the digestive tract
DESCRIPTION
THE DIGESTIVE TRACT. JACKI HOUGHTON, DC. 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 - PowerPoint PPT PresentationTRANSCRIPT
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THE DIGESTIVE TRACT
JACKI HOUGHTON, DC
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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
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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
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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
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Those regions again! The more common
Plus: epigastric periumbilical suprapubic flank
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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
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Inner layer: the mucosa* (mucous membrane)
Three sub-layers1. Lining epithelium
2. Lamina propria
3. Muscularis mucosae
*
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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
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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
*
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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
*
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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
*
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Smooth muscle
•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
6 major locations: 1. inside the eye 2. walls of vessels 3. respiratory tubes 4. digestive tubes 5. urinary organs 6. reproductive organs
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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
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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
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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
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Mesenteries
Note dorsal, ventral and formation of retroperitoneal position
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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)
*
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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…
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Note mesenteries: falciform ligament, lesser omentum, greater omentum
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Note: greater omentum, lesser omentum, falciform ligament, transverse mesocolon, mesentery, sigmoid mesocolon
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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”)
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Mouth = oral cavity Lining: thick stratified
squamous epithelium Lips- orbicularis oris
muscle Cheeks – buccinator
muscle
The Mouth
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“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
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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
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Teeth
Called “dentition” (like dentist)
Teeth live in sockets (alveoli) in the gum-covered 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
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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)
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Teeth are classified according to shape and function
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
incisor
canine
premolar
molar
Cusps are surface bumps
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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
B
C
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Tooth structure
Cementum – bone layer of tooth root
Attaches tooth to periodontal ligament
Periodontal ligament Anchors tooth in boney
socket of the jaw Continuous with gingiva
(gums) Cavities or caries - rot Plaque – film of sugar,
bacteria and debris
A
B
C
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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
External to mouth Ducts to mouth Secrete saliva only right
before or during eating
Saliva: mixture of water, ions, mucus, enzymeskeep mouth moistdissolves food so can be tastedmoistens foodstarts enzymatic digestionbuffers acidantibacterial and antiviral
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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 branchesTubo = tubesAlveolar = sacs
*
#
+
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Pharynx
Oropharynx and laryngopharynx Stratified squamous
epithelium
Three constrictor muscles* Sequentially squeeze
bolus of food into esophagus
Are skeletal muscles Voluntary action Vagus nerve (X)
___oropharynx
___laryngopharynx
*
*
*
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Esophagus Continuation of pharynx in mid
neck Muscular tube collapsed when
lumen empty Descends through thorax
On anterior surface of vertebral column
Behind (posterior to) trachea
Esophagus___________
*
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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)
*
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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
Microscopic anatomy of esophagus
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Esophagus histology
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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)
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Stomach
Lies mostly in LUQ 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 Anchored at both ends but
mobile in between Main regions in drawing to
right-------------------------------- Capacity: 1.5 L food; max
capacity 4L (1 gallon)
epigastrium
junction with esophagus
funnel shaped
contains pyloric sphincter
dome
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Stomach Regions
Cardiac region Fundus (dome shaped) Body
Greater curvature Lesser curvature
Pyloric region Antrum Canal Sphincter
junction with esophagus
funnel shaped
contains pyloric sphincter
dome
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Rugae: longitudinal folds on internal surface (helps distensibility)
Muscularis: additional innermost oblique layer (along with circular and longitudinal layers)
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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
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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 RLQSmall intestine___________
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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 looseDuodenum receives
bile from liver and gallbladder via bile duct*enzymes from pancreas via main pancreatic duct*
*
*
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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
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
*
Absorptivie cell with microvilli to increase surface area & many mitochondria: nutrient uptake is energy-demanding
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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
•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 energy-demanding
-produce mucus
Duodenal glands* *
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General histology of digestive tract
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Large intestine
Subdivisions
Cecum
Appendix
Colon
Rectum
Anal canal
Digested residue reaches itMain function: to absorb water and electrolytes
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Three special features
1. Teniae coli (3 longitudinal muscle strips)
2. Haustra (puckering into sacs)3. Epiploic appendages (omental or fat
pouches)
1.
2.
3.
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Between ileum and cecum
1st part
Blind tube
Colon has segments: ascending, transverse and descending colon; then sigmoid colon Right angle turns: hepatic flexure* in RUQ and splenic flexure* in LUQ
*
*
S-shaped
Movement sluggish and weak except for a few “mass peristaltic movements” per day to force feces toward rectum powerfully
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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
*
*
*
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Defecation1. 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)
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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
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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
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anterior
posterior
Fissure on visceral surfacePorta hepatis: major vessels and nerves
enter and leave - see picsLigamentum teres: remnant of umbilical vein in fetus, attaches to navel – see next slide
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Fetal circulation
Umbilical vein ___________
Ligamentum teres__________
Navel_______
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What the liver does…
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
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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)
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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
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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
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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.
*
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Pancreas(exocrine and endocrine)
Lies in LUQ kind of behind stomachIs retroperitonealHas a head, body and tailHead is in C-shaped curve of duodenumTail extends left to touch spleen
Main pancreatic duct runs the length of the pancreas, joins bile duct
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Pancreatic exocrine function Compound acinar (sac-
like) 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
one acinus
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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)
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Endocrine cells: