W2: NUTRIENTS, STRUCT + FUNCTION, GASTRIC SECRETION, MOTILITY & GI ORGANS Flashcards
Describe the digestive processes for conversion of complex carbohydrates to monosaccharides
disacc.: breakdown by brush border enzymes
polysacch.: a-amylose starch => glucose chains amylopectin => amylase: a-1,4 cellulose => unbranched linear glucose: B-1,4 glycogen: via pancreatic amylase: a-1,4
Describe the mechanisms whereby monosaccharides are absorbed across intestinal epithelial cells
GLUT 5 transporter (passive diffusion) - nil osmotic effect
SGLT1 (2º active transport); 1Na+glucose/galactose = change (Na gradient established by K-Na-ATPase
=>
GLUT2
=> osmotic gradient, H20 travels into small intestines
Describe the digestive processes for converting proteins to small peptides and amino acids.
endopeptidase = interior A.A exopeptidase = terminal A.A aminopeptidase = NH2 end of A.A carboxypeptidase = COOH end of A.A
Describe the mechanisms of absorption for amino acids and small peptides.
Na-A.A transporter + Na independent carrier: Na gradient established by K-Na-ATPase =>osmotic gradient
PEPT1 (dipeptide/penicillin tripeptide) is H+ dependent. H+ microenvironment established by NHE3 (Na in; H+ out) d/t Na-K-ATP Na gradient in cell.
Describe the digestive processes for conversion of fats to triacylglycerols and monoglycerides.
smc contraction + emulsifying gent + bile => emulsification of lipid droplets (acidic environment d/t H+ balances FA charge)
pancreatic lipase = monoglyceride + 2 F.A
• Explain the requirement for emulsification of ingested fats.
Increases SA and accessibility of lipase action
- FA + monoclycerol enter sER = triacylglycerides reform
- emulsified by ampiphatic protein + vesicles transported to golgi
- exocytosed into ECF as CHYLOMICRONS
Describe the role of bile salts in the production of emulsification droplets and micelles
ampiphatic therefore binds to respective parts of lipids
=> repulsion and prevents reformation
• Describe the absorption of fat soluble and water soluble vitamins.
Vit B, C, Folic Acid (water soluble)
=> diffusion / carried-med transport
vit A, D, E, K
=> sER > Golgi > Chylomicrons (ECF) > thoracic Duct > Vena Cava
Describe the absorption of the important dietary minerals
B12: intrinsic factor in stomach = complex; absorbed in distal ileum
Fe: abs via DMT (duodenal enterocytes) > ferritin complex (intracellular store)
Vs
unbound fe via IREG1. bound to transferrin in blood.
- Ferritin exp regulated via own Fe-levels
Describe the anatomy of the stomach and oesophagus and assign general physiological functions to each of its components
stomach is the site of intrinsic factor production essential for B12 abs.
Describe the features of the stomach and oseophagus canal wall structure
OESOPHAGUS WALL mucosa: statified squamous submucosa: glands for lubrication muscularis ext: superior 1/3 skeletal; rest smc adventitia
STOMACH WALL lumenal mucus cells: gastric pits cont. glands mucosa & submucosa: rugae m. ext.: longitudinal, circular, oblique serosa
What cells are found in the gastric glands
mucus neck
parietal: HCL + intrinsic factor
chief cells: pepsinogen
Describe the reflex control mechanism involved in swallowing.
- voluntary oral phase
- pharyngeal phase (medulla) d/t bolus presence
propulsion to somach via peristalsis
receptive relaxation of stomach; vagal reflex: relaxation of thin elastic smc + fundus and body
Describe the structure and various functions of the stomach.
FUNDUS = storage
BODY = storage, mucus, HCl, pepsinogen, intrinsic factor
ANTRUM = mixing/grinding, gastrin (neuroendocrine Gcells)
Explain the importance of gastric acid and pepsin secretion and describe the hormonal and neural mechanisms responsible for their control
gastric acid secr. (+) cephalic phase (senses+thought) 1. CN. X 2. +ACh (parietal cells) 3. +Gastrin (G cells) > parietal cells 4. Gastrin/ACh (enterochromatin-like cells) > histamine > parietal
GASTRIC PHASE
+stomach distension = CN X => ACh (P cells)
+ peptides in lumen = Gastrin (g cells)
+ Gastrin/Ach (ECL) > Histamine > P Cells
PEPSINOGEN secr (chief cells) : zymogen; pH 3+lover; controlled by Gastrin | Histamine | ACh
HCl secretion in the stomach
Parietal cells
CO2 > carbonic acid > breakdown > H+ lumen; bicarb blood > Cl- enters and leaves cell to lumen > H20 drawn into lumen
HKATPase
(+) Gastrin via PKC
(+) Histamine via cAMP pkA
(+) ACh via CN X cholinergic M3 receptors > PKC
(-) Prostaglandin E3 receptor > Gi inhibits cAMP
Explain the essential role of intrinsic factor in vitamin B12 absorption
IF secreted by parietal cells found in the gastric body
* IF/B12 complex absorbed in ILEUM
Explain the functions of gastric mucus
(surface epith. and mucus neck)
*protective (pH7); lubricant;
Inhibition of Gastric Acid secretion
(-) cephalic phase
(-) gastric phase
INTESTINAL PHASE: ENTEROGASTRIC SPLANCHNIC REFLEX
1. ACID in duodenum
=> SECRETIN (duodenal mucosal S cells)
bicarb secr.
=> ↓gastrin + ↓gastrin stimulation of P cells
- FAT + CHO in duodenum
=> GIP
=> ↓ gastric secr. + HCl secr.
Role of enterogastrogens
protective and prevents acidic dmg to duodenum
inhibits: gastric acid section
and
↓↓gastric acid emptying via motility/pyloric sphincter
Describe the organisation of the enteric nervous system & peristalsis
meissners plexus = submucosal
myenteric plexus = muscularis propria
pacemaker cells in LONGITUDINAL MUSCLE LAYER
slow wave; basic electrical rythm
conducted via gap junctions
gastrin(+) stim to threshold & AP
distension (+)
presence of producs/tonicity (-)
Explain the mechanisms involved in the neutralisation of gastric acid in the duodenum, including control of bicarbonate secretion in the duodenum
BRUNNERS GLANDS (submucosal duodenal) HCO3
(+) duodenal acid via CN X & ENS
(+) secretin (PANCREAS + LIVER)
-ve feedback; acid neutr. = inhibits secretin release + self-limiting
Explain the functions of the pancreas
duct cells = bicarb
acinar cells = zymogen storage
sphincter of oddi @ hepatopancreatic ampulla = control of pancr. secr. and biliary output
Describe the actions of secretin and cholecystokinin (CCK) on pancreatic and bile secretion and the stimuli which will cause their release
(+) SECRETIN (acid in duoden) => HCO3
(+) CCK (fat/A.A duoden) => zymogen stimulation
(+) Vagal/local (organic mol. presence)
Describe the control of pancreatic enzyme secretion and the role of zymogens
zymogens released from acinar cells; lobules connected to intercalated ducts via CCK
zymogens are activated at the duoden brush border by
ENTEROKINASE
Describe the structure of the exocrine pancreas
ACINAR LOBULES > INTERCALATED DUCT> INTERLOBULAR DUCT > PANCREATIC DUCT
A + B CELLS > PANCREATIC ISLET > TO BLOODSTREAM
Describe the structure and function of the liver.
posta: connective tissue capsule allows branching + support
portal triad: hepatic portal vein; hep art.; hepatic duct
lobule: central hepatic vein => IVC
Hepatic blood flow
alimentary canal => hepatic portal vein => hepatic sinusoid
Hepatic Arteries => hepatic sinusoid
hepatocytes (bile synth, nutrient storage) ⇔ hepatic sinusoid
Hepatic sinusoid => central vein => hepatic vein => RHS
Describe the composition and function of bile, the mechanisms controlling its storage and release from the gallbladder and the mechanism whereby it is reabsorbed.
Liver: prod and secr. of bile + bile components (bile acid from cholesterol)
Bile is reabs and recycled @ileum via enterohepatic circulation/ excreted into urine / modified by bacteria
Falciform ligament significance
part of embryological foregut and forms a connection between the ventral abdominal wall and the liver.
Describe the structure and function of the small intestine
DUODENUM
-acid neutr; digestion; Fe abs
JEJENUM
-abs
ILEUM
- NaCl/H20 abs => chryme dehydration
Describe the basic electrical rhythm and explain the difference between segmentation and peristalsis
segmentation: cicular muscles contraction
- B.Elec.Rhythm determines frequency of segmentation
- moves chyme up+down and brings into contact w/ abs surface and enzymes
peristalsis: rythmic contraction of longitudinal muscles
- pacemaker cycle through spontaneous depol->repol in longitudinal
- sub-threshold > threshold => variable AP generated
CN.X => ↑contraction VS Symp. ↓contraction
Describe the features which act to increase absorptive surface area along the alimentary canal
- PLICAE CIRCULARES: fold channels chyme follow
- villi projections
- lacteal core; peyer’s; goblet cells
- Crypt secretion of H20 via apical CFTR brings secretes Cl-
=> cell is -ve d/t 3Na pumped out for 2K (Na/K Cl co-transporter)
-> Cl- gr builds up
State the structure and functions of the large intestine
simple columnar flat mucosa w/ large crypts + Goblets
(stratified squamous epithleium @ anal canal
- complete circular; incomplete longitudinal = TENIAE COLI = haustra pouches
- COLON: active transport of Na from lumen->blood therefore abs of H20. Chyme dehydration.
Describe the importance of intestinal bacteria in the digestive process
Bacterial fermentation of undigested carbs
• SCFA
• Vitamin K synth
• Flatulus
Describe the rectum, anal canal and rectal sphincters
Rectum: storage of stool until evacuation
internal and external anal sphincters are rings of muscle at the opening of the anus. Keeps the anus closed as stool collects in the rectum. Increasing pressure on the rectum wall causes the internal anal sphincter to relax.
external anal sphincter = skeletal muscle!
Describe the mechanisms controlling defecation
MASS MOVEMENT CONTRACTION => progressive distension of rectal wall (mechanoreceptors)
Defection reflex (para. pelvic splanchnic nerves)
- Rectal contraction
- sphincters (int. relax ext constrict.) => descending pathways!
- pressure on exp. sphincter
Describe the mechanisms which cause secretory diarrhoea and explain how this can be treated with simple salt/sugar solutions
enterotoxigenic bacteria (vibro cholerae; E. coli)
activate Cl- secretion => H20 secretion
↑cAMP, cGMP, Calcium
villus absorptive capacity hindered
=>Na/Glucose solution drive H20 abs => reabs
passive sodium-glucose cotransport (symporter)
