S5: absorption and digestion

Question 1

Describe the key properties of chyme leaving the stomach

Answer 1

Hypertonic
Acidic
Partially digested

Question 2

Describe how duodenum deals with chyme

Answer 2

Relatively permeable to water – movement of water from circulation/ECF into duodenum (Brunner’s glands)
Chyme is isotonic when it leaves duodenum (generally)

Question 3

List the secretions of the exocrine pancreas

Answer 3

Acinus – produces enzymes: amylases/lipases (active), proteases (inactive): trypsin, chymotrypsin, elastase, carboxypeptidase
Duct cells – alkaline aqueous component of pancreatic secretions

Question 4

Describe the mechanism of secretion of alkaline juice

Answer 4

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)

Question 5

Describe the function of zymogen granules

Answer 5

Membrane bound
Contain zymogen, inactive pre-cursor of an enzyme (avoids digesting pancreas)
If pancreatic enzymes appear in blood, it signifies pancreatic damage

Question 6

List the main functions of the liver

Answer 6

Energy metabolism
Detoxification
Plasma protein production
Secretion of bile

Question 7

Describe the structure and function of hepatocytes

Answer 7

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

Question 8

Describe the structural unit of a liver lobule

Answer 8

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

Question 9

Describe the two components of bile

Answer 9

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

Question 10

List the two primary bile acids. What is the difference between bile acids and bile salts?

Answer 10

2 primary bile acids: cholic acid & chenodeoxycholic acid

Bile salts = bile acids that are conjugated with the amino acids glycine/taurine

Question 11

Why are bile salts used instead of bile acids?

Answer 11

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)

Question 12

Describe the digestion of fats

Answer 12

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

Question 13

Describe the enterohepatic circulation of bile acids

Answer 13

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

Question 14

Describe the function of the gallbladder

Answer 14

Stores bile 
Concentrates bile (this can lead to gallstones) 
CCK released from duodenum stimulates gallbladder contraction – sphincter of Oddi is relaxed

Question 15

Describe steatorrhea

Answer 15

If bile acids or pancreatic lipases are not secreted in adequate amounts, fat appears in the faeces
Faeces are pale, floating & foul smelling

Question 16

Describe jaundice

Answer 16

Accumulation of bilirubin in the blood if it cannot be excreted

Question 17

Describe starch metabolism

Answer 17

Amylase breaks down amylose into maltose (maltase breaks this down into glucose)
Amylopectin -> alphadextrins -> amylose -> glucose (isomaltase breaks down the alphadextrins into amylose)

Question 18

Describe lactose and sucrose metabolism

Answer 18

Lactose -> glucose + galactose (lactase – brush border enzyme)
Sucrose -> glucose + fructose (sucrase)

Question 19

Describe the absorption of sugars into the blood

Answer 19

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

Question 20

Describe protein digestion

Answer 20

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

Question 21

Describe the basis of oral rehydration therapy

Answer 21

Contain Na+ and monosaccharides -> this will drive the maximum influx of osmotically active substances into the cell
Water will then follow

Question 22

Describe the causes and symptoms of vitamin B12 deficiency

Answer 22

Causes – lack of intrinsic factor, hypochlorhydria, not enough B12 in food & inflammation of terminal ileum
Symptoms – neurological problems & megaloblastic anaemia

Question 23

Describe lactose intolerance

Answer 23

Deficiency of lactase enzyme

In colon – fermented -> gases released -> flatus & bloating

Question 24

Describe irritable bowel syndrome

Answer 24

Diagnosed in absence of documented abnormalities

Symptoms: abdominal pain, bloating, flatulence, diarrhoea/constipation & rectal urgency

Question 25

Describe coeliac disease

Answer 25

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

Question 26

Describe the symptoms, investigations and treatment of coeliac disease

Answer 26

Symptoms – diarrhoea, weight loss, flatulence, anaemia & neurological symptoms
Investigations – blood tests (IgA), upper GI endoscopy
Treatment – gluten free diet