Digestive System Extra Credit Assignment Assignment: You have

Digestive System Extra Credit Assignment
**The following Suggested Answer is based on an Extra Credit submission by a former student. I
couldn’t have said it better myself!
Assignment: You have just eaten a balanced meal. The meal contained the
macromolecules protein, carbohydrate, and fat as well as a vitamin B 12 and a
vitamin D (fat soluble) supplement. Describe what will happen to this food and
the supplements in the digestive tract.
Anatomy: structures
Oral cavity (lips, tongue, teeth, hard and soft palate, uvula, muscles of mastication)
oropharynxlaryngopharynxupper esophageal sphincteresophaguscardiac sphincter
(lower esophageal sphincter)stomach (cardiacfundusbodypyloric regions: pyloric antrum,
pyloric canal, pyloris)pyloric sphinctersmall intestine (duodenumjejunumileum)assisted
by secretions of accessory organs liver, gallbladder, & pancreas
nutrients are absorbed (usually facilitated diffusion) by epithelial cells in small intestineblood
capillarieshepatic portal veinlivermixed with hepatic arterial bloodkupffer cellshepatic
veininferior vena cavaheartlungsheartcells in body
what is not absorbed by epithelial cells in small intestine ileocecal sphincterlarge
intestinececumascending colon transverse colondescending colonsigmoid
colonrectumanal canalanusinternal anal sphincterexternal anal sphincter
Physiology: mechanical and chemical
CEPHALIC PHASE:
 stimulated by thinking, smelling, and tasting food
 parasympathetic (PNS)  stimulates G cells to release
Gastrin
releases ACh which stimulates Parietal cell to release HCLGastrin stimulates Parietal cell to release HCL Salivary glands activated (salivary amylase)
Oral Cavity:
 teeth grind nutrient particles increases nutrient surface area
decreases particle size for ease of swallowing
 salivary glands submandibular, parotid, sublingual
Salivary Amylase to begin breakdown of carbohydrate
Lingual lipase to begin breakdown of lipids
Mucous to assist in nutrient particles sticking together to form bolus,
assists in movement of bolus
Swallowing:
 Buccal phase: voluntary initiation of swallowing, tongue and mastication muscles push
food to pharynx
 Pharyngeal phase:
mechanoreceptors at oropharyngeal and layrngopharyngeal opening initiate
swallowing
reflex


raising of :
uvula to close off nasopharynx; epiglottis to close off trachea
soft palate
hyoid bone
larynx
longitudinal pharyngeal muscles contract, pulling pharynx to bolus; food moves via
peristalsis into esophagus
Esophageal phase:
inferior constrictor muscle relaxes to allow esophageal opening
superior muscle contracts, bolus s forced into esophagus
Esophagus:
 mechanoreceptors of enteric nervous system (ENS) stimulates upper esophageal
sphincter to relax, bolus enters esophagus
 upper esophageal sphincter muscle contracts to prevent reflux
 smooth muscle of esophagus through peristalis and gravity, push bolus to lower
esophagus
 mechanoreceptors at lower esophageal sphincter (cardiac sphincter) stimulate sphincter
to relax, bolus enters stomach
 lower esophageal sphincter contracts to prevent reflux
GASTRIC PHASE:
Stomach:
 bolus distends stomach wall, mechanoreceptors stimulate ENS and PNS which
release
G release Gastrin
stimulates PNS, which stimulate Chief cell to release Pepsinogen
stimulates smooth musculature, increases action potential and
force of
contraction
ACh stimulate: ECL cells to release Histamine which stimulate Parietal cells
increase secretion of HCLincrease secretion of Intrinsic factor which attaches to
vitamin B12 to allow absorption (in ileum)
 increase of HCL- decreases stomach pH which stimulates:
mucous cells to release mucous and HCO-3
Chief cells to release Pepsinogen which is activated into Pepsin by HCLPepsin begins breakdown of protein
 presence of protein (peptides) in stomach stimulates release of Gastrin
 bolus is mixed with gastric juices forming chyme
 secretion of Somatostatin by glands in pyloris which will suppress release of Gastrin,
CCK, Secretin,
INTESTINAL PHASE:
Duodenum of Small Intestine (SI):
 retropulsion, before nutrient intake, pyloric sphincter is closed to prevent reflux (s)
 mechano/chemoreceptors at pyloric sphincter detect duodenal stretch, chyme, pH, liquid,
solid, lipid and initiate :
gastroenteric reflex pyloric sphincter relaxes, contracts, relaxes, contracts to
allow for slow
passage of chyme into duodenum, slowing of gastric motility but increase of
intestinal motility
PNS innervation of smooth muscle of stomach to decrease peristalis, allowing for
slower gastric
emptying

low pH of chyme stimulates secretion by:
duodenal S cells Secretin which:
stimulates liver, pancreas, and duodenum to secrete HCO-3
which increases
pH (decreases free H+) in duodenum
duodenal D cells secrete Somatostatin which:
decreases peristalis in stomach
decrease parietal cell secretion of HCL
decrease ECL cell secretion of Histamine
increases gastric D cell secretion of Somatostatin which
decreases
secretions of:
gastrin
CCK
Secretin
which will slow gastric emptying
duodenal I cells CCK which:
causes contraction of gallbladder and relaxes hepatopancreatic
sphincter
stimulates Pancreas to secrete Lipase, Amylase,Trypsinogen,
Proelastase,
Chymotrypsin, Procarboxypeptidase A & B

exocrine secretions from pancreas and gallbladder enter duodenum through
hepatopancreatic ducts
Digestion (most takes place in the Duodenum & Jejunum; some occurs in Ileum):
 increase Mucous secretion from goblet cells
 epithelial cells (intestinal cells; enterocytes) secrete:
Enterokinase converts Trypsinogen into Trypsin which then activates
Proelastase
into Elastase,Chymotrypsinogen into Chymotrypsin,
Procarboxypeptidase A & B
into Carboxypeptidase A & B, all of which break down protein into
amino acids
Sucrase, Maltase, Lactasebreaks down complex carbohydrates into
disaccharides
and monosaccharides
Intestinal lipasebreaks down:
triglycerides (TG) to glycerol and fatty acids (FA)
cholesterol to cholesterol and FA
phospholipids to FA
 all of which are emulsified with bile salts to form micelles
Absorption (most takes place in the distal Duodenum & Jejunum; some occurs in Ileum):
 microvilli, villi, plicae on epithelial lumen surface increase surface absorption area
 micelles diffuse to epithelial cells ( including vitamins A, D, E, K)
FA and monoglycerides are released from bile salts and enter enterocytes
FA and monoglycerides enter the smooth ER and are converted back into TGs
TGs are encased in protein along with cholesterol and phospholipis to form
Chylomicrons
Chylomicrons are exported from enterocytes into the interstitial fluid and absorbed
into lacteals
Lacteals -> cisterna chyli -> thoracic duct -> subclavian vein
all other nutrient particles are taken up by facilitated diffusion, and what is not utilized by
cell is transported into surrounding capillaries where it is taken to the liver by way of
hepatic portal vein
such as:
+
 amino acids are taken up by epithelial cell with Na - amino acid cotransporter
and what is not
utilized by cell is diffused into capillaries surrounding SI by facilitated diffusion
 carbohydrates are taken up by epithelial cells with Na+ - glucose cotransporter
and what is not
utilized by cell is diffused into capillaries surrounding SI by facilitated diffusion
 bile salts are taken up by Na+ - bile salt cotransporter, diffused to capillaries and
returned to
liver through hepatic portal vein
 vitamin B12-Intrinsic Factor is absorbed

Musculature in SI:
 stretching of duodenum, mechanoreceptors signal ENS, and myenteric and submucosal
plexus
 chemoreceptors detect fats in SI, stimulate epithelial cells to release Somatostain which
decreases motility to allow for digestion and absorption
 segmented contractions as nutrients pass through, small segment contracts, dividing
chyme, then relaxes to allow chyme to remix, allows for chyme to mix with digestive
juices in SI
 peristalic contractions smooth muscle behind chyme contracts while smooth muscle in
front of chyme relaxes, propelling chyme forward
 Gastroileal reflex: ileocecal sphincter muscle is usually contracted when not digesting.
 mechanoreceptors detect undigested matter, trigger PNS and ENS to relax ileocecal
sphincter
 undigested matter enters large intestine LI, ileocecal sphincter contracts preventing reflux
Large Intestine (LI):
 gastrocolic reflex mechanoreceptors detect distension of stomach wall as bolus enters,
stimulates PNS to release CCK and Gastrin which increase LI motility, frequency of mass
movement
 goblet cells are stimulated to release Mucous to ease mass movement
 musculaturemechanoreceptors stimulate PNS & ENS which control segmentation and
peristalis, very slowly (1-3x contractions per day)
 absorption Na+-ion active transport in LI epithelial call wall transports needed
electrolyes into blood
H2O is reabsorbed by osmosis to blood
 bacteria breakdown of molecules that cannot be digested by human enzymes, i.e.,
Cellulose
 mechanoreceptors detect stretch caused by fecal matter as it enters rectum stimulates
PNS & ENS, involuntary control,
rectum smooth muscle contracts
internal anal sphincter relaxes; external anal sphincter contracts
 somatic nervous system voluntary relaxation of external anal sphincter
muscle of glottis contracts, increasing abdominal pressure
fecal matter is eliminated