chapter 15 - digestive system

gastrointestinal tract

a tube that stretches from the mouth to anus

accessory glands

teeth, tongue, salivary glands, pancreas, liver, gall bladder

digestive tract functions

1. ingestion

2. digestion — large molecules are broken down into smaller molecules

   • mechanical digestion → chewing, churning of food, segmentation of food, peristalsis

   • chemical digestions → enzymes and acid secretion function to break down food

3. absorption — end products of digestion are taken up into the blood and lymph

4. defecation — elimination of waste and undigested materials

visceral smooth muscle

• composed of thin and thick myofilaments

• cells are spindle-shaped and contain one centrally located nucleus

• arranged as sheets and operates as a unit called a “syncytium”

• cells are connected to one another by gap junctions

peritoneum

serous membrane of the abdominal cavity

• visceral → organ wall

• parietal → wall of abdominal cavity

• cavity → filled with serous fluid

mesentery

a fused sheet of 2 serous membranes on top of one another (double layer of peritoneum) that extends to digestive organs from the wall of the abdominal cavity

• provides a necessary route for blood cells, lymphatics, and nerves to reach the viscera

• hold organs in place

• store lipids

• binds both the small and large intestines to the wall of the abdominal cavity

omentum

folds of the visceral peritoneum

greater omentum

folds of the visceral peritoneum that covers the transverse colon and small intestine

• provides protection, source of stored energy, and insulation

lesser omentum

folds of the visceral peritoneum that connects the stomach to the duodenum to the liver

peritonitis

the inflammation of the peritoneum

retroperitoneal organs

organs located posterior to the peritoneum

adventitia

connective tissue located on the posterior surface of retroperitoneal organs

histology of the digestive tract

1. serosa/adventitia

   • made of areolar connective tissue and simple squamous epithelium

   • outermost, protective layer of organs that are located in the peritoneal cavity (visceral peritoneum)

     • ex. stomach, lower small intestine, most of the large intestine, liver, gallbladder

   • retroperitoneal organs contain both serosa and adventitia

     • anterior surface → from peritoneum

     • posterior surface → from adventitia

     • ex. pancreas, duodenum

2. muscularis externa

   • layer of visceral smooth muscle (can't control)

   • contractions are responsible for motility — moves and mixes food along the digestive tract

   • composed of an inner circular layer (furthest away from serosa) and an outer longitudinal layer

     • myenteric nerve plexus located in between

3. submucosa

   • composed of areolar connective tissue

   • binds mucosa to muscularis externa

   • contains lymph and blood vessels

   • submucosal nerve plexus is located here

4. mucosa

   • innermost (collectively) layer; faces the lumen

   • composed of mucous membrane

   • composed of 3 layers:

     i. muscularis mucosa

     • located nearest to the submucosa

     • composed of (more) smooth muscle — permits motility in the mucosa

     ii. lamina propria

     • composed of areolar connective tissue

     • located in the centre of the mucosal alyer

     • contains blood (to absorb nutrients), lymphatic vessels and tissues (for lipids)

     iii. mucosal epithelium

     • mouth, pharynx, esophagus, and anal canal are composed of stratified squamous epithelium — against abrasion

     • stomach, small intestine, and large intestine are composed of simple columnar epithelium — for absorption

     • many goblet cells here — for mucous production

oral cavity

the mounth

• lined by:

  • mucosa — stratified squamous epithelium

  • skeletal muscle

• includes:

  1. lips (anteriorly)

  2. cheeks (laterally)

  3. palate (superiorly)

  4. tongue (inferiorly)

  5. salivary glands

  6. teeth

hard palate

an anterior component of the roof of the mouth, formed from maxilla and palatine bones

soft palete

a posterior component of the roof of the mouth, formed from uvula and skeletal muscle arch

tongue

a muscular organ located inferiorly in the oral cavity

• composed of skeletal muscle

• covered by mucosa on the surface

• attached to the hyoid bone

• superior surface → contains projections of lamina propria (“papillae”)

  • 3 types

  • some contain taste buds

• dorsal surface → contains lingual glands

  • contain fluid and lingual lipase that digest fats in the mouth

parotid gland

a salivary gland located both inferior and anterior to the ears

• produces thin saliva that lacks mucus but rich in salivary amylase

mumps

a disease that results from viral infection/inflammation of one or both parotid glands

submandibular gland

a salivary gland located at the bottom of the mouth

• produces a fluid mix that contains amylase and mucus

sublingual gland

a salivary gland located below the tongue on the floor of the mouth

• produces mucus

lysozyme

an enzyme that digests important bacterial structures (by cutting peptidoglycan, cell wall around bacteria), preventing oral infections

teeth

strucutres in found in the gum-covered margins of maxillae and mandible

• function to mechanically breakdown food into smaller pieces, allowing swallowing to occur with ease

  • also allows chemical digestion to happen more efficiently bc the food surface area is increased

decidous/primary teeth

teeth found in children

secondary/permanent teeth

teeth found in adutts

crown

a part of a tooth that is located above the gum; composed of dentin with an enamel overlay

neck

a part of the tooth that is located at the boundary between the enamel and cementum at gum levelr

root

a part of the tooth that is located within the gum; composed of dentin with a cementum overlay

periodontal ligaments

ligaments that attach the root of the tooth to the jaw bones

pulp cavity

a part of that tooth that is located centrally, containing pulp (blood vessels, nerves, connective tissue, lymph vessels) enclosed by dentin

root canal

a part of the tooth that is an extension of the pulp cavity into the root

dentin

calcified connective tissue very similar to bone but avascular; makes up majority of the tooth

enamel

acellular, avascular, highly calcified; protects the tooth from wear and tear

• hardest substance found in the body

cementum

calcified, avascular connective tissue that attached the root of the tooth to the periodontal ligament

pharynx

• only the oropharynx (oral) and laryngopharynx (larynx) — imbedded within tissues of the neck

• composed of only mucosal layer and muscularis external — in contact with food

  • mucosa → stratified squamous epithelium; bc food is abrasive

    • muscularis externa → skeletal muscle; have full control, able to choose when to swallow

esophagus

located posterior to the trachea in the mediastinum — passes thru the diaphragm, entering into the abdominal cavity; connects the pharynx to the stomach

• composed of fibrous connective tissue (adventitia); no serosa

• mucosal layer → stratified squamous epithelium

• submucosa layer → layer of connective tissue

• upper 1/3 to the middle section (so 2/3) contains skeletal muscle

  • middle section transitions into smooth muscle (1/3 overlap)

  • continues thru the bottom 1/3 to the stomach (so 2/3)

stomach

organ that is located in the left quadrant of the abdominal cavity

• mucosa → composed of simple columnar epithelium, containing goblet cells

• contains rugae

• muscularis externa is composed of 3 layers:

  1. outer layer → longitudinal layer

  2. middle layer → circular layer

  3. inner layer → oblique layer; found predominantly on the body of the stomach

• there are gastric glands formed by mucosal folds and secrete gastric juice into the lumen from gastric pits/depressions

components of the stomach

1. cardiac region — separated from the esophagus by the “cardiac sphincter”

   • only close to the heart, but has no association

2. fundus — tip

3. body — main section; bulk of the structure

4. pyloric canal — closed off by the “pyloric sphincter”; prevents overwhelming of the intestine

   • controlling ejection of chyme little by little for adequate digestion

5. greater (outside/concave surface) and lesser (inside/convex surface) curvature

   • for landmarking

rugae

large folds formed from the contraction of muscularis mucosa

cell types of gastric glands

1. chief cells — secrete pepsinogen (digest pepsin protein) and gastric lipase (digest lipids)

2. parietal cells — secrete HCl (acid)

3. mucous neck cells — goblet cells that secrete mucous

4. G cells — secrete hormone gastrin; controls digestion (mediator)

chyme

mixture of food and gastric juice that leaves the stomach

small intestine

• stretches from the pyloric sphincter to the ileocaecal sphincter

• epithelial layer of mucosa contains:

  1. enterocytes — absorptive cells

     • mature intestinal cells that specialize in absorption

  2. goblet cells

     • secrete mucus

• epithelial cells have microvilli

• mucosal layer is highly folded to increase intestinal surface area for absorption — “villi”

  • project into the lumen of the small intestine

  • each contains a lymphatic capillary (lacteal) and blood capillaries

• at the base between each vili, there are “crypts of lieberkuhn” — pits that open into intestinal glands

• muscularis externa → both circular (inner layer) and longitudinal (outer layer) layers

brush border

a blurry fuzzy line formed from intestinal microvilli. that can be seen under the microscope

crypts of leiberkuhn

pits at the base between each villi that open into intestinal glands

cells in the epithelium of crytps

1. undifferentiated stem cells — give rise to all of the cell types that are found on the surface of vili and inside crypts

2. goblet cells — secrete mucus

3. eneteroendocrine cells — some secrete secretin, others secrete cholecystokinin (CKK); kinda like B cells

4. paneth cells — released immune modulators such as defesins (pokes holes in membranes of invaders) and antibiotic enzyme lysozyme

5. secretory intestinal cells — immature enterocytes

plicae circulares

permanent structures in which submucosa and mucosa are folded tgt; circular folds

parts of the small intestine

1. duodenum

   • first section

   • receives partially digested (acidified/denatured) material from the stomach

   • retroperitoneal

   • ~25 cm in length

2. jejunum

   • middle section

   • ~1 m in length

3. ileum

   • inferior section

   • ~2 m in length

   • joins the large intestine at the ileocaecal sphincter

peyer’s patches

groups of lymph nodes located within the ileum that serve in immune defenses against pathogens encountered along the digestive tract

pancreas

• retroperitoneal

• contains a head, body, and tail

• has both exocrine and endocrine glands

acini cells

glandular epithelium that secretes pancreatic juice into ducts

• pancreatic juice is alkaline and contains enzymes (for fully body digestion)

• buffers acidity of chyme that’s entering from the stomach — protect from ulcers

islets of langerhans

endocrine cells in the pancreas that functions to secrete insulin (hyperglycemic) and glucagon (hypoglycemic)

• serve to regulate blood sugar levels

• in direct contact with blood

hepatocytes (hepatic cells)

cells of the liver that function to produce bile

liver

• works to filter materials that have entered the body via the GI tract before they exit and travel to the remaining cells of the body

  • this incl nutrients and toxins, such as drugs and alcohol

gal bladder

a muscular sac located on the surface of the liver that stores and concentrates bile

• composed of simple columnar epithelium

• lacks submucosal layer

• contains rugae

hepatopancreatic duct

duct that drains both bile and pancreatic juice into the duodenum of the small intestine

large intestine

stretches from the ileocaecal valve to the anus

• composed of 4 basic layers:

  1. mucosal layer

     • composed of columnar epithelium (except within anal canal)

     • contains large quantity of goblet cells

  2. submucosa — connective tissue layer

  3. muscularis externa

     1. circular layer → completely extending around its circumference

     2. longitudinal bands → reduced to only 3 layers; incomplete

        • called “teniae coli”

        • teniael contraction (pulls + tension) gives rise to haustra (bunches/pockets)

        • slow movement of undigested material thru the small intestine allows water to be removed

parts of the large intestine

1. caecum and appendix

2. colon — consists of ascending, transverse, descending, and sigmoid portions; unidirectional

3. hepatic (right colic) flexure — between ascending and transverse colon

4. splenic (left colic) flexure — between transverse and descending colon

5. haustra — pocket-like sacs that form along the length of the colon

6. anal canal — final 3 cm

   • external sphincter → skeletal muscle

   • internal sphincter → smooth muscle

7. anus

external surface of the large instestine

contains teniae coli, haustra, epiploic appendages

internal surface of the large instestine

contains intestinal glands composed of goblet, absorptive, and secretory cells

• doesn't have the same villi and folds in small intestine — no nutrient absorption occurs, so not much surface area needed

portal system

blood vessels located between 2 capillary beds

• blood first passes from one capillary bed directly into another, w/o first returning to the heart