THE DIGESTIVE SYSTEM
I. Digestion: all the processes by which the condition of food is
changed so it can be absorbed
A. Types of digestion
1. Mechanical: physical breakdown
a. takes place mostly in the mouth
2. Chemical: enzymatic breakdown
a. mouth
b. stomach
c. small intestine
B. Gastrointestinal (GI) tract/Digestive tract
1. organs that food passes through
2. GI tract + accessory organs = digestive system
C. Pathway of food/functions of structures
1. mouth
a. mechanical digestion
1) teeth for mastication
2) tongue for deglutition (swallowing)
--with nervous system, forms words
3) cheeks.lips—holds food in, helps in
swallowing
4) hard & soft palate
a) uvula—end of soft palate
b. chemical digestions
1. salivary glands—3 pairs
a) produce salivary amylase
1) hydrolyzes starch and glycogen
into sugars
2.
mucus
a) lubricates food in mouth
b) binds food into a bolus
c) contains antimicrobial agents
II. The digestive process
A. Mouth
1. Mechanical aspect mostly accomplished by teeth
2. Tooth structure is more complex than realized
a. enamel: hardest substance in our bodies; covers
teeth above gumline & is primarily calcium
b. cementum: bonelike tissue forming outer
surface of root; is continuous w/ enamel below
gumline
1) is bound to bony socket by periodontal
membrane
c. dentin: a calcium compound, hard as bone, that
surrounds the pulp cavity
d. pulp cavity: contains blood vessels, nerves
diff’t kinds of teeth for diff’t functions
a. incisors
b. canines—aka, eyeteeth or cuspids
c. premolars
d. molars
1) most vertebrates have at least one set of
deciduous teeth
B. Esophagus: 25 cm (10 in.) muscular tube
1. composed of smooth muscle
2. undergoes rhythmic contractions which move food
through it called peristalsis
3. is closed except when food passes through it
a. bolus passes into pharynx after deglutition
1) uvula covers air passage to nose
2) epiglottis covers glottis (opening into
trachea)
b. food passes into esophagus which moves the
bolus by peristalsis
c. goblet cells in lining of esophagus secrete mucus
for lubrication into the lumen of the esophagus
d. cardiac sphincter between esophagus and
stomach regulates movement of food between
1) usuallyl closed to prevent substances from
backing into the esophagus (including stomach acid)
**reflux here is referred to as ‘heartburn’ or acid
reflux
C. Stomach—distensible, j-shaped sac about the size of your
fist, where food stays 2-6 hours
1. stores & mixes food w/ gastric secretions
2. helps regulate the movement of food into the small
intestine
3.
3.
composed of layers of smooth muscle
a. creates peristaltic movements (“churning”)
4. has an inner mucosal lining (replaced approx. every 6
weeks)
5. lining is folded into rugae
a. inner folds visible when empty, smooth out when
stomach is full
6. mucosal cells which secrete gastric juices
a. chief cells—secrete pepsinogen, which is
converted to pepsin for protein hydrolysis
b. mucus cells—secretes mucin to coat and protect
stomach; lubricates food (aka, goblet cells)
c. parietal cells—secretes HCl (pH 1-3) which
changes the 3-D structure of proteins
d. other cells
1) secrete gastrin ( a hormone) which
stimulates production/release of HCl and pepsinogen
7. chyme=food after about 3 hours, when all gastric
juices are done with it
D. Pyloric sphincter
1. opening between stomach and small intestine
2. regulates entry of chyme into small intesting
E. Small intestine—about 6 m (21 ft.) long, 1 inch in
diameter
1. most chemical digestion and much absorption occurs
here
2. has 3 distinct regions (based on function)
a. duodenum—first 10 inches
1) intestinal fluid mixes w/ chyme here
2) fluid includes: carbohydrases, proteases,
lipases, nucleases, hormones, mucus and acid
3) products are from accessory organs enter
here as well
a) pancreas—produces pancreatic juice for
all foods, bicarb to neutralize acid; enters small
intestine through the pancreatic/common duct
b. jejunum—middle 7 or so feet
1) absorption of nutrients through villi
c. ileum—last 15 or so feet
1) absorption of vitamin B12 and bile salts
2) opens into large intestine
3. 90% of digested food and 10% of water is absorbed
in the small intestine
a. facilitated by large surface area (folded inner
lining)
b. villi increase the surface area from about 200
2
cm to 2,000,000 cm2
F. Large Intestine
1. responsible for absorption of water remaining in chyme
a. too much absorbed = contstipation
b. too little absorbed = diarrhea
2. some minerals reabsorbed too
3. houses commensal bacteria
a. produces vitamin K, biotin, some amino acids, which
would otherwise be wasted
4. prepares waste for defecation
5. has several distinct regions
a. ascending colon—upward along right side
b. transverse colon—above small intestine
c. descending colon—down along left side
1) lower end = sigmoid colon
d.
rectum
1) muscular cavity at end of large intestine
2) stores feces
3) opening = anus, controlled by anal sphincter
e. cecum—blind pouch below opening into small
intestine
1) ends in veriforms appendix, which may have
some lymphoid properties