Digestion of Carbohydrates and Proteins Flashcards
What is the functional unit of the exocrine pancreas made up of?
an acinus and its draining ductule
What does the exocrine pancreas contain?
- enzymes (acinus)
- electrolytes (ductule)
- mucin (goblet)
What does the endocrine pancreas produce?
- insulin
- glucagon
- somatostatin
- pancreatic polypeptide
What do acinar cells in the pancreas secrete?
peptidases, lipases, alpha-amylases and nucleases
What do ductal cells secrete?
1.2-1.5L/day of pancreatic juice
Why does pancreatic juice have a high HCO3- concentration?
to regulate the pH of the upper intestine
What can failure to produce pancreatic juice lead to?
duodenal ulcers
What does the aqueous component of pancreatic juice contain?
- same amount of sodium and potassium as the ECF
- more HCO3- than ECF
How is an acid tide formed in the plasma?
the HCO3- concentration increases as the rate of secretion increases so there is less time for HCO3-/Cl- exchange
What are the 3 enzymatic components of the pancreatic juice?
- pancreatic α-amylase
- pancreatic lipases secreted in their active forms
- pancreatic proteases secreted in inactive zymogen forms
What does pancreatic α-amylase do?
- hydrolyse glycogen and starch
- complex carbohydrates (except cellulose) into disaccharides
Give examples of pancreatic proteases
trypsin, chymotrypsin and carboxypeptidase A and B
What do water-insoluble esters require?
bile salts
What is pancreatic secretion stimulated and inhibited by respectively?
- stimulated by parasympathetic system
- inhibited by sympathetic system
What is the cephalic phase of pancreatic secretion activated and enhanced by?
activated by thought, sight, smell or taste of food and enhanced by vagal stimulation
What happens in the cephalic phase?
secretion of pancreatic juice
How is the gastric phase of pancreatic secretion enhanced?
by gastric distention and food breakdown products to release gastrin which produces low-volume high-enzyme pancreatic secretion
What is the most important phase of pancreatic secretion?
intestinal phase (responsible for 70% of total secretion)
What is the intestinal phase of pancreatic secretion stimulated by?
CCK and secretin released from endocrine cells in the duodenum and upper jejunum
What is secretin used for?
to increase HCO3- secretion by the pancreas
How does ACh affect pancreatic secretion?
it potentiates the effects of both CCK and secretin
When is vagal stimulation more potent in stimulating pancreatic secretion?
when CCK and secretin are present in plasma
When are CCK and secretin secreted?
in response to the entrance of chyme into the small intestine and a decrease in pH (<4.5)
What are the major stimulants for CCK secretion?
amino acids (mainly phenylalanine and tryptophan), fatty acids and monoglycerides
What must happen to CH2O before absorption?
it must be digested into monosaccharides
Where does almost all digestion occur?
within the small intestine
Where is pancreatic α-amylase most concentrated and what does it do?
in the duodenum and digests carbohydrates into a variety of oligosaccharides
How are oligosaccharides converted to monosaccharides?
by brush border disaccharidases
What is the rate limiting step in carbohydrate absorption?
the conversion of oligosaccharides to monosaccharides
What are the end products of carbohydrate digestion?
fructose, glucose and galactose
What parts of the GIT hasve the highest capacity to absorb sugars?
duodenum and upper jejunum
What are glucose, galactose and xylose absorbed by?
a common Na+-dependent transport system
What is fructose absorbed by?
facilitated transport – most of the fructose is rapidly converted into glucose and lactic acid within epithelial cells to maintain a gradient for diffusion
How are monosaccharides transported across the basolateral membrane?
by facilitated transport
What does failure to absorb carbohydrates result in?
diarrhoea and intestinal gas
What is lactase sensitive to?
infectious and inflammatory diseases that affect the intestine
What happens to proteins before they are absorbed?
they are digested into small polypeptides and amino acids
What are the 3 enzymes involved in protein digestion?
- gastric pepsin
- pancreatic proteases
- enterocytes peptidases
How much protein is digested and absorbed by the duodenum and jejunum?
50%
How much protein reaches the ileum and colon respectively?
- 20-50% reaches ileum
- 10% reaches colon
What is the protein in stool from?
bacterial and cellular debris
What are Na+-dependent transport systems for?
tripeptides, dipeptides and L-amino acids
