Gastrointestinal system 8 Flashcards
Describe basic absorption
Net passage of substance from GI lumen across lining of intestine into interstitial fluid and into blood or lymph
Where does absorption occur
Mouth esophagus stomach Small intestine (90% of water and sodium) Large intestine (9% of water and sodium)
What are the 4 factors affecting absorption
Motility
Surface area available for absorption
Transport across epithelium
Removal from interstitial fluid
How does motility affect absorption
Exposure of products of digestion to absorptive surfaces
Peristalsis and segmentation
How does surface area affect absorption
Rate of absorption proportional to surface area
What are the adaptions for maximising surface area
Length of intestine
Circular folds
Villi
Microvilli
What is the problem with transport of molecules across epithelium
Lumen of intestine continuous with outside world
Intestinal epithelium is a barrier
What are the two pathways of transport across epithelium
Paracellular - ‘gap’ between cells
Cellular - Across cell membrane and through cytoplasm
Describe paracellular pathways
Solutes don’t cross membrane - tight junctions binding cells together - relatively non-selective if small enough - passive
Describe the cellular pathway
Solutes cross two cell membranes - lipid bilayers
How can you maximise absorption across surface area
Reduces nutrients into smallest possible unit
Transport proteins - active transport
Describe the removal of substance from interstitial fluid
Large blood flow to intestine
Arrangement in villi of blood vessels and lacteal
Prevents build up in interstitial fluid
How much water do we secrete into the GI tract
Salivary - 1.5 L per day Gastric - 3 L per day Pancreatic - 1.5 L per day Biliary - 0.5 L per day SI - 1.5 L per day
What is the total water delivered to Small intestine
9-10 L per day
we do not replace losses and reabsorb the secreted water
Describe Osmosis
Passive movement of water from lumen to blood
set up by absorption of salts and nutrients
What types of transport does Na+ undergo
Passive movement - paracellular
Active transport - Transcellular - across membrane
Describe mechanism of Na+ active transport mechanism
Na+ transport alone
Na+ coupled transport to monosaccharides
Na+ coupled to amino acids
Describe absorption of Na+ alone
Down cell gradient (high-low)
What is needed in Na+ absorption alone
Na+ channel
Na+/H+ exchanger
Active energy - ATP needed - Na+, K+-ATPase
Describe carbohydrate absorption
Cotransport with Na+ - cellular pathway - monosaccharides
Across cell membrane
Apical - Na+ glucose cotransporter
Basolateral - Glucose carrier
What two mechanisms do amino acids absorb through
Passive absorption
Active absorption
Describe absorption of products of fat digestion
Lipid soluble - diffuse across cell membrane - delivered to brush boarder by micelles (not absorbed)
What happens during absorption of fat digestion in the cell
Resynthesized into triglycerides
Packaged into chylomicrons - exit and enter the lacteal
When are bile salts absorbed
After fat absorption is complete in ileum - active transport
In colon - passive absorption
95% absorbed
What are the fat soluble vitamins and how are they absorbed
A, D, E and K - absorbed with fats
What is the water soluble vitamin and how is it absorbed
Vitamin C
Na+ dependant absorption
Describe Vitamin B12 absorption
In ileum
Binds to intrinsic factor - in stomach - specific transporter
Pernicious anemia
Do we absorb everything
No - faeces formed in large intestine
