Digestive System Notes
Digestive system- tubular tract that digests/absorbs food
GI tract- gastrointestinal tract; extends from mouth to anus in 9 meter long tube
Includes oral cavity, pharynx, esophagus, stomach, small/large intestine
Accessory organs- teeth, tongue, salivary glands, liver, gall bladder & pancreas
ANATOMY
Membranes of GI tract
Lines cavities & covers organs
Made of simple squamous epithelial with some connective tissue
Periosteum- serous membranes of abdominal cavity
Parietal peritoneum- lines wall & forms mesentery (apron of connective tissue that
supports GI tract); lines abdominal wall
Visceral peritoneum- covers the organs
Peritoneal cavity- space between 2 membranes that fills with lubricating fluid
Layers of GI tract
Mucosa- lining that has both absorptive/secretory functions
Made of simple columnar epithelia with connective tissue support
Where goblet cells are found
Submucosa- thick highly vascular connective tissue layer
Absorbed molecules enter blood here
Contains nerves/glands
Muscularis- 2 layers of smooth muscle that contract/move food through GI tract
Inner circular & outer longitudinal layer
Serosa- outer layer
Protective layer of connective tissue & simple squamous
Food enters digestive system through mouth
Mouth Receptacle for food so digestion can begin
Oral cavity formed by cheeks, lips, hard/soft palate
Cheeks- skin, subcutaneous fat & muscle
Lips- fleshy highly mobile organs attached to inner surface by labial frenulum
Color due to blood vessels being close to surface
Palate- roof of cavity made of bony hard palate & muscular soft palate
Uvula is a projection of the soft palate
Tongue
Highly muscular organ needed for swallowing, chewing & speech
Made of skeletal muscle covered by mucus membrane
On surface are papillae which contain taste buds
Attached to floor of mouth by lingual frenulum
Teeth Function to masticate food to increase surface area of food so digestive fluids have more
surface area to work on
Heterodont dentition
Incisors- 4 pr front teeth for biting; chisel shaped
Canines- 2 pr cone-shaped teeth for tearing food
Premolars- 4 pr of teeth that cut/shear food
Molars- 6 pr of teeth that grind/crush/pulverize food; not part of baby teeth; broad
crowns with rounded cusps
Diphyodont dentition- 2 sets of teeth in lifetime
Deciduous teeth- 20; incisors, canines & premolars
Permanent teeth- 32; incisors, canines, premolars & molars
Anatomy of a tooth
Crown- exposed part above gumline
Neck- at gumline where tooth is anchored into jaw
Cementum- bonelike material on root of tooth
Most of tooth is made of dentin & covered with enamel
In 1930s National Institute of Health reported people who grew up in areas with water that is
naturally high in fluoride had fewer cavities. In 1945 cities first began to put fluoride in
public water supplies; advocates claim it is a safe/cost effective way to promote healthy
teeth; opponents are concerned about health risks & also view it as forced medication;
today fluoride is added to water, used in toothpaste & beverages are made with
fluoridated water
Salivary glands
3 pairs of glands that secrete saliva
Controlled by autonomic nervous system (simple reflex)
Begins some digestion but mostly for moistening
Daily amounts secreted 1-1.5 L
Parotid- largest salivary gland; between ear/masseter; drains into upper oral cavity
mumps results when swollen & infected
Submandibular- below jaw & empties into floor of mouth
Sublingual- under tongue
Pharynx
Passageway behind oral cavity that is shared by respiratory/digestive systems
Tonsils found here
Epiglottis- covers trachea so that food does not enter respiratory system
Uvula- muscular flap hanging from soft palate that prevents food/drink from entering nasal
cavity
Esophagus
Collapsible muscular tube connecting pharynx to stomach
10” long
Lies behind trachea & heart
Passes through diaphragm
Has tough protective covering over 2 muscles layers which contract/relax to move food
Muscles layers are skeletal as well as smooth
Stomach
J shaped sac that lies below diaphragm
Filled through the esophagus (cardiac sphincter) & empties into small intestine (pyloric sphincter
Regions- fundus (rounded top), body with greater/lesser curvatures, cardia, pylorus (funnel
shaped end)
Layers
Periotoneum- outside covering made of parietal/visceral membranes
Mucosa- lines stomach’s interior; folds (rugae) gradually smooth out & disappear as
stomach distends/stretches with food; contains gastric pits/gastric glands;
secretes mucus to protect against HCl & enzymes
Muscle- three smooth muscle layers forming middle
Small intestine
Between stomach & large intestine
Where digestion ends & absorption begins
12’ long & 1” wide
Held in place by mesentery
Only 2 muscle layers
Regions
Duodenum- 10”C shaped tube immediately after stomach; receives secretions from
liver, gall bladder, & pancreas; has glands in the submucosa to secrete mucus
Jejunum- 3’ long middle section; has more internal folds than last part
Ileum- 6-7’ long terminal section that empties into large intestine; has lymph nodes in
its walls
Lining has many folds, villi & microvilli to give it a velvety texture & to increase surface area for
absorption; also contains intestinal glands & brush border (microvilli); villi are functional
units of digestive system because absorption occurs here
Large intestine
Last part of GI tract
5’ in length & 2.5” diameter
Function in absorption & formation of fecal matter
No villi but does have goblet cells in mucosal layer
Regions
Cecum- pouch off large intestine; appendix is attached here
Colon- 4 parts (ascending, transverse, descending, sigmoid)
Rectum- 7.5” terminal end of GI tract
Anal canal- very end of tract that opens externally through anus
PHYSIOLOGY
Digestive system performs vital function of preparing ingested food for cells to absorb/metabolize
Digestion
Process of altering the physical state & chemical composition of food so that it can be absorbed
& utilized by cells of body
Accomplished by acid/enzyme action in digestive tract
Chemical/mechanical processes of food break down
Requires catabolic chemical reactions
Absorption- small molecules can pass through cells of intestinal tract to enter blood/lymph
Metabolism- process by which foods are finally utilized by cells
Major food constituents
Nutrients- proteins, carbohydrates, fats; only ones that are changed by digestion
Minerals- macro & micro
Vitamins
Water
Digestion of Nutrients
Proteins
Start out as long polypeptide chains of amino acids
Split into smaller peptide chains which can be broken into oligopeptides, dipeptides,
tripeptides or amino acids
Carbohydrates
Start out as long branched polysaccharide chains of sugars which are broken into
oligosaccharides & then disaccharides & finally into monosaccharides such as
glucose
Fats
Triglycerides are broken into glycerol, monoglycerides & fatty acids
Cholesterol esters are broken down into fatty acids/cholesterol
Take the longest to digest
Food enters digestive system through mouth
Tongue helps in swallowing/chewing
Teeth masticate food to increase surface area
Saliva softens/lubricates food as it starts to digest it
Digestion in mouth
Mechanical- chewing = mastication
Teeth come together & squeeze food out from them; as jaw lowers, lips/cheeks draw
food inward & tongue spreads outward to sides to push food back between
teeth to be rechewed; chewing helps break down plant materials so digestive
juices can act on parts of food inaccessible because they are enclosed in cells
Chemical
Salivary amylase enzyme in saliva can break starch into simpler sugar products &
continues to work on food mass up to half an hour after reaching stomach;
Saliva cleanses mouth, moistens food, dissolves chemicals for tasting
Saliva contains electrolytes, antibodies, mucus, amylase & water
Swallowing (deglutition)
Tip of tongue arches slightly & starts to push food toward back of mouth
Tongue forces food against hard palate as uvula/soft palate closes nasopharynx so food won’t
enter nose
Peristalsis
Involuntary contractions of smooth muscles that push food down esophagus & opens cardiac
sphincter of stomach for food to pass through
Occurs all through GI tract from esophagus to rectum
Esophagus
While food is here, bolus travels at about 1 inch/sec
No digestive enzymes are secreted so only digestion occurring here is extension of amylase in
saliva
Stomach
Layers Peritoneum- outside layer
Mucosa- lines stomach interior; folds or rugae gradually smooth out & disappear as
stomach fills with food; single layer of columnar epithelial cells which extend
downward into gastric pits where gastric glands are found; layer is replaced
every 3 days
Muscle- smooth muscles forming middle layer
Gastric juice
Secreted by 3-5 million tiny glands & deposited into gastric pits
Contains 0.5% HCl, enzymes, mucus
About 0.5 L produced per meal
Gastric pits include
Chief cells that secrete pepsin
Parietal cells that secrete HCl
Goblet cells that secrete mucus
G cells that secrete gastrin (hormone) into bloodstream
Functions of stomach
Storing food until it can be accommodated by small intestine
Mixing food with gastric juices to form chime (semifluid mixture)
Emptying food from stomach to small intestine
Begins protein digestion & denatures proteins
Secretion of gastric juice
Food enters stomach & since there is little muscle tone the stomach can bulge to hold a lot
Capacity 1-1.5 L
Food stays here about 3-4 hours with liquids passing through faster
Little food absorbed here but these can be: glucose, water, some salts, alcohol, aspirin
Absorption rate depends on volume of stomach contents
Mechanical digestion
Churning- forward/backward movement of gastric contents which mixes food/juices
Peristaltic waves- sweeps stomach contents toward pyloric sphincter; opening of
sphincter & emptying of stomach controlled by consistency of chime
Chemical digestion
Gastric juice- mostly hydrochloric acid (HCl) which can destroy tissue (pH 1-3.5)
Hydrochloric acid necessary because
Needed for gastric protease formation
Need acidity for protease to digest proteins
Destroys bacteria that enter with food
Activates other enzymes secreted in stomach such as pepsin
Pepsin- powerful protein digesting enzyme (only digestive enzyme secreted in stomach)
Stomach is emptied by peristaltic waves at rate of 3 waves/min
Each peristaltic wave pushes several mm of chime into intestine & rate of this action is
regulated by stimulus from stomach caused by volume of stomach’s contents;
Emptying is opposed by enterogastric reflex
Strong nervous stimuli sent from intestine to stomach
Controlled by fullness of intestine, acidity of intestinal chime & products of
protein/fat digestion in intestine
Will be slowed by fatty food
Inhibits rate at which stomach empties
Accessory organs
Liver Plays vital role in digestion of fats by secreting bile which breaks fat globules into
smaller droplets (emulsification) to increase surface area for digestive enzymes
to act on
Makes about 0.5 quart of bile daily
Gall bladder
Stores/concentrates bile from liver
Releases bile into small intestine through common bile duct
Pancreas
Secretes pancreatic fluid which contains enzymes to help break down chime
Produces 1.5 quart of fluid/day
Fluid enters small intestine through pancreatic duct
Fluid contains sodium bicarbonate to neutralize stomach acid, pancreatic amylase to
split polysaccharides, pancreatic lipase to break down fats, trypsin to split
proteins plus other enzymes to break down proteins/fats
Small intestine
Most chemical digestion occurs here
Food may be here 8-12 hours
Chemical digestion
Peptidase (protease) breaks down proteins
Maltase, lactase, & sucrose split disaccharides
Lipase splits fats
End products of digestion then absorbed into circulatory system through villi/microvilli by
process of diffusion or active transport
Nearly 100% of all digested carbohydrates absorbed, 95% of all digested proteins/fats
Lacteals absorb fats & other lipid soluble products of digestion
Large intestine
Food stays here 1-3 days
Material entering here contains an abundance of water/minerals that body reabsorbs to form
fecal matter
Reabsorbs 90% of water that enters from small intestine
Some Disorders of Digestive System
Constipation
Most common gastrointestinal complaint
Difficult/painful bowel movements, bloating, discomfort, sluggishness
Colon absorbs too much water resulting in hard, dry stools
Can be caused by low fiber intake or emotional stress
Diarrhea
Intestines either lose their ability to absorb salt/water or intestines secrete excess fluid/salt
into waste
May be triggered by intestinal inflammation, diabetes, stress, some antibiotics
Most common result of food poisoning
Vomiting
Caused by digestive tract irritation, overfilling, overexcitement or rapid changes in position
Take a deep breath, raise hyoid bone so larynx pulls esophagus open, closing glottis & lifting
soft palate to close nose
Powerful contraction of diaphragm occurs along with contractions of abdominal muscles
Ulcer Stomach lining has sore due to acids/enzymes working on stomach cells
May also form as a result of the regurgitation of bile salts from small intestine (this removes
mucus from stomach walls)
Some may be caused by bacterial infections
Symptoms: sharp abdominal pain or bleeding
Treatments: antacids, antibiotics
Heartburn
Gastroesophageal reflex
Movement of food from stomach into esophagus
Stomach acids injure esophagus to cause the pain
Occurs in infants because their sphincter isn’t working properly
Ways to prevent: stop smoking, lose weight, don’t eat greasy food, don’t drink alcohol,
eat less, do not overfill stomach by eating/drinking too much
Gall stones
Form when cholesterol & other substances that are normally suspended in fluid crystallize/grow
into a rock-like material
Risk factors: gender (females twice as likely as males), pregnancy, use of birth control or
estrogen replacement therapy, overweight, native Americans or Mexican Americans,
people over 60
Prevention methods: lose weight, eat regularly (4 small meals/day), exercise, eat high fiber diet
eat calcium foods, limit saturated fats
Treatments: surgery, stone dissolving with oral medication or shattering stones with ultrasonic
waves