BIO211 Weekly Guide #7
DIGESTIVE SYSTEM:
GASTROINTESTINAL TRACT
After completing this laboratory you should be able to:
1) Distinguish between the overall processes of ingestion, digestion, absorption, and egestion
2) Identify the major regions and organs of the alimentary canal in anatomical models, providing the major digestive functions of each
3) Describe in correct sequence the four layers and various sublayers of the wall of the alimentary canal
4) Recognize how the wall layers structurally vary throughout the alimentary canal and use this to recognize digestive organs in histological section
5) Understand the structure of the peritoneal cavity, identifying visceral and parietal peritoneum, mesenteries, the mesocolon, and the greater and lesser omenta
6) Identify the muscles of mastication in the head
7) Describe the mechanisms and anatomical bases for mastication, deglutition (swallowing), peristalsis, haustration, and defecation
Guide to Gross Anatomy Guide to Histology Guide to Physiology
Outline
I. Digestive System Overview
A. Basic Functions [FAP 24-1]
ingestion
digestion
mechanical
chemical
secretion
absorption
egestion/excretion
B. Major regions and organs [FAP Fig 24-1]
oral cavity, salivary glands, pharynx, esophagus, stomach, small intestine,
liver, gall bladder, pancreas, large intestine, rectum
C. Organizational scheme - tube w glandular outpocketings
D. Basic wall structure of lower tube (esophagus to colon) - 4 layers [FAP Fig 24-5]
mucosa
epithelium - type is indicative of GI region
lamina propria - C.T. with lymphoid aggregations at transition zones
muscularis mucosa - inner circular and outer longitudinal bands
submucosa
C.T.
blood and lymph vessels
submucosal glands (in some regions)
submucosal nerve plexus (Meissner's)
muscularis
inner circular muscular layer
myenteric nerve plexus (Auerbach's)
outer longitudinal muscular layer
serosa (adventitia for esophagus)
loose C.T.
blood and lymph vessels
adipose tissue
mesothelium (except esophagus)
E. Movement
chewing (mastication) [FAP p893]
swallowing (deglutition) [AP Fig 24-11]
peristalsis [FAP p907]
haustration [FAP p921]
defecation [FAP Fig24-26]
II. Oral Cavity [FAP 24-2]
A. Boundaries
B. Lips
stratified squamous epithelium
"vermillion border"
keratinized on outside
moist on inside
C. Vestibule
D. Teeth
E. Tongue
intrinsic musculature - herringbone pattern
frenulum
papillae on upper surface
innervation
taste - NVII,NIX
general sensation - NV mandibular
motor - NXII
F. Salivary glands (next week)
G. Lingual tonsils
III. Pharynx [FAP 24-3]
A. Oropharynx
boundaries
shared with respiratory system
moist stratified squamous epithelium
muscular walls
palatine tonsils
uvula
B. Epiglottis - function in swallowing
C. Laryngeopharynx
boundaries
moist stratified squamous epithelium
muscular walls
IV. Esophagus [FAP 24-4]
A. Location and boundaries
B. Histological features
mucosa - moist stratified squamous epithelium
submucosa - deep mucous glands
muscularis
skeletal muscle in upper 1/3
mixed in middle 1/3
smooth muscle in lower 1/3
adventitia (above diaphragm) vs. serosa (below diaphragm)
C. Esophageal (cardiac) sphincter
V. Stomach [FAP 24-5]
A. Regions
cardia, fundus, body, antrum, pyloris, greater and lesser curvatures
B. Histological features
mucosa
simple columnar epithelium
mucous glands in cardia and pyloris
gastric pits and glands in fundus and body
mucous cells --> mucus
parietal cells --> HCl
chief cells --> pepsinogen
submucosa - folds form rugae
muscularis
inner oblique, middle circular, outer longitudinal layers
C. Pyloric sphincter
D. Gastrin (next week)
E. Functions of stomach
VI. Small Intestine (Enteron) [FAP 24-6]
A. Regions
duodenum, jejunum, ileum
B. Histological features
mucosa
simple columnar epithelium with microvilli and goblet cells
folds form crypts of Lieberkuhn and villi
lacteals
Peyer's patches in ileum lamina propria
submucosa
Brunner's glands in duodenum
folds form plicae circularis (valves of Kierkerung)
C. CCK and secretin (next week)
D. Extrinsic glands - liver and pancreas (next week)
D. Functions of small intestine
VII. Large Intestine (Colon) [FAP 24-7]
A. Regions
ileocecal valve, cecum, ascending colon, hepatic flexure, transverse colon,
splenic flexure, descending colon, sigmoid colon
B. Histological features
mucosa
simple columnar epithelium with microvilli and lots of goblet cells
no villi
muscularis mucosa
very prominent
submucosa
no folds
muscularis
outer longitudinal layer is in 3 bands - taenia coli
C. Haustra
D. Vermiform appendix
blind sac off of cecum
highly infolded walls, lots of lymphoid tissue
E. Functions of colon
VIII. Rectum [FAP 24-7]
A. Features
moist stratified squamous epithelium
muscular walls
inner and outer anal sphincters
B. Function
Gross Anatomy List
labia (lips)
vermilion border
superior and inferior frenula
oral cavity
vestibule
hard palate
soft palate
tongue
frenulum
lingual tonsil
uvula
oropharynx
larygeopharynx
esophagus
stomach
curvatures: greater & lesser
regions: cardia, fundus, body,
antrum, pyloris
rugae
pyloric sphincter
enteron (small intestine)
regions: duodenum, jejunum, ileum
plicae circularis
villi
duodenal papillae
ileocecal junction
ileocecal valve
vermiform appendix
colon (large intestine)
regions: cecum
ascending colon
hepatic flexure
transverse colon
splenic flexure
descending colon
sigmoid colon
haustra
taenia coli
epiploic appendages
rectum
columns
recto-anal border
peritoneal cavity
peritoneum
parietal & visceral layers
mesenteries
greater and lesser omenta
mesocolon
Key: Know location and function of all structures
Guide to Gross Anatomy
[APL Exercise 24-1]
The alimentary canal of the gastrointestinal (GI) system is simply a hollow tube that extends from one external aperture, the mouth, to another, the anus. Along the way it performs five basic functions:
1) Ingestion - the intake of food
2) Digestion - the solubilizing of food and its mechanical and chemical breakdown into simple biomolecules
3) Secretion - the addition of materials such as enzymes and waste products to the canal
4) Absorption - the passage of those simple molecules into the bloodstream
5) Egestion/Elimination - the elimination of undigested food and some body products as feces
Oral Cavity [APL Fig 24.8]
The oral cavity is involved in the first two functions.
a) The vestibule of the oral cavity lies between the labia (lips) and the gingiva (gums).
- Palpate the upper and lower labial frenula with your tongue.
b) The palate separates the oral and nasal cavities.
- What four bones contribute to the anterior portion, the hard palate? Palpate the hard and soft palates with your tongue. What is a "cleft palate"?
- What is the function of the uvula?
c) Study the tongue in the models, charts, and your mouth. The criss-crossed muscular fibers give it a remarkable range of motion, for manipulating food, deglutition (swallowing), and articulating speech.
- Locate the lingual frenulum. What condition arises when the frenulum restricts the motion of the tongue?
- Try to locate the lingual tonsil on the posterior surface.
- What four cranial nerves innervate the tongue, and what are their functions?
Pharynx [APL Fig 24.8]
Recall that the pharynx has 3 parts, the nasopharynx, the oropharynx, and the laryngeopharynx, and that only the latter two are part of the digestive system.
a) Study the oropharynx on the charts and models. With what other "passages" does it connect? Locate the palatine tonsils on the lateral walls between the palatoglossal and palatopharyngeal arches.
b) The laryngeopharynx runs posterior to the larynx and connects the oropharynx to the esophagus.
c) Notice that the respiratory and digestive tracts cross in the pharynx. What structures prevent food from entering the nasopharynx and larynx during swallowing? What allows free passage of air from the nose to the larynx when the oral cavity if full of fluid (during nursing, for example)?
Esophagus [APL Fig 24.8, 24.9]
The esophagus is unique in that it runs through the neck, thorax, and upper abdominal cavity. It is located just posterior to the trachea, and is recognizable by the prominent outer longitudinal muscle fibers.
a) Locate the esophageal hiatus through the diaphragm. What is its location relative to the other two major apertures in the diaphragm?
b) What important blood vessel crosses the esophagus in the thoracic cavity?
c) List the mechanisms of food transport through the esophagus.
Stomach [APL Fig 24.9]
The stomach is the most dilated part of the digestive system. It is primarily involved in the digestive function, although a few substances are absorbed as well.
a) Locate the following stomach regions and structures in the models and charts:
cardia pylorus pyloric orifice
fundus greater curvature pyloric sphincter
body lesser curvature rugae
antrum esophageal (cardiac) orifice
- The several regions of the stomach walls produce different products, and have differerent functions in digestion. What are these products?
- Note that the pyloric sphincter, which regulates the size of the pyloric orifice, is a prominent circular muscular ring. By contrast, the esophageal sphincter is a "physiological sphincter", meaning that the most caudal region of the circular muscle of the esophagus acts like a sphincter, but does not form a distinct anatomical feature. When does the esophageal sphincter allow reflux of stomach contents back into the esophagus?
b) List the mechanisms of food transport through the stomach.
Small Intestine [APL Fig 24.10]
The major part of the processes of digestion and absorption take place in the small intestine. The three subdivisions are the duodenum (25 cm.), the jejunum (2.5 m.), and the ileum (3.5 m). Food traveling through the duodenum is in the form of a liquid mass, the chyme. This liquid has been supplied by all of the previous regions of the digestive tract.
a) Locate the duodenum on the charts and models. Note that from its proximal end at the pyloric orifice, it loops posteriorly, and runs retroperitoneally for much of its length.
- Note the well developed pyloric sphincter at the proximal end of the duodenum. Why is it important that stomach and duodenal contents not freely intermix?
- If possible, locate the duodenal papilla on the charts. What important glandular organs empty their products into the duodenum here?
- Notice that the proximal region of the duodenum lacks the prominent plicae circularis of the rest of the small intestine (see below).
b) Locate the jejunum in the charts and models. The exact borders with the duodenum and ileum are not obvious in gross anatomy, so just pick the approximate center of the coiled mass of the small intestine.
- Small, closely spaced ring-like wall folds extending into the lumen. These are the plicae circularis or "valves of Kierkerung", folds of the subucosa which increase the luminal surface area of the small intestine.
- If you were to examine the luminal surface very closely, you would just be able to detect that the surface is made up of densely packed tiny bumps. These are the intestinal villi, folds of the mucosa itself which greatly increase the surface area.
c) Locate the ileum on the charts and models.
- The plicae circularis in the ileum are larger and more widely spaced than in the jejunum.
- What lymphatic structures are especially prominent in the ileum wall?
d) List the mechanisms of food transport through the small intestine.
Colon [APL Fig 24.11]
The large intestine, or colon, has the function of absorbing most of the water remaining in the food residue, forming that residue into feces, and storing the feces in its distal section prior to defecation. It passes around the circumference of the coiled small intestinal mass and terminates in the rectum, a muscular canal which effects defecation.
a) Identify the following regions and structures of the colon on the charts and models:
cecum transverse colon rectum
vermiform appendix splenic flexure haustra
iliocecal valve descending colon taenia coli
ascending colon sigmoid colon epiploic appendages
hepatic flexure
- The diameter of the colon is largest at the cecum, and diminishes steadily to the proximal rectum.
- Locate the vermiform ("worm-shaped") appendix. Notice that it is a rather narrow, blind-ended tube. The function of the appendix in man is not really known. It is often regarded as simply a vestigial organ. However, it probably plays an important role in the immune system.
- One distinguishing feature of the large intestine is that the outer longitudinal smooth muscle layer is in the form of three separate bands - the taenia coli (literally "worms of the intestine").
- Notice that the taenia coli draw the colon up into a series of pouches or segments - the haustra. This sacculated appearance is very different from the smooth surface of the small intestine.
- Fat tabs, or epiploic appendages, hang off of the colon and further distinguish it from the small intestine. These are prominent in the real colon, but are often omitted in models.
b) List the mechanisms of chyme and stool transport through the colon.
c) Stools that have formed in the colon are stored in the sigmoid colon, then forcibly ejected by the rectum through the anal canal.
- The luminal wall (specifically the submucosa) of the rectum is thrown into a series of longitudinal folds - the columns of the rectum.
- Egestion of feces from the body is regulated by an internal (smooth muscle) and an external (skeletal muscle) sphincter surrounding the anal canal.
Abdominal Cavity & Peritoneum [APL Fig 24.6, 24.7]
The peritoneum is the lining of the abdominal cavity. Its relationship to the abdominal organs is analagous to that of the pericardium and heart or pleurae and lungs. However, due to the complicated folds of the lower GI tract, the peritoneal topography is much more complex.
a) Study the diagram of the midsagittal section showing the pattern of infolding of the peritoneum. Identify the following:
parietal peritoneum mesenteries greater omentum
visceral peritoneum transverse mesocolon lesser omentum
- As with the pericardium and plurae, the parietal peritoneum is adherent to the abdominal walls, and the visceral peritoneum is adherent to the organs.
- As each intestinal loop pushes into the peritoneal sac during development, it carries a doubled layer of the peritoneum with it. This doubled layer forms a mesentery (for the enteron or small intestine) and the mesocolon (for the colon or large intestine). The omenta may be understood as doubled folds in the mesenteries, or quadruple layered sheets of peritoneum.
- The abdominal aorta, inferior vena cava, and spinal cord are all retroperitoneal, that is, dorsal to and outside the peritoneal cavity. This means that blood vessels and nerves traveling to and from the intestines pass between the two layers of the peritoneum making up the mesenteries and mesocolon.
- It is important to recognize that once you have cut into the abdominal cavity, you are looking at the peritoneal cavity, the inside of the peritoneal sac. In the living person, this space is of very small volume and contains only serous peritoneal fluid.
b) It would be a good idea at this point to review the circulatory system in the abdominal cavity. For each organ of the lower digestive tract, you should be able to name the artery(s) which supplies it and the vein(s) which drains it.
c) List the abdominal GI structures which are retroperitoneal.
d) The strong abdominal muscles can place a great deal of pressure on the abdominal organs during muscular exertions. This pressure is occasionally sufficient to force part of an organ out through the abdominal wall. This process is called herniation and the end result is a hernia. Herniation usually occurs at a weak place in the abdominal wall. Some of the more common hernias are:
inguinal hernia - a intestinal loop is forced through the internal inguinal ring of
the inguinal canal
direct inguinal hernia - the loop passes directly through the abdominal aponeurosis
in the vicinity of the inguinal canal, sometimes exiting through
the external inguinal ring into the scrotum or labia
indirect (oblique) inguinal hernia - the loop passes through both the internal and
external inguinal rings and into the scrotum or labia
femoral hernia - an intestinal loop is forced through the femoral ring into the open area
of the femoral triangle at the proximal anterior thigh
hiatal hernia - a portion of the cardia of the stomach is forced through the esophageal
hiatus of the diaphragm and into the thoracic cavity
umbilical hernia - a portion of the omentum, intestine, or umbilical ligaments is forced
through the gap in the abdominal aponeurosis in the vicinity of the
umbilicus
strangulated hernia - any of the above hernias in which the blood supply is cut off to
the herniated organ region
- Hernias often reflect a congenital weakness in the abdominal wall, and may be present at birth. The immediate precipitating factor is generally strain on the abdominal wall due to lifting, child-bearing, or child-birth
- Why are inguinal hernias more common in men and femoral and umbilical hernias more common in women?
Guide to Histology
[APL Exercise 24-2]
Again, the alimentary canal is basically a tube running from mouth to anus. After working through these slides you should be able to:
1) Distinguish between the following regions of the gastrointestinal tract on the basis of histology: esophagus, stomach, small intestine (duodenum, jejunum, and ileum), colon, rectum. Describe the major digestive functions of each region.
2) Distinguish these four basic wall layers in each region of the gastrointestinal (GI) tract: mucosa, submucosa, muscularis, serosa (for most abdominal regions) or adventitia (for thoracic esophagus and duodenum). Briefly describe the structure and major subdivisions of each layer.
3) Identify the major glandular structures found in the wall of each GI region, and state the products and functions of those glands.
We will not deal directly here with the tissues of the mouth and pharynx. You are welcome to review the slide of the developing tooth, but you will not be responsible for this.
a) Oral Cavity
The oral cavity is lined with stratified squamous moist (non-keritinizing) epithelium.
- Observe the slide of the vermillion border of the lip. Can you distinguish the keratinizing from non-keratinizing epithelium?
- Observe the slide of the developing tooth. How do enamel, dentine, and pulp differ structurally? How does each develop? Which most closely resembles compact bone?
b) Esophagus [APL Fig 24.17]
The esophagus may be recognized by its smooth stratified squamous epithelium (about 6-10 cells thick) and the deep mucous glands of the submucosa.
- Identify the four layers of the esophageal wall.
- Classify the epithelium of the esophagus. What lubricates its apical surface? Can you identify any of the deep mucous glands of the submucosa?
- Identify the smooth and/or striated muscle fibers of the muscularis. What is the difference in the distribution of these two types of muscle along the length of the esophagus? Identify and note the fiber orientations of the inner and outer muscular layers.
- In the slide of the gastroesophageal border, locate the border between the esophagus and stomach. Contrast the esophageal epithelium with that of the stomach. What different functions do these epithelia serve?
- Note the lymphoid aggregations in the lamina propria of this border region.
c) Stomach [APL Fig 24.18]
The stomach may be recognized by the simple cuboidal epithelium with no surface modifications and by the deep, extremely narrow gastric pits and glands.
- Identify the mucosa, submucosa, muscularis, and serosa of the stomach wall and review their components.
- Classify the epithelium of the stomach. Note that it is deeply infolded. Identify gastric pits and gastric glands. Try to distinguish mucous cells, chief cells, and parietal cells on the basis of their locations, staining, and form. What does each of these cell types produce? In what region of the stomach is each of these cell types most prevalent?
- Locate the thin muscularis mucosae which separates the mucosal C.T. from that of the submucosa.
- Note that the thick muscularis has three sublayers of muscle, rather than the usual two. What is the fiber orientation of each of these layers?
d) Small Intestine [APL Fig 24.19]
The small intestine (or enteron) may be recognized by its characteristic epithelium, the prominent mucosal villi and intervening crypts, and the larger foldings of the submucosa - the plicae circularis.
- Identify the four wall layers in the duodenum, jejunum, and ileum slides.
- Compare the slides of the duodenum, jejunum, and ileum. The duodenum is distinguished by the Brunner's glands and lack of plicae; the jejunum by its dense, short plicae; the ileum by its Peyer's patches and large, sparse plicae.
- Fully classify the epithelium. Identify the goblet cells, villi, and crypts of Lieberkuhn. Note that the core of each villus is an extension of the C.T. lamina propria of the mucosal layer. What is a lacteal?
- Identify the characteristic Peyer's patches of the ileum. What is the predominant cell type in these structures? In what layer of the wall are they found?
- What are the functions of the Brunner's glands, which are unique to the duodenal submucosa?
- Identify the plicae circularis, especially in the jejunum. Note that the core of each of these circular rings is an extension of the submucosal C.T.
- Look between the layers of the muscularis and try to find neurons of Auerbach's myenteric plexus. Of which division of the autonomic nervous system is this plexus a part? Can you find Meissner's submucosal plexus? Which plexus mediates primarily secretion? WHich plexus mediates primarily motility (muscle contractions)?
e) Appendix
The vermiform (worm-shaped) appendix is an extension of the caecum near the iliocecal junction. It is characterized by an extremely high density if lymphoid aggregates in the wall.
- What criteria could you use to histologically identify the appendix? How would you classify its epithelium? What are all those tightly-packed dark dots in clusters in the submucosa?
- What is the immune function of the appendix? Why do you suppose it tends to become inflamed? Is it really just a vestigial nuisance as the AMA would have you believe?
f) Large Intestine
The large intestine (colon) is distinguished by its characteristic epithelium with very numerous goblet cells, and the lack of villi.
- Again, identify the four wall layers in the colon slide. Histologically, how do the large and small intestines differ?
- Why are there so many goblet cells in the colon? I.e. what function do they serve?
- Observe the slide of the recto-anal junction. Classify the two epithelia at this botrder. Which is rectal and which is anal?
Guide to Physiology
There is no real physiological component to this week's lab. APL Exercise 4 in the lab text is a good review of both the anatomy and physiology of the digestive system.