prevent from untimely defecation, by keeping the external anal sphincter contracted, defecation occurs when this sphincter relaxes.
Vomitting
Heart rate & salvation increases, food enters the stomach via reverse peristalsis, & the pyloric sphincter relaxes to allow food to fill the stomach. Involuntary retching occurs, alongside muscle contraction in abdomen & diaphragm forcing the contents into the oesophagus, as the epiglottis closes, & vomiting occurs. Epiglottis remains closed, for a period after.
Fat digestion
Fatty acids pass through epithelial cell layer, where they’re converted to triglycerides. These combine with cholesterol, phospholipids & protein to form chylomicron, which is water soluble & enters lacteal, then to the bloodstream.
Hepatic First Pass Effect
During first passage through liver, a fraction of amount of drug absorbed is transformed into metabolite(s), which can reduce bioavailability if the metabolites are inactive.
Efflux Pumps
Cellular proteins which can prevent intracellular accumulation of drugs by pumping drugs that enters cell right back out. An example is P-glycoprotein, which requires an inhibitor for some drugs to be absorbed.
Stomach - Degradation & Metabolism
Hydrochloric acid – degrades some drugs, so gastro-resistant coatings are used to protect the drug & delay release. Pepsin – a powerful digestive protease. Destroys polypeptide drugs, but small peptides survive.
Duodenum - Degradation & Metabolism
Enzymes which degrade some large proteins to small peptides.
Small intestine - Degradation & Metabolism
High expression of cytochrome P450 enzymes (& others), in the intestinal epithelial cells. Highest [in] the villus tips of the duodenum& jejunum of the intestine.
Colon - Degradation & Metabolism
Metabolic activity gradually decreases from the duodenum to the colon. Absorption remains good, however, potential to bypass metabolic degradation & target colonic delivery.
Gut flora can metabolize & inactivate drugs.
Gastric Emptying:
The stomach gradually releases its contents into the small intestine. Numerous factors affect gastric emptying and subsequent drug absorption:
- Meal size: bigger meal, faster initial emptying
- Type of meal: Fatty acids (chain length dependent), carbs & proteins ([dependent]) reduce emptying rate
- Physical state: smaller empty faster
- Chemicals: acids & [high] alkalis reduce emptying rate ([dependent]) while [low] alkalis increase emptying rate
- Drugs: some reduce emptying rate e.g. narcotics
- Body position: lying on left side reduces
- Disease: emptying rate reduced by ulcers & in some diabetics.
- Exercise: reduces emptying rate.
Gastric transit & drug absorption :
Passage of drug through pylorus is an important element with respect to rate of GI absorption
- Drug A : Drug is rapidly released from dosage form into gastric fluid. Drug can rapidly leave through the pylorus in a fasting subject.
- Drug B: Gastro-resistant dosage forms too large to pass the pylorus, must wait for «house-keeper» wave, causing a lag-time before the drug arrives at the intestines.
