Digestion and Absorption Flashcards
Saliva Secretion
When released from acinar cells, saliva is isotonic. However, in ducts, Na and Cl are pumped out, some HCO3 pumped in, and saliva become hypotonic. Rich in salivary amylase and lingual lipase.
How does bicarbonate concentration change as salivary flow increases?
It increases!
What happens when food arrives in the stomach
Vagal activity, Gastric distension, gastrin release, histamine release. Causes parietal cell release of HCl, Chief cell secretion of pepsinogen.
Activation of parietal cell and second messengers.
Parietal cells are in fundus Gastrin binds to CCK B increases Ca, Histamine increases cAMP, binds to H2. ACh binds to M3, releases Ca. When second messengers increase, H-K ATPases sitting in tubulovesicles are inserted into the canalicular membrane.
How is HCl secreted from parietal cells?
As K is moved in from the lumen, H is moved out. Cl passes through a chloride channel. These things are brought into the cell from the Na K ATPase, and the Cl/HCO3 exchanger at the basolateral membrane.
What cells are in gastric fundus? Where are G cells located?
Chief cells, parietal cells,ECL cells. G cells in antrum, secrete gastrin to circulation.
How does distension cause gastric secretion
Stretch receptors send info to Dorsal Vagal Complex in brain, which activates the release of GRP onto G cells, which secrete gastrin into circulation, causing release of gastric secretions.
Pancreatic duct cell secretion machinery
At apical membrane, CFTR for release of Cl, also Cl HCO3 exchanger for release of HCO3.
How does secretin increase HCO3 release?
Increases CFTR, which increases CL HCO3 exchange. Increase HCO3 brought into the cell at the basolateral membrane with HCO3 Na cotransporter.
How does pancreatic bicarbonate concentration change as pancreatic flow increases?
It increases!
How is release from pancreatic acinar cells mediated?
VIP and secretin increase cAMP. GRP, ACh, CCK increase Ca, all promote vesicle fusion and release.
Formation of Bile Acids (primary vs secondary)
From cholesterol. Primary bile acids from the liver are conjugated with glycine and taurine and secreted into bile. Reabsorbed in the terminal ileum. Secondary bile acids are formed in the colon from primary bile acids, when bacteria conjugate them). They are absorbed, changed by liver and stored in gallbladder.
Why conjugate bile acids?
Lowers pKa, which enhances hydrophilicity to reduce passive diffusion during transport in biliary tree.
What are micelles?
Pockets of hydrophobic and hydrophilic interactions that are formed by bile acids. Take up FA’s and stuff.
What happens to micelles transporting FAs.
They are taken up into enterocytes and changed into chylomicrons, which then enter lymph. Micelles are recycled.
Where are epithelial cells born and where do they migrate?
They’re born at the bottom of the intestinal crype and migrate up villus. This process takes 5-6 days.
How much volume is presented to the SI?
8000mL, 6000 Absorbed. 1500 absorbed in colon, 100-200 excreted in fecal matter.
Mechanism of H2O absorption
During fasting, no spaces exist between mucosal cells. During absorption, spaces form. Absorbate transferred to nearest capillary.
Solute transport produces an osmotic gradient. H20 moves passively down osmotic gradient, absorbate is isotonic. H2O moves into capillary down hydrostatic gradient.
Differences in passive permeability between jejunum, ileum, colon.
Jenunum has widest holes, then ileum, then colon.
Transport of sodium in intestine.
The major ion that drives absorption. Taken into a cell actively by 3 mechanisms: 1) solute coupled sodium transport, where Na is coupled with glucose or amino acids SGLT1. 2) Sodium-hydrogen exchanger that’s coupled with Cl (in) - HCO3 pump. HCO3 and H produce H2O and CO2 in intestine. 3) Electrogenic Na channel diffusion promoted by aldosterone.
SGLT 1
Na and Glucose transported into cell. Na pumped out of basolateral membrane by NaK pump, glu pumped out via GLUT2. This creates a hypertonic environment that water will migrate to dilute.
How does aldosterone affect sodium uptake from lumen?
Increases it by increasing activity of Na channel. Also, NAK pump creates negative charge inside cell, which pulls sodium in.
How is K transported?
Passively via concentration gradients. Mostly into lumen due to electrochemical gradient.
Transport of chloride in intestine.
Major ion that drives secretion of fluid. Follows electrochemical gradient generated by Na transport and via the Cl HCO3 exchange. Enters basolaterally via the NA K 2Cl pump, exits via CFTR, which is increased by anything that activates cAMP cGMP intracellular Calcium.
Osmotic diarrhea
Lactose intolerance. Non absorbable solute enters lumen, water enters lumen in the small bowel. Solute and water load exceeds absorptive capacity, but mucosal transport processes are intact. Stool volume decreases with fasting
Secretory diarrhea
Stimulation of normal secretory processes. Absorptive processes intact but overwhelmed. cAMP/cGMP Ca, etc increasing CFTR secretion of Cl. Salt and water depletion. Stool volume persists despite fasting. Also blocks Na transport into cell.
Motility diarrhea
Caused by insufficient time for absorption due to hypermotility.
Where does digestion occur?
In the lumen or in the brush border (premucosal)(
Where does absorption occur?
In the mucosa
Where is iron absorbed?
In the duodenum
Where are carbs absorbed?
Mostly in duodenum and jejunum, some in ileum.
Where are protein lipids sodium and water absorbed?
All throughout the small intestine.
Where is B12 absorbed?
In the ileum
Where are bile acids absorbed.
Mostly in the ileum, very little before.
How are carbohydrates digested? What are the components of carbohydrates?
Must be hydrolyzed to monosaccharides. Starch 50%, Sucrose- 30%, Lactose-10%
Amylase
Converts starch to maltose
Maltase
In brush border membrane, converts maltose to glucose units
Sucrase
In intestine brush boarder, converts sucrose to glucose and fructose
How is glucose/galactose absorbed?
Actively transported into cell w/ sodium via SGLT1, pumped out, then diffuse to portal vein. Anything unabsorbed gets converted to short chain fatty acids.
How is fructose absorbed?
Using GLUT5
How is protein digested?
Hydrolysis to oligopeptides or amino acids.
Where does protein digestion occur?
In stomach, where HCl denatures protein and pepsin hydrolyzes it to polypeptides. In duodenum and pancreas, other proteases are released. Trypsin continues to cleave pancreatic proenzymes. Small intestine- broken down further.
How are proteins absorbed?
Amino acids and oligopeptides are absorbed and transported to capillaries into portal vein.
Pancreatic zymogens
Trypsin created from trypsinogen by enteropeptidase. Trypsin cleaves zymogens to create chymotrypsin, elastase, carboxypeptidase.
How are lipids digested?
Emulsified, lipolysis by gastric lipase, packaged into micelles.
How are lipids absorbed?
Passive diffusion of mixed micelles across bbm into cell. Resynthesized as chylomicrons and VLDL and exported into lacteals. Medium chain tryglycerides are absorbed into portal vein.
How is medium chain triglyceride absorption different from that of other fats?
MCTGs diffuse to portal vein, all others into lacteals.
Fat soluble vitamins
ADEK
Water soluble vitamins
BC
How is cobalamin absorbed?
Along with intrinsic factor made by parietal cells. Absorbed in ileum.
Where are most minerals absorbed?
In proximal small intestine (duodenum, jejunum).
How is iron absorbed?
With acid, it changes its oxidation state and is absorbable.
