1. The Digestive Tract

2. 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

3. The Digestive Process Ingestion Propulsion (movement of food)
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

4. 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

5. Ways to divide…. The more common Plus: epigastric periumbilical
suprapubic
flank

6. Histology of alimentary canal wall Same four layers from esophagus to anal canal
Mucosa
Submucosa
Muscularis externa
Serosa
from lumen (inside) out

7. Inner layer: the mucosa* (mucous membrane)
Three sub-layers
Lining epithelium
Lamina propria
Muscularis mucosae *

8. 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

9. 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 *

10. 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 *

11. 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 *

12. 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
6 major locations:
inside the eye 2. walls of vessels 3. respiratory tubes
4. digestive tubes 5. urinary organs 6. reproductive organs

13. 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

14. 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

15. 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

16. Mesenteries
Note dorsal, ventral and formation of retroperitoneal position

17. 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) *

18. 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…

19. Note mesenteries: falciform ligament, lesser omentum, greater omentum

20. Note: greater omentum, lesser omentum, falciform ligament, transverse mesocolon, mesentery, sigmoid mesocolon

21. 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”)

22. The Mouth Mouth = oral cavity Lips- orbicularis oris muscle
Lining: thick stratified squamous epithelium
Lips- orbicularis oris muscle
Cheeks – buccinator muscle

23. “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

24. 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

25. 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

26. Teeth Two sets Primary or deciduous Permanent: 32 total
“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)

27. 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

28. Tooth structure Two main regions Enamel
Crown (exposed)
Root (in socket)
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

29. 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 C B

30. 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, enzymes
keep mouth moist
dissolves food so can be tasted
moistens food
starts enzymatic digestion
buffers acid
antibacterial and antiviral

31. 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 #

32. Pharynx Oropharynx and laryngopharynx Three constrictor muscles* *
Stratified squamous epithelium
Three constrictor muscles*
Sequentially squeeze bolus of food into esophagus
Are skeletal muscles
Voluntary action
Vagus nerve (X)
___laryngopharynx * * *

33. 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___________ *

34. 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) * *

35. Contains all 4 layers (see right)
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

36. Esophagus histology

37. 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)

38. Stomach Lies mostly in LUQ Just inferior to (below) diaphragm
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
dome
junction with esophagus
contains pyloric sphincter
funnel shaped

40. 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

41. Rugae: longitudinal folds on internal surface (helps distensibility)
Muscularis: additional innermost oblique layer (along with circular and longitudinal layers)

42. 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

43. 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___________

44. * * 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* * *

45. * 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 energy-demanding *
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

46. * * 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
-have many mitochondria: nutrient uptake is energy-demanding
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) *
-produce mucus

47. General histology of digestive tract

50. Large intestine Subdivisions Cecum Appendix Colon Rectum Anal canal
Digested residue reaches it
Main function: to absorb water and electrolytes
Subdivisions
Cecum
Appendix
Colon
Rectum
Anal canal

51. Three special features
Teniae coli (3 longitudinal muscle strips)
Haustra (puckering into sacs)
Epiploic appendages (omental or fat pouches)
Three special features 3. 2. 1.

52. 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
1st part
S-shaped
Blind tube
Movement sluggish and weak except for a few “mass peristaltic movements” per day to force feces toward rectum powerfully

53. Rectum Anal canal * * * 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 * * *

54. Defecation
Triggered by stretching of wall, mediated by spinal cord parasympathetic reflex
Stimulates contraction of smooth muscle in wall and relaxation of internal anal sphincter
If convenient to defecate voluntary motor neurons stimulate relaxation of external anal sphincter
(aided by diaphragm and abdominal wall muscles -called Valsalva maneuver)

55. 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

56. 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

57. Fissure on visceral surface Porta hepatis: major vessels and nerves
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

58. Fetal circulation Ligamentum teres__________ Umbilical vein
___________
Navel_______

60. 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

61. 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)

62. Portal triad Liver sinusoids Kupffer cells 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

64. 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 ml bile each day
Secrete into bile canaliculi (little channels) then ducts
Regeneration capacity through liver stem cells

65. 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. *

66. Pancreas (exocrine and endocrine)
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)

68. Pancreatic exocrine function
one acinus
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

69. 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)

70. Endocrine cells: