Digestive System Anatomy
Digestive System Anatomy
Overview of Digestive Anatomy
[Section 23.1: Overview of the Digestive System]
[Section 23.1.1: Digestive System Organs]
[Figure 23.2: Components of the Digestive System]
The organs of the digestive system are divided into 2 main groups:
1. the alimentary canal (GI tract)
· Tube that runs from the mouth to the anus
· organs include: pharynx, esophagus, stomach, intestines, rectum, anus
· the lumen of the alimentary canal opens to the external environment at both ends and therefore anything inside the lumen is considered external to the body
2. accessory organs
· these organs are not part of the tube, i.e. they are not hollow organs that food passes through
· secrete substances into the tube
· many of these substances are required for digestion or enhance digestion
· include: teeth, tongue, salivary glands, liver, gallbladder, and pancreas
· the liver/gallbladder and pancreas will be considered separately at the end of this lab
Histology of the Alimentary Canal
[Section 23.1.2: Histology of the Alimentary Canal]
[Figure 23.3: Layers of the Alimentary Canal]
MSMS = mucosa, submucosa, muscularis, serosa
1. Mucosa
a. epithelium
i. in mouth, pharynx, and esophagus the epithelium is stratified squamous
ii. in stomach, small intestine and large intestine, the epithelium is simple columnar
iii. in anal canal the epithelium becomes stratified squamous again
iv. the epithelium has a high rate of turnover (rate of mitosis); some cells live only a few days, others up to a week
b. lamina propria
i. loose connective tissue containing blood vessels and lymphatic tissues
2. Submucosa
a. dense connective tissue
b. blood vessels and lymphatic vessels for transporting absorbed nutrients from food to rest of body
3. Muscularis (aka, muscularis externa)
a. in the mouth, pharynx, esophagus, and anal canal this muscular layer is skeletal muscle, providing some voluntary control
b. the small intestine has a basic 2-layer organization of smooth muscle (named for direction of fibers):
i. inner layer called the circular layer
ii. outer layer called longitudinal
c. the 2-layer organization is modified in the stomach with an additional layer
i. the oblique layer is superficial to the longitudinal layer in the stomach wall
d. the large intestine technically has 2 layers, but the outer longitudinal layer is separated into three, narrow bands called tenia coli
4. Serosa
a. present only on organs within the abdominal cavity
b. covers the tubular organs in this cavity
c. doubles back on itself to form the mesenteries that hold the alimentary canal in place within the abdominal cavity
Six Major Processes of Digestion
[Section 23.2: Digestive System Processes; Section 23.2.1: Digestive Processes]
[Figure 23.5: Peristalsis; Figure 23.6: Digestive Processes]
1. Ingestion of food into the mouth
2. Movement of food through the digestive tract
· swallowing and peristalsis
3. Mechanical digestion of food
· process of physically grinding bites of food and separating them in to smaller pieces
· this increases the surface area of the food particles to increase contact with digestive juices
4. Chemical digestion of food
· unlike mechanical digestion, chemical digestion requires enzymes
· enzymatic breakdown of large food polymers into their monomers (proteins into amino acids, for example)
5. Absorption of nutrients in the blood
6. Formation and elimination of indigestible materials and waste.
GI Tract: Mouth and Oral Cavity
· Mainly mechanical digestion
· Limited amount of chemical digestion with salivary amylase and lingual lipase
· hard & soft palates – form the roof of the mouth
· tongue – forms the floor of the mouth
· gingiva – gums
· superior and inferior labial frenulum – attaches the lips to the gums
· vestibule - region between teeth and cheek.
· lingual frenulum – attaches the tongue to the gums
o read more about ankyloglossia in the “Tongue” section of your text
o it can impair speech, but it can also severely impair a newborn’s ability to latch and breastfeed
· uvula – oval process that hangs down the posterior portion of the oval cavity – prevents food or liquid from moving into the nasal cavity
Tongue
muscles of the tongue perform 3 important functions:
1. position food while chewing
2. form and shape food into a bolus
3. position food for swallowing
surface covered with papillae
· most of the surface of the tongue is covered by two types of papillae
1. fungiform that contain the microscopic structures known as taste buds
2. filiform papillae that have touch receptors and create an abrasive surface for moving food around in the mouth
Salivary glands
[Section 23.3.3: Salivary Glands]
[Figure 23.9: Salivary Glands]
1. Parotid (serous gland)
2. Submandibular (mucous + serous gland)
3. Sublingual (mucous + serous gland)
Teeth
[Figure 23.10: Permanent and Deciduous Teeth; Figure 23.11: Anatomy of a Tooth]
· Tooth is anchored to the jaw bone by periodontal ligament that lines the embedded part of the tooth, the root.
· Crown is the portion of the tooth above gingiva or gum
· Only inner pulp cavity is filled with living tissue, the pulp.
· Surrounding pulp cavity is dentin, which makes up most of the structural mass of the tooth.
· The exposed crown is covered with enamel, the hardest substance produced by living organisms.
GI Tract: Esophagus
[Section 23.3.6: The Esophagus]
[Section 23.3.6.1: Passage of Food through the Esophagus]
· food moves from the mouth into the oropharynx, laryngopharynx, and then into the esophagus
· it is a closed tube
· a food lump, bolus, is moved through the esophagus by skeletal muscles – peristalsis
· upper and lower esophageal sphincters
GI Tract: Stomach
[Section 23.4.1: Stomach Structure]
· Four areas of stomach:
1. cardia
2. fundus
3. body
4. pylorus
· Esophageal or cardiac sphincter – prevents stomach contents from moving into the esophagus
o remember, the mucosa of the esophagus has a much thinner lamina propria, so acids from stomach can severely erode the esophagus
o GERD = gastroesophageal reflux disorder (or disease) can be painful and severe, but is also associated with an increased risk of esophageal cancers
· Cardia – where food enters from esophagus
· Z-line – epithelial cells change from squamous to columnar
· Fundus – serves as a temporary holding area for food
· Rugae – large ridges that allow the stomach to be stretched
· Body– the main, central region of stomach
· Pylorus – the lower region of stomach
· Pyloric sphincter – smooth muscle that allows release of stomach contents into the first part of the small intestine
GI Tract: Small Intestine
[Section 23.5: The Small and Large Intestines]
[Section 23.5.1: The Small Intestine]
[Figure 23.18: Small Intestine]
[Figure 23.19: Histology of the Small Intesine]
· primary functions = chemical digestion and nutrient absorption
· Has circular folds, villi, and microvilli to increase surface area
· ileocecal sphincter separates the small and large intestines
· Three distinct regions:
1. Duodenum
o first 25 cm of small intestine
o C-shaped structure that begins at the pyloric sphincter of the stomach
2. Jejunum
o approximately 2 meters
o has a thick layer smooth muscle
o Site of most nutrient absorption
3. Ileum
o approximately 3 meters
o contains dense numbers of Peyer’s patches (clusters of immune cells) to aid in immune defense
§ read more about MALT in chapter 21, the paragraph just above figure 21.11
o Empties into cecum of large intestine through ileocecal sphincter
GI Tract: Large Intestine
[Section 23.5.2: The Large Intestine]
[Figure 23.21: Large Intestine]
[Figure 23.23: Teniae Coli, Haustra, and Epiploic Appendages]
· approximately 1.4 m long
· major function is absorption of water, vitamins, and solutes & the formation of feces
· Composed of 3 main regions:
1. cecum
a. appendix: may play an immunological role or may serve as bacterial reservoir to replenish colon bacterial after severe diarrhea
2. colon (4 subdivisions):
a. ascending colon
b. transverse colon
c. descending colon
d. sigmoid colon
3. rectum
a. anal canal
b. internal anal sphincter is smooth muscle and is involuntary
c. external anal sphincter is skeletal muscle and is voluntary
Accessory Organ: Liver
[Section 23.6: Accessory Organs in Digestion: The Liver, Pancreas, and Gallbladder]
[Figure 23.24: Accessory Organs; Figure 23.25: Microscopic Anatomy of the Liver; Figure 23.27: Gallbladder]
· Liver has 4 lobes
o left and right lobes can both be seen from the anterior view
o on the posterior/inferior view the caudate and quadrate lobes can also be seen
· Each lobe is organized into microscopic columns called lobules.
o Primarily made of cells called hepatocytes
o hepatocytes arranged in hexagonal columns around central vein
· Bile is produced by hepatocytes
o Bile flows through a series of small ducts called canaliculi
o Canaliculi ducts drain into the right and left hepatic ducts (better seen in figure 23.27)
o These join to form the common hepatic duct
o The cystic duct of the gallbladder and common hepatic duct join to form the common bile duct
o This joins with the pancreatic duct in the pancreas before draining into the duodenum of small intestine
Accessory Organ: Gall Bladder
[Section 23.6.3: The Gallbladder]
[Figure 23.24: Accessory Organs; Figure 23.27: Gallbladder]
· Small organ (~3”x1.5”)
· Excess bile is pushed up the cystic duct by peristalsis
· Bile is stored and concentrated until needed
· Microvilli on innermost epithelial layer absorb water
· Muscle layer pushes bile back into cystic duct
Accessory Organ: Pancreas
[Section 23.6.2: The Pancreas]
[Figure 23.24: Accessory Organs; Figure 23.26: Exocrine and Endocrine Pancreas]
· ~6” long
· Connects to duodenum
· Endocrine gland (produces hormones)
o Insulin
o Glucogon
· Exocrine function
o Produces digestive enzymes
o See, but don’t memorize, Table 23.8 for an overview of the major digestive enzymes. Notice how many of the total are produced and secreted by the pancreas