Digestive System

Alimentary Canal or Gastrointestinal Tract

- muscular tube that winds from the mouth to the anus

Accessory Digestive Organs

- teeth, tongue and salivary glands
- gallbladder, liver and pancreas

Digestion Process

1. Ingestion
2. Propulsion
3. Digestion
4. Absorption
5. Defecation

Ingestion

- first step of the digestive process
- taking food into the mouth

Propulsion

- second step of the digestive process
- movement of food through the GI tract

Digestion

- third step of the digestive process
- mechanical and chemical processes that break down food

Absorption

- fourth step of the digestive process
- movement of food into blood and lymph capillaries

Defecation

- fifth step of the digestive process
- elimination of waste materials

Peristalsis

- ripple-like wave of muscular contractions that move food along the GI tract

Segmentation

- churning and mixing to help disperse material and combine it with digestive secretions

Tissue Layers of the Alimentary Canal

1. Mucosa
2. Submucosa
3. Muscularis
4. Serosa

Mucosa

- innermost layer of the wall of the alimentary canal
- consists of three sublayers: epithelium, lamina propria and muscularis mucosae

Epithelium

- top layer of the mucosa
- consisting of squamous columnar epithelium
- different functions depending on the location

Lamina Propria

- second layer of the mucosa
- loose areolar connective tissue (blood vessels)
- capillaries nourish epithelium and absorb digested nutrients

Muscularis Mucosae

- bottom layer of the mucosa
- thin layer of smooth muscle that produces local movements of the mucosa

Submucosa

- layer of connective tissue surrounding the mucosa containing major blood and lymphatic vessels, nerves
- elastic fibers enable canal to return to original shape after passage of food

Muscularis

- two layers of smooth muscle surrounding the submucosa
- inner circular and outer longitudinal - responsible for peristalsis

Exceptions of the Muscularis

- esophagus has a mixture of smooth and skeletal muscle
- stomach contains three layers of smooth muscle

Serosa

- outer layer of the alimentary canal consisting of simple squamous epithelium
- areolar connective tissue (blood vessels)
- also called adventitia or visceral peritoneum

Teeth

- humans normally have 32 permanent
- incisors, canines, premolars, molars

Incisors

- nipping off pieces of food (8)
- front teeth

Canines

- tearing and piercing (4)
- after incisors

Premolars and Molars

- grinding of food
- 20 total at the back of the mouth

Intrinsic Gland

- type of salivary gland consisting of many small glands that keep oral cavity moist

Extrinsic Glands

- type of salivary gland that lie external to mouth but connect through ducts
- active when feeding

Pharynx

- swallowed food is passed posteriorly into oropharynx and laryngopharynx
- makes use of pharyngeal constrictor skeletal muscles

Voluntary Phase

- first phase of swallowing
- bolus pushed by tongue against hard palate and to oropharynx

Pharyngeal Phase

- second phase of swallowing
- soft palate and uvula close off nasopharynx
- epiglottis covers laryngeal opening

Esophageal Phase

- final phase of swallowing
- peristalsis pushes bolus towards stomach

Esophagus

- muscular tube that propels food to stomach
- passes through esophageal hiatus in diaphragm
- contains the four layers of the GI tract
- epithelium is stratified squamous epithelium
- highly folded when empty
- lots of mucous glands that are triggered by food bolus
- muscularis changes from skeletal to smooth as it approaches stomach

Cardiac Sphincter

- prevents stomach acid from re-entering the esophagus

Stomach

- temporary storage organ in which food is churned into chyme
- divided into various regions
- internal surface contains numerous longitudinal folds
called rugae
- surface of the mucosa is columnar epithelia
- extending down into lamina propria are gastric glands
- produce very acidic solution with digestive enzymes (gastic juice)

Protection of the Stomach from Gastric Juice

- lots of mucous
- epithelial cells have tight junctions
- damaged cells replaced quickly

Peptic Ulcer

- craterlike erosion of the mucosa in any part of the alimentary canal exposed to stomach secretions
- about 98% occur in pyloric region of stomach (gastric ulcer) or first part of duodenum (duodenal ulcers)

Perforated Ulcer

- stomach contents leaked into the peritoneal cavity

Small Intestine

- longest part of alimentary canal
- site of most enzymatic digestion and nearly all absorption
- large surface area
- food spends at least 12 hours here where chemical digestion takes place
- 3 regions

Duodenum

- first section of the small intestine emerging from the stomach
- 25 cm in length
- where bile and pancreatic secretions enter the intestine

Jejunum

- middle of the small intestine
- 1m long
- primary region for chemical digestion and nutrient absorption

Ileum

- last region of the small intestine that empties into the large intestine
- sphincter controls material entering into large intestine
- responsible for absorption of vitamin B12 and bile salts
- patches of lymph tissue

Small Intestine Histology

- internally get circular folds with villi increasing the surface area of intestinal lining
- each contain an arteriole, venule, capillaries and lacteal
- microscopic microvilli are found on the surface of each villus

Intestinal Glands

- invaginations into mucosa found between villi
- secrete intestinal juice, digestive enzymes, hormones, mucous, etc.
- diseases associated with these glands are colorectal caner, Crohn's diseas and cryptitis

Large Intestine

- 1.5m extending from the ileum to anus
- frames the small intestine on three sides
- consists of the cecum, ascending colon, hepatic flexure, transverse colon, splenic flexure, descending colon, rectosigmoid colon and rectum

Teniae Coli

- three separate longitudinal bands of smooth muscle on the outside of the ascending, transverse, descending and sigmoid colons
- thickenings of the muscularis

Haustra

- saccules in the colon that give it its segmented appearance
- series of pouches caused by the contraction of teniae coli

Appendicitis

- blockage traps bacteria in lumen of appendix
- begins to swell with mucous and leads to an infection

Diverticulosis

- refers to presence of small pockets (diverticula) in large intestine
- in some cases, spasms, cramping, diarrhea, constipation

Diverticulitis

- inflamed diverticula become infected and perforate

Rectum

- portion of the large intestine
- no teniae coli
- well developed muscularis to generate strong contractions

Anal Canal

- last 3 cm of the large intestine
- internal sphincter consists of smooth muscle
- external sphincter consists of skeletal muscle

Hemorrhoids

- stretched and inflamed veins that bulge into the lumen of the anal canal
- can be caused by straining to defecate or delivering a baby

Liver

- accessory organ of digestion
- liver is largest gland of body
- comprised of four incompletely separated lobes
- falciform ligament completely divides right from left lobe
- digestive function is to produce bile

Gallbladder

- bile is then stored in gallbladder
- dumped into duodenum via bile duct

Cirrhosis

- chronic inflammation of the liver
- usually the result of hepatitis or alcoholism
- main cell types of liver damaged, replaced by scar tissue
- only effective treatment are liver transplants with a good survival rate but few donors available

Hepatitis

- inflammation of the liver
- often due to viral infection
- six different viruses known so far (A-F)
- type A from contaminated food, water and feces
- type B from blood transfusions, contaminated needles and sexually transmitted fluids

Gallstones

- if there is too much cholesterol for bile to break down, it can become crystalized
- cause substantial pain if they block cystic duct

Pancreas

- accessory organ in digestion
- produce most of enzymes that digest food in small intestine (pancreatic juice secreted through two ducts)
- also lots of bicarbonate to neutralize the acid from stomach
- main pancreatic duct merges with common bile duct
- accessory duct enters duodenum proximally

Pancreatitis

- pancreatic enzymes activated in pancreas instead of small intestine
- abdominal pain, nausea, vomiting
- acute lasts a few days and is normally caused by gallstones
- chronic persists over many years and is often related to high consumption of alcohol

Pancreatic Cancer

- dangerous to screen for, yet deadly if ignored
- pancreas is very sensitive and biopsies can lead to serious complications
- cancer is usually detected too late
- five-year survival rate of less than 5%
- emphasis on earlier detection