GI and Liver Flashcards Preview

- Store and mix food

- Dissolve + continue digestion

- Regulate emptying into duodenum

- Kill microbes

- Secrete intrinsic factor

- Activate proteases

- Lubrication

- Mucosal protection

- Produces chyme

This is an active process. H+ ions are pumped into the lumen of the stomach from parietal cells via a H+/K+ proton pump. It is important that H+ and K+ have the same charge.

- Cephalic phase

- Gastric phase

- Also protein in stomach. Proteins in the lumen cause pH to rise by mopping up H+ ions. This decreases somastatin secretion, which increases parietal cell activity (as somastatin inhibits parietal cells)

- Parasympathetic nervous system stimulated by sight, smell + taste of food 

- Acetylcholine released. This acts directly on parietal cells + triggers release of gastrin + histamine. The net effect is increased acid production

- Gastric distension, presence of amino acids + peptides

- Gastrin release. This acts directly on ECF-like + triggers the release of histamine. Histamine acts directly on parietal cells. Net effect = increased acid production

- Gastric phase

- Intestinial phase

- Low luminal pH (high H+)

- This directly inhibits gastrin secretion from G cells. This indirectly inhibits histamine secretion (via gastrin). 
- This low pH stimulates stomastatin release which inhibits parietal cell activity

- In the duodenum there is:

- Distension

- Low luminal pH

- Hypertonic luminal contents

- Amino acids + fatty acids

- These factors all decrease HCl secretion via:

- Parasympathetic nerve inhibition (less ACh = less gastrin + histamine release) 

- Triggering the release of enterogastrones, e.g. secretion + CKK. These inhibit gastrin release + promote somastatin release

- Controlled by the brain, stomach + duodenum

- 1 parasympathetic neurotransmitter (ACh)

- 1 hormone (gastrin)

- 2 paracrine factors (histamine, somatostatin)

- 2 key enterogastrones (secretin, CCK)

- Alkaline mucus on luminal surface 

- Tight junctions between epithelial cells

- Rapid cell replacement of damaged cells by stem cells present in base of pits

- Feedback loops for regulation of gastric acid secretion

- Consequence of insufficient defense = peptic ulcers

- Causes of peptic ulcers:

- Helicobacted pylori infection (damages gastric epithelium, can be treated by 1 proton pump inhibitor + 2 antibiotics, e.g. amoxicillin)

- NSAIDs (non-steroidal anti-inflammatory drugs, inhibit cyclo-oxygenase 1 which causes reduced mucosal defense. Treated by prostaglandin analogues, e.g. misoprotol, reduced acid secretion with proton pump inhibitors or H2-receptor antagonists

- Chemical irritants, e.g. alcohol, bile salts

- Gastrinoma (tumours of G cells, secret gastrin in an unregulated way). Worth noting that G cells are enterochromaffin-like cells 

- Pepsinogen, an inactive zymogen is secreted by chief cells. This secretion is stimulated by ACh (parasympathetic)

- Pepsinogen's secretion parallel HCl secretion from parietal cells

- Pepsinogen --> pepsin. Converted by HCl and pepsin (pepsin catalyses the reaction as it is a positive feedback loop)

- Most efficient conversion when pH < 2 (as HCl needed)

- Pepsin accounts for ~20% of protein digestion

- Pepsin increases surface area for later digestion 

- Empty stomach volume = 50ml, maximum volume after eating = 1.5L. This is achieved by receptive relaxation of muscles in body + fundus of stomach

- Mediated by:

- ACh (parasympathetic - Vagus nerve)

- Nitric oxide + serotonin (releases by enteric nerves)

- Peristalsis is a series of wave-like muscle contractions that moves food. It is produced in response to arriving food

- Ripple movement begins in body

- More powerful contraction wave in antrum 

- Pyloric sphincter closes (little chyme can enter duodenum)

- Antral contents forced back to body - mixing 

- Frequency of peristaltic waves determined by pacemaker cells called the interstitial cells of Cajal and is constant (3x per minute)

- Depolarisation waves transmitted through gap junctions to adjacent smooth muscle cells 

- The strength of peristaltic waves varies (excitatory hormones + neurotransmitters further depolarise membranes, action potential generated when threshold reached)

- Gastrin 

- Gastric distension (mediated by mechanoreceptors)

- Duodenal distension

- Increase in duodenal fat

- Increase in duodenal osmolarity 

- Decrease in duodenal pH

- Increased sympathetic NS stimulation

- Decreased parasympathetic NS stimulation 

- Gastroparesis = delayed gastric emptying, makes people feel nauseous etc.

- Caused by:

- Gastrointestinal agents, e.g. H2 receptor antagonists, proton pump inhibitors 

- Anticholinergic medications, e.g. diphenhydramine (Benadryl)

- Miscellanous, e.g. calcium channel blockers

- Detoxification = filters + cleans blood of waste products

- Immune functions = fights infections + diseases

- Involved in synthesis of clotting factors, proteins, enzymes, glycogen + fats

- Production of bile + breakdown of bilirubin

- Energy storage (glycogen + fats)

- Regulation of fat metabolism

- Ability to regenerate  

- Carbohydrates and fats

- Regulated by endocrine glands, e.g. pancreas. Also regulated by nerves

- Lipds are esters of fatty acids + glycerol or other compounds, e.g. cholesterol. Insoluble in water and variety of structures and functions

- 90% of lipids are triglycerides, others phospholipids etc. Triglycerides = 1 glycerol molecule esterified to 3 fatty acids

- Storage areas of fat include adipocytes and hepatocytes

- Saturated = line up

- Unsaturated = one or more kink

- Energy reserve, almost all energy required is provided through oxidation or lipids, carbohydrates + proteins (liver is main storage place for glycogen)

- Part of cell membranes

- Hormone metabolism

- Portal vein

- Hepatic artery 

- Lymphatics

- Lipids are often transported as triglycerides or fatty acids bound to albumin, within lipoproteins or as chylomicron remnants 

- Triglycerides can't diffuse through cell membranes, so lipoprotein lipase breaks down triglycerides to release free fatty acids which can diffuse through bilayer

- They can be stored + the fatty acids are re-esterified to triglycerides. Adipocytes are the main store of lipids, but hepatocytes also store some

- Hormone sensitive lipase releases free fatty acids

- Hepatic lipase enables the uptake of free fatty acids into hepatocytes