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Biol 2402 – Human Anatomy & Physiology - Sloan
THE DIGESTIVE SYSTEM
Chapter 14
March 2009
I. Anatomy and Physiology of the Digestive System
A. Alimentary Canal - Gastrointestinal tract - continuous, coiled, hollow, muscular tube that winds
through the ventral body cavity; 30 ft long in cadaver; mouth pharynx, esophagus, stomach,
small intestine, large intestine
1. Mouth - oral cavity
a. ingestion
b. digestion
1) mechanical digestion - mastication(chewing), mixing of food w/saliva by tongue
2) chemical digestion (see salivary glands)
c. very little absorption
d. Parts
1.) lips - protects anterior opening
2.) cheeks - form lateral walls
3.) hard palate - forms anterior roof
4.) soft palate - forms posterior roof
5.) vestibule - space between lips and cheeks and teeth and gums
6.) tongue - floor of mouth
7.) frenulum - attaches front of tongue to floor of mouth
2. Pharynx – throat
a. two muscular layers, longitudinal and circular
b. starts peristalsis (wave of muscle contraction and relaxation)
c. only propulsion here
3. Esophagus (gullet)
a. 10 inches long
b. for propulsion of food by peristalsis to stomach
c. passes through diaphragm
d. walls from here through large intestine made of four layers (pg 436)
1.) Mucosa - innermost layer, moist lining of lumen, made of epithelium plus a small amount
of connective tissue and smooth muscle
2.) Submucosa - just beneath the mucosa, connective tissue containing blood vessels,
nerve endings and lymphatic vessels
3,) Muscularis Externa - circular inner layer, longitudinal outer layer of muscle
4.) Serosa - outermost layer, single layer of serous fluid producing cells, the visceral
peritoneum, is continuous with parietal peritoneum which lines abdominopelvic cavity
(see page 438)
a) Greater omentum - extension of outer stomach layers, forms an apron over the
abdominal organs and attaches to the transverse colon, is where fat deposits in
abdomen
b) Lesser omentum - serosa between stomach and bottom of liver
c) Mesentary - serosa from posterior abdominal wall, covers small intestines
d) Peritonitis - inflammation of serosa in abdominopelvic cavity
4. Stomach - C shaped, on left under liver and diaphragm (see pg 437)
a. Functions - 4 hrs
1. stores food
2. mechanical digestion - thick muscles (3 layers) to churn food
3. chemical digestion - a, b, d, and e are called gastric juice
a. Parietal cells make HCl - pH 1
b. Chief Cells produce Pepsinogen (pepsin when activated) - protease requiring acid pH
to function
c. gastrin - hormone released with low pH to stimulate production of pepsin
d. mucus - protects stomach lining
e. rennin - milk digesting enzyme present in infants
4. very little absorption (alcohol and aspirin)
5. Other
a. Intrinsic factor – for absorption of vit B12
b. Mucous neck cells produce mucus to practice stomach mucosa
c. Gastrin – hormone that helps coordinate activities
b. Structures
1. cardioesophageal sphincter - muscle controlling entrance to stomach (if acid splashes
back up through this into esophagus which has very little mucus it burns the esophagus
= heartburn
2. fundus - upper part of stomach, can sometimes poke up above diaphragm = hiatal hernia
3. body - main part of stomach
4. pylorus - bottom, right part of stomach
5. pyloric sphincter - controls exit from stomach, food is now creamy = chyme
6. rugae - folds of mucosa in stomach, “unfold” to stretch stomach
7. Gastric pits - depressions in lining that contain chief cells, gastric glands, parietal cells,
mucous neck cells
5. Small intestine
a. Structure
1. starts at the pyloric sphincter
2. duodenum
a) 10 inches long
b) retroperitoneal (behind parietal peritoneum)
c) ducts from liver (gallbladder) & pancreas enter GI tract here at
hepatopancreatic/duodenal ampulla)
3. jejunum (8ft long)
4. ileum (12ft long)
5. ileocecal valve - controls entrance to large intestine
6. Lining folded into circular folds called plicae circulares
7. Lining has projections called villi - contain capillaries and lacteals
8. Cells of lining have many microvilli called brush border
6, 7, and 8 greatly increase surface area for absorption, all three decrease toward end of
small intestine, while Peyer;s patches increase in number(lymphatic tissue in
submucosa)
b. Functions - 4 to 8 hrs
1. Chemical digestion
a. Brush border enzymes - from microvilli cells, hydrolyse carbo’s
b. Pancreatic enzymes
1. amylase
2. proteanases - trypsin, chymotrypsin, carboxypeptidase
3. lipases
2.
3.
4.
5.
4. nucleases
c. Pancreatic bicarbonate - neutralizes acid, makes contents alkaline, which inactivates
some enzymes and activates others
Hormone production - by mucosal cells of duodenum from chyme in duodenum, cause
release of pancreatic juices
a. secretin - causes liver to secrete bile
b. cholecystokinin (CCK) - causes gallbladder to contract
Mechanical digestion
a. Bile - emulsifies fat into thousands of tiny droplets, allows for absorption of vitamins
K, D, & A
b. segmentation
Propulsion - peristalsis
Absorption - along entire length, mostly by active transport, except lipids, which diffuse
through to lacteals
6. Large intestine
a. Structures
1) ileocecal valve - right lower abdomen
2) cecum - saclike area, off of which wormlike appendix hangs
3) ascending colon - from cecum up right side toward liver
4) hepatic flexure – turn at liver
5) transverse colon - across top of abdomen beneath stomach toward spleen
6) splenic flexure - turn by spleen
7) descending colon - down left side of body
8) sigmoid colon - S shaped part in pelvis
9) rectum - very stretchable
10) anal canal - surrounds anus, surrounded by
a. internal involuntary sphincter
b. external voluntary sphincter
11) lining has no villi, but lots of goblet cells to produce mucus for lubrication
12) longitudinal muscle layer reduced to three bands, forms puckers called haustra
b. Functions- 12 to 24 hrs
1. absorption
a. water
b. vitamins made by bacteria living there
c. some ions
2. Defecation - rids body of feces (undigested food, mucus, bacteria, water), walls of
sigmoid colon and rectum contract and internal sphincter relaxes after mass movements
3. Bacteria living there metabolize some of remaining nutrients, produce methane and
hydrogen sulfide gas, make vit K and some B vit
4. Propulsion
a. Mass movements - long, slow-moving, powerful contractile waves that move over
colon 3 or 4 times per day to force contents to rectum, usually after meals, initiates
defecation reflex
b. Peristalsis
B. Accessory Digestive Organs - aid GI tract in accomplishing its task
1. Pancreas - soft, pink, triangular, wormy-looking, retroperitoneal, from spleen to duodenum
a. exocrine - secretes alkaline enzymatic fluid through common bile duct into duodenum
b. endocrine - secretes insulin and glucagon into blood stream
2. Liver
a. largest gland in body, under diaphragm, mostly on right
b. Filters blood after it picks up products of digestion from small intestine
c. Produces bile (greenish yellow) into hepatic duct to gallbladder
d. Gallbladder
1) small muscular sac embedded in under surface of liver where bile is stored
2) if bile stays too long it crystallizes and causes gall stones
3) contracts to squeeze bile into common bile duct to duodenum
e. Jaundice - backup of bile into bloodstream, is deposited into skin and sclera, makes yellow
coloring, can cause brain damage in infants
f. Hepatitis - acute inflammation of liver, most frequently viral
g. Cirrhosis - chronic inflammation, causing scarring, commonly due to alcoholism
3. Salivary glands
a. produce saliva, secreted through ducts into mouth
1. saliva - mucus and serous fluids
2. salivary amylase - breaks down carbohydrates into sugars
3. lysozyme
4. IgA
5. water - dissolves food to be tasted and to form bolus
a. Three pair
1. Parotid glands - in front of ears
2. Submandibular glands
3. Sublingual glands
4. Teeth - to masticate (chew) food
a. Types
1. Cutting - incisors - center teeth
2. Tearing - canines - vampire teeth
3. Tearing/Grinding - premolars
4. Grinding - molars - back teeth
b. Two sets
1. Deciduous - 20 baby teeth
2. Permanent - 32
c. Structure (see drawing pg 765)
1. Tooth parts - enamel, dentin, pulp cavity (blood vessels, nerves), root canal, cementum
2. Gums - gingiva
3. Bone - periodontal membrane
II. Functions of the Digestive System
A. General
1. Ingestion - active, voluntary process - food into mouth
2. Digestion - breakdown of food
a. Mechanical - physical crushing of food into smaller particles
1. Mouth - teeth & tongue
2. Stomach - muscular churning
3. Small intestine - segmentation - alternating areas contract simultaneously to mix food
w/juices
b. Chemical - enzymatic or chemical breakdown of molecules into smaller molecules
1. Carbohydrates - starches into sugars
2. Proteins - polypeptides into amino acids
3. Lipids - fats and oils into fatty acids and glycerol
3. Propulsion - food movement
a. Peristalsis - alternating waves of contraction and relaxation of circular muscles
b. Swallowing (deglutition) - tongue, soft palate, pharynx, esophagus
1. Buccal phase - voluntary
2. Pharyngeal-esophageal phase
a. tongue closes off mouth
b. soft palate closes nasopharynx
c. epiglottis closes larynx
d. peristalsis - moves food into esophagus
4. Absorption - movement of molecules by diffusion or active transport from lumen into lining cells
into capillaries or lacteals
5. Defecation - elimination of indigestible substances from body via anus in the form of feces
B. Control of Digestion - reflexes via parasympathetic division of autonomic NS
1. Stimuli
a. Stretch of organ by food in its lumen
b. pH of the contents
c. Presence of certain breakdown products of digestion
2. Responses - activation of inhibition of
a. glands that secrete digestive juices into lumen or hormones into blood
b. smooth muscles
C. Abnormalities of Digestion
1. Nausea/Vomiting - emetic center in brain, causes backwards peristalsis & contraction of
abdominal wall muscles & diaphragm
2. Constipation - feces in colon too long, loses too much water; dry, hard feces
3. Diarrhea - feces pass through colon too quickly, don’t lose enough water; loose, watery feces
4. Acid reflux - “heartburn”, stomach acid comes up into esophagus through cardiac sphincter
5. Hiatal hernia - part of stomach comes up into chest because of enlargement of hole in diaphragm
6. Peptic Ulcers - craterlike erosion in the mucosa of the stomach or duodenum, associated with an
acid-resistant bacteria
7. Appendicitis - inflammation of appendix, common because of twisted nature of organ
8. Cleft palate - hard palate incompletely fused in midline, allows food and liquids into nasal cavity
9. Gallstones - crystallization of bile in gallbladder, can block cystic duct
10. Pancreatitis - caused by activation of pancreatic enzymes within pancreatic duct
11. Diverticulosis - outpocketings of colon caused by too little bulk in diet; colon narrows and its
circular muscles contract more forcefully, which increases pressure on walls and mucosa
expands out in small areas; can easily become inflamed, resulting in diverticulitis