Lecture 27 GI physio Absorption Flashcards
whats the process of absorption?
net passage of substances from lumen across lining of intestine into the interstitial fluid and then into the blood or lymph
wheres the main site of absorption?
Small intestine
90% water and sodium
All nutrients
what are the sites of absorption?
Mouth, esophagus and stomach
Small intestine
Large intestine
whats the absorption of Mouth, esophagus and stomach?
Minimal
Lipid soluble substances
whats the absorption of Small intestine?
Main site
90% water and sodium
All nutrients
whats the absorption of Large intestine?
9% of water and sodium
what factors affect absorption?
motility
SA
transport
removal
what do you need for motility?
correct rate of propulsion for digestion and absorption
exposure to absorptive surfaces
what do you need for available SA?
Rate of absorption proportional to SA
Anatomical adaptations maximise SA (villi, epithelial cells, microvilli)
whats the problem of transport of molecule across epithelium?
intestinal epithelium is a barrier to absorption
what are the 2 pathways for transport of molecules across epithelium?
Paracellular (between cells)
Transcellular (across cell membranes, through cytoplasm)
whats the paracellular pathway?
Passive (requires gradient) non selective (need to be small enough)
solutes dont cross cell membranes
only barrier is tight junction that binds cells together
whats the transcellular pathway?
solutes must cross 2 cell membranes lipid bilayer (solutes not lipid soluble need transport proteins)
whats transport proteins?
selective
allow active transport
how do you Maximise absorption across epithelium?
Reduction in size
- Chemical digestion
Specific transport proteins
- Selective absorption
how do you remove substances from interstitial space?
Large blood flow
arrangement of vessels in villi
position of lacteal
what are the specific substances absorbed?
- Water
- Na
- Bile salts
- Vitamins
- Carbs
- Proteins
- Lipids
How much water has to be absorbed?
Drink ≈ 1.5 L day-1
Total water delivered to SI ≈ 9-10 L/day
whats the mechanism of water absorption?
osmosis
passive
movement of water from lumen into blood
what sets up osmotic gradient?
absorption of salts and nutrients
whats the mechanism for Na absorption?
passive via paracellular pathway
active vis transcellular
whats the Transport mechanisms for active absorption of Na+?
Na+ transport alone
Na+ absorption coupled to glucose
Na+ absorption coupled to amino acids
Na will move INTO cells (high to low concentration) but how does Na get OUT of cells?
uses energy (ATP) i.e. ACTIVE
whats the 2 mechanisms for carbohydrate absorption (products of carb digestion)?
passive or active absorption
whats the passive absorption of carbs?
monosaccharides - glucose, galactose, fructose
diffuse down gradient via paracellular pathway
whats the active absorption of carbs?
cotransport with Na+
monosaccharides glucose and galactose
via transcellular pathway
Na+ coupled absorption of products of carbohydrate digestion
whats the 2 mechanisms for protein absorption (products of protein digestion)?
passive or active absorption
whats the passive absorption of proteins?
amino acids
diffuse down gradient via paracellular pathway
whats the active absorption of proteins?
co transport with Na+
Amino acids coupled to Na+
via transcellular pathway
similar process that occurs for glucose
Products of fat digestion are lipid soluble. whats the absorption of lipids?
FFA and monoglycerides diffuse across cell membrane into cell
Dont need a transporter
Delivered to brush border via micelles
absorption of products of lipid digestion. is the whole micelle absorbed?
no
fatty acids and monoglycerides diffuse into cell
absorption of products of lipid digestion
what happens to FFA and monoglycerides in the cell?
synthesized into triglycerides
packaged into chylomicrons
chylomicrons exit cell via exocytosis, enter lacteals.
How do chylomicrons exit cell?
via exocytosis, enter lacteals.
where does Bile salt absorption occur?
distal portions of small intestine to promote fat absorption.
Is the Majority of secreted bile salts reabsorbed?
yes
whats bile sale salt absorption?
Active transport in terminal ileum.
- Apical Na+-dependent bile acid transporter.
Passive absorption in jejunum.
where does Passive absorption of bile salt occur?
jejunem
where does Active transport of bile salt absorption occur?
terminal ileum.
- Apical Na+-dependent bile acid transporter.
what are fat soluble vitamins and hows it absorbed?
Vitamins A, D, E & K
Absorbed with fats
what are water soluble vitamins and hows it absorbed?
Vitamin C and B12
Na+ dependent absorption
Hows Vitamin B12 absorbed?
distal ileum
Would be digested as passes through duodenum/jejunum so needs Intrinsic Factor (produced in stomach) bound to it to prevent digestion.
whats elimination?
dont absorb it all
Expulsion of residues of digestion
eg faeces
wheres faeces formed and hows it eliminated?
LI
Transferred to rectum via peristaltic contractions
Faeces eliminated from the body via defecation reflex.
vitamin B12 would be digested as it passes through duodenum/jejunum, what does it need to prevent digestion?
Intrinsic Factor (produced in stomach) bound to it
