The Digestive Tract

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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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Ways to divide….

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 muscleSmooth 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 hormones6 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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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

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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: