Lecture 32 - Intestinal Function Flashcards
Where are the sites of absorption in the GI tract?
mouth and esophagus
stomach
intestine
What absorption happens in the small intestine?
bulk absorption of isosmotic fluid
nutrients
What absorption happens in the proximal colon of the large intestine?
absorption of products of fermentation
fluid and electrolytes
What absorption happens in the distal colon of the large intestine?
selective absorption of fluid and electrolytes
What is the nutrient dependent Na+ absorption in the small intestine?
Na+ coupled solute absorption of monosaccharides and amino acids
Specialised absorption
- bile salts
- vitamins
What is the nutrient dependent Na+ absorption in the proximal colon?
Na+ dependent absorption of short chain fatty acids
Where do we have the nutrient independent Na+ absorption?
small intestine
large intestine
What channels are involved in the nutrient dependent Na+ absorption in the small intestine?
Na+-coupled solute absorption
(SGLT1, amino acids)
What channels are involved in the nutrient independent Na+ and Cl- absorption in the small intestine?
Na+/H+ exchanger (NHE3)
Cl-/HCO3- exchanger
What occurs in the proximal colon?
Na+ absorption
Fermentation of indigestible sugars and proteins
How do we have the fermentation of indigestible sugars and proteins?
generation of short chain fatty acids
- acetate, propionate and butyrate
What is Na+ absorption associated with in proximal colon?
absorption of short chain fatty acid
What is Na+ absorption in the proximal colon?
Electroneutral NaCl absorption
- paired Na+/H+ and Cl-/HCO3- exchangers as in small intestine
What transporter for SCFA is found in the apical membrane of the proximal colon?
Sodium monocarboxylate transporter
What is SCFA?
short chain fatty acid
What is SCFA used by?
Colonocytes for intracellular metabolism
What is the function of the distal colon?
Regulation of composition of faeces
What does Na+ absorption variation depend on?
Fluid and electrolyte status
What are the plasma levels of aldosterone in salt depleted conditions?
High
What are the plasma levels of aldosterone in salt replete conditions?
Low
What is the normal volume of intestinal secretions per day?
1-2 L
What type of secretion occurs in the duodenum?
Cl- secretion
HCO3- secretion
What type of secretion occurs in the jejunum?
Cl- secretion
What type of secretion occurs in the ileum and proximal colon?
Cl- and HCO3- secretion
What type of secretion occurs in the distal colon?
Cl- secretion
Where is the main mechanism for electrogenic Cl- secretion in intestinal secretion?
small intestine & colon
What does active Cl- secretion drive?
Active Cl- secretion drives movement of Na+ and water
- Result: isotonic solution of NaCl
What transporter is found in the basolateral membrane of the small intestine and colon in the secretion of Cl-?
NKCC1cotransporter
- accumulates Clin the cell above electrochemical equilibrium
What transporter is found in the apical membrane of the small intestine and colon in the secretion of Cl-?
Cl- channel - CFTR
- exit pathway for Cl-
What is CFTR activated by?
cAMP
What is vibrio cholerae?
Bacteria that multiples in small intestine that secretes cholera toxin
What is the mechanism of cholera toxin?
Binds to intestine
Enters villous and crypt cells
Stimulates adenylate cyclase irreversibly
Elevation of cAMP
Stimulates Cl- secretion in crypts
Inhibits nutrient-independent Na+ absorption in villi
Secretory diarrhea
- Loss of isotonic fluid
The absorption of Na+ and water in the small intestine
A. is mainly facilitated by channels.
B. is nutrient dependent.
C. is entirely nutrient independent.
D. is facilitated by CFTR.
B. is nutrient dependent.
What are the different types of dietary sugars>
polysaccharides - glycogen and starch
disaccharides - sucrose, lactose and maltose
monosaccharides - glucose, fructose
What percentage of dietary sugars do polysaccharides comprise?
45-60%
What percentage of dietary sugars do disaccharides comprise?
30-40%
What percentage of dietary sugars do monosaccharides comprise?
10%
What dietary sugar is absorbed?
Monosaccharides
What are the main monosaccharides absorbed?
Glucose, galactose, fructose
What is glucose and galactose absorbed by?
Na+-dependent mechanism (SGLT1)
What is fructose absorbed by?
Na+ -independent mechanism (GLUT5)
What are the two steps in the digestion of sugars to release monosaccharides?
Luminal digestion
Contact or brush boarder digestion
What is the luminal digestion of sugar?
Mouth - a amylase
Small intestine – pancreatic amylase
Products:
- a-limit dextrins
- short oligosaccharides and some disaccharides
What is the contact or brush border digestion of sugar?
Enzymes attached to apical membrane of epithelial cells
Release monosaccharides
- glucose, galactose, fructose
The absorption of glucose via SGLT1 is against the
concentration gradient, BECAUSE basolateral glucose
absorption is mediated by facilitated diffusion.
A - Both true and causally related
Why do we have the digestion and absorption of proteins?
NOT for energy
Required for amino acids
What are our sources of proteins (%)?
50% diet
25% sloughed cells
25% digestive enzymes
What are the stages of digestion and absorption of fat?
Emulsification and stabilization
Digestion
Formation of micelles
Absorption
Re-esterification – synthesis of triglycerides
Formation of chylomicrons
What is the emulsification and stabilization of fat?
Reduction in size to droplets
- motility
- important as it increases the surface area available for digestion
What is the emulsification and stabilization of fat in the stomach?
crude emulsion
What is the emulsification and stabilization of fat in the small intestine?
Emulsion stabilized by emulsification agents
- bile salts, phospholipids, cholesterol, lecithin
What are the minor enzymes in digestion?
gastric and lingual lipases
What are the major enzymes in digestion?
pancreatic lipase/colipase
secreted by the pancreas into the small intestine
What are the fat soluble vitamins and how are they absorbed?
Fat soluble vitamins A,D & E
- absorbed in the same fashion as fats
What are examples of water soluble vitamins?
Vitamin C
Vitamin B12
What is the transport process of vitamin B12?
IF is released in stomach
In duodenum B12 binds to IF
IF/B12 complex attaches to specific receptors in terminal ileum
Complex absorbed by endocytosis and degraded to release B12 into portal blood
Protein
A. digestion starts in the mouth.
B. digestion occurs mainly in the stomach.
C. digestion is reduced during the gastric phase.
D. absorption is facilitated by brush border membrane peptidases.
D. absorption is facilitated by brush border membrane peptidases.
