S5: absorption and digestion Flashcards
Describe the key properties of chyme leaving the stomach
Hypertonic
Acidic
Partially digested
Describe how duodenum deals with chyme
Relatively permeable to water – movement of water from circulation/ECF into duodenum (Brunner’s glands)
Chyme is isotonic when it leaves duodenum (generally)
List the secretions of the exocrine pancreas
Acinus – produces enzymes: amylases/lipases (active), proteases (inactive): trypsin, chymotrypsin, elastase, carboxypeptidase
Duct cells – alkaline aqueous component of pancreatic secretions
Describe the mechanism of secretion of alkaline juice
Enzymes:
1) Formed on RER
2) Moved to Golgi complex – vacuoles are condensed
3) Concentrated in zymogen granules
4) Released with appropriate stimulus (parasympathetic/CCK)
Describe the function of zymogen granules
Membrane bound
Contain zymogen, inactive pre-cursor of an enzyme (avoids digesting pancreas)
If pancreatic enzymes appear in blood, it signifies pancreatic damage
List the main functions of the liver
Energy metabolism
Detoxification
Plasma protein production
Secretion of bile
Describe the structure and function of hepatocytes
Chief functional cell of the liver Very active at producing proteins/lipids for export -contains lots of RER/SER -stacks of Golgi apparatus Contain a lot of glycogen
Describe the structural unit of a liver lobule
Triad at corners – portal vein (venous drainage of gut), hepatic artery & bile duct
Central vein in the middle
Blood flowing in: venous portal blood & arterial blood flowing towards central vein which then drains into hepatic veins -> vena cava
Bile flows out along canaliculi -> bile ducts to duodenum
Describe the two components of bile
Bile acid dependent – contains bile acids and pigments
-secreted into canaliculi by hepatocytes
Bile acid independent – similar alkaline solution to pancreatic duct cells
-secreted by duct cells & stimulated by secretin
List the two primary bile acids. What is the difference between bile acids and bile salts?
2 primary bile acids: cholic acid & chenodeoxycholic acid
Bile salts = bile acids that are conjugated with the amino acids glycine/taurine
Why are bile salts used instead of bile acids?
Bile acids – not always soluble at duodenal pHs (bile salts generally are)
Amphipathic structure – can act at oil/water interface (crucial for the emulsification of dietary lipids)
Describe the digestion of fats
Bile acids emulsify fat into smaller units – increases surface area for lipases to act
Bile acids then form micelles with products of lipid breakdown
Micelles – vehicle to carry hydrophobic molecules through an aqueous medium: products of lipid digestion
-diffuse with products to brush border of epithelial cells
Inside cell, they are re-esterified to triglycerides, phospholipids & cholesterol
Packaged with apoproteins & exocytosis from basolateral membrane & enter lymph capillaries
Describe the enterohepatic circulation of bile acids
Bile salts don’t enter gut epithelial cells with lipids
Remain in gut lumen
Reabsorbed in terminal ileum -> return to liver in portal blood & extracted by liver = liver reuses bile acids
Describe the function of the gallbladder
Stores bile Concentrates bile (this can lead to gallstones) CCK released from duodenum stimulates gallbladder contraction – sphincter of Oddi is relaxed
Describe steatorrhea
If bile acids or pancreatic lipases are not secreted in adequate amounts, fat appears in the faeces
Faeces are pale, floating & foul smelling
Describe jaundice
Accumulation of bilirubin in the blood if it cannot be excreted
Describe starch metabolism
Amylase breaks down amylose into maltose (maltase breaks this down into glucose)
Amylopectin -> alphadextrins -> amylose -> glucose (isomaltase breaks down the alphadextrins into amylose)
Describe lactose and sucrose metabolism
Lactose -> glucose + galactose (lactase – brush border enzyme)
Sucrose -> glucose + fructose (sucrase)
Describe the absorption of sugars into the blood
Apical membrane:
-SGLT1 – Na+ & glucose/galactose co-transported into cell
-GLUT5 – fructose is transported into the cell
Basolateral membrane:
-sodium-potassium ATPase
-GLUT2 – glucose, galactose & fructose is transported out of the cell
Describe protein digestion
Stomach – pepsin breaks down proteins into oligopeptides + amino acids
Intestinal lumen – endopeptidases and exopeptidases continue breaking down proteins into amino acids
Brush border – Na+ and amino acids co-transported into cells, dipeptides & tripeptides are transported via PepT1; Na+/K+/ATPase on basolateral membrane
Cytosolic peptidases – some dipeptides and tripeptides are acted on by cytosolic peptidases to make amino acids, but others can be directly absorbed into the blood
Describe the basis of oral rehydration therapy
Contain Na+ and monosaccharides -> this will drive the maximum influx of osmotically active substances into the cell
Water will then follow
Describe the causes and symptoms of vitamin B12 deficiency
Causes – lack of intrinsic factor, hypochlorhydria, not enough B12 in food & inflammation of terminal ileum
Symptoms – neurological problems & megaloblastic anaemia
Describe lactose intolerance
Deficiency of lactase enzyme
In colon – fermented -> gases released -> flatus & bloating
Describe irritable bowel syndrome
Diagnosed in absence of documented abnormalities
Symptoms: abdominal pain, bloating, flatulence, diarrhoea/constipation & rectal urgency
Describe coeliac disease
Immunological response to gluten – damages mucosa (impaired digestion + malabsorption)
-flattening of villi
-hypertrophy of crypts
-lymphocytes in epithelium/LP
Causes not understood but genetic factor is thought to have a role
Describe the symptoms, investigations and treatment of coeliac disease
Symptoms – diarrhoea, weight loss, flatulence, anaemia & neurological symptoms
Investigations – blood tests (IgA), upper GI endoscopy
Treatment – gluten free diet
