Histology of Digestive Tract I+ II Flashcards
(76 cards)
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Functions of the digestive system
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- Ingestion
- Propulsion
- Mechanical digestion
- Chemical digestion
- Absorption
- Defecation
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What are the main divisions of the digestive system? What are the organ components of each?
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Alimentary Canal
- Organs: Mouth, Pharynx, Esophagus, Stomach, Small and Large Intestines
Accessory Digestive Organs
- Organs: Teeth, Tongue, Gallbladder, Salivary Glands, Liver, and Pancrease
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The ________ is a continuous, muscular digestive tube winding thorughout the body.
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Alimentary Canal
*Component that digests and absorbs food particles
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What are the four basic processes of the digestive system?
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Digestion
- Mechanical and chemical breakdown of food
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Motility
- Movement of material along the GI tract
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Secretion
- Release of substances from salivary glands, GI epithelial cells, hepatocytes or pancreatic acinar cells into the GI tract lumen or ECF
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Absorption
- Active or passive transfer of substances from the lumen of the GI tract to ECF
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Which components of the GI tract added chyme to food?
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GI epithelium, liver and pancreas
NOTE: Chyme is soupy mixture designed to assist with secretion
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Anatomy of the Digestive System
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General Structure of Wall of the GI Tract
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What are the four layers of the GI tract?
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Mucosa
- Secretory and absorptive layer
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Submucosa
- Absorption is complete when nutrients enter blood vessels of the submucosa
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Muscularis externa
- Segmental contractions and peristalic movements
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Serosa
- Visceral peritoneum in abdominal region
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General Organization and Structure of the Digestive/Alimentary Tube
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Function of the esophagus
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- Conducts food from oral cavity to stomach (using peristalsis)
- Secretes mucus
NOTE: If acid from the stomach gets in here that’s heartburn.
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Function of the stomach
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- Fragmentation of food
- Initiates digestion
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Function of the small intestine
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- Digestion is completed
- Main absorption occurs
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Function of large intestine
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- Water absorption (20%) to reduce liquid content of solids
- Production and absorption of Vitamins B and K
- Synthesized by bacterial flora
- A holding chamber for feces prior to defecation via the anal canal
- Elimination
- Bacterial digestion
- Ferments carbohydrates
- Protein breakdown
- Concentrates waste
NOTE: 80% of water reabsorption occurs in the small intestine
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Where does chemical digestion occur?
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Mouth, stomach, and small intestine
NOTE: Brush-border enzymes of the small intestine complete the final hydrolysis of carbohydrates, proteins and nucleotides prior to their absorption
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Which enzymes break down carbohydrates in chemical digestion and where?
* **Salivary amylase**
* ****Mouth
* **Pancreatic amylase**
* ****Small intestine
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Which enzymes break down proteins in chemical digestion and where?
* **Pepsin**
* ****Stomach
* **Pancreatic proteases**
* Small intestine
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Which enzymes break down lipids in chemical digestion and where?
* **Lingual lipase**
* ****Mouth
* **Gastric Lipase**
* ****Stomach
* **Pancreatic Lipase**
* ****Small intestine
**NOTE:** Bile significantly increases the rate of hydrolysis
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Which enzymes break down nucleic acids in chemical digestion and where?
* **Pancreatic nucleases**
* Small intestine
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What are the 3 stages of the digestive system?
1. **Cephalic**
2. **Gastric**
3. **Interstinal**
\*Stages names based on location of the food
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Function of Cholecystokinin
* _Slows down_ emptying of the stomach by acting on the **pyloric sphincter**
* _Stimulates_ **bile release** from the gall bladder and the **secretion of pancreatic enzymes**
* _Inhibits_ gastric acid secretion
**NOTE:** Site of production is duodenum and jejunum
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Function of secretin
* _Stimulate_ **pancreatic bicarbonate secretion**
* _Enhances_ **insulin secretion** by B cells of the islet of Langerhans
* _Inhibits_ gastric acid secretion
**NOTE:** Site of production id duodenum
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Function of gastrin
* _Stimulate_ the **secretion of HCl** by parietal cells
* _Stimulates_ **insulin secretion** by B cells of Islet of Langerhans
* _Stimulates_ **gastric motility** **and growth of the mucosal cells**
**NOTE:** Site of production is stomach
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How does contraction the muscularis externa cause motility?
* **Peristalsis**
* ****Occurs over **long** distances is the **esophagus** to move food from the pharynx to the stomach and within the **stomach** where it contributes to the mixing of food
* Occurs over a **short** distance in the **small intestine**
* **Segmentation**
* ****characterized by short segments of the s**mall and large intestines alternately contracting and relaxing** which **mixes contents and keeps them in contact with absorptive epithelium**
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Peristalsis
**Adjacent areas** of the GI tract alternatively contract and relax which keeps moving food along the tract distally (propulsion).
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Segmentation
**Nonadjacent segments** of the intestine alternatively contract and relax, moving the food forward and backward.
**NOTE:** This results in **food mixing** rather than food propulsion.
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What are the major feautures of the esophagus?
* **Skeletal muscle**
* (upper third for swallowing)
* **Smooth muscle**
* (lower third) for peristalsis
* **Esophageal glands**
* produce mucus to lubricate bolus
* **Esophageal sphincter**
* prevents backflow into oral cavity
* **Cardiac sphincter**
* prevents backflow into esophagus
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Food found in the stomach is called \_\_\_\_\_\_\_\_.
chyme
**NOTE:** The stomach can hold 2 L of food and water when completely filled.
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When the stomach is empty, the mucosa folds into \_\_\_\_\_\_\_\_\_\_\_\_\_.
rugae
**NOTE:** When filled, the expanded wall of the stomach causes these folds to disappear (flatten).
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The stomach is divided into 3 regions based on what?
The type of mucosal glands found in each
* **Cardiac glands**
* ****Surface mucus-secreting cells and mucus-secreting, tubular, coiled glands
* **Pyloric glands**
* Pits deeper than in cardiac and fundic region
* Glands lined mostly with mucous cell
* **Fundus (gastric) glands**
* **Chief cells and parietal cells of gastric glands** secrete substances into the lumen of the stomach which combine to make gastric juice
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Function of Chief Cells
* RER in basal cytoplasm – **pepsinogen synthesis and packaging**
* Exocytosis of pepsinogen is **stimulated by acetylcholine** after feeding
* Pepsinogen is converted in acid environment to pepsin, a proteolytic enzyme
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How is acid secretion regulated in parietal cells?
* **Parasympathetic stimulation** and the hormone gastrin leads to fusion of the tubulo-vesicles with the canaliculi, and an increase in the secretion of HCl acid
* Histamine potentiates these effects
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Where are enteroendocrine cells located?
Dispersed in **mucosal epithelium** throughout GI tract
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Function of enterendocrine cells?
* Produce peptide hormones that regulate GI tract
* Enzyme secretion and electrolyte metabolism
* Gastrointestinal motility & mucosal growth
* Release of other peptide hormones
**NOTE:** The secretions are released basally and enter the blood
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Hepatic and pancreatic secretions enter the lumen of the small intestine at the duodenum through a duct which is guarded by the contracted \_\_\_\_\_\_\_\_\_\_\_.
**Sphinctor of Oddi**
**NOTE:** The Sphinctor of Oddi functions to keep the pancreated and hepatic secretions from entering the small intestine except during the **intestinal phase of digestion**
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The presence of food in the small intestine detected by ______ stimulates a combination of segmental and peristaltic contractions.
**Mechanoreceptors**
**NOTE:** The stimulation of segmental and peristaltic contractions mixes the chyme with pancreatic juice and bile and moves it toward the large intestine
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Small intestines are roughly ______ long.
7 meters
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Function of villi in instenine walls?
To increase surface
\*The villi are covered in microvilli which further increases surface area for absorption
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Crypts of Lieberkuhn
Secrete intestinal juice and special lysozymes that protect against bacteria
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Peyer's Patches
Aggregated lymphoid tissues containing lymphocytes
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Which four compoents amplify the total surface of the mucosa?
1. Plicae
1. Evagination of the mucosa and submucosa
2. Villi
1. Evagination of the mucosa
3. Glands or crypts of Lieberkuhn
4. Microvilli
1. Evagination of apical domain of the enterocytes
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Structure of the Small Intestine
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Individual epithelial cells of the small intestinal mucosa have highly folded apical cell membrane. Each fold is called a \_\_\_\_\_\_\_\_\_.
Microvillus
**NOTE:** Each microvillus increases the # of integral membrane proteins for digestion and absorption that can be exposed to the membrane
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The intestinal mucosa is lined by simple columnar epithelium. What the four main types of cells found in the simple columnar epithelium?
* Absorptive cells (enterocytes)
* Goblet cells
* Paneth cells
* Enteroendocrine cells
**NOTE:** The paneth cells, enteroendocrine cells and stem cells are found _only_ in the **crypts of Lieberkuhn**
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Functions of Lacteals
Uptake and transport of **lipoproteins and chylomicrons**
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Where does apoptosis occur in the small intestine?
Apoptosis of the intestinal epithelial cells occurs at the **tips of the villi**
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Where does mitosis of stem cells occur within the epithelial cells in the small intestine?
Mitosis of stem cells occurs within the **mucosal glands/crypts of Lieberkuhn**
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Functions of the duodenum
* To receive chyme from stomach
* To neutralize acids before they can damage surfaces of the small intestine
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How does the jejunum differ from the duodenum?
* Few or no submucosal glands
* Submucosa is folded into **plicae circulari**
* The musosal glands (**crypts of Lieberkuhn)** are much less prominent in the jejunum
**NOTE:** Both the duodenum and the jejunum have villi to increase surface area
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This part the small intestine is the location of the most chemical digestion and nutrient absorption.
Jejunum
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Lymphocytes in intestinal epithelium of peyer's patches perform immunlological surveillance of luminal antigens presented by ___ cells.
M
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Mechanism by which M cells play a role in immune response
1. M cells take up antigen from intestinal lumen
2. B-Lymphocytes occupy M cell cytoplasm. Antigen binds to Ig receptors on the sruface of the B-lymphocytes
3. B-Lymphocytes exposed to antigen from the SI lumen, migrate to regional lymph nodes
4. Migration to thoracic duct
5. Migration to blood stream and circulation
6. B- cells home to the lima propria of the gut, develop into plasma cells, secrete IgA
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Ileocecal junction
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Teniae Coli
Bands of smooth muscle that create pcoket-like sacs in the large intestine
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What type of epithelium is found in the large intestine?
Simple columnar epithelium
**EXCEPT:** In the anal canal where there is **stratified squamous**
**NOTE:** There are no villi or digestive-secreting cells in the large intestine
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Components of the epithelial cells in the mucosal lining of the large intestine
* Few absorptive cells
* Numerous goblet cells
* Enteroendocrine cells
* Stem cells
* NO Paneth cells
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Innervation of the GI tract
* Submucosal plexus of Meissner
* Myenteric plexus of Auerbach
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