Phys 3 Flashcards
how much bile does the liver produce?
up to 1L per day
what is bile production stimulated by?
- secretin
- enterohepatic bile salt return
what is the function of bile?
- elimination of wastes
- emulsification of intestinal lipids
what stimulates the intestinal hormone secretin?
acidic chyme in the duodenum
What are the actions of secretin on the stomach?
decrease gastric secretion and motility
what is the action of secretion on the liver?
bile production
what is the composition of bile?
- 97% water
- bile salts
- electrolytes
- FA
- cholesterol
- phospholipids
- bilirubin
what is bile salt?
- conjugated bile acid in deprotonated form (cations)
- it is not excreted but returned to the liver
- contain bile acids
bile acids
- produced by normal flora
- produced by cytochrome p450 mediated oxidation of cholesterol
- insoluble unless congugated
what must bile acids be conjugated with in order to be soluble?
- taurine or
- glycine
bilirubin
- breakdown product of heme
- released from splenic macrophages
- congugated in liver to glucuronid which is more water soluble
- eliminated in feces
- some returns to liver
- can also be eliminated in urine
gall bladder
- thin walled muscular sac that lays in the ventral surface of the liver
- functions to store and concentrate bile (up to 10X)
- capacity: 30-40 mL
describe the route of the biliary tree
- left and right hepatic ducts merge to form common hepatic duct
- common hepatic duct and cystic duct merge to form common bile duct
- common bile duct and main pancreatic duct merge at hepatopancreatic ampulla in duodenum
what are the 2 sources of gall bladder contraction and therefore bile release?
- vagal stimulation (weakly)
- CCK (mainly)
CCK actions
- gall bladder contraction
- sphincter of Oddi relaxation
- pancreatic secretions
what stimulates the action of CCK?
fatty chyme in the duodendum
What happens after bile is released into the duodenum?
- it mixes w/ chyme and intestinal juices
- bile salts emulsify fats for absorption
- remainder of bile is eliminated in feces
monomer
- monoshaccharides
- glucose, fructose, galactose
polymers
- disaccharides
- lactose, maltose, sucrose
starches
- glucose polymers
- amylopectin
- salivary amylase
- pancreatic amylase
What is the important take away for digestion/absorption of carbohydrates?
there is an enzyme for each specific thing
i. e:
- lactose enzyme = lactase (SI)
- maltose enzyme = maltase (SI)
- sucrose enzyme = sucrase (SI)
Where does carb absorption occur?
along the length of the SI
In terms of carb absorption, what happens at the apical surface of SI cells?
-Glucose, galactose and fructose all move in to the cell
How are glucose and galactose transported into the epithelial cells along the SI?
- name of transporter
- type of transport
- by SGLT-1
- Na is high in the lume, moves along concentration gradient into cell
- secondary active transport
How is fructose moved into the SI cells?
- name of transporter
- type of transport
- GLUT-5
- facilitated transport
In terms of carb absorption, what happens at the basal surface of cells in SI?
- contents are transported to the interstitium
- use GLUT-2 transporter
Where does protein digestion begin?
stomach
Describe the beginning of protein digesting in the stomach
- chief cells produce pepsinogen (prohormone)
- low pH activates pepsinogen to pepsin which releases polypeptides
What is the function of the polypeptides released by pepsin in protein absorption?
- enteropeptidase activates trypsinogen to trypsin
- trypsin activates pancreatic carboxypeptidases at SI brush border
- trypsin activates chymotrypsinogen to chymotrypsin
What are the structures when peptides are further broken down into “smaller chunks”
oligopeptides
What works on the oligopeptides?
brush border enzymes (SI)
brush border enzymes from the SI:
- aminopeptidase
- dipeptidase
- tri and di monomers released
Describe the physiology of trimer and monomer digestion/absorption
trimer:
-through PepT1 proton cotransporter
-cytosolic preteases release aa
monomer:
-aa-Na cotransporters
-duodenum and jejunum distribution
How does infantile protein absorption occur?
by endocytosis d/t immaturity of intestinal mucosa
How are nucleic acids digested down after ingesting them?
- into pentose sugars
- nitrogenous bases
- phosphate ions
- all done by nucleases
What are the nucleases that digest nucleic acids?
- pancreatic nucleases: RNA and DNA nucleotides
- intestinal mucosa nucleases
what happens to the pentoses and bases during nucleic acid absorption
- pentoses absorbed w/ other sugars
- bases absorbed by active transport
Where does most lipid digestion occur?
duodenum
lipid digestion by lipase
- TG digested to 2 FFA + monoglyceride
- use lingual, gastric and pancreatic lipases
What is the function of cholesterol esterase?
-hydrolyze cholesterol esters
What does cholesterol esterification do to the cholesterol?
increases hydrophobicity
Function of bile salts in terms of lipid digestion/absorption
to emulsify lipid droplets which organizes them into micelles
What all lipids can diffuse across the brush border membrane? (4)
- FFA
- monoglycerides
- cholesterol
- fat-soluble vitamins
chylomicron
-lipoprotein composed of TG, FFA, chol, and vitamins
Where does chylomicron formation occur?
golgi
Explain the path of the chylomicron
- extruded from basal membrane
- enters lacteal
- travels vis lymph system to enter venous circulation at the thoracic duct
What are the fat soluble vitamins?
- AKED
- are absorbed w/ lipids
Where are water soluble vitamins absorbed?
in the upper SI by diffusion
absorption of vitamin B12
-binds to intrinsic factor
which is produced by gastric parietal cells
-absorbed in the ileum
absorption of K and B vitamins
- produced by normal flora
- absorbed in the colon
What is the source of electrolytes?
- foods
- GI secretion
cation and anion digestion/absorption
cations:
- Na: cotransported w/ glc and aa
- K: dimple diffusion; absorbed in SI, secreted in LI
anions:
- Cl
- HCO3
- both follow electrochemical gradients
What is the cofactor for Ca absoprtion
vitamin D
What happens if there is a decrease in blood Ca?
- parathyroid gland secrete parathyroid hormone
- stimulates bone to release Ca
- increases kidney reabsorption of Ca
- increases renal activation of vit. D increasing intestinal absorption
What are our sources of iron
- heme iron from hgb and myoglobin (only 10-15% but higher bioavailability)
- non heme iron from cereals, veggies, fruits etc
how is iron absorbed?
- ferric reductase enzyme on brush border reduced ferric to Fe2+
- DMT1 transporter actively transports divalent metals into enterocyte
- binds to ferritin = mucosal iron barrier
What happens when Fe is needed?
it is absorbed and carried by transferrin
What happens when Fe amount is adequate?
it is sloughed off and eliminated
How much fluid enters the intestine daily? And what is the breakdown?
about 10 L
- 8 L secreted
- 1-1.5 as saliva
- 2-3 by stomach
- 1 as bile
- 1 as pancreatic secretions
- 2 secreted by SI
How much fluid makes it to the colon?
- about 1 L
- 90% is absorbed across the SI epithelium
how much fluid is lost in feces daily?
- about 150 mL
- the rest is absorbed by colon
intestinal absorption of water
- passive process
- requires movement of solutes
