Lecture 22 - The Digestive Tract.ppt
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Transcript Lecture 22 - The Digestive Tract.ppt
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