GI PHYSIOLOGY IV - ABSORPTION Flashcards
TRUE/FALSE: absorption of the products of digestion for all nutrients always require a driving force
true: there’s always a driving force to help the nutrients move whether through the cells or in between the cells
briefly describe the general pathway of absorption
GI lumen —— lining of the intestine —–> interstitial fluid —> blood/lymph
what are the sites of absorption across the GI tract, what do they absorb and
Mouth, esophagus, stomach: minimal of lipid soluble substances
Small intestine: 90% of water and sodium + all nutrients
Large intestine:9% of water and sodium
what are the factors that affect the absorption?
- motility
- SA available for absorption
- transportation across epithelium
- reduction in size of chemical digestion
- membrane transporters
- removal from interstitial fluid
what are the 2 things tha mobility affect absorption?
- peristalsis: correct rate of propulsion and storage
- segmentation for exposure of products
what are the anatomical adaptations that maximises the surface area
- length of intestine
- plicae circulares
- villi
- microvilli
since the lumen of the intestine is continuous with the outside world, the intestinal epithelium therefore a barrie, but there are 2 pathways of transporting substances through the layer, what are they?
- paracellular: between cells
- transcellular: across the cell membrane and through the cytoplasm
what are the traits of paracellular pathway and why is it easy to paracellularly transport?
- only barrier is the tight junction binding the cells together which makes it relatively non-selective because any small enough molecule would be able to cross
- passive: requires a gradient
what are the traits of the transcellular pathway and why is it harder?
- solutes must cross 2 cell membrane
- the membrane is made of phospholipid –> if solute is not lipid soluble than it requires a transport protein
what are the 2 mechanisms that help maximise the absorption?
- reduce the nutrient into smallest possible units
- transport protein
what is interstitial fluid?
the fluid that fills the space between cells
what are the substances that we absorb everyday?
- water
- sodium
- carbs
- proteins
- lipids
-bile salts - vitamins
what is the amount of water that is delivered to small intestine per day?
9-10L –> replacement of the loss and reabsorption of secreted water is very important
what is the mechanism of water absorption?
osmosis by the paracellular pathway
what are the 2 mechanisms of Na+ absorption
- passive movement via paracellular pathway
- active transport via transcellular pathway
how does Na+ transport into the cells?
- Na+ transport alone from high to low concentration
- Na+ transport coupled to monosaccharides
- Na+ transport coupled to amino
- Na+ channels or Na+/H+ exchanger
how does Na+ transport out of the cells?
active transport: use energy - ATP with Na+/K+ ATP
why do we remove substances from the interstitial fluid and how do we remove it?
- to prevent build up in interstitial fluid
- using blood vessels or lymphatics (lacteal)
what are the 2 mechanisms for carbohydrate absorption? describe?
Passive and active
- Passive: monosaccharides passively diffuse via concentration gradient
Active -driving force: Na+/K+ ATPase:
- Co-transport with Na+ via cellular pathway
- Monosaccharides
- Apical membrane: Na+ glucose cotransporter
- Basolateral membrane: glucose carrier
- Driving force: Na+/K+ ATPase
what are the 2 mechanisms for amino acid absorption?
Passive and active
Passive : amino acids diffuse down concentration gradient via paracellular pathway
Active -driving force: Na+/K+ ATPase:
- Co-transport with Na+ via cellular pathway (same as glucose)
- Apical membrane: Na+ amino acid cotransporter
- Basolateral membrane: amino acid carrier
how are small peptides absorbed?
Active transport across cell membrane with Na+/K+ ATPas as driving force:
- Apical membrane: H+/peptide cotransporter
- Cytoplasmic peptidases digest small peptides into AA’s
- Basolateral membrane has amino acid carrier
what are the 3 stages of products of fat digestion absorption?
Lumen and apical membrane
Free fatty acids and monoglycerides move by simple diffusion from micelle into the cell
Intracellular
- Transported to the ER where they are synthesised to triglycerides to maintain a gradient from apical diffusion.
- These are then secreted from the golgi as chylomicrons and sent to the basolateral membrane
**Basolateral membrane **
Exocytosed as chylomicrons, then these enter lymph via lacteals of villi
what is chylomicrons?
droplets of triglyceride and other small lipids enclosed by membrane from
TRUE/FALSE: absorption of the products of fat digestion does not require a driving force
false
- absorption of products of fat digestion does indeed REQUIRES a driving force (triglyceride gets taken into ER, the reasembly and driving away of the molecules means there’s always a concentration –> there’s always driving force)
- however, we absorption of products of fat digestion occurs by passive diffusion
define apical and basolateral membrane?
- apical membrane facing an “outside” lumen
- a basolateral membrane facing neighboring cells and the basal lamina
when does the bile salts in micelles absorbed and where does that occurs?
- after fat absorption complete
- happens in the ileum: active transport (apical Na+ dependent bile acid cotransporter)
- happens in colon: passive absorption
Fat soluble vitamins are absorbed with what?
fats
Water soluble vitamins (B group and C) are absorbed using what mechanism?
Na+ dependent
What is the special case of vitamin absorption?
Vitamin B12:
- binds to the intrinsic factor in stomach
- there is specific transporter for intrinsic factor vitamin B12 in the ileum
how is the residues of digestions be repulsed?
form faeces and being transfered to recum via peristaltic waves and eliminated from the body by defecation reflex
