Digestion and Absorption of Lipids Flashcards
What are the functions of bile?
- digestion and absorption of fats
- excretion of water-insoluble substances
What is bile formed by?
hepatocytes from the liver and ductal cells from the bile duct
Where is bile stored during the interdigestive period?
in the gallbladder
How is bile released?
chyme causes release of CCK which causes the gallbladder to contract
What are primary bile acids?
cholic and chenodeoxycholic acids synthesised from cholesterol but are more water soluble than cholesterol
Where are primary bile acids actively reabsorbed?
at the ileum
What are secondary bile acids?
deoxycholic and lithocholic acids produced by deconjugation and dehydroxylation of 1º bile acids by intestinal bacteria
Give examples of bile pigments
bilirubin and biliverdin
What are bilirubin and biliverdin?
yellow metabolites of haemoglobin used for excretion
What happens to bilirubin?
it is converted to brown urobilin by bacteria
What are phospholipids of bile mostly?
lecithins (second most abundant organic bile compounds)
What do lecithins do?
increase cholesterol solubilisation in bile micelles
What is the major route for cholesterol excretion?
bile micelles
How are bile salts produced?
bile acids are conjugated with glycine or taurine
What does bile acid conjugation do?
- make them more amphipathic
- make them less susceptible to hydrolysis by pancreatic enzymes
How does bile acid conjugation aid fat absorption?
it is easier to form micelles which reabsorb poorly and stay in the gut for longer
What does enterohepatic circulation do?
recycle bile from the small intestine to the liver and back
Where are bile salts reabsorbed?
only in the terminal ileum
How much bile salt does each meal require?
4-8g so the total pool must recirculate twice per meal to facilitate it
What is the rate of synthesis of bile determined by?
the rate of return to the liver
What are the 3 phases of assimilation of lipids?
- digestive phase
- absorptive phase
- post-absorptive phase
What does intragastric lipolysis account for?
20-30% of total lipid digestion
What are lingual lipases responsible for?
0.015% of gastric lipase activity
How does intra-intestinal digestion of fats work?
- alkaline pH
- adequate Ca2+
- bile salts
- lecithin
- lipolytic enzymes
What are the 3 lipid soluble lipases of the pancreas?
- glycerol ester hydrolase (main lipase)
- cholesterol esterase
- phospholipase A2
Why must fats be soluble in water?
for digestion and absorption
What does lecithin do to bile?
increase its detergent power
Where are micelles formed and what do they do?
in the lumen of the small intestine to help transport lipids into the intestinal cells or enterocytes
What do micelles contain?
20-30 molecules of lipids and bile salts
Why are micelles extremely hydrophobic?
due to the cholesterol and fat-soluble vitamins within
When are lipids absorbed from micelles?
once they come into contact with the microvilli
What is the rate limiting step of fat absorption?
migration of the micelle from chyme to the microvilli surface
What is fat in the stool from?
colonic bacteria and desquamated intestinal cells
What happens in the post-absorptive phase of fat absorption?
- re-esterification
- formation of chylomicrons for re-esterified lipids
- lymphatic transport
Why is there no re-esterification for medium or short-chain fatty acids?
they are bacterial sources and don’t form micelles or chylomicrons
What are the 2 types of GIT disorders?
functional and structural
What are common signs and symptoms of GIT disorders?
- blood in your stool
- very narrow stool
- changes in bowel habits
- unusual or severe abdominal pain
- unintentional/unexplained weight loss
- heartburn not relieved by antacids
- fatigue
What are primary causes of functional GIT disorders?
10
- low fibre diet
- not enough exercise
- travelling or other changes in routine
- eating large amounts of dairy products
- stress
- resisting the urge to have a bowel movement
- overusing laxatives
- taking certain medicines
- pregnant
- ageing
Give examples of functional GIT disorders
- gastroesophageal reflux disease
- peptic ulcers
- IBS
- constipation
What happens in gastroesophageal reflux disease?
weakening of lower oesophageal sphincter and reflux of gastric acid irritates the oesophagus, damages the lining and causes inflammation
What are causes of gastroesophageal reflux disease?
pregnancy, alcohol, certain foods and medication
What are the 2 types of peptic ulcers and what are they caused by?
- gastric ulcer caused by helicobacter pylori infection, alcohol or drugs e.g. NSAIDs
- duodenal ulcer cause by stress or overproduction of gastric acid
What is IBS caused by?
certain foods, medicines or emotional stress
What are potential remedies of IBS?
decreased caffeine/alcohol, minimised stress, increased dietary fibre and medication
What is constipation?
difficult or infrequent passage of stools caused by inadequate fibre or disruption of regular routine
What are treatments of constipation?
increased dietary fibre, regular exercise and laxatives
Give examples of GIT structural disorders
- haemorrhoids
- diverticulosis
- colon polyps and cancer
- liver diseases
What are haemorrhoids and what are they caused by?
swollen blood vessels that line anal opening caused by chronic excess pressure from straining during bowel movement, persistent diarrhoea and pregnancy
What are treatments of haemorrhoids?
improving bowel habits and surgery
What is diverticulosis?
small out-pouching in muscular wall of large intestine common in individuals over 60
What is diverticulosis caused by?
high pressure over weakened area
What are treatments of diverticulosis?
antibiotics and increased fluids
What do almost all colorectal cancers begin as?
polyps
Give examples of liver diseases
- fatty liver
- hepatitis
- fibrosis
- cirrhosis
- cancer
