Physiology 2 Flashcards
What are the two main types of digestion occurring in the small intestine?
Luminal digestion - mediated by pancreatic enzymes secreted into the duodenum
Membrane digestion –> mediated by enzymes situated at the brush border of epithelial cells
Which cells control absorption from the lumen in the small intestine and what are heir two membranes/borders?
Enterocytes
- apical membrane (brush border) faces lumen
- basolateral membrane faces interstitium
What are they different classes of carbohydrate?
Polysaccharides
Oligosaccharides
Monosaccharides
What are the different types of polysaccharides?
Starch (plant)
- amylose: straight chain
- amylopectin: branched chain
Glycogen (more highly branched than amylopectin)
What are the different types of oligosaccharides and what are they made up of?
Sucrose = glucose + fructose Lactose = glucose + galactose
What are the different types of monosaccharides?
Glucose
Fructose
Galactose (not from diet)
In what form can dietary carbohydrates be absorbed?
As monosaccharides
Where and how are polysaccharides converted to oligosaccharides?
Intraluminal hydrolysis
By alpha-amylase (salivary and pancreatic)
Where and how are oligosaccharides converted to monosaccharides?
Membrane digestion at the brush border
By lactase, maltase, sucrase-isomaltase
Where are carbohydrates absorbed in the small intestine?
Duodenum and jejunum
How are the monosaccharides absorbed over the apical membrane (brush border)?
Glucose and galactose –> SGLT1
Fructose –> GLUT5
How are the monosaccharides transported through the basolateral membrane?
GLUT2
In what form can protein be absorbed from the intestinal lumen?
Oligopeptides
Amino acids
How are some proteins digested in the stomach?
HCl denatures proteins
Pepsin cleaves proteins into peptides
(not essential for protein digestions)
How does protein digestions occur in the duodenum?
5 pancreatic proteases are secreted as proenzymes from the acinar cells of the exocrine pancreas
(converted into active form in duodenum)
What are the 5 pancreatic proteases?
Trypsin Chymotrypsin Elastase Procarboxypeptidase A Procarboxypeptidase B
Where are additional proteases present?
Brush border
Within cytoplasm of the enterocyte
In what form can proteins be absorbed over the basolateral membrane?
Amino acids only
Why are fats (lipids) more difficult to digest?
They are insoluble/poorly soluble in water
–> must be converted from solid fat to emulsion of fat droplets in order for enzymes to break it down
How is fat broken down in the stomach?
Gastric lipase (modest importance in adults)
How is fat broken down in the duodenum?
Pancreatic lipase
What is the role of bile salts in fat digestion?
Bile salts released into duodenum in response to CCK
Act as detergents to help emulsify large lipid droplets into small droplets
–> increase surface area for attack by pancreatic lipase
What happens when there is a failure to secrete bile salts?
Lipid malabsorption - steatorrhoea
Secondary vitamin deficiency due to failure to absorb fat soluble vitamins
Which vitamins are fat soluble?
A, D, E and K
How are fatty acids and monoglycerides transported across the enterocyte?
Short chain fatty acids –> diffuse across both membranes
Long chain fatty acids and monoglycerides are resynthesised to triglycerides in the enterocyte before crossing the basolateral membrane
How is cholesterol absorbed from the intestinal lumen?
Via the NPC1L1 protein
Which drugs blocks the NPC1L1 protein in order to reduce absorption of cholesterol?
Ezetimibe
What form is dietary iron usually in, and what form can be absorbed by duodenal enterocytes?
Dietary –> Fe3+
Absorbed –> Fe2+
Describe the conversion of Fe3+ to absorbable Fe2+
Fe3+ is reduced (accepts an electron) which is promoted by:
- HCl in stomach
- vitamin C
- ferric reductase
Fe2+ then binds to gastroferrin (from gastric parietal cells)
How are where is vitamin B12 absorbed?
Intrinsic factor released from gastric parietal cells
B12 binds to intrinsic factor in the small intestine
B12-instrinsic factor complex is absorbed in the terminal ileum
Which diet predisposed to B12 deficiency?
Vegan diet –> B12 not present in vegetables
Which two hormones are responsible for reporting levels of stored fat to the brain?
Leptin (made and released from fat cells)
Insulin
–> inform hypothalamus to eat less and increase energy burn
This malfunctions in obesity
What happens if someone has a genetic deficiency of leptin?
Mimics starvation in brain –> obsessive eating –> obesity
Can give therapeutic leptin injections
In normal, diet induced obesity, what happens to leptin levels?
High leptin levels
BUT leptin resistance
Which drug is licenced for weight loss and how does it work?
Orlistat
- inhibits pancreatic lipase, decreasing triglyceride absorption
What are the side effects of Orlistat?
Cramping, bloating, flatulence, abdominal pain and diarrhoea
Need to take supplements for fat soluble vitamins
Tend to get rebound weight gain
What are the effects of bariatric surgery?
Gastric bypass surgery
- -> substantial, sustainable weight loss
- -> significantly reduced mortality
- -> often complete resolution of T2DM
