Digestive

What does the digestive system do?

Converts food into form body can use

What are the four basic processes of digestion?

1. Motility

2. Secretion

3. Digestion

4. Absorption

What is motility?

Mixing/stirring & propulsion

Is motility regulated?

Yes

What are the two types of secretion? Give an example of each

Exocrine: digestive juice

Endocrine: hormones

What are the two types of digestion? Give an example for each

Mechanical: chewing

Chemical: enzymes

What is the purpose of absorption?

It is designed to maximize

In chemical digestion of carbohydrates, what are the three products of digestion?

• Glucose

• Fructose

• Galactose

What is the difference between mechanical and chemical digestion?

Mechanical is the physical breakdown of food into smaller pieces

Chemical is the chemical breakdown of food into its molecular components

What is the purpose of mechanical digestion?

To increase SA of food for enzyme action

What is the mechanism of mechanical digestion?

Involves physical forces such as chewing, grinding, and churning

What are the key structures involved in mechanical digestion?

Teeth, tongue, stomach, muscles, intestines

Where does mechanical digestion start and end?

Begins in the mouth and continues into the stomach and small intestine

What is the action of the longitudinal muscle?

Propulsion

What are the products of digestion of proteins?

20 amino acids

What are the products of digestion of triglycerides?

Monoglyceride + 2 free fatty acids

What does the submucosal plexus control?

Secretions

What is the function of the circular muscle?

Mixing food

What does the submucosa layer contain?

Glands

What does the myenteric plexus control?

Motility

What does the serosa layer contain/do?

Connective tissues; makes fluid to reduce friction

What do the layers of the GI tract do?

They are highly innervated to regulate/coordinate the enteric NS

Which Nervous System does intrinsic control of the GI tract? What kind of reflexes does it contain?

Enteric Nervous System; Short Reflexes

Which Nervous System does extrinsic control of the GI tract? What kind of reflexes does it contain?

Autonomic NS (PSNS & SNS); Long Reflex

List the layers of the GI tract in order (outside to inside)

Serosa, Muscularis Externa, Submucosa, Mucosa

What does the muscularis externa contain?

• Circular muscle

• Myenteric Plexus

• Longitudinal muscle

What does the submucosa contain?

• Submucosal Plexus

• Major blood and lymphatic vessels

What does the mucosa contain?

• Muscularis mucosa

• Lamina propina

• Epithelium

When the PSNS is active what happens to digestive activity?

It increases due to “rest and digest”

• Motility increases

• Secretions increase

• Sphincters relax

When the SNS is active what happens to digestive activity?

It decreases

• Motility decreases

• Secretions decrease

• Sphincters contract

What is the action/contraction of longitudinal muscle referred to as? Where does it occur?

Peristalsis contractions

Occurs in the esophagus, stomach, and small intestine

What is the action/contraction of circular muscle referred to as? Where does it occur?

Segmentation contractions

Occurs in small & large intestines

What are the 3 phases of GI control?

Cephalic, Gastric, Intestinal

What is the stimulus for the cephalic phase?

Sight, smell, thought, or taste of food

What does the cephalic system activate? Through what mechanism/structure?

The extrinsic NS, and increases PSNS activity

Via the vagus nerve

What does activation of the cephalic phase result in?

Increased exocrine secretions in the mouth & stomach

Increased gastric motility

What is the stimulus for the gastric phase?

Stretch receptors, acidity, amino acids in stomach

Occurs when food enters the stomach

What Nervous System activation occurs as result of entering the gastric phase?

Extrinsic NS (increases PSNS activity) & enteric nervous system

What else is secreted as a result of food entering the stomach? Why?

Histamine & gastrin from stomach to stimulate acid and enzyme secretion

What is the result of activating the gastric phase?

Increased gastric secretions, increased gastric motility, increased large intestine motility

What is the intestinal phase also known as? When does it occur?

The enterogastric reflex

Occurs when food enters the small intestine

What stimulates the intestinal phase?

Stretch, acidity, osmolarity in the duodenum

What occurs neurologically in the intestinal phase?

PSNS activity decreases, SNS activity increases

What hormones are secreted during the intestinal phase?

Secretin & CCK are secreted from the duodenum

What occurs as a result of stimulating the intestinal phase?

Gastric secretion decrease

Gastric motility decreases

Secretions & motility of the small intestine increase

Why do gastric secretions & motility decrease during the intestinal phase?

To slow stomach emptying to give the duodenum time to work

What is Xerostomia? What does it result in?

No saliva; results in difficulty chewing, speaking, swallowing, halitosis, and tooth decay - NO disgestion issues

When are secretions in the mouth activated? What neurological mechanisms stimulate it?

During the cephalic phase; stimulated by both SNS and PSNS

What the the function of water in saliva?

Moisten food & dissolve molecules to facilitate chemoreception

What is the function of mucous in saliva?

Lubricate food and facilitates formation of a bolus for swallowing

What is the function of HCO3- in saliva?

Neutralizes acids in foods and bacterial metabolites

What is the function of lysozymes in saliva?

Kills bacteria to maintain health of gums and teeth

What is the function of amylase in saliva?

Begins the digestion of polysaccarides

Salivary amylase

What is the function of lipase in saliva?

Begins the digestion of triglycerides

Lingual lipase (activated n stomach)

Where is mucous secreted from in the mouth/pharynx/esophagus?

Esophageal glands

What is the function of mucous in the mouth/pharynx/esophagus?

Forms a slippery protective surface and buffers pH

What does mucous help to digest?

Minor carbohydrate digestion via salivary amylase

What does mucous help to absorb?

Virtually NO foods, some drugs (nitroglycerine)

What is nitroglycerine?

A vasodilator used to treat angina

What is the purpose of chewing/mastication?

For taste & to prevent choking

Breaks up food, increases SA, mixes with saliva, chemoreception, voluntary (skeletal) but coordinated by pattern generator in brainstem

What is swallowing also known as?

Deglutition

What are the 3 stages of swallowing?

1. Voluntary

2. Pharyngeal

3. Esophageal

What occurs during the voluntary stage of deglutination?

Initiates swallowing process

What occurs during the pharyngeal stage of deglutination?

Passage of bolus through pharynx into esophagus

What occurs during the esophageal phase of deglutination?

Passage of food from pharynx to stomach

What is the swallowing mechanism? (5 steps)

1. Voluntary movements of the tongue push bolus into pharynx

2. Becomes irreversible when bolus in pharynx

3. Uvula blocks nasopharynx

4. Upper esophageal sphincter relaxes

5. Peristalsis

What occurs as result of a weak lower esophageal/cardiac sphincter?

Acid reflux or heartburn

What do the oblique muscles of the stomach do?

Churn the food

What is the function of the pyloric sphincter?

Controls movement into the duodenum

What is the function of the gastric rugae?

Increase surface area & allows for distension

What do mucous cells secrete? What is their function?

Mucous; for protection

What do parietal cells secrete?

HCL & Intrinsic factor

What do chief cells secrete?

Pepsinogen

What do enteroendocrine (G cells) secrete

Gastrin

What do ECL cells secrete?

Histamine

What do D cells secrete?

Somatostatin

How much secretion comes from the gastric pits per day?

2L

What makes up chyme?

Gastric juice & food

What are the 3 functions of HCL?

1. Dissolves food/denatures protein (mechanical digestion)

2. Kills bacteria

3. Activates enzymes such as → lingual lipase, pepsin, gastric lipase

What things regulate the secretion of HCL? What neural component drive these? Which is most important and why?

• Gastrin: gastric hormone

• Ach: neurotransmitter

• Histamine (+) & Somatostatin (-): paracrine agents

PSNS

Histamine is the most important because it increases the effects of gastrin & Ach

When is somatostatin secreted?

If you don’t actually eat after the cephalic phase has been initiated. Stops HCL secretion

During what phase does HCL secretion stop? Why?

Intestinal phase due to the enterogastric reflex

SNS activity increases & PSNS decreases to slow stomach emptying and give duodenum time to work

Duodenum secrets CCK, & secretin

When pepsinogen comes into contract with HCL what is produced? Where does this occur?

Pepsin; in the stomach

What is the function of pepsin?

Pepsin breaks down proteins into smaller peptides during digestion

What kind of pH is needed for pepsin to work?

A low or acidic pH

What is the function of intrinsic factor? Where does it work?

Binds to and facilitates absorption of vitamin B12

Distal ileum

If there is no intrinsic factor what will occur? Why?

Pernicious anemia; this happens because intrinsic factor is needed for RBC production

What is receptive relaxation?

A state that prepares stomach to receive food

What mediates receptive relaxation? Where are these substances released from?

Serotonin & nitric oxide released from enteric neurons

What are peristaltic waves?

Waves of smooth muscle contraction that mix & propel food via longitudinal muscle

Why are contractions stronger in the pyloric region of the stomach?

There is more muscle there

How much chyme is released into the duodenum after each wave?

A small amount

What is gastric emptying?

Moving chyme from stomach into the duodenum?

What is gastric emptying stimulated by?

Stomach distension & gastrin

What is gastric emptying inhibited by?

Enterogastric reflex

What does the inhibiting of gastric emptying & the enterogastric reflex allow for?

Adequate time for digestion and absorption in the duodenum & prevents over filling

What is mechanical digestion in the stomach?

Physical grindage, churning & HCL

In chemical digestion in the stomach how much protein digestion occurs and what digests it?

15%; Pepsin

In chemical digestion in the stomach how much fat digestion occurs? What digests it?

10-15%; Gastric lipase & Lingual lipase

Are carbohydrates digested in the stomach?

No ; salivary amylase does not like low pH