Ch 35 - GI Function

What are the major structures included within the GI tract?

* Mouth
* Pharynx
* Esophagus
* Stomach
* Small intestine
* Large intestine

What are the major structures outside of the GI tract?

* Salivary glands
* Pancreas
* Biliary system (liver, gallbladder, and bile ducts)

How long is the GI system from the mouth to the anus?

7 meters

What are the 5 main functions of the GI system?

* Motility
* Secretion
* Digestion
* Absorption
* Elimination

What is another name for the oral cavity?

Mouth

What is the significant of the oral cavity?

* Point of entry for nutrients
* Site of initial breakdown of nutrients into a usable form
* Food is broken down and moistened by saliva

What are the main functions of saliva?

1. Moistens food and allows the tongue to convert a mouthful of food into a bolus that can be swallowed easily
2. Allows for taste perception
3. Contains salivary amylase/ptyalin (a digestive enzyme) that initiates carbohydrate digestion by breaking down polysaccharides (AKA starch) into dextrin and maltose

What are the three types of salivary glands?

1. Parotid
2. Submandibular
3. Sublingual

What is another name for the pharynx?

Throat

What are the three parts of the pharynx?

1. Nasopharynx
2. Oropharynx
3. Laryngopharynx

What structure separates the nasopharynx from the oropharynx?

soft palate

What are the functions of the pharynx?

* Serves as the entry way for the respiratory and GI systems
* Transports food to the esophagus
* Protects airway from aspiration of food particles via epiglottis
* Swallowing via peristaltic waves

What is the length of the pharynx?

12 cm

What is the length of the esophagus?

25 cm

What is the function of the esophagus?

Initiates the progress of food through the gut after ingestion

What is food facilitated by in the esophagus?

Mucus secreted by cells in the epithelial lining

What kind of epithelium is present in the esophagus?

Stratified squamous epithelium

What propels nutrients to the stomach?

Strong muscular contractions and gravity (when the body is in an upright position)

Where is the lower esophageal sphincter (LES) located?

At the lower end of the esophagus, about 2 to 5 cm above its juncture with the stomach

Why does the LES remain tonically constricted, in contrast to the middle and upper portions of the esophagus?

To prevent the highly acidic gastric contents from moving in a retrograde motion (refluxing) back into the esophagus

What can happen if the LES is not working properly?

It can lead to reflux (GERD) or heartburn/dyspepsia

What is the function of the stomach?

An elastic food reservoir for food, mixing, and initial digestion of proteins (pepsin)

What is the normal capacity of the stomach?

1000 to 1500 mL

What areas of the stomach absorb nutrients?

* Cardia
* Fundus
* Body
* Antrum
* Pylorus: pyloric sphincter?

What area of the stomach does not have rugae?

the antrum

What is the function of the pyloric sphincter?

It is a muscular sphincter between the stomach and duodenum that serves to control gastric emptying and limits the reflux of bile from the small intestine

What are the 4 specialized cells in the stomach?

1. Chief cells
2. Parietal cells
3. Mucous cells
4. Gastrin cells

What is the function of chief cells?

Produce pepsinogen (the inactive form of the enzyme pepsin)

What is the function of parietal cells?

Produce hydrochloric acid and intrinsic factor

What is the function of mucous cells?

Produce a layer of alkaline mucus to shield the stomach wall and neutralize the acidity in the immediate area of the lining

What is the function of gastrin cells?

Have surface microvilli that monitor intragastrical pH

How long is the small intestine?

\~5 to 6 meters long (the longest portion of the GI tract)

What is the function of the small intestine?

Digestion and absorption

What 2 enzymes are secreted in the small intestine?

Secretin and cholecystokinin

What are the 3 sections of the small intestine in order?

1. Duodenum
2. Jejunum
3. Ileum

What is the largest section of the small intestine?

Ileum

What is the smallest section of the small intestine?

Duodenum

In the duodenum, what is the relation to the pancreas?

The pancreas delivers digestive enzymes and bicarbonate to the duodenum

In the duodenum, what is the relation to the gallbladder?

The gallbladder delivers bile salts to the duodenum

What is the function of the ileocecal sphincter?

• Normally closes so that contents of the large intestine cannot reflux back into the small intestine

• Opens in response to peristaltic contractions moving intestinal contents towards the large intestine

What is plicae circulares?

• Circular folds of a mucous membrane on the entire inner wall of the small intestine

• Permanent ridges that do not lose their elasticity when the intestine is distended.

What is intestinal villi?

Fingerlike projections in the lining of the small intestine

What is the function of intestinal villi?

Serve to increase the surface of the intestine for digestion and absorption of nutrients

What are microvilli?

Microscopic projections on each of the intestinal villi

What do microvilli contain?

They are covered by a fuzzy coat called a brush border, which contains many digestive enzymes

What are the Crypts of Lieberkuhn?

• AKA intestinal glands

• Found between the intestinal villi

• Secretes fluid into the lumen of the intestine, which is quickly reabsorbed by the villi

How much fluid do the Crypts of Lieberkuhn secrete a day?

About 2 L of fluid daily

What reabsorbs the fluid that the Crypts of Lieberkuhn secrete?

The villi of the small intestine

What do goblet cells and Brunner cells secrete?

Large amounts of mucus

What is the large intestine?

A muscular tube 1.5 m long and 6.5 cm in diameters that forms a frame around the small intestine

What is the function of the large intestine?

What is the appendix?

The colon is considered what part of the large intestine?

From the cecum to the rectum

Does the colon contain villi? Digestive enzymes?

No to both

What does the large intestine absorb?

Water and electrolytes

What cells in the large intestine produce mucus?

Goblet cells lining the glandular crypts in the surface epithelium of the colon

What is peristalsis? Is it a propulsive or mixing movement? Is it in one or two directions? What muscles are involved? Where does it occur?

• The basic propulsive movement of the GI tract

• Moves in one direction → downward

• Involves longitudinal and circular muscles

• Occurs in the whole GI tract

What is segmentation? Is it a propulsive or mixing movement? Is it in one or two directions? What muscles are involved? Where does it occur?

• Mixing movement

• Both directions

• Involves circular muscle

• Occurs in stomach, small intestine, and large intestine

Does the PNS or the SNS increase motility/secretions?

• PNS increases motility/secretions

• CNS decreases motility/secretions

What is also known as the “second brain”?

GI motility is regulated by...

the enteric nervous system, the ANS, and hormonal mediators

The myenteric branch of the enteric nervous system contains what plexus?

The Auerbach plexus

Where is the myenteric branch located?

Between the longitudinal and circular muscular layers

What does the myenteric branch control? How?

• GI movements

• Stimulation of the myenteric plexus → activity of GI tract increases →

  • Tonic contracts of the intestinal wall increase

  • Rhythmic contracts increase in intensity and rate

  • Velocity of conduction of excitatory waves along the intestinal wall increases

The submucosal branch of the enteric nervous system contains what plexus?

The Meissner plexus

Where is the submucosal branch located?

In the submucosa

What does the submucosal branch control?

Secretion and sensory functions

What is gastrin secreted by and when?

The stomach mucosa (G cells) when food enters the stomach

What is the function of gastrin? What does it cause?

• Increases stomach motility

• Stimulates the exocrine (secretory) cells of the gastric glands to produce their specific products (HCl)

• Causes increased constriction of the LES

What cells does gastrin primarily provoke? What do these provoked cells release?

• Enterochromaffin-like cells (ELC)

• Provokes ELCs to release histamine → induces parietal cells to release acid

What is cholecystokinin secreted by?

Jejunum mucosa (I cells)

When is cholecystokinin secreted?

When fat enters jejunum

What does cholecystokinin stimulate/what is its function?

• Stimulates pancreatic secretion

• Helps regulate gastric emptying and bowel motility

• Induces satiety

• Increase contractility of gallbladder

What is secretin secreted by?

The mucosa of the duodenum

When is secretin secreted?

In response to the entry of acidic gastric juice from the stomach

What does secretin stimulate/what is its function?

Stimulates pancreatic fluid and bicarbonate secretion → neutralizes acidity of intestinal contents

Does secretin increase or decrease motility?

Mild inhibitory effect on motility in most of the GI tract (decreases motility)

Where is gastric inhibitory peptide (GIP) released from?

The upper portion of small intestine

What is the release of gastric inhibitory peptide (GIP) in response to?

Fat in chyme and carbohydrates

Does gastric inhibitory peptide (GIP) increase or decrease stomach motility?

Decreases stomach motility

What is ghrelin secreted by?

The stomach

What are the function(s) of ghrelin?

• Increases appetite

• Stimulates growth hormone secretion

• Produces weight gain

• Regulates mealtime hunger and meal intitiation

Salivary amylase is also known as what?

Ptyalin

What are the 3 major functions of the salivary glands?

1. Moistening actions allows tongue to convert food into a bolus to allow easy swallowing by moistening

2. Moistening action changes dry food into a solute form to allow for taste perception by tongue

3. Digestive enzyme (salivary amylase) initiates carbohydrate digestion by breaking down polysaccharides (starch) into dextrin and maltose

What 2 cells types fall under the category of gastric glands?

Chief cells and parietal cells

What do chief cells secrete?

Pepsinogen

What do parietal cells secrete?

• Hydrochloric acid (HCl) → converts pepsinogen to pepsin

• Intrinsic factor (IF) → increase vitamin B12 absorption

Parietal cell secretion is stimulated by what 3 hormones?

• Acetylcholine

• Gastrin

• Histamine

What is the function of peptidase?

Digests proteins

What is the function of lipase?

Digests lipids

What is the function of lactase?

Digests sugars

In the pancreas, what is the function of amylase?

Digests sugars

In the pancreas, what is the function of trypsin?

Digests proteins

In the pancreas, what is the function of chymotrypsin?

Digests proteins

In the pancreas, what is the function of lipase?

Digests lipids

In the pancreas, what is the function of bicarbonate?

Neutralizes stomach acid

In the gallbladder, what is the function of bile?

Digests lipids

What is digestion?

The process of converting large molecules to simpler forms

What two processes make up digestion?

Mechanical and enzymatic (chemical)

Why is digestion necessary?

It is a necessary prelude to absorption → only simple molecules can cross the intestinal epithelia