Chapter 16 - The Digestive System
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Introduction
● Digestive system consists of
the muscular digestive tract
and various accessory organs
● Functions include: ingestion,
mechanical processing,
secretion, absorption, and
excretion
● Tract includes the oral cavity,
pharynx, esophagus,
stomach, small intestine,
large intestine, rectum, and
anus
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Composition of The Digestive System
● Epithelium and underlying
connective tissue
● Lamina propria, form the
mucosa (mucous membrane)
of the digestive tract
● Submucosa, the muscularis
externa, and the adventitia
● Serosa, a serous membrane
● Mesenteries suspend
portions of the digestive
tract
● Neurons that innervate the
smooth muscle involuntary 4
Movement
● Muscularis
externa propels
materials through
the esophagus by
means of the
contractions of
peristalsis
● Segmentation
movements in
areas of the small
intestine churn
digestive
materials
Peristalsis
Segmentation
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Oral Cavity
● Functions of the oral cavity:
○ 1) sensory analysis of
potential foods
○ 2) mechanical processing
using the teeth, tongue, and
palatal surfaces
○ 3) lubrication of food by
mixing with mucus and
salivary secretions
○ 4) digestion by salivary
enzymes
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Oral Cavity
● Also called the buccal
cavity, is lined by oral
mucosa
● Hard palate and soft
palate form its roof
● Tongue forms the floor
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Tongue
● Primary functions of the
tongue:
○ 1) mechanical processing
○ 2) manipulation to assist in
chewing and swallowing
○ 3) sensory analysis
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Glands
● Parotid, sublingual, and
submandibular salivary glands
discharge their secretions into
the oral cavity
● Saliva lubricates the mouth,
dissolves chemicals, flushes
the oral surfaces, and helps
control bacteria
● Salivation is usually controlled
by the ANS
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Chewing
● Mastication (chewing) occurs
through the contact of the
opposing surfaces of the teeth
● The periodontal ligament anchors
each tooth in a bony socket
● Dentin forms the basic structure of
the tooth
● Crown is coated with the enamel
● Root is covered with cementum
● The 20 primary teeth, or
deciduous teeth, are replaced by
the 32 teeth of the secondary
dentition during development
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Passageway Between the Oral Cavity and Esophagus
● Pharynx serves as a
common passageway for
solid food, liquids, and air
● Pharyngeal muscle
contractions propel food
mass
● Esophagus carries solids and
liquids from the pharynx to
the stomach
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Swallowing
● Deglutition (swallowing) can be
divided into oral, pharyngeal, and
esophageal phases
● Begins with the compaction of a
bolus and its movement into the
pharynx
● Epiglottis covers the larynx as the
bolus moves into the esophagus
● After opening of the upper
esophageal sphincter, peristalsis
moves the bolus down the
esophagus to the lower esophageal
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sphincter
Stomach
● Four major functions:
○ 1) temporary storage of
ingested food
○ 2) mechanical breakdown of
food
○ 3) breakage of chemical
bonds by acids and enzymes
○ 4) production of intrinsic
factor (absorbs vitamin B12)
● Chyme forms in the stomach as
gastric and salivary secretions
are mixed with food
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Stomach
● Four regions of the
stomach: cardia,
fundus, body, and
pylorus
● Pyloric sphincter
guards the exit out of
the stomach
● In a relaxed state the
stomach lining contains
numerous rugae
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● Within the gastric
glands, parietal cells
secrete intrinsic
factor and
hydrochloric acid
● Chief cells secrete
pepsinogen, which
acids in the gastric
lumen convert to the
enzyme pepsin
● Gastric gland
endocrine cells
secrete the hormone
gastrin
Stomach
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Stomach
● Gastric secretion includes
○ (1) the cephalic phase, which
prepares the stomach to
receive ingested materials;
○ (2) the gastric phase, which
begins with the arrival of
food in the stomach
○ (3) the intestinal phase,
which controls the rate of
gastric emptying
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Small Intestine
● Includes the duodenum, the
jejunum, and the ileum
● The ileocecal valve, a sphincter,
marks the junction between the
small and large intestines
● Intestinal mucosa bears
transverse folds called plicae
circularis and small projections
called intestinal villi
● Both structures increase surface
area for absorption
● Each villus contains a lymphatic
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capillary called a lacteal
Small Intestine Digests and Absorbs Nutrients
● Some of the smooth muscle cells in the
muscularis externa of the small intestine
contract periodically, without stimulation
● Brief, localized peristaltic contractions that
slowly move materials along the tract
● Intestinal glands secrete intestinal juice,
mucus, and hormones
● Intestinal juice moistens chyme, helps
buffer acids, and dissolves digestive
enzymes and the products of digestion
● Most of the important digestive and
absorptive functions occur in the small
intestine
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Pancreas and Gallbladder
● Pancreatic duct penetrates the
wall of the duodenum, where it
delivers the secretions of the
pancreas
● Pancreas has both an endocrine
function (secreting insulin and
glucagon) and an exocrine
function (secreting water, ions,
and digestive enzymes into the
small intestine)
● Pancreatic enzymes include
carbohydrases, lipases,
nucleases, and proteases
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● Produce pancreatic juice
Liver
● Liver is the largest visceral organ in the
body and performs over 200 known
functions
● Made up of four unequally sized lobes: the
left, right, caudate, and quadratic lobes
● Blood is supplied to the lobules by
branches of the hepatic artery and hepatic
portal vein
● Major functions include: 1) metabolic
regulation, 2) hematological regulation, and
the 3) production of bile
● Gallbladder stores and concentrates bile
for release into the duodenum
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Large Intestine
● Functions are to 1) reabsorb water and
compact feces, 2) absorb vitamins
made by bacteria, and 3) store fecal
material prior to defecation
● Has three parts: the cecum, colon, and
rectum
● Cecum - collects and stores material
from the ileum and begins the process
of compaction; appendix is attached
● Colon - has a larger diameter and a
thinner wall
● It bears haustra (pouches) and taeniae
coli (longitudinal bands of muscle)
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Large Intestine
● Large intestine reabsorbs water and
other substances, such as vitamins,
bile salts, organic wastes, and toxins
● Bacteria residing in the large
intestine are responsible for
intestinal gas, or flatus
● Distension of the stomach and
duodenum stimulates peristalsis, or
mass movements, of feces from the
colon into the rectum
● Muscular sphincters control the
passage of fecal material to the anus
● Distension of the rectal wall triggers
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the defecation reflex
References
● Whats Up Dude. (2020, April 23,). Digestive System [Video]. Youtube.
https://www.youtube.com/channel/UCG7WdmLGzacESLc9DMyZb-A