Class notes
Bio 173 Unit 6 Notes
- Course
- Bio Human Antomy
- Institution
- Kirkwood Community College
This is a comprehensive and detailed note on Unit six objectives - digestive system, nutrition and metabolism for Bio 173. *Essential!!
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UNIT SIX OBJECTIVESChapter 26: The Digestive System
Introduction to the Digestive System
1. Six general functions of the digestive system
a. Ingestion- introduction of solid and liquid nutrients into the oral cavity (mouth), first step
in the process of digesting and absorbing nutrients
b. Motility- general term for describing both voluntary and involuntary muscular
contractions for mixing and moving materials throughout the GI tract
c. Secretion- process of producing and releasing substances, like digestive enzymes, acid,
and bile into the GI tract
i. These secretions facilitate digestion
d. Digestion- breakdown of ingested food into smaller structures that may be absorbed
from the GI tract
i. Mechanical digestion- occurs when ingested material is physically broken down
into smaller units by chewing and mixing without changing their chemical
structure
ii. Chemical digestion- involves the activity of specific enzymes to break chemical
bonds to change larger complex molecules into smaller molecules that can be
absorbed
e. Absorption- involves membrane transport of digested molecules, electrolytes, vitamins,
and water from the GI tract into the blood or lymph
f. Elimination- the expulsion of indigestible components that are not absorbed
2. Six organs that make up the GI tract
a. Oral cavity (mouth), pharynx (throat), esophagus, stomach, small intestine, and large
intestine
b. Forms a continuous tube from the mouth to the anus
3. Accessory digestive organs
a. Accessory digestive glands- salivary glands, liver, and pancreas
i. Produce secretions that empty into the GI tract
b. Other accessory organs
i. Teeth and tongue- participate in the chewing and swallowing of food
ii. Gallbladder- concentrates and stores the secretions of the liver
4. Four layers of the GI tract wall (inner to outer)
a. Mucosa- typically consists of epithelium, an underlying lamina propria, and a thin layer
of muscularis mucosae
i. Epithelium- in contact with contents of the lumen and is simple columnar for
most of the GI tract (stomach and small/large intestine)—this type of epithelium
allows for secretion and absorption
1. The portions of the GI tract that must withstand abrasion (like the
esophagus) are lined by a nonkeratanized, stratified squamous
epithelium
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, ii. Lamina propria- consists of areolar CT that contains small blood vessels and
nerves; absorption occurs when substances are moved through epithelial cells
that line the GI tract wall, and are absorbed into blood or lymphatic capillaries
located within the lamina propria
iii. Muscularis mucosa- thin layer of smooth muscle
1. Contractions of this layer cause slight movements in the mucosa, which
facilitate the release of secretions from the mucosa into the lumen and
increase contact of materials in the lumen with the mucosa
b. Submucosa- composed of areolar and dense CT, the relative amounts of each depend on
the region of the GI tract
i. Contains many large blood vessels, lymph vessels, nerves, and glands
ii. Fine branches of nerves extend into the mucosa and along with their associated
autonomic ganglia are collectively referred to as the submucosal nerve plexus—
these nerves innervate both the smooth muscle and glands of the mucosa and
submucosa
c. Muscularis- usually contains two layers of smooth muscle
i. Inner circular layer- smooth muscle cells of the inner layer are oriented
circumferentially within the GI tract
1. Greatly thickened at several locations along the GI tract to form a
sphincter, which closes of the lumen, controlling the movement of
materials into the next section of the GI tract
ii. Outer longitudinal layer- cells of the outer layer are oriented lengthwise
iii. Enteric nervous system- submucosal nerve plexus and the myenteric nerve
plexus combined
1. Sensory neurons detect both changes in the GI tract wall and chemical
makeup of the contents of the lumen—composed of both motor
neurons from the ANS and visceral sensory neurons
iv. Function of the muscularis- to mix and propel the contents within the GI tract
(contractions of the circular layer constrict the lumen and contractions of the
longitudinal layer shorten the tube
v. Contraction of the two smooth muscle layers are associated with 2 primary
types of motility
1. Peristalsis- the alternating contraction sequence of both the inner
circular layer and outer longitudinal layer for the purpose of propelling
ingested materials through the GI tract
2. Mixing- the backwards and forwards, or kneading, motion that occurs at
any point in time within different regions but lacks directional
movement—for the purpose of blending ingested materials with the
secretions within the GI tract
d. Adventitia or serosa
i. Adventitia- composed of areolar CT with disperse collagen and elastic fibers and
is associated with portions of the GI tract that are outside of the peritoneal
cavity
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, ii. Serosa- same composition as the adventitia but is completely covered by a
serous membrane called the visceral peritoneum and is associated with
portions of the GI tract within the peritoneal cavity
5. Process of absorption
a. Digestive substances are transported from the lumen of the GI tract through the
epithelium—substances only need to cross the epithelium of the mucosa through
membrane transport processes to be absorbed into the blood capillaries or lymphatic
capillaries
b. Most nutrients are absorbed into the blood capillaries; lipids and lipid-soluble vitamins
are absorbed into the lymphatic capillaries
6. Serous membranes associated with the GI tract
a. Parietal pericardium- serous membrane that lies the inner surface of the abdominal wall
b. Visceral pericardium- serous membrane that reflects over and covers the surface of
internal organs
c. Peritoneal cavity- space between the two serous membranes; potential space in which
both peritoneal layers secrete a lubricating serous fluid, this fluid lubricates both the
internal abdominal wall and external organ surfaces and allows the abdominal organs to
move freely and reduces friction
7. Intraperitoneal vs retroperitoneal organs
a. Intraperitoneal organs- organs within the abdomen that are completely surrounded by
visceral pericardium
i. Stomach, most of small intestine, and parts of large intestine
b. Retroperitoneal organs- organs that lie outside the parietal peritoneum directly against
the posterior abdominal wall, so only their anterolateral portions are covered with the
parietal pericardium
i. Most of the duodenum (first part of small intestine), pancreas, ascending and
descending colon (parts of large intestine), and the rectum
8. Mesentery- general term refers to the double layer of peritoneum that supports, suspends, and
stabilizes the intraperitoneal GI tract organs; blood vessels, lymph vessels, and nerves that
supply the GI tract are sandwiched between the two folds
a. Greater omentum- extends inferiorly like an apron from the inferolateral surface of the
stomach (greater curvature) and covers most of the abdominal organs; it often
accumulates large amounts of adipose CT, so it’s referred to as the fatty apron
i. Functions- insulate the abdominal organs and storage for excess fats
b. Lesser omentum- connects the superomedial surface of the stomach (lesser curvature)
and the proximal end of the duodenum to the liver
c. Falciform ligament- flat, thin, crescent-shaped peritoneal fold that attaches the liver to
the internal surface of the anterior abdominal wall
d. Mesentery proper- fan-shaped fold of peritoneum that suspends most of the small
intestine (jejunum and ileum) from the internal surface of the posterior abdominal wall
e. Mesocolon- fold of the peritoneum that attaches parts of the large intestine to the
posterior abdominal wall; several distinct sections for the portion of the colon it
suspends
i. Transverse mesocolon—transverse colon; sigmoid mesocolon— sigmoid colon
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