Chapter 24
Digestive System
Digestive Tract
top to bottom
Buccal cavity - between teeth and cheeks
Oral cavity - inside the arch of teeth
Fauces - opening of oral cavity into pharynx
Pharynx - ‘throat’ … in particular the oropharynx
Esophagus - muscular tube leading from pharynx to stomach
Digestive tract...
Stomach
Small Intestine (3 regions)
duodenum
jejunum
ileum
Large Intestine
colon
rectum
anus
Accessory Digestive Glands
Salivary - moisten & lubricate food, begin digestion
of carbohydrates
Liver - secretes bile for emulsifying fat (bile stored in
gall bladder
(g.b. not actually a gland, no secretions of its own
)
Pancreas - production of several digestive enzymes which
are secreted into duodenum
Wall of Digestive Tract
(a generalized description)
• Serosa - outer layer of connective tissue surrounding the
gut
• Muscularis - smooth muscle layers arranged both
circularly and longitudinally
• Submucosa - thick connective tissue layer with
glands, blood vessels, nerves and other components
• Mucosa - innermost layer - lined with stratified squamous
or simple columnar epithelium (mucous epithelium), a
loose connective tissue layer (lamina propria) and a thin
layer of muscle (muscularis)
Beginning Digestion
Ingestion - taking in food, usually by mouth
Mastication - chewing. Tearing, crushing food with teeth
Deglutition - swallowing… a single lump of food is
referred to as a bolus
Muscular Movement of Food
Propulsion of food through the digestive system occurs
by regular, pulse-like muscle contractions called peristalsis
Mixing/Grinding muscle contractions occur in stomach
these are referred to as segmental contractions
Rugae - muscular ridges in the wall of the stomach help
with the grinding process
But first...
Mastication - chewing of food with teeth
Humans are diphytodont (2 sets of teeth)
32 adult teeth & 20 deciduous (baby) teeth
Heterodont - different types of teeth specialized for
different tasks
Crown (outer layer) of enamel, hardest material in body
Dentin - layer chemically similar to bone but no internal cells
Pulp cavity - containing blood vessels & nerves
Salivary Glands 3 pairs
Secretory cells can be either serous (watery secretion) rich
in digestive enzymes (amylase) or mucous (mucoid secretion)
for lubrication
Parotid -located in front of the ear, serous glands producing
primarily serous saliva. Duct opens near upper M2
Submandibular - located under the jaw these glands are
roughly equal serous & mucous. Duct under tongue near
frenulum.
Gleek!
Sublingual - located under tongue, mixed but primarily
mucous secretion. No obvious duct, rather several
openings
Digestion begins
Chemical breakdown of carbohydrates begins in the mouth
with the enzyme Salivary Amylase
Food travels down esophagus below the laryngopharynx
carried along by peristalsis
Entry into esophagus controlled by upper esophageal sphincter
Lower esophageal sphincter (cardiac sphincter) at connection
of esophagus to stomach keeps digestive juices of the stomach
from reaching into the esophagus
Histology of Esophagus
Lining - stratified squamous epithelium - multi-layered
because this takes a lot of wear & tear, lots of swallowing &
some assault from gastric juices at lower end.
Thick muscularis mucosa to propel food by peristalsis
You can actually swallow food while standing on your head
TAA - tell Wally Schirra’s
astronaut & martini story
Anatomy of Stomach
4 major regions
Cardiac portion - where esophagus enters
Fundus - enlarged swelling at upper end (above cardiac region)
Body below the funds, makes up about 1/2 of the total area
Pyloric region - the funnel shaped lower end where the
stomach joins the duodenum (first part of small intestine)
Pyloric Sphincter - ring of muscle at the base of the pylorus
regulates the flow of material out of the stomach
Stomach
Lined with simple columnar epithelium (this lining continues for
the remainder of the digestive tract)
Tunica Muscularis arranged in 3 layers:
Longitudinal layer – outermost layer
Circular layer – middle layer
Oblique layer – innermost layer
Circular layer also forms a sphincter at the lower end of the stomach:
Pyloric Sphincter – regulates flow of food out of stomach
Digestion
Carbohydrate digestion, which began in mouth (salivary amylase)
is temporarily halted in stomach
Protein digestion begins in stomach
Food mixes with gastric juices to form chyme
Stomach Mucosa
Surface of mucosa (internal lining of stomach) arranged in gastric pits
Gastric pits contain:
Mucous cells – produce mucous (protective)
Enteroendrocrine cells – produce hormone gastrin (regulate)
Parietal Cells – secrete hydrochloric acid (HCl)
Chief Cells – secrete pepsinogen
Activation of pepsin
for protein digestion
Parietal cells
Chief cells
HCl
Pepsinogen
Q: Why not secrete active pepsin?
A: It would digest stomach lining if it
had nothing else to work on!
Pepsin
digestion of proteins
Movie Break
Ulcers!
Small Intestine
Histology of Duodenum (fig 24.11)
four tunics: serosa, muscularis, submucosa & mucosa (out to in)
Mucosal lining arranged in circular folds (increases surface area)
Inner mucosa is lined with villi – finger-like projections
Villi (singular = villus) – contain lacteal of lymphatic system for
absorption of fat. Also contain capillary network
Surface of villi further divided into microvilli which further increase
surface area forming a brush border
Cell types in Duodenal Mucosa
& Submucosa
Absorptive Cells – produce digestive enzymes & absorb nutrients
Goblet Cells – produce mucous (protective)
Granular Cells – immune response to bacteria?
Endocrine Cells – produce hormones that regulate digestion
Crypts of Lieberkühn – site where above cell types originate
Submucosa contains mucous glands called Brunner’s Glands
(duodenal glands) which secrete a mucous that helps
lubricate food bolus
Jejunum & ileum
Fewer Brunner’s Glands (mucous accini) – why?
These two sections are major sites of absorption
Lymph nodules called Peyer’s Patches numerous in mucosa &
submucosa of ileum
Overall structure similar to duodenum but a gradual narrowing is seen
Ileocecal junction – connection between ileum & large intestine = ring
of smooth muscle (sphincter)
Colon (large intestine)
Divisions of colon:
Ascending – up right side of abdomen
Transverse – across top of abdomen R L
Descending – down left side
Sigmoid – over to center and down to rectum
Rectum & anus – storage of feces & exit
Colon/Rectum
Longitudinal muscle layer greatly reduced. Exists as bands of
muscle called teniae coli.
Most colon musculature is circular muscle
Crypts of Lieberkühn exist but limited to mucous production
Rectum – last reservoir for feces prior to anal canal
Anal canal protected by stratified squamous epithelium
Two sphincters control anal canal:
Internal Sphincter - involuntary
External Sphincter - voluntary (to a point)
Colon/Rectum…
Anal canal ridged with anal columns (folds) which contain an artery
and vein
These veins can become weakened forming a sac-like structure - hemorrhoids
Hemorrhoids – often caused by poor diet  straining too hard to
void dense feces
Sometimes occur during late pregnancy & childbirth due to increased
abdominal pressures & straining
Defecation may occur as a result of gastrocolic reflex
‘full stomach = GOTTA GO!’ (massive peristalsis)
Liver
Divided into sections called lobes (which are subdivided into lobules)
Lobules are hexagonal and are composed of cords of simple cuboidal
epithelium called hepatic cords
Cords separated by two cavities
Capillary sinusoids – capillaries of circulatory system
Bile canaliculi – transport of bile
Border of lobule = site of hepatic triad
• branch of bile duct – collects bile from canaliculi
• branch of hepatic artery – oxygenated blood
• branch of portal vein – blood from stomach/intestine
Bile
Produced in liver lobules by hepatocytes (liver cells)
Drains from bile canaliculi – bile duct – hepatic duct –
cystic duct – gall blader
Bile – drains from gall bladder into duodenum through the
common bile duct … from liver into CBD from Common hepatic duct
Bile emulsifies fat
Movie
Break!
Gall Stones
Other Liver Functions
Produces blood proteins – fibrinogen, albumin, prothrombin
Stores minerals – zinc, magnesium, iron, copper
Stores vitamins A, D, E & K
Stores glycogen ‘animal starch’
Nutrient conversion – fat to glucose, glucose to fat, protein to fat,
proteins to glucose
Detoxifies poisons including alcohol
More functions of Liver
Breaks down bilirubin (from hemoglobin) into biliverdin for secretion
Von Kupffer’s Cells – part of mononuclear phagocytic system
line the sinusoids
Pancreas
Elongated organ runing along curve of duodenum. Contains both
endocrine and exocrine glands
Endocrine portion in isolated spots called Islets of Langerhans
produce insulin
Exocrine portion – Acinar glands – secrete digestive enzymes which
drain via pancreatic duct into the duodenum
Cells along duct secrete bicarbonate ions which serve as a buffer
to help neutralize stomach acid
Examine
Models!
Final
Thursday May 3
8:00 – 10:00 a.m.
Phases (3) of Deglutition
swallowing
1. Voluntary – a bolus (lump) of food is formed by jaw/tongue
action and is pushed against the roof of the mouth.
This action forces the bolus backward into the orpharynx
Pharyngeal Phase of Swallowing
2. Pharyngeal Phase – touch activated receptors in oropharynx are
triggered when contacted by food bolus. Cranial nerves V and IX
signal swallowing center in medulla. Nerves V, IX, X and XI signal
muscles in soft palate and pharynx.
• soft palate lifts to seal entry to nasopharynx (nothing up your nose!)
• pharynx rises to accept bolus
• superior, middle and then inferior pharyngeal muscles contract to
move bolus downward toward and then into esophagus
• Epiglottis (flap closing larynx) pushed back/down so it covers larynx
(don’t breathe your food!)
Swallowing continues
3. Esophageal phase – moves food between pharynx & stomach
peristaltic waves of the muscles in the esophageal wall cause bolus
to proceed down length of esophagus.
As one region contracts, the area immediately below relaxes
this set of involuntary contractions is mediated by the
intramural plexus (a network of nerve fibers) and by tactile
receptors in the esophageal wall which send impulses via
the vagus to medulla then back through vagus to muscles in the
esophagus
Stomach Regulation
Three Phases
Cephalic Phase
Gastric Phase
Intestinal Phase
histamine analogs such as
Tagamet® and Zantac®
bind to histamine receptors
on parietal
but dofluid
not
Function – prepare stomach for arrival of food
by increasing
stimulate release of HCl.
volume.
Cephalic Phases
Duration – short (minutes)
These
are competitive
Triggered by smell, taste, texture and thoughts
of food.
inhibitors that function
Mechanism – Neural, via preganglionic fibers
in vaguslike
synapsing
something
a ‘broken
with nerve plexus in stomach. Stimulates stomach
key’ cells
Actions:
• Parietal cells – release hydrochloric acid
• Chief cells – release pepsinogen
• Endocrine cells – release gastrin (hormone), which triggers
further release of HCl & pepsinogen
• Mast cells – release histamine triggering parietal cells to secrete
even more HCl
Gastric Phase
Functions: Enhance secretion started in cephalic stage,
homogenize and acidify chyme, initiate digestion of proteins by
pepsin
Duration: long (3-4 hours)
Mechanisms:
•Neural: short reflexes triggered by stimulation of stretch
receptors as stomach fills
stimulation of chemoreceptors as pH changes
•Hormonal: stimulation of gastrin released from endocrine cells
•Local: release of histamine by mast cells as stomach fills
Actions: increased acid & pepsinogen produced, increased motility
beginning of mixing waves of muscle contraction
Intestinal Phase
Function: control rate of chyme entry into duodenum
Duration: long (hours)
Mechanisms:
Neural: short reflexes (enterogastric) triggered by distension of
duodenum.
Hormonal: (two components)
1. stimulation of cholecystokinikn (CCK), gastrointestinal
inhibitory peptide (GIP), and secretin all released in response to
presence of acid, carbohydrates & lipids
2. release of gastrin released in response to presence of
undigested proteins
Actions: inhibit production of gastric acid and pepsinogen
(by feedback inhibition) and reduce motility of stomach
Mixing of stomach contents
Muscular contractions mix food with stomach secretions to form
chyme
Mixing Waves – peristaltic contractions about every 20 seconds
from body toward pylorus
Peristaltic waves – more powerful, less frequent, force chyme toward
pyloric sphincter. Simultaneously push more solid materials back
toward cardiac region to continue processing
Stomach Emptying
Generally 1 ½ - 2 ½ hours after entering stomach, materials leave
via pyloric sphincter by actions of pyloric pumping
Peristaltic contractions of pylorus forces a small volume of chyme
through the pyloric sphincter – action repeats until stomach is empty
of chyme
Digestion of Carbohydrates
begins in mouth with salivary amylase, reduces macromolecules to
polysaccharides and a few disaccharides – temporarily halts in stomach
continues in intestine with pancreatic amylase which reduces these
molecules to disacharides. Disaccharides are broken down by enzymes
(disaccharideases) on intestinal microvilli in the lining of the small
intestine. these enzymes convert the disaccharides into monosaccharides
Disaccharides include: lactose, sucrose, maltose, isomaltose
enzymes that digest these are xxxase where xxx = sugar name
prefix
e.g. sucrose cleaved by sucrase
Digestion of Lipids
Begins with emulsification by bile in duodenum – this breaks the
lipids into small droplets increasing surface area so enzymes will have
more area to work
Lipase secreted by pancreas converts triacylglycerols to free fatty acids
and glycerol
Digested lipids together with bile salts form microscopic droplets
called micelles which are absorbed when they come in contact with
the epithelium of the small intestine
Transport of Lipids
Once inside intestinal epithelium, free fatty acids combine with
gylcerol to form triacylglycerol.
Proteins (made in intestinal epithelium) attach to surface of lipid
molecules and form a ‘package’ called chylomicrons
Chylomicrons leave intestinal epithelium and enter lymph lacteal
for transport to circulatory system
These protein coats are necessary because lipids are insoluble in water
(thus insoluble in blood)
Classes of lipoproteins include: VLDL, LDL, and HDL
very low density, low density and high density lipoproteins
Excess lipid removal
Excess lipids packaged as HDLs for transport to liver where they
are recycled or disposed
Why HDL as packaging for transport – increased solubility in
blood makes for easier transport
Digestion of Proteins
Begins in stomach with pepsin – cuts proteins into short polypeptide
chains and some dipeptides. (10% - 20% digestion by pepsin)
Duodenum – pancreatic enzymes continue digestion (trypsin,
chymotrypsin, & carboxypeptidase) into smaller pieces
Intestinal lining – enzymes (aminopeptidase, peptidase) in microvilli
continue processing into tri/dipeptides and amino acids
Digested amino acids and small peptides carried into epithelium by
active transport on carrier molecules (more peptides than amino acids)
Once internal to epithelial cells the small peptides are cleaved by
dipeptidases or tripeptidases into amino acids.
Examine
Models!