Lecture 22 - The Digestive Tract.ppt

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The Digestive Tract
 The GI tract
(gastrointestinal tract)
The muscular alimentary canal
 Mouth
 Pharynx
 Esophagus
 Stomach
 Small intestine
 Large intestine
 Anus
 The accessory
digestive organs
Supply secretions contributing
to the breakdown of food
 Teeth & tongue
 Salivary glands
 Gallbladder
 Liver
 Pancreas
2
The Digestive Process
 Ingestion
 Taking in food through the mouth
 Propulsion (movement of food)
 Swallowing
 Peristalsis – propulsion by alternate
contraction &relaxation
 Mechanical digestion
 Chewing
 Churning in stomach
 Mixing by segmentation
 Chemical digestion
 By secreted enzymes: see later
 Absorption
 Transport of digested end products into
blood and lymph in wall of canal
 Defecation
 Elimination of indigestible substances from
body as feces
3
 Chemical digestion
 Complex food molecules (carbohydrates,
proteins and lipids) broken down into
chemical building blocks (simple sugars,
amino acids, and fatty acids and glycerol)
 Carried out by enzymes secreted by digestive
glands into lumen of the alimentary canal
4
Ways to divide….
The more common
Plus:
epigastric
periumbilical
suprapubic
flank
5
Histology of alimentary canal wall
Same four layers from esophagus to anal canal
1. Mucosa
2. Submucosa
3. Muscularis
externa
4. Serosa
from lumen (inside) out
6
Inner layer: the mucosa*
(mucous membrane)
Three sub-layers
*
1. Lining epithelium
2. Lamina propria
3. Muscularis
mucosae
7
More about the mucosa
 Epithelium: absorbs nutrients, secretes mucus
 Continuous with ducts and secretory cells of intrinsic
digestive glands (those within the wall)
 Extrinsic (accessory) glands: the larger ones such as
liver and pancreas
 Lamina propria
 Loose connective tissue with nourishing and
absorbing capillaries
 Contains most of mucosa-associated lymphoid tissue
(MALT)
 Muscularis mucosae
 Thin layer of muscle producing only local movements 8
Second layer: the submucosa*
*
 Connective tissue
containing major
blood and
lymphatic vessels
and nerves
 Many elastic fibers
so gut can regain
shape after food
passes
9
Next in, the muscularis externa*
(AKA just “muscularis”)
Two layers of smooth
muscle responsible
for peristalsis and
segmentation
 Inner circular layer
(circumferential)
*



Squeezes
In some places forms
sphincters (act as
valves)
Outer longitudinal
layer: shortens gut
10
Last (outer), the serosa*
(the visceral peritoneum)
 Simple squamous
epithelium
(mesothelium)
 Thin layer of areolar
connective tissue
underneath
 Exceptions:
*
 Parts not in peritoneal
cavity have adventitia,
lack serosa
 Some have both, e.g.
retroperitoneal organs
11
Smooth muscle
Smooth muscle
6 major locations:
•Muscles are spindle-shaped cells
•One central nucleus
•Grouped into sheets: often running
perpendicular to each other
•Peristalsis
•No striations (no sarcomeres)
•Contractions are slow, sustained and
resistant to fatigue
•Does not always require a nervous signal:
can be stimulated by stretching or hormones
1. inside the eye 2. walls of vessels 3. respiratory tubes
4. digestive tubes 5. urinary organs 6. reproductive organs
12
Nerves
 Enteric nervous system: the gut’s own
 Visceral plexuses within gut wall controlling the
muscles, glands and having sensory info
 Myenteric: in muscularis
 Submucosal
 100 million neurons! (as many as the spinal cord)
 Autonomic input: speeds or slows the system
 Parasympathetic
 Stimulates digestive functions
 Sympathetic
 Inhibits digestion
 Largely automatic
13
Review of some definitions….
 Peritoneum: serous membranes of the
abdominopelvic cavity
 Visceral peritoneum: covers external
surfaces of most digestive organs
 Parietal peritoneum: lines body wall
 Peritoneal cavity: slit-like potential space
between visceral and parietal peritoneum
 Serous fluid – lubricating
14
New definitions
 Mesentery





Double layer of peritoneum
Extends to digestive organs from body wall
Hold organs in place
Sites of fat storage
Route by which circulatory vessels and nerves reach
organs
 Most are dorsal
 Extend dorsally from gut to posterior abdominal wall
 Ventral mesentery – from stomach and liver to
anterior abdominal wall
 Some mesenteries are called “ligaments” though not
technically such
15
Mesenteries
 Note dorsal, ventral and formation of
retroperitoneal position
16
Mesenteries
 Two ventral mesenteries
 Falciform “ligament”
*
 Binds anterior aspect of liver
to anterior abdominal wall
and diaphragm
 Lesser omentum (=“fatty
skin”) – see diagram*
 All other mesenteries are
dorsal (posterior)
17
Mesenteries continued (all these are dorsal)
 Greater omentum





Connects stomach to posterior abdominal wall – very roundabout
Wraps around spleen: gastrosplenic ligament
Continues dorsally as splenorenal ligament
A lot of fat
Limits spread of infection by wrapping around inflamed e.g. appendix
 “Mesentery” or mesentery proper
 Supports long coils of jejunum and ileum (parts of small intestine)
 Transverse mesocolon
 Transverse colon held to posterior abdominal wall
 Nearly horizontal sheet fused to underside of greater omentum
 Sigmoid mesocolon
 Connects sigmoid colon to posterior abdominal wall
see next slides for pics…
18
Note mesenteries: falciform ligament, lesser
omentum, greater omentum
19
Note: greater omentum, lesser omentum, falciform ligament,
transverse mesocolon, mesentery, sigmoid mesocolon
20
Some organs are “retroperitoneal”




Are “behind the peritoneum”
Fused to posterior (dorsal) abdominal wall
Lack a mesentery
Include:





Most of duodenum (1st part of small intestine)
Ascending colon
Descending colon
Rectum
Pancreas
 Tend to cause back pain, instead of abdominal pain
(This is as opposed to the organs which are intraperitoneal,
or just “peritoneal”)
21
The Mouth
 Mouth = oral cavity
 Lining: thick
stratified squamous
epithelium
 Lips- orbicularis
oris muscle
 Cheeks –
buccinator muscle
22
 “Vermillion border” or red
border
 Between highly
keratinized skin of face
and mucosa of mouth
 Needs moisture
 Note frenulums (folds of
mucosa)
 Palate – roof of mouth
 Hard plate anteriorly
 Soft palate posterioly
 Uvula
23

Tongue
Mostly muscles







Grip and reposition food
Forms “bolus” of food (lump)
Help in swallowing
Speech – help form some consonants
Note frenulum on previous slide: can be too tight
Taste buds contained by circumvallate and fungiform papillae
Lingual tonsil – back of tongue
24
Teeth
 Called “dentition” (like dentist)
 Teeth live in sockets (alveoli) in the gumcovered margins of the mandible and
maxilla
 Chewing: raising and lowering the
mandible and moving it from side to side
while tongue positions food between teeth
25
Teeth
 Two sets
 Primary or deciduous
 “Baby” teeth
 Start at 6 months
 20 are out by about 2 years
 Fall out between 2-6 years
 Permanent: 32 total
 All but 3rd set of molars by
end of adolescence
 3rd set = “wisdom teeth”
– Variable
 Some can be “impacted”
(imbedded in bone)
26
Teeth are classified according to
shape and function
incisor
canine
premolar
molar
 Incisors: chisel-shaped for
chopping off pieces
 Canines: cone shaped to
tear and pierce
 Premolars (bicuspids) and
 Molars - broad crowns
with 4-5 rounded cusps for
grinding
Cusps are surface bumps
27
Tooth structure

Two main regions
A. Crown (exposed)
B. Root (in socket)
C. Meet at neck

Enamel




99% calcium crystals
Hardest substance in
body
Dentin – bulk of the
tooth (bone-like but
harder than bone, with
collagen and mineral)
Pulp cavity with vessels
and nerves

Root canal: the part of
the pulp in the root
A
C
B
28
Tooth structure

Cementum – bone
layer of tooth root


C
Periodontal ligament




Attaches tooth to
periodontal ligament
A
Anchors tooth in boney
socket of the jaw
Continuous with gingiva
(gums)
B
Cavities or caries - rot
Plaque – film of sugar,
bacteria and debris
29
Salivary glands
(tuboalveolar glands)
 Intrinsic salivary glands
– within mucosa
 Secrete saliva all the time
to keep mouth moist
 Extrinsic salivary glands
 Paired (2 each)
 Parotid
 Submandibular
 Sublingual
Saliva: mixture of water, ions, mucus, enzymes
keep mouth moist
dissolves food so can be tasted
moistens food
starts enzymatic digestion
buffers acid
antibacterial and antiviral
 External to mouth
 Ducts to mouth
 Secrete saliva only right
before or during eating
30
Extrinsic salivary glands
 Parotids* - largest (think mumps)
 Facial nerve branch at risk during surgery here
 Submandibular # - medial surface mandible
 Sublingual + - under tongue; floor of mouth
*
+
#
Compound = duct branches
Tubo = tubes
Alveolar = sacs
31
Pharynx
___oropharynx
___laryngopharynx
 Oropharynx and
laryngopharynx
 Stratified squamous
epithelium
 Three constrictor
muscles*
 Sequentially squeeze
bolus of food into
esophagus
 Are skeletal muscles
*
*
 Voluntary action
 Vagus nerve (X)
*
32
Esophagus
 Continuation of pharynx in
mid neck
 Muscular tube collapsed
when lumen empty
Esophagus___________
 Descends through thorax
 On anterior surface of
vertebral column
 Behind (posterior to) trachea
*
33
Esophagus continued
 Passes through “esophageal hiatus” in the diaphragm to
enter the abdomen
 Abdominal part only 2 cm long
 Joins stomach at cardiac orifice*
 Cardiac sphincter at cardiac orifice to prevent regurgitation (food
coming back up into esophagus)
 Gastroesophageal junction and GERD
___________________esophageal hiatus
(hiatus means opening)
*
34
Microscopic anatomy of esophagus
Contains all 4
layers (see right)

Epithelium: nonkeratinized stratified squamous epithelium
 At GE junction – thin simple columnar epithelium


Mucus glands in wall
Muscle (muscularis externa) changes as it goes down
 Superior 1/3 of esophagus: skeletal muscle (like pharynx)
 Middle 1/3 mixture of skeletal and smooth muscle
 Inferior 1/3 smooth muscle (as in stomach and intestines)

When empty, mucosa and submucosa lie in longitudinal folds
35
Esophagus histology
36
Stomach
 J-shaped; widest part of alimentary canal
 Temporary storage and mixing – 4 hours
 Into “chyme”
 Starts food breakdown
 Pepsin (protein-digesting enzyme needing acid
environment)
 HCl (hydrochloric acid) helps kill bacteria
 Stomach tolerates high acid content but esophagus
doesn’t – why it hurts so much when stomach
contents refluxes into esophagus (heartburn; GERD)
 Most nutrients wait until get to small intestine to
be absorbed; exceptions are:
 Water, electrolytes, some drugs like aspirin and
alcohol (absorbed through stomach)
37
Stomach
 Lies mostly in LUQ
epigastrium
 But pain can be epigastric or
lower
 Just inferior to (below)
diaphragm
 Anterior (in front of) spleen
and pancreas
 Tucked under left lower
margin of liver
junction with
 Anchored at both ends but
esophagus
mobile in between
contains pyloric
 Main regions in drawing to
sphincter
right------------------------------- Capacity: 1.5 L food; max funnel shaped
capacity 4L (1 gallon)
dome
38
39
Stomach Regions
 Cardiac region
 Fundus (dome shaped)
 Body
 Greater curvature
 Lesser curvature
 Pyloric region
 Antrum
 Canal
 Sphincter
dome
junction with
esophagus
contains pyloric
sphincter
funnel shaped
40
 Rugae: longitudinal folds
on internal surface (helps
distensibility)
 Muscularis: additional
innermost oblique layer
(along with circular and
longitudinal layers)
41
Histology of
stomach
 Simple columnar
epithelium: secrete
bicarbonate-buffered
mucus
 Gastric pits opening
into gastric glands
 Mucus neck cells
 Parietal cells
 HCL
 Intrinsic factor (for
B12 absorption)
 Chief cells
 Pepsinogen
(activated to pepsin
with HCL)
 Stimulated by gastrin:
a stomach hormone
42
Small intestine
 Longest part of alimentary canal (2.7-5 m)
 Most enzymatic digestion occurs here
 Most enzymes secreted by pancreas, not
small intestine
 Almost all absorption of nutrients
 3-6 hour process
 Runs from pyloric sphincter
to RLQ
Small intestine___________
43
 Small intestine has 3 subdivisions
 Duodenum – 5% of length
 Jejunum – almost 40%
 Ileum – almost 60%
Blood supply: superior
mesenteric artery;
Veins drain into hepatic
portal vein
Duodenum is retroperitoneal (stuck down under peritoneum); others are loose
Duodenum receives
bile from liver and gallbladder via bile duct*
enzymes from pancreas via main pancreatic duct*
*
*
44
 Small intestine designed for absorption
 Huge surface area because of great length
 Structural modifications also increase absorptive area
 Circular folds (plicae circulares)
 Villi (fingerlike projections) 1 mm high – simple columnar epithelium: velvety
 Microvilli
*
Absorptivie cell
with microvilli to
increase surface
area & many
mitochondria:
nutrient uptake is
energydemanding
Lacteal*: network of blood
and lymph capillaries
-Carbs and proteins into blood to
liver via hepatic portal vein
-Fat into lymph: fat-soluble toxins
e.g. pesticides circulate
systemically before going to liver
for detoxification
45

Intestinal crypts
* (of Lieberkuhn) inbetween villi
 Cells here divide every 3-6 days to renew epithelium (most rapidly dividing cells of the
body)
 Secrete watery intestinal juice which mixes with chyme (the paste that food becomes
after stomach churns it)

Intestinal flora – the permanent normal bacteria
 Manufacture some vitamins, e.g. K, which get absorbed
*
Duodenal glands
•Mucus to counteract acidity
from stomach
•Hormones:
Cholecystokinin (stimulates GB
to release stored bile, also pancreas)
Secretin (stimulates pancreatic
ducts to release acid neutralizer)
*
*
-have many
mitochondria:
nutrient uptake
is energydemanding
-produce
mucus
46
General histology of digestive tract
47
48
49
Large intestine
Digested residue reaches it
Main function: to absorb water
and electrolytes
Subdivisions
Cecum
Appendix
Colon
Rectum
Anal canal
50
Three special
features
1. Teniae coli (3 longitudinal
muscle strips)
2. Haustra (puckering into sacs)
3. Epiploic appendages (omental
or fat pouches)
3.
2.
1.
51
Colon has segments: ascending, transverse and descending colon; then sigmoid colon
Right angle turns: hepatic flexure* in RUQ and splenic flexure* in LUQ
*
*
Between ileum
and cecum
S-shaped
1st part
Blind tube
Movement sluggish and weak except for a few “mass peristaltic
movements” per day to force feces toward rectum powerfully
52
 Rectum
 In pelvis
 No teniae
 Strong longitudinal muscle
layer
 Has valves
 Anal canal
 Pectinate line*
 Inferior to it: sensitive to
pain
 Hemorrhoids (enlarged
veins)
 Superior to pectinate
line: internal
 Inferior to pectinate line:
external
*
*
 Sphincters (close opening)
 Internal*
– smooth muscle
– involuntary
*
 External*
– skeletal muscle
– voluntary
53

Defecation
1. Triggered by stretching of
wall, mediated by spinal
cord parasympathetic reflex
2. Stimulates contraction of
smooth muscle in wall and
relaxation of internal anal
sphincter
3. If convenient to defecate
voluntary motor neurons
stimulate relaxation of
external anal sphincter
(aided by diaphragm and
abdominal wall muscles called Valsalva maneuver)
54
Histology – large intestine
 No villi
 Fewer nutrients
absorbed
 “Columnar cells” in pic
= absorptive cells
 Take in water and
electrolytes
 A lot of goblet cells for
mucus
 Lubricates stool
 More lymphoid tissue
 A lot of bacteria in stool
55
The Liver
 Largest gland in the body
(about 3 pounds)
 Over 500 functions
 Inferior to diaphragm in
RUQ and epigastric area
protected by ribs
 R and L lobes
 Plus 2 smaller lobes
 Falciform ligament
 Mesentery binding liver to
anterior abdominal wall
 2 surfaces
 Diaphragmatic
 Visceral
 Covered by peritoneum
 Except “bare area” fused to
diaphragm
56
posterior
Fissure on visceral surface
Porta hepatis: major vessels and nerves
enter and leave - see pics
Ligamentum teres: remnant of
umbilical vein in fetus, attaches to navel
– see next slide
anterior
57
Fetal circulation
Umbilical vein
___________
Ligamentum
teres__________
Navel_______
58
59
Just some of the liver’s repertoire







Produces bile
Picks up glucose from blood
Stores glucose as glycogen
Processes fats and amino acids
Stores some vitamins
Detoxifies poisons and drugs
Makes the blood proteins
60
Liver histology
 Liver lobules (about one million of them)
 Hexagonal solid made of sheets of hepatocytes (liver
cells) around a central vein
 Corners of lobules have “portal triads”
(see next pic)
61
 Portal triad
 Portal arteriole
 Portal venule
 Branch of hepatic
portal vein
 Delivers substances
from intestines for
processing by
hepatocytes
 Bile duct
 Carries bile away
 Liver sinusoids
 Large capillaries
between plates of
hepatocytes
 Contribute to central
vein and ultimately to
hepatic veins and IVC
 Kupffer cells
 Liver macrophages
 Old blood cells and
microorganisms
removed
62
63
Hepatocytes (liver cells)
 Many organelles
 Rough ER – manufactures blood proteins
 Smooth ER – help produce bile salts and detoxifies
blood-borne poisons
 Peroxisomes – detoxify other poisons, including
alcohol
 Golgi apparatus – packages
 Mitochondria – a lot of energy needed for all this
 Glycosomes - role in storing sugar and regulation of
blood glucose (sugar) levels
 Produce 500-1000 ml bile each day
 Secrete into bile canaliculi (little channels) then ducts
 Regeneration capacity through liver stem cells
64
Gallbladder*





Bile is produced in the liver
Bile is stored in the gallbladder
Bile is excreted into the
duodenum when needed (fatty
meal)
Bile helps dissolve fat and
cholesterol
If bile salts crystallize, gall
stones are formed
 Intermittent pain: ball valve
effect causing intermittent
obstruction
 Or infection and a lot of pain,
fever, vomiting, etc.
*
65
Lies in LUQ kind of behind stomach
Is retroperitoneal
Has a head, body and tail
Head is in C-shaped curve of duodenum
Tail extends left to touch spleen
Main pancreatic duct runs the length of the
pancreas, joins bile duct
Pancreas
(exocrine and
endocrine)
66
67
one acinus
Pancreatic
exocrine function
 Compound acinar (saclike) glands opening into
large ducts (therefore
exocrine)
 Acinar cells make 22
kinds of enzymes
 Stored in zymogen
granules
 Grape-like arrangement
 Enzymes to duodenum,
where activated
68
Pancreatic endocrine function
(hormones released into blood)
 Islets of Langerhans (AKA “islet cells”)
are the hormone secreting cells
 Insulin (from beta cells)
 Lowers blood glucose (sugar)
 Glucagon (from from alpha cells)
 Raises blood glucose (sugar)
(more later)
69
Endocrine cells:
70

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