Vander's GIT Flashcards
Only substance that can cross the epithelium of the gastric wall
Water
Intake vs output in digestive system
CNS receives info from GIT
Afferent input
CNS has an influence on GIT
Efferent input
Structure of GIT wall
Contraction of circular muscle
Produces a narrowing of the lumen
Contraction of longitudinal muscle
Shortens the tube
Function of Peyer’s Patches and immune cells in SI
Secrete inflammatory mediators (e.g. cytokines) and alter motility
Where do carboxypeptidases come from
The pancreas
Where are aminopeptidases found
On the luminal membranes of epithelial cells
How do free AAs enter the epithelial cells
// Secondary active transport coupled to Na+
How are short chains of 2 or 3 AAs absorbed
By a secondary active transport coupled to H+ gradient
Within the epithelial cell, these di- and tri-peptides are hydrolysed to AAs, which then leave the cell and enter the blood through a facilitated diffusion carrier in the basolateral membranes
Difference between CHO and protein absorption
With CHO absorption, molecules larger than monosaccharides are not absorbed
With protein absorption, short chains of 2/3 AAs are absorbed by a secondary active transport coupled to H+ gradient
How do the SMALL NUMBER of intact proteins cross the intestinal epithelium and gain access to interstitial fluid
Endocytosis & exocytosis
(absorptive capacity for intact proteins is much greater in infants than in adults - antibodies)
Action of lipase
catalyses the splitting of bonds linking FAs to the 1st and 3rd C atoms of glycerol
Action of phospholipids on fat digestion
They are amphipathic molecules consisting of 2 non-polar FA chains attached to glycerol with a charged phosphate grp located on glycerol’s 3rd carbon
EMULSIFYING AGENT
What are bile salts formed from
Cholesterol in the liver - amphipathic
How do we over come the empairment of the accessibility of water-soluble lipase to its substrate
- COLIPASE from pancreas, which is amphipathic, lodges on the lipid droplet surface
- COLIPASE binds the lipase enzyme, holding it on the surface of the lipid droplet
Components of micelles
- Bile salts
- FAs (low solubility in water)
- Monoglycerides (low solubility in water)
- Phospholipids
- During their passage through the epithelial cells, what are FAs and monoglycerides re-synthesised to
- How is a diffusion gradient for these molecules maintained
- What is the function of these packages
- Triglycerides
- This process lowers the conc of cytosolic free FAs and monoglycerides and thus maintains a diffusion gradient for these molecules into the cell
- The re-synthesised fat aggregates into small droplets coated with amphipathic proteins that perform an emulsifying function similar to that of bile salts
How are the fat droplets released into IS fluid
Vesicles containing the droplet pinch off the ER, are processed through the golgi apparatus and eventually fuse with the plasma membrane
These are known as CHYLOMICRONS
Content of chylomicrons
Triglycerides
Other lipids
Where do the chylomicrons pass into
- Chylomicrons released from epithelial cells pass into the lacteal rather than the blood capillaries because the BASEMENT MEMBRANE (an EC glycoprotein layer) at the outer surface of the capillary provides a BARRIER to the diffusion of large chylomicrons
- Lacteals have large, slit like pores between their endothelial cells
- Lymph eventually empties into system veins via thoracic duct
Overview of fat digestion and absorption
What will a vit D malabsorption result in
Decrease in calcium absorption in GIT
How is vit B12 absorbed
With intrinsic factor by ENDOCYTOSIS
- erythrocyte formation
- pernicious anaemia
What accounts for much of the actively transported solute
Na+ ions - most abundant solute in chyme
- Na+ absorption is a primary active process using the Na+/K+ ATPase pumps
- Cl- and HCO3- are absorbed with Na+ ions
How is iron transported into intestinal epithelial cells
Fe is incorporated into ferritin, the protein-iron complex that functions as an IC Fe store
Most of Fe bound to ferritin in the epithelial cells is released back into the intestinal lumen when the cells at the tips of the villi disintegrate, and the iron is then excreted in the faeces
What happens to the absorbed iron that does not bind to ferritin
It’s released on the blood side where it circulates throughout the body bound to plasma protein TRANSFERRIN
What does iron absorption depend on
Types of food ingested because it binds to many -vely charged ions in food which can retard its absorption
e.g. iron in ingested liver is much more absorbable than iron in egg yolk, due to the phosphates that bind the iron to form an insoluble and unabsorbable complex
Absorption typical of most trace metals
- Cellular storage proteins and plasma carrier proteins are involved
- The control of absorption, rather than urinary excretion is the major mechanism for homeostatic control of the body’s content of Fe
Luminal stimuli that initiate GI reflexes
- Distension of wall by the volume of the luminal contents
- Chyme osmolarity (total solute conc)
- Chyme acidity
- Chyme concentration (of digestive products)
2 nerve networks of the enteric nervous system
- Myenteric plexus – SM activity
- Submucosal plexus – secretory activity
2 types of neural reflex arcs
- Short reflexes – from receptors through the nerve plexuses to effector cells
- Long reflexes from receptors in the tract to the CNS by way of afferent nerves, and back to the nerve plexuses and effector cells by way of autonomic nerve fibres
What stimulates the secretion of CCK
Presence of FAs and AAs in the SI triggers CCK secretion from cells in the SI into blood
Effect of CCK
Circ CCK stimulates the pancreas to increase secretion of digestive enzymes
CCK causes the gallbladder to contract, delivering to the intestine the bile salts required for micelle formation
How is the stimuli for CCK release removed
Fat and AAs are absorbed
Potentiation of secretin and CCK
- Secretin strongly stimulates pancreatic bicarbonate secretion WHEREAS CCK is a weak stimulus of bicarbonate secretion
- HOWEVER both hormones together stimulate pancreatic bicarbonate secretion more strongly than would be predicted by the sum of their individual stimulatory effects