Unit 15 - digestive system pt 1

1)     Gastrointestinal (GI) tract

-  tube from mouth to anus

2)     Accessory organs

-  teeth, tongue, salivary glands

- pancreas

- liver, gall bladder

- Ingestion (food into oral cavity)

- Digestion (mechanical and chemical (enzymes and acid secretions))

- absorption (end products of digestion enter blood or lymph)

- defecation(elimination of undigested material)

—  lined by a mucosa (mucous membrane) made of a stratified squamous epithelium and lamina propria

—  includes:

a)      lips

b)     cheeks

c)      palate (hard and soft)

d)     tongue

- hard palate (2 maxillae and 2 palatine bones)

-soft palate (posterior to hard, skeletal muscle, posterior projection is uvula to close nasopharynx when swallowing)

—  attached to hyoid bone

—  skeletal muscle

—  projections of mucosa = papillae (taste buds)

3 pairs:

a)      parotid 

—  inferior and anterior to ears

—  mumps = inflammation of 1 or both parotids

b)     submandibular

—  floor of mouth

c)      sublingual 

—  below tongue on floor of mouth

o   99.5% water

o   0.5% solutes (e.g. enzymes)

- in maxillae and mandible

- child dentition are primary dentition – deciduous (“baby”) teeth

- adult dentition are secondary dentition – permanent teeth

crown, root, neck, periodontal ligaments, root canal extends to pulp cavity

—  above the gum

—  dentin forms the majority of tooth

—  enamel overlay is acellular, highly calcified – hard!

root: dentin with cementum overlay

note: dentin, enamel, and cementum are similar to bone but avascular

a)      neck

—  enamel and cementum boundary (gums)

b)     periodontal ligaments

—  attach root to bones

—  contains connective tissue, blood/lymph vessels, and nerves

—  only muscularis externa (skeletal muscle) and stratified squamous epithelium

mucosa, submucosa, muscularis externa, and serosa or adventitia (double walled membrane)

a)      epithelium with numerous goblet cells

—  stratified squamous: esophagus, anal canal

—  simple columnar: stomach, small and large intestines, rectum

b)     lamina propria (areolar connective tissue)

—  contains blood, lymph vessels, lymph nodules/tissues (immune)

c)      muscularis mucosa

—  smooth muscle - allows movement of mucosa

—  areolar CT

—  contains: blood, lymphatic vessels, and a network of nerve cells

- two layers of smooth muscle separated by a second network of nerve cells

o   inner circular layer (contraction constricts the lumen)

o   outer longitudinal layer (contraction shortens gut length)

- contractions controlled by the nerve network cause motility (mixing and movement)

—  serous membrane that lines the abdominopelvic cavity and most of the abdominal organs

—  specializations: omenta and mesentry

—  located posterior to the parietal peritoneum

—  peritoneum lines only one side of the organ

—  e.g. pancreas, duodenum

o   anterior surface is covered by parietal peritoneum

o   posterior surface is covered by adventitia that connects the organ to the body wall

1)     visceral peritoneum (against organ wall)

2)     parietal peritoneum (against abdominal cavity wall)

3)     peritoneal cavity (space between parietal and visceral peritoneum)

- filled with serous fluid to lessen friction between parts digestive tract during motility

—  folds of serosa between organs made of a sheet of 2 fused layers of visceral peritoneum

—  contains blood/lymph vessels and nerves

—  examples:

- greater omentum (“fatty apron”) (connects stomach to transverse colon, forms large fold that hangs down over transverse colon and small intestine)

- lesser omentum (connects liver to stomach)

—  fold of serosa between the posterior abdominal cavity wall and the small/large intestine made of a sheet of 2 fused layers of parietal peritoneum

—  entry and exit point for blood vessels, nerves, and lymphatic vessels supplying digestive organs

Esophagus, stomach, small intestine, large intestine,

—  posterior to trachea

—  passes through the diaphragm entering into the abdominal cavity

a)      muscularis externa

—  upper 1/3 = skeletal muscle

—  middle 1/3 = skeletal and smooth muscle

—  lower 1/3 = smooth muscle

b)     the outer layer is adventitia within thoracic cavity and serosa within the abdominal cavity

—  Stores, partially digests, and regulates emptying of chyme (food and gastric juices) into the small intestine

—  4 regions: cardiac region (cardia), fundus, body, pyloric region (pylorus)

- cardiac region (cardia): attached to inferior esophagus

- fundus: superior to esophageal entrance

- body: middle portion

- pyloric region (pylorus): inferior portion of the stomach

the pyloric sphincter regulates release of stomach contents into small intestine

the mucosal surface is:

- formed exclusively by mucous cells

- has millions of gastric pits (invaginations of the epithelium) that are connected to the underlying gastric glands

- exocrine glands that secrete gastric juice into the gastric pits where it then enters the lumen of the stomach

- goblet cells

- chief cells (secrete enzymes for protein and fat digestion)

- parietal cells (secrete hydrochloric acid (HCl) which lowers the pH of the stomach)

- G cells (enteroendocrine cells) (secrete a hormone into the blood that regulates activity of parietal cells and other digestive processes.)

- are folds of the mucosa and submucosa due to contraction of muscularis mucosa

- visible when stomach is empty

- allows expansion of stomach without tearing

o   function = churning chyme

o   3 layers instead of 2:

i.           inner oblique

ii.           middle circular

iii.           outer longitudinal

- pyloric sphincter to ileocaecal valve

- where most food digestion/absorption occurs

- 3 segments: duodenum, jejunum (middle section), ileum

simple columnar epithelium 3 main cell types

1. enterocytes: simple columnar cells (absorptive cells that form most of the mucosa)

2. goblet cells: secrete mucus

3. enteroendocrine cells: located within intestinal glands secrete hormones into the blood that help regulate digestive processes

—  first fold (short) – is retroperitoneal

—  extra glands here secrete alkaline mucous to protect against stomach acid

—  ducts of accessory organs (liver, gall bladder, pancreas) enter the digestive system here

- last portion of small intestine before large intestine 

- attached to caecum (part of large intestine)

- has groups of lymph nodules called Peyer’s patches (prevent infection of small intestine and prevent bacteria from entering blood)

- plicae circulares: submucosa thrown into large folds

- villi: projections of mucosa into lumen of small intestine (contains blood capillaries, lacteals (are lymphatic capillaries that absorb fats))

- microvilli: projections of the enterocyte cell membranes that extend into the lumen of the small intestine, forming a fuzzy “brush border” on the surface of the mucosa

- from ileocaecal valve to anus

- basic functions: absorption of water, electrolytes, vitamins and formation and temporary storage of feces

- contains: caecum, appendix, colon, rectum, and anal canal

—  mucosa has a smooth surface with no folds or villi

connected to ileum by ileocaecal valve

- last segment of large intestine, but external to the abdominopelvic cavity

- opening and closing of the inferior anal canal during defecation is controlled by two anal sphincters: internal anal sphincter (smooth muscle), and external anal sphincter (skeletal muscle – voluntary control)

- opening at inferior end of anal canal is the anus

- histology: mucosa transitions to stratified squamous epithelium

—  longitudinal layer of muscularis externa is reduced to three bands of muscle called teniae coli (contraction of the teniae coli forms pouches called haustra)

- ascending colon (right side of abdominal cavity)

- hepatic flexure

- transverse colon

- splenic flexure

- descending colon (left side of abdominal cavity)

- sigmoid colon

Large intestine