Chapter 24

The Digestive System

Digestive system contains

• Alimentary canal

• GI tract

• Accessory organs

Alimentary canal

Tube from mouth to anus (technically inside of tube is “outside the body”!)

Gastrointestinal (GI) tract

stomach and intestines

Accessory organs include 

• Liver

• Pancreas

• Gallbladder

Alimentary Canal Tissue Layers (from deepest to most superficial)

• Mucosa

  • Epithelial tissue layer

  • Lamina propria loose connective tissue

  • Muscularis mucosae – smooth muscle

• Submucosa

  • Loose connective tissue with vessels and nerve

• Muscularis externa

  • Smooth muscle for propulsion and mixing

  • Inner circular layer and outer longitudinal layer

• Serosa

  • Areolar tissue topped with simple squamous mesothelium

Nervous System of Alimentary Canal 

• Extensive nerves from the CNS 

• Enteric nervous system (ENS) 

Enteric nervous system (ENS)

• Is the nervous network to esophagus, stomach, intestines containing multiple parasympathetic nerve fibers

• Two plexuses
  

What are the two plexus that the Enteric nervous system has?

• Submucosal plexus

  • In submucosal layer

• Myenteric plexus

  • Between muscles of muscularis externa

Circulation of the Alimentary Canal 

• Foregut 

  • esophageal arteries

  • Celiac trunk

• Midgut

  • superior mesenteric artery

• Hindgut

  • inferior mesenteric artery

• Hepatic portal system

Peritoneum includes

Mesenteries

• connective tissue sheets holding abdominal viscera in place

• Posterior and anterior mesentery

Posterior and anterior mesentery

• two-layered membranes that may hang freely or connect organs together or to abdominal wall

  • lesser omentum

  • greater omentum

  • Mesocolon

Lesser Omentum

Extends from lesser curvature of stomach to liver

Greater Omentum

Hangs down like an apron from stomach’s greater curvature

Mesocolon

Mesentery of the colon

Mesenteries locations

The mouth (Oral (buccal) cavity) includes

Cheeks and lips

• Labial frenulum – median fold attaching lip to gum

• Vestibule – space just inside lips and cheeks

Palate

• Hard (bony)

• Soft – includes uvula

The tongue includes 

• lingual papillae 

• body 

  • anterior two-thirds

• Root

  • posterior one-third

• Lingual Frenulum

Lingual Frenulum

• Attachment to floor of mouth

• “Tongue-tied”

Contains both intrinsic and extrinsic muscles to mix food with saliva

• Chemical and mechanical digestion
  

The teeth (dentition) includes

• Mastication- chewing

• Incisors- slicers (chisels)

• Canines- Punctures and shred

• Premolars and molars- crush and grind

• 20 deciduous vs 32 permanent teeth

Tooth includes

• crown, neck, root

• Embedded in mandible or maxilla

  • Alveolus- socket 

  • Periodontal ligament 

  • cement of root 

• Gingiva- surrounds neck 

• Enamel covers crown 

• Internal structures 

  • dentin 

• Pulp cavity, root canal 

  • spaces for nerves and vessels 

What are the main salivary glands, their locations, and opening? 

• Parotid – near ear

  • Duct opens at 2nd upper molar

• Submandibular – under jaw

  • Duct opens at lower central incisors

• Sublingual – under tongue

  • Several duct openings under tongue.

Pharynx

• Oropharynx and Laryngopharynx pass food

  • Walls contain skeletal muscle that we control for swallowing

Esophagus

Esophagus is posterior to trachea

• Starts behind larynx and ends at lower esophageal sphincter located at the cardiac orifice to stomach

Stomachs anatomy External Anatomy “Gross Anatomy”

• lesser curvature

• greater curvature

stomach “gross” anatomy divided into portions called 

• Cardiac/Cardial

•  Fundus

• Body

• Pylorus/Pyloric part contains the 

  • Antrum

  • Pyloric canal

  • Pyloric sphincter

stomach “gross” anatomy of the gastric rugae 

• internally

  •  Longitudinal folds in empty stomach

The Stomach: Microscopic Anatomy

Gastric pits of mucosa with glands/cells

• Mucous cells

• Regenerative (stem) cells

• Parietal cells

• Chief cells

• Enteroendocrine cells

Mucous cells

• secrete mucus that protects the stomach lining and aids in digestion.

Regenerative (stem) cells

• make new cells for the gastric epithelium, replacing those lost through wear and tear.

• Epithelial cells are frequently replaced (they

  live only 3 to 6 days)
  

Chief cells

pepsinogen (enzyme that breaks down proteins)

Enteroendocrine cells

digestive hormones

Mucous coat

for protection

• Is thick and highly alkaline (basic) to counteract the stomach acid
  

Tight junctions

between epithelial cells

• Prevent seepage of acidic gastric juice
  

Small Intestine: Duodenum length and contains?

• First 25 cm (10 in.) of SI

• Contains circular folds (plicae circulares) to increase surface area for absorption

Small intestines: Duodenum function

• Receives and mixes stomach contents, pancreatic juice, and bile

• Major duodenal papilla for bile and pancreatic duct secretions

• Minor duodenal papilla for accessory pancreatic duct secretions

• Duodenojejunal flexure

Small Intestine: Jejunum location and function

• Region of SI following the duodenum

• Lies mostly within umbilical region

• Most digestion and absorption occur here

• Very prominent circular folds (plicae circulares)

Small Intestine: Ileum location, contains, an function

Location: Last portion of the SI, sparse circular folds

contains: lymphatic nodules called Peyer’s patches

Function: connects to the cecum of the large intestine

• ileocecal junction

  • ileal papilla

small intestine microscopic anatomy includes

• Intestinal Villi

  • Enterocytes

  • Goblet cells

  • Lacteals

  • Capillary bed

• Intestinal crypts

  • goblet cells and enterocytes

  • paneth cells

Enterocytes

• (absorptive cells) each have tiny microvilli projections to increase absorption

Goblet cells

secrete mucus

Lacteals

lymphatics for lipid absorption

Paneth cells

enteroendocrine cells in crypts

(Colon)-Cecum (pouch)

• First part of the LI

• Ileum attaches to cecum at the ileocecal valve

• Contains the appendix – a blind tube that contains lymphatic tissue

The Large Intestine (Colon) CONTAINS

• Ascending colon

• transverse colon

• left colic (Splenic) flexure

• descending colon

• sigmoid colon

Ascending colon location

On right side of body

The large intestines include the

colon, rectum, and anal canal.

Right colic (Hepatic) flexure location 

Bend in LI between ascending and transverse colon

Transverse colon location

Runs right to left horizontally

Left colic (Splenic) flexure location

Bend in LI between transverse and descending colon

Descending colon location

On left side of body

Sigmoid colon location

• Last portion

• S-shaped

Other features of the large intestine (colon) 

• Taeniae Coli

  (longitudinal m.)

• Haustra/haustum (pouch/pouches)

• Omental appendices (fat)

• Diverticula/um

Rectum

Transverse rectal folds (valves) – holds and retains feces

Anal canal

~3 cm long

• Anal columns – folds

• Anal sinuses – depressions between folds

• Internal (smooth m.) and external (skeletal m.) anal sphincters

  • Internal sphincter – involuntary

  • External sphincter - voluntary

Large Intestine: Microscopic Anatomy type of epithelium?

Epithelium is mostly simple columnar with many goblet cells

• Exception: anal canal has stratified squamous

Large Intestine: Microscopic Anatomy contains 

• Has intestinal crypts but no villi or circular folds

• Abundant lymphatic tissue

• Mucosa specialized for fluid and electrolyte absorption

The Liver function

Body’s largest gland

Digestive function is bile production

Other functions:

• Makes bilirubin – decomposition of hemoglobin

  • Urobilinogen

    • Metabolized bilirubin

    • Makes feces brown and urine yellow

The livers gross anatomy lobes?

right (largest), left, quadrate, and caudate

The livers gross anatomy ligament?

• Falciform ligament – separates R. lobe from L. lobe

• Round ligament- remnant of umbilical vein 

The livers gross anatomy hepatis?

Porta hepatis – area between quadrate and caudate lobes

• Contains hepatic artery, hepatic portal vein, bile duct

The livers Hepatic lobules?

small cylinders with central vein with radiating plates of hepatocytes

• hepatic sinusoids

• stellate macrophages (Kupfer cells)

• Bile canaliculi → bile ductules → right/left hepatic ducts

• Hepatic sinusoids

• leaky capillaries

• Stellate macrophages (Kupfer cells)

• clean blood running through sinusoids

Livers Circulation

• Hepatic portal vein

• hepatic artery

• hepatic veins

Hepatic portal vein

Brings nutrient-rich blood from veins of GI tract to liver

Hepatic artery

• Brings arterial blood

• Aorta→ celiac trunk→ common hepatic a. → hepatic artery proper →hepatic a.

Hepatic veins

Exit from top of liver and empty into inferior vena cava

The Gallbladder location, function, and anatomy?

Location: Sac on underside of liver

Function: Stores and concentrates bile

Anatomy: Has fundus (head), cervix (neck)

• Cystic duct – from gallbladder merges with common hepatic duct

Bile passages 

• Two hepatic ducts merge to form the common hepatic duct

• Common hepatic duct merges with cystic duct to form bile duct

• Bile duct merges with main pancreatic duct to form hepatopancreatic ampulla at major duodenal papilla of the duodenum

The Pancreas location and function?

Function: Lies posterior to stomach

• Head (near duodenum), body, and tail (near spleen)

Function:

• Acinar cells (Exocrine) secrete pancreatic juice for digestion

  • Enzymes, zymogens (inactive precursors of enzymes), sodium bicarbonate (alkaline fluid), water, and electrolytes

• Pancreas islets (Endocrine) secrete hormones

The pancreas Branching Ducts

• Pancreatic duct merges with Bile duct to form hepatopancreatic ampulla at major duodenal papilla

• Accessory pancreatic duct – opens at minor duodenal papilla

Appendicitis

Inflammation of the appendix, with swelling, pain, and a threat of gangrene, perforation, and peritonitis.

Ascites

Accumulation of serous fluid in the peritoneal cavity, often causing extreme distension of the abdomen. Most often caused by cirrhosis of the liver and frequently associated with alcoholism. The diseased liver "weeps" fluid into the abdomen.

Ulcerative colitis

Chronic inflammation resulting in ulceration of the large intestine, especially the sigmoid colon and rectum. Tends to be hereditary but exact causes are not well known.

Crohn’s disease

Inflammation of small and large intestines, similar to ulcerative colitis in symptoms and hereditary predisposition. Produces granular lesions and fibrosis of intestine; diarrhea; and lower abdominal pain.