BIOL 373 Exam Flashcards
what are the functions of the GI system?
Digestion- break down macromolecules into forms that ca be transported across the epithelium
absorption- transport nutrients, water, ions, vitamins across the epithelium
motility- keep the gut contents moving
secretion- release on enzymes into the gut lumen
maintain water balance- balance between secretion and reabsorption
what are some problems faced by the GI tract regarding its function?
- need to digest macromolecules but not itself
- need to allow entry of digested nutrients but not pathogens
-GI lining is largest area of contact between external and internal environments
-protection from pathogens by various barriers:
-epithelial barrier
-mucus
-digestive enzymes
-acid
-GALT
anatomy of GI tract
stomach
small intestine- duodenum, jejunum, ileum
large intestine- colon, rectum
what is the mucosa surface anatomy?
- mucosa- epithelium, lamina propria, muscularis mucosa
- submucosa- messsiners submucosal plexus
- smooth muscle layers- circular muscle, longitudinal muscle, Auerbach’s myenteric plexus
- serosa
differences between small intestine and stomach anatomy
stomach: gastric glands, oblique muscles
small intestine: villi, crypt, peyers patch (in mucosa)
what features increase surface area?
stomach- gastric gland
small intestine- crypts
What are the two major patterns of contraction for gut motility?
peristalsis: moving food from mouth to anus, contraction and relax to move food forward
segmental contractions: mixing/churning, maximizes exposure to digestive enzymes and epithelium (little or no net movement forward)
these both occur during/after a meal
Tonic vs. phasic gut motility contractions
-most gut muscle is single unit smooth muscle, connected by gap junctions
-certain regions are tonically contracted for minutes to hours: smooth muscle sphincters keeps food from moving backwards
-other regions undergo phasic contractions for a few seconds: posterior stomach, small intestine
migrating motor complexes
this occurs between meals
sweep slowly down tract, roughly 90 minutes from stomach to large intestine
slow wave potentials
below threshold= no contraction
above threshold= opening of voltage gated Ca channels-> action potentials-> contraction
the degree of the contraction relates to the amount of Ca that enters
longer wave= more time for Ca to enter= larger contraction
amplitude and duration of contraction influenced by: hormones, paracine factors, neurotransmitters
Wave frequency and interstitial cells of cajal
slow wave frequency varies in different regions of the tract
more frequent in duodenum versus the stomach
set by pacemakers cells between smooth muscle layers
what is secreted in the GI tract
-water and ions (secreted into lumen then reabsorbed)
-enzymes
-mucus
-saliva
How are water and ions secreted in GI tract?
-mostly through membrane transporters
-water follows osmotic gradient
-water and ions in some regions can pass between cells: paracellular pathway
-similar channels and transporters to kidney but expressed on apical vs. basolateral membranes
transporters:
Na/k ATPase, NKCC cotransporter, H+/K+ ATPase
exchanger- Na+/H+, Cl-/HCO3-
Ion channels:
ENaC, K+ channels, Cl- channels (CFTR)
How is acid secreted?
acid is secreted by parietal cells
H+ is a million times higher in the lumen
- inside the cell CA forms bicarb from OH and CO2
- on the basolateral side: HCO3- out and Cl- into the cell
- On the apical side: H+ out and K+ in using the H+/K+ ATPase. Cl- out of the cell through Cl- channel
bicarb moving out of the cell is absorbed by blood, alkaline tide can be measured after a meal
How is NaCl secreted?
it is secreted from small intestine, colon, salivary glands
- Na+, K+, 2Cl- enter the cell on the basolateral side through NKCC
- Cl- enters the cell lumen on apical side through CFTR channel
- Na+ is reabsorbed on the basolateral side through Na+/K+ ATPase
- Negative Cl- in lumen attracts Na+ by paracellular pathway and water follows. goes from basolateral to apical side
crypt cells in small intestine and colon secrete ‘isotonic saline’ that mixes with mucus secreted by goblet cells to lubricate gut contents
Bicarb secretion from pancreas into duodenum
bicarb secreted into duodenum neutralizes acid coming from the stomach
acinar cells of pancreas secrete enzymes
epithelial cells lining ducts secrete bicarb solution
How is bicarb secreted?
secreted from epithelial cells lining ducts-> secreted into duodenum to neutralize stomach acid
- CA creates bicarb inside the cell (CO2 + H20 = HCO3- + H+
- basolateral side: Cl- enters the cell through the NKCC
- apical side: HCO3- leaves the cell through HCO3-/ Cl- exchanger, Cl out of the cell through CFTR channel then it reenters the cell through HCO3-/Cl- exchanger
How does cystic fibrosis effect the pancreas?
-mutation in gene that encodes in CFTR channel
-leads to a defect in the Cl- channel, and water transport
-fluid filled cysts and fibrosis (named for changes in the pancreas)
1. Cl- not transported into the ducts
2. there are various effects, water and Na+ do not transport into ducts since Cl- doesn’t
3. mucus is still produced but is greatly thickened due to lack of water
4. blockage of pancreatic ducts
5. exocrine secretions of pancreas does not get released
6. back pressure/ inflammation causing damage to pancreas
How are enzymes secreted?
-enzymes are secreted by exocrine glands or epithelial cells of stomach and small intestine
-they are synthesized by rough ER, packaged by golgi into vesicles, stored in cell under signal for release by exocytosis
-enzymes sometimes remain linked to apical membranes by protein or lipid stalks
often released as inactive precursor to prevent-auto digestion (zymogens)
-secretion regulated by neural, hormonal, paracrine signals (usually stimulated by parasympathetic)
How is mucus secreted?
-mucus consists of mucins mixture of glycoproteins
-produced by exocrine cells:
-serous cells in salivary gland
-mucous cells in stomach
-goblet cells in intestine
-signals for secretion:
-parasympathetic stimulation
-many neuropeptides
-cytokines
-infection and inflammation increase mucus secretion
How is saliva secreted?
-the salivary glands are arranged similar to exocrine pancreas (acini and ducts)
-saliva is a complex hypo-osmotic fluid
-it is a two step process:
1. fluid is secreted by acinar cells similar to ECF
2. as it passes through ducts, epithelial cells take back Na+ and secrete K+, so it eventually resembles intracellular fluid
-ducts cells have low water permeability, so water remains in saliva-> hypo-osmotic
-signals for secretion is parasympathetic, inhibited by sympathetic
organization of hepatic lobule. how is bile secreted?
hepatocytes-> bile canaliculi-> bile ductules-> common hepatic duct-> common bile duct (gall bladder)-> duodenum
how does blood flow work in the liver?
-most absorbed nutrients move into capillaries than into villi, then into hepatic portal vein
-xenobiotics must first pass through the liver before systemic circulation
75% hepatic portal vein, 25% hepatic artery-> sinusoids-> central vein-> hepatic vein
what are the key components of bile
-bile salt
-bile pigments
-cholesterol
-drugs and other xenobiotics being processed in liver and excreted in kidneys
Iron and RBC turnover: bilirubin
- spleen converts Hb to bilirubin
- liver metabolizes bilirubin and excretes it in bile
- bilirubin metabolites excreted in urine and feces
what are bilirubin metabolites responsible for
-normal colour of feces and urine
-yellow colour of bruising and yellow pigment of jaundice
what is digestion?
combination of mechanical and enzymatic processes
-occurs int he mouth, stomach, small intestine
-chewing, churning, expose more surface area to enzymes
what is absorption?
crossing the gut epithelium
-mostly in small intestine
-use many of the same transporters as in the kidney
are digestion and absorption regulated?
not directly regulated
-influenced by motility and secretion, which are regulated
how does absorption in the small intestine occur?
lumen-> apical membrane-> epithelial cell-> basolateral membrane-> interstitial-> capillary or lymph
carbohydrates
-constitute about half caloric intake, mostly starch and glucose
-can only be absorbed through membrane transporter
-we only have transporters for monosaccharides
-artificial sweetners can interact with sweet receptors but can not be digested as a way to cross into enterocytes
how are carbohydrates digested?
glucose polymers-> (amylase)-> disaccharide-> monosaccharide
how are carbohydrates absorbed?
- glucose/ galactose enter with Na+ on SGLT apical side and exit through GLUT2 on the basolateral side
- fructose enter through GLUT5 on apical side and exit through GLUT2 on basolateral side
how is protein absorbed?
proteins break down into peptides
-di and tri peptides transport with H+
-amino acids transport with Na+
how is protein digested?
In two different ways, through endopeptidase and exopeptidase
- endopeptidase
-endopeptidase digests internal peptide bond
-making smaller peptides
-examples are trypin, pepsin, chymptrypisn - exopeptidase
-digests terminal peptide bonds to release amino acids
-examples are aminopeptidase, carboxypeptidase
products of protein digestion are amino acids, di and tri peptides