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The Digestive Tract.

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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:

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