W2: NUTRIENTS, STRUCT + FUNCTION, GASTRIC SECRETION, MOTILITY & GI ORGANS

Question 1

Describe the digestive processes for conversion of complex carbohydrates to monosaccharides

Answer 1

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

Question 2

Describe the mechanisms whereby monosaccharides are absorbed across intestinal epithelial cells

Answer 2

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

Question 3

Describe the digestive processes for converting proteins to small peptides and amino acids.

Answer 3

endopeptidase = interior A.A
exopeptidase = terminal A.A
aminopeptidase = NH2 end of A.A
carboxypeptidase = COOH end of A.A

Question 4

Describe the mechanisms of absorption for amino acids and small peptides.

Answer 4

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.

Question 5

Describe the digestive processes for conversion of fats to triacylglycerols and monoglycerides.

Answer 5

smc contraction + emulsifying gent + bile => emulsification of lipid droplets (acidic environment d/t H+ balances FA charge)

pancreatic lipase = monoglyceride + 2 F.A

Question 6

• Explain the requirement for emulsification of ingested fats.

Answer 6

Increases SA and accessibility of lipase action

1. FA + monoclycerol enter sER = triacylglycerides reform
2. emulsified by ampiphatic protein + vesicles transported to golgi
3. exocytosed into ECF as CHYLOMICRONS

Question 7

Describe the role of bile salts in the production of emulsification droplets and micelles

Answer 7

ampiphatic therefore binds to respective parts of lipids

=> repulsion and prevents reformation

Question 8

• Describe the absorption of fat soluble and water soluble vitamins.

Answer 8

Vit B, C, Folic Acid (water soluble)
=> diffusion / carried-med transport

vit A, D, E, K
=> sER > Golgi > Chylomicrons (ECF) > thoracic Duct > Vena Cava

Question 9

Describe the absorption of the important dietary minerals

Answer 9

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

Question 10

Describe the anatomy of the stomach and oesophagus and assign general physiological functions to each of its components

Answer 10

stomach is the site of intrinsic factor production essential for B12 abs.

Question 11

Describe the features of the stomach and oseophagus canal wall structure

Answer 11

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

Question 12

What cells are found in the gastric glands

Answer 12

mucus neck
parietal: HCL + intrinsic factor
chief cells: pepsinogen

Question 13

Describe the reflex control mechanism involved in swallowing.

Answer 13

1. voluntary oral phase
2. 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

Question 14

Describe the structure and various functions of the stomach.

Answer 14

FUNDUS = storage

BODY = storage, mucus, HCl, pepsinogen, intrinsic factor

ANTRUM = mixing/grinding, gastrin (neuroendocrine Gcells)

Question 15

Explain the importance of gastric acid and pepsin secretion and describe the hormonal and neural mechanisms responsible for their control

Answer 15

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

Question 16

HCl secretion in the stomach

Answer 16

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

Question 17

Explain the essential role of intrinsic factor in vitamin B12 absorption

Answer 17

IF secreted by parietal cells found in the gastric body

* IF/B12 complex absorbed in ILEUM

Question 18

Explain the functions of gastric mucus

Answer 18

(surface epith. and mucus neck)

*protective (pH7); lubricant;

Question 19

Inhibition of Gastric Acid secretion

Answer 19

(-) 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

2. FAT + CHO in duodenum
   => GIP
   => ↓ gastric secr. + HCl secr.

Question 20

Role of enterogastrogens

Answer 20

protective and prevents acidic dmg to duodenum
inhibits: gastric acid section
and
↓↓gastric acid emptying via motility/pyloric sphincter

Question 21

Describe the organisation of the enteric nervous system & peristalsis

Answer 21

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 (-)

Question 22

Explain the mechanisms involved in the neutralisation of gastric acid in the duodenum, including control of bicarbonate secretion in the duodenum

Answer 22

BRUNNERS GLANDS (submucosal duodenal)
HCO3

(+) duodenal acid via CN X & ENS
(+) secretin (PANCREAS + LIVER)

-ve feedback; acid neutr. = inhibits secretin release + self-limiting

Question 23

Explain the functions of the pancreas

Answer 23

duct cells = bicarb
acinar cells = zymogen storage

sphincter of oddi @ hepatopancreatic ampulla = control of pancr. secr. and biliary output

Question 24

Describe the actions of secretin and cholecystokinin (CCK) on pancreatic and bile secretion and the stimuli which will cause their release

Answer 24

(+) SECRETIN (acid in duoden) => HCO3
(+) CCK (fat/A.A duoden) => zymogen stimulation
(+) Vagal/local (organic mol. presence)

Question 25

Describe the control of pancreatic enzyme secretion and the role of zymogens

Answer 25

zymogens released from acinar cells; lobules connected to intercalated ducts via CCK zymogens are activated at the duoden brush border by ENTEROKINASE

Question 26

Describe the structure of the exocrine pancreas

Answer 26

ACINAR LOBULES > INTERCALATED DUCT> INTERLOBULAR DUCT > PANCREATIC DUCT A + B CELLS > PANCREATIC ISLET > TO BLOODSTREAM

Question 27

Describe the structure and function of the liver.

Answer 27

posta: connective tissue capsule allows branching + support portal triad: hepatic portal vein; hep art.; hepatic duct lobule: central hepatic vein => IVC

Question 28

Hepatic blood flow

Answer 28

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

Question 29

Describe the composition and function of bile, the mechanisms controlling its storage and release from the gallbladder and the mechanism whereby it is reabsorbed.

Answer 29

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

Question 30

Falciform ligament significance

Answer 30

part of embryological foregut and forms a connection between the ventral abdominal wall and the liver.

Question 31

Describe the structure and function of the small intestine

Answer 31

DUODENUM -acid neutr; digestion; Fe abs JEJENUM -abs ILEUM - NaCl/H20 abs => chryme dehydration

Question 32

Describe the basic electrical rhythm and explain the difference between segmentation and peristalsis

Answer 32

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

Question 33

Describe the features which act to increase absorptive surface area along the alimentary canal

Answer 33

- 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

Question 34

State the structure and functions of the large intestine

Answer 34

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.

Question 35

Describe the importance of intestinal bacteria in the digestive process

Answer 35

Bacterial fermentation of undigested carbs • SCFA • Vitamin K synth • Flatulus

Question 36

Describe the rectum, anal canal and rectal sphincters

Answer 36

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!

Question 37

Describe the mechanisms controlling defecation

Answer 37

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

Question 38

Describe the mechanisms which cause secretory diarrhoea and explain how this can be treated with simple salt/sugar solutions

Answer 38

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)