BSC2086 LECTURE 1
THE DIGESTIVE SYSTEM
Hernan Aviles
Main topics
• General anatomy and digestive
processes
• Mouth through esophagus
• Stomach
• Liver, gallbladder and pancreas
• Small intestine
• Chemical digestion and absorption
• Large intestine
1
Overview
• nutrients that we eat must be
broken into smaller components
(amino acids, sugars)
• proteins such as myosin (beef) and
others can not be absorbed and
incorporated into your muscles, it
must be broken down into amino
acids before it can be used to make
human proteins
• digestive system is a disassembly
line that breaks food into molecules
that can be used by the body, it also
absorbs and distribute those
molecules to tissues
• Gastroenterology: study of digestive
system and its disorders
2
Digestive Functions
• ingestion: intake of food
• digestion: breakdown
– mechanical (teeth, contractions):
physical breakdown (cutting,
grinding) that expose more food to
digestive enzymes
– chemical (enzymes from saliva,
stomach, pancreas, intestines ):
break macromolecules into their
monomers (poly into
monosaccharides, proteins into
amino acids, fats into glycerol and
fatty acids
• absorption: uptake nutrients into
blood/lymph
• defecation: elimination of undigested
material
3
Anatomy of the Digestive System
• two subdivisions and is open in
both ends
– most material like feces never go
inside the body
• digestive tract: 30 foot long tube
extending from mouth to anus
– mouth, esophagus, stomach,
small and large intestines
– gastrointestinal (GI) tract:
stomach and intestines
• accessory organs: teeth,
tongue, liver, gallbladder,
pancreas, salivary glands
4
Tissue Layers of GI Tract
• mucosa: epithelium (simple columnar in all GI except in mouth,
esophagus and anus that have stratified squamous), lamina propria,
muscularis mucosae)
• submucosa
• muscularis externa: inner circular layer, outer longitudinal layer
• adventitia or serosa: areolar tissue or mesothelium
5
Enteric Nervous Control
• regulates moltility,
secretion and blood flow
• function independently of
CNS, two nerve networks
– submucosal plexus
• glandular secretion
of mucosa &
contractions of
muscularis
mucosae
– myenteric plexus
• peristalsis &
contractions of
muscularis externa
6
Relationship to Peritoneum
• organs are covered by peritoneum, because of contractions, organs
need freedom to move (suspended by sheets of connective tissue called
mesenteries)
• dorsal mesentery suspends GI tract and forms serosa (visceral
peritoneum) of stomach and intestines
• ventral mesentery forms lesser and greater omentum (contains
lymph nodes, lymphatic vessels, blood vessels)
• duodenum, pancreas and parts of large intestine are retroperitoneal
7
Lesser and Greater Omentum
• Lesser: attaches stomach to liver
• Greater: covers small intestines like an apron, extends
from greater curvature of stomach to transverse colon
8
Mesentery and Mesocolon
• Mesentery of small intestines holds many blood vessels
• Mesocolon anchors colon to posterior body wall
9
Regulation of Digestive Tract
• digestive processes such as motility (contractions that break up food,
mix it with enzymes and move it along), secretion, membrane transport
(absorption of nutrients) are controlled by 3 mechanisms
• neural control: short myenteric (swallowing) & long vagovagal
reflexes (motility and secretion)
• hormones (gastrin, secretin): messengers diffuse into bloodstream,
distant targets
• paracrine secretions (histamine, protaglandins): messengers diffuse to
nearby target cells
10
Features of Oral Cavity
• cheeks and lips: keep food
between teeth for chewing;
essential for speech and
suckling in infants
– vestibule: space between
teeth and cheeks
– lips: cutaneous (mustache)
and red area or vermilion
(lipstick)
• tongue is sensitive, muscular
manipulator of food
– papillae and taste buds on
dorsal surface (vallate,
foliate, filiform, fungiform)
– lingual glands secrete
saliva, tonsils in root
• hard and soft palate: allow
breathing and chewing at
same time
– palatoglossal and
palatopharyngeal arches
11
The teeth (Dentition)
• Baby or milk or deciduous teeth erupt from 6-30 months and by 2
years (20); Adult or permanent (32) between 6 and 25 years, third
molars (wisdom) erupt 17-25 years, because of evolution, jaws
become shorter (no space for them, become impacted)
– incisors: cutting teeth
– canines: puncture and shred food
– premolars and molars: crushing and grinding
• oclussion: meeting of teeth when mouth closes
• cusps: rounded bumps located in the occlusal surfaces (premolars have
2, bicuspids; molars have 4-5 cusps)
12
Tooth Structure
• each tooth anchors into a cavity
alveolus
• periodontal ligament anchors
teeth into alveolus
• dentin: tissue of teeth (material)
– cementum covers root and neck
– enamel covers crown
– damaged dentin and cementum
can regenerate, enamel does
not
• root canal leads into pulp cavity
– nerves and blood vessels
• gingiva or gum covers the
alveolar bone
– crown: above the gum; root:
below the gum and neck: place
where crown, root and gum meet
– gingival sulcus: space between
tooth and gum
13
Tooth & gum disease, mastication or chewing
• mouth has ~700 species (bacteria), bacteria
and sugars form plaque, if not removed
bacteria acts on sugars to produce lactic acid
(dissolve minerals and form cavities)
•
If penetrate the pulp cavity (either remove or
root canal therapy to remove pulp and replace
with inert material)
• periodontal disease (~90% of tooth loss):
plaque calcifies (tartar) which separates tooth
and gum followed by bacterial infection
• mastication: breaks food into smaller pieces
to be swallowed
– ↑ surface area exposed to digestive enzymes
– contact of food with sensory receptors triggers
chewing reflex
– tongue, buccinator and orbicularis oris
manipulate food
– masseter and temporalis elevate the teeth to
crush food
– medial and lateral pterygoids swing teeth in
side-to-side (grinding action of molars)
14
Saliva and Salivary Glands
• functions: moisten, begins starch and fat
digestion, cleanse teeth, inhibit bacteria,
bind food together into bolus, pH 6.8 to 7.0
• components: 97-99.5% water and solutes
such as amylase (starch digestion); lipase
(digests fat, activated by stomach acid);
mucus (aids in swallowing); lysozyme (kills
bacteria); immunoglobulin A (inhibits
bacterial growth); electrolytes such as Na+,
K+, Cl-, phosphate and bicarbonate
• salivary glands: (intrinsic and extrinsic)
– intrinsic: small glands (lingual, tongue; labial,
lips; buccal, cheeks), small amounts of saliva
– extrinsic: 3 pairs connected to oral cavity by
ducts
• parotid: anterior to earlobe, duct empties in
the upper second molar (mumps, virus)
• submandibular: duct empties close to the
lower incisors
• sublingual: floor of the mouth, multiple ducts
empty posterior to submandibular duct
15
Salivation
• total of 1 to 1.5 L of saliva per day
• cells filter water from blood and add other substances
(amylase, mucin, lysozyme)
• food stimulates receptors in mouth that signal salivatory
nuclei in medulla and pons, nuclei send signals to glands
using facial and glossopharyngeal nerves
– parasympathetic stimulation → salivary glands produce
thin saliva, rich in enzymes
– sympathetic stimulation → produce less abundant, thicker
saliva, with more mucus
– stress and dehydration reduce salivation
• higher brain centers also stimulate salivatory nuclei so
sight, smell and thought of food cause salivation
16
Pharynx
• skeletal muscle
– deep layer: longitudinal
– superficial layer: circular
• superior, middle and inferior
pharyngeal constrictors
• inferior constrictor (upper
esophageal sphincter) remains
closed when not swallowing, it
is physiological sphincter
(disappears in dead people)
17
Esophagus
• muscular tube of 25-30 cm long
– skeletal muscle in upper part,
mixed in the middle and
smooth in bottom
– nonkeratinized stratified
squamous epithelium
– glands in submucosa
• extends from pharynx, passes
through esophageal hiatus in
diaphragm and finishes in
stomach (cardiac sphincter)
• upper sphincter (inferior
pharyngeal constrictor) excludes
air from esophagus
• lower sphincter closes orifice
helping diaphragm to avoid reflux
• heartburn: nothing to do with
heart (acid reflux)
18
Esophagus
19
Swallowing (Deglutition)
• series of muscular contractions (22
muscles) coordinated by the swallowing
center (medulla and pons), center send
signals using cranial nerves V, VII, IX and XII
• buccal phase: tongue collects food,
presses it against palate (bolus) and pushes
it back into oropharynx
• pharyngeal-esophageal phase: 3 actions
avoid food going to mouth, nose or larynx
– tongue (blocks mouth), soft palate rises
(blocks nasopharynx), infrahyoid muscles lift
larynx; epiglottis folds back (block larynx)
– pharyngeal constrictors push bolus down and
triggers peristalsis (wave of contractions
that pushes bolus ahead of it)
– food uses gravity to swallow when standing,
peristalsis can swallow in any position,
liquids 1-2 sec, bolus 4-8 sec
– lower esophageal sphincter relaxes to let
food pass into stomach
20
Swallowing
21
Stomach
• food storage, empty: 50 mL, after meal: 1-1.5 L, extremely full: 4 L
• mechanically breaks up food, liquefies it and begins chemical digestion
of protein and fat resulting in a soupy or pasty mixture (chyme)
• absorbs only small amount of nutrients, absorbs aspirin and some drugs
• gross anatomy: curvatures: greater and lesser; regions: cardia (small
area inside cardiac orifice); fundus (dome-shaped area on top); body (greatest
part); pyloric region subdivided into antrum and pyloric canal, which finishes in
the pylorus (narrow passage into duodenum); pylorus surrounded by a muscle
(pyloric sphincter)
Esophagus
Pyloric
Antrum
Cardia
Pyloric
Canal
Fundus
Pyloric
Orifice
Body
Duodenum
25-23
Stomach
Esophag
us
Cardia
Fundus
Body
Pylorus
Duodenu
m
23
Innervation and Circulation
• innervation by parasympathetic (vagus); sympathetic fibers (celiac plexus)
• blood is supplied by arterial branches of celiac trunk, used blood from
stomach enters hepatic portal circulation and is filtered through liver before
returning to heart
24
Unique Features of Stomach Wall & Cells
• mucosa: simple columnar epithelium,
pocked with depressions (gastric pits)
• mucosa and submucosa (empty
stomach) form wrinkles (rugae)
• lamina propria: tubular glands
• muscularis externa: 3 layers (outer
longitudinal, middle circular and inner
oblique)
• 2 or 3 glands open in bottom of each
gastric pit (cardiac, pyloric or gastric
glands), cells of glands are:
– mucous: secrete mucus
– regenerative: divide fast, produce
new cells that migrate to surface
– parietal: HCl and intrinsic factor
– chief (greater #): pepsinogen, and
chymosin and lipase in infancy
– enteroendocrine (G): hormones and
paracrine messengers
– cardiac and pyloric (mucus); gastric
(acids & enzymes); both (hormones)
25
Gastric Secretions
• 2 to 3L juice/day (H2O, HCl, pepsin)
• HCl functions: activates enzymes,
breaks up tissue, converts ferric
(Fe3+) to ferrous ions (Fe2+) used for
Hb synthesis, destroys bacteria and
pathogens
• HCl production: parietal cells
contain carbonic anhydrase (CAH)
CAH
– CO2 + H2O → H2CO3 → HCO3- + H+
– H+ is pumped into stomach lumen
by H+K+ ATPase, antiporter uses
ATP to pump H+ out and K+ in
– HCO3- goes to blood and is
exchanged for Cl- (chloride shift), Clpumped out joins H+ forming HCl, ↑
HCO3- in blood causes blood pH ↑
(alkaline tide)
• pepsin: protein digestion (partial),
secreted by chief cells as
pepsinogen (inactive), HCl
converts it to pepsin (active)
27
Other Enzymes, Intrinsic Factor, Chemicals
• gastric lipase and chymosin (infants’s chief
cells), lipase digests butterfat of milk,
chymosin curdles milk by coagulating proteins
• intrinsic factor (parietal): essential for B12
absorption by small intestine; if not B12, Hb
can not be produced (pernicious anemia)
• chemical messengers (gastrin, serotonin,
histamine, somatostatin): many produced by
enteroendocrine (G) cells
– hormones enter blood → distant cells
– paracrine secretions → neighboring cells
• gut-brain peptides: signaling molecules
produced in digestive tract and CNS
– substance P, vasoactive intestinal peptide
(VIP), gastric inhibitory peptide (GIP),
cholecystokinin, neuropeptide Y (NPY)
27
28
Gastric Motility & Vomiting
• motility: swallowing center signals stomach to relax
• food stretches stomach activating a receptive-relaxation response
– resists stretching briefly, but relaxes to hold more food
• soon, stomach shows a rhythm of peristalsis controlled by pacemaker
cells in longitudinal muscle layer
– gentle ripple of contraction happens every 20 seconds, churns and
mixes food with gastric juice, after 30 mins it becomes strong
– stronger contraction at pyloric region; ejects 3 ml of chyme at a time
(this give time to duodenum to neutralize and digest nutrients)
– typical meal emptied from stomach in 4 hours (less if liquid and ~ 6
hours if high in fat)
• vomiting: excessive stretching of stomach, psychological stimuli,
chemical irritants (alcohol, bacterial toxins), diaphragm and abdominal
muscles and not stomach are involved
– emetic center in medulla causes lower esophageal sphincter to relax while
diaphragm and abdominal muscles are stimulated to contract
• most digestion and nutrient absorption occur when chyme passes into
small intestine (alcohol is absorbed in small intestine)
29
Healthy Mucosa and Peptic Ulcer
• protection: living stomach is protected in 3 ways: thick mucus resists
acids and enzymes, cell replacement every 3-6 days and tight junctions
• peptic ulcer: breakdown of protection can lead to inflammation and ulcer
(if untreated, ulcer can perforate organ causing bleeding or peritonitis)
• causes: no evidence of stress, many involve Helicobacter pylori, other
risk factors are smoking and nonsteroidal anti-inflammatory drugs
(aspirin), they block prostaglandin production (mucus and bicarbonate)
• treatment: cimetidine
(tagamet) which reduces
acid secretion (20-30%
success)
• it is an H2 receptor blocker,
histamine stimulates acid
secretion in parietal cells by
binding to H2 receptors
• lately, antibiotics against H.
pylori + Pepto-Bismol (90%
success)
30
Regulation of Gastric Function
• cephalic phase: sight, smell, taste
or thought of food; vagus
stimulates secretion and motility
• gastric phase: food or semidigested protein activate gastric
activity by stretch and ↑ in pH
– stretch: activates short
(myoenteric) and long reflexes
(vagus)
– secretion is stimulated by ACh
(parasympathetic from both,
short and long), histamine
(enteroendocrine cells) and
gastrin (pyloric G cells)
– 3 have receptors on
parietal (HCl & intrinsic
factor) and chief cells
(pepsinogen, mucus)
31
Regulation of Gastric Function
• intestinal phase: duodenum regulates
gastric activity through hormones and
nervous reflexes
– at first gastric activity increases (if
duodenum is stretched or amino acids
in chyme cause gastrin release)
– enterogastric reflex: duodenum inhibits
activity
• caused by acid and semi-digested
fats in duodenum
– chyme stimulates duodenal cells to
release secretin, cholecystokinin
(CCK) and gastric inhibitory peptide
(GIP)
• all 3 suppress gastric secretion
and motility
• gastrin decreases, pyloric
sphincter contracts (no more food
passes)
32
Regulation of Gastric Secretion
33
Positive Feedback Control- Gastric Secretion
34