Digestive System
Function: To break down food into usable (absorbable) form
Processes involved:
1. Ingestion
2. Movement of food along GIT – voluntary (ie: swallowing); involuntary (ie: peristalsis)
3. Digestion
a. Mechanical (i.e.: mastication, emulsification, mixing waves, segmentation)
b. Chemical
- carbohydrates  disaccharides  monosaccharides
- proteins  amino acids
- lipids  fatty acids + glycerol
4. Absorption
5. Excretion (defecation)
Organs involved:
A. Gastrointestinal tract (GIT) – mouth, pharynx, esophagus, stomach, small intestine, large
intestine
B. Accessory structures – teeth, tongue, salivary glands, liver, gall bladder, pancreas
Histology of GIT –
1. Mucosa –
a. epithelium
b.lamina propria (CT)
c.muscularis mucosa (smooth muscle)
2. Submucosa
3. Muscularis externa
4. Serosa (aka- visceral peritoneum)
Peritoneum & Mesenteries –
- parietal & visceral peritoneum
- Greater & lesser omentum
- Mesentery proper
Mouth (oral/buccal cavity)
A. Tongue –
- intrinsic/extrinsic muscles
- papillae – filiform, fungiform, circumvallate
B. Salivary glands
- parotid, submandibular, sublingual
-saliva
A. Teeth –Involved in mastication
Pharynx
-nasopharynx, oropharynx, laryngopharynx, uvula, epiglottis
Esophagus
-functions in “deglutition”
-variations in muscularis externa
-pierces through diaphragm (esophageal hiatus), ends at cardiac sphincter of stomach
Stomach
-cardia, fundus, body, pylorus, cardiac & pyloric sphincters, greater & lesser curvatures
-mucosa has rugae, simple columnar epithelium with gastric pits containing gastric glands
-gastric glands contain:
a. peptic (chief) cells – secrete pepsinogen
b. parietal cells- secrete HCL & intrinsic factor
c. mucous cells – secrete mucous
d. enteroendocrine (“G”) cells- secrete (among other things) gastrin
-muscularis externa – 3 layers of smooth muscle (outer longitudinal/middle circular/inner oblique)
-main function – protein digestion
Small Intestine
-1’’ diameter, 15-25’ long
-duodenum -receives chyme from stomach, secretions from liver, gall bladder & pancreas
-jejunum -most digestion & absorption occurs here
-ileum – connects to large intestine at ileocecal sphincter/valve
-modifications in mucosa & submucosa increase effectiveness of absorption:
a. plicae circulares (circular folds)
b. villi with capillary network and lacteal
c. mucosal epithelium is simple columnar with microvilli (“absorptive cells”) and has intestinal crypts containing
intestinal glands that secrete “intestinal juice”
Pancreas
-retroperitoneal
-head (by duodenum), body, tail (by spleen)
-endocrine (pancreatic islets of Langerhans) and exocrine gland (pancreatic acini)
-“pancreatic juice” containing digestive enzymes ( amylase, trypsin, chymotrypsin, carboxypeptidase, lipase, nucleases)
and bicarbonate secreted into pancreatic duct & released into duodenum
Liver
-2 main lobes (right & left) divided by “falciform ligament”, 2 smaller lobes –quadrate & caudate
-lobes of liver divided into functional “lobules” containing rows of hepatic cells (“hepatocytes”) arranged radially around a
“central vein”. Hepatocytes surround blood “sinusoids” which are partially lined by phagocytic “stellate reticuloendothelial
(Kupffer) cells”
-bile – produced by hepatocytes, secreted into bile canaliculi which merge to eventually form rt. & lt. Hepatic ducts which
unite to form common hepatic duct which merges with cystic duct from gall bladder to form common bile duct which enters
into duodenum
-bile functions in “emulsification” of lipids, absorption of fats, and as an excretory substance
-functions of liver include (but are not limited to): bile production & excretion, interconversion of nutrients, detoxification &
removal of drugs, toxins & hormones from blood, storage of fat soluble vitamins & minerals, storage of glycogen,
phagocytosis of worn out RBC’s, bacteria, & other pathogens
-blood supply to liver – hepatic artery & hepatic portal vein into liver; hepatic veins from liver into IVC
Gall bladder
-stores & concentrates bile
-cystic duct – merges with common hepatic duct to form common bile duct into duodenum
Large Intestine
-2-1/2 – 3” diameter, 5’ long
-regions include:
a. cecum – joined to ileum at iliececal sphincter, vermiform appendix
b. colon – ascending transverse  descending  sigmoid
c. rectum – becomes anal canal which opens at anus, internal & external anal sphincters
-modifications in muscularis externa forming “taeniae coli” which create “haustra”
-serosa forms “epiploic appendages”
-main function – H2O reabsorption
-no chemical digestion but some bacterial digestion
-formation of feces for excretion
Secretions of the GI Tract
Organ
Secretion
Function
Mouth
(Salivary glands)
Saliva (water, mucins, salivary
amylase)
Starch (polysaccharide)  Maltose (disaccharide)
Esophagus
Mucus
Lubrication
1. Peptic (Chief) cells
Pepsinogen
Proteins  polypeptides
2. Parietal cells
HCL
intrinsic factor
Converts pepsinogen  pepsin, maintains acidic pH
Vitamin B12 absorption
3. Mucous cells
Mucus
Protection
4. Enteroendocrine (“G” ) cells
Gastrin (hormone)
Stimulates secretion of gastric juice (pepsinogen,
HCL, mucus); increases stomach motility
Liver
Bile (bile salts)
Emulsification of fats, absorption of fatty acids & fat
soluble vitamins (A,D,E,K)
Pancreas
Pancreatic amylase
Starch (polysaccharide)  maltose (disaccharide)
Trypsin, chymotrypsin,
carboxypeptidase
Proteins & polypeptides  smaller polypeptides &
dipeptides
Lipase
Lipids (triglycerides)  fatty acids + glycerol
Nucleases
Digests nucleic acids (RNA & DNA)
Bicarbonate ions
Neutralizes acidic chyme
Disaccharidases (maltase,
sucrase, lactase)
Disaccharides  monosaccharides (glucose,
fructose, galactose)
Peptidases
Polypeptides  dipeptides  amino acids
“Intestinal juice”
Provides medium for absorption of nutrients
Mucus
Lubrication
Stomach
Small intestine
Large intestine