Module 6: The digestive system

What are the 4 phases of digestion?

Interdigestive

Cephalic

Gastric

Intestinal

When is the digestive tract quiet?

Interdigestive phase

Low pH in the stomach ___ gastrin secretion and ____ somatostatin secretion during the interdigestive phase

inhibits, stimulates

What does somatostatin inhibit (interdigestive phase)?

gastring & HCI secretion

What does the sight, smell & taste of food activate?

Vagal efferents (cephalic phase)

What does ACh stimulate (cephalic phase)?

Secretion of HCI, pepsinogen, gastrin, & pancreatic enzymes

What is inhibited during the cephalic phase?

Somatostatin

What begins to enter the jejenum (cephalic phase)?

Some pancreatic juice

When is the "stomach working"?

Gastric phase -> highest acid secretion & highest plasma gastrin

What stimulates gastrin secretion in the gastric phase?

Peptides & amino acids

What does the entry of food stimulate (gastric phase)?

Mechano- & chemoreceptors of the stomach wall

What is activated during the gastric phase?

Vagus & enteric (intrinsic to the GI) nervous system

What does the enteric nervous system cause?

Peristalsis

Peristalsis

Involuntary waves of muscle contraction that keep food moving along in one direction through the digestive system.

What do amino acids, peptides & digested fats stimulate (intestinal phase)?

The release of CCK

What does CCK responsible for (intestinal phase)?

Slows the entry of chyme

Stimulates pancreatic juice

Stimulates bile release

What is the function of gastric inhibitory peptide (GIP)? & when is it active?

Decreases stomach secretions, motility & emptying during the intestinal phase

As food leaves the stomach, ___ buffers available

less

What happens in the stomach as less buffers are available?

pH decreases

gastrin is inhibited

somatostatin increases

somatostatin increases

What does the low pH stimulate (intestinal phase)?

The secretion of secretin

What does the secretion of secretin do (intestinal phase)?

Stimulates the flow of pancreatic juice & inhibits the secretion of gastric juice

What do secretin & CCK enhance (intestinal phase)?

The secretion of digestive enzymes from pancreas & bile salts from the gall bladder

What happens when digested food feedbacks into stomach (intestinal phase)?

The feedback inhibits gastric acid production

Mouth GI Tract Function

Mastication, swallow

Pharynx & esophagus GI Tract Function

Transport

Stomach GI Tract Function

Mechanical disruption, absorption of water & alcohol

Small intestine GI Tract Function

Chemical & mechanical digestion & absorption

Large intestine GI Tract Function

Absorb electrolytes & vitamins (B & K)

Rectum & anus GI Tract function

Defecation

What are the 2 major divisions of the GI tract?

Upper GI Tract

Lower GI Tract

Functional segments of GI Tract

Mouth, esophagus, stomach, small intestine, & large intestine

Accessory structures of GI tract

Teeth, tongue, salivary glands, liver, gallbladder & pancreas

the GI tract is a tube open at both ends for the ___ and ___ of food

transit, processing

organization of the GI tract (order)

Mouth -> pharynx -> esophagus -> stomach -> small intestine (duodenum -> jejunum -> ileum) -> large intestine/colon (cecum -> ascending/right colon -> transverse colon -> descending/left colon -> sigmoid colon) -> rectum -> anus

6 basic processes of the digestive system

1. ingestion

2. secretion

3. mixing & propulsion

4. digestion (mechanical & chemical)

5. absorption of nutrients

6. defecation

Ingestion

Taking food into mouth

Secretion

Release of water, acids, buffers & enzymes INTO the GI tract

Mixing & propulsion

Churning & propelling food through GI tract

Mechanical digestion

movements of the GI tract that aid chemical digestion

Chemical digestion

catabolic (hydrolysis) reactions that break down large molecules into smaller, usable molecules

absorption of nutrients

passage of nutrients from GI tract into blood or lymph for distribution to cells

defecation

elimination of feces from the GI tract

What's in the oral cavity?

Gingiva, hard palate, soft palate

Gingiva

gums

Palates

roof of the mouth

Hard palate

anterior, formed by maxilla & palatine bones, bony separation between oral & nasal cavities

Soft palate

Muscular arch posterior to hard palate

Tooth structure (superficial to deep)

crown -> neck -> roots -> pulp cavity

What forms of digestion happen in the mouth?

Mechanical and chemical digestion

How does mechanical digestion happen in the mouth?

Through mastication/chewing -> breaks the food into pieces

What does the pieces of food mixed with saliva become?

Bolus

What forms of chemical digestion happen in the mouth?

Amylase and lingual lipase

Amylase

Begins starch digestion at pH of 6.5 or 7.0 in the mouth

Lingual lipase

An enzyme secreted by glands in the tongue that begins the breakdown of triglycerides into fatty acids & glycerol

What enzyme are carbs broken down by in the mouth?

Amylase

What enzyme are triglycerides (fats) broken down by in the mouth?

Lingual Lipase

Saliva composition

99.5% water (medium for dissolving) and 0.5% solutes

What are the solutes in saliva?

Ions (Na, K, Cl, bicarbonate, phosphate), lingual lipase, salivary amylase, lysozyme

What does lysozyme do?

Break down bacteria in the mouth

Epiglottis

Catilaginous, leaf like structure that prevents fluid/food entry into the trachea during swalling

Where is the epiglottis?

Above the glottis

Esophagus

A collapsible muscular tube posterior to the trachea that connects the pharynx to the stomach

How long is the esophagus?

~10 inches long

What does the esophagus do?

Secretes mucous and transports food to stomach

Peristalsis

involuntary movements that push the bolus towards the stomach

Gastroesophageal Reflux Disease

(GERD) stomach acids enter esophagus & cause heartburn because the lower esophageal sphincter fails to close

What happens if the lower esophageal sphincter fails to open?

Distention of esophagus, which feels like chest pain/heart attack

What happens if the lower esophageal sphincter fails to close?

GERD

What can make the lower esophageal sphincter relax (which worsens GERD)?

Smoking and alcohol

What foods should be avoided to prevent GERD?

Coffee, chocolate, tomatoes, fatty foods, onions & mint

What should you take 60 minutes before eating to help with GERD?

Tagamet HB or Pepcid AC

What is used to neutralize existing stomach acids?

tums

What are the 3 stomach regions?

Cardia, Fundus, Body

Cardia stomach region

Upper, narrow region just below lower esophageal sphincter

Fundus stomach region

Dome shaped portion in contact w/ diaphragm that is superior to & left of cardia

Body stomach region

Main, central portion

What is the role of the stomach?

Acts as a reservoir (mixing & holding) for ingested food & is where the formation of chyme occurs which is then delivered to small intestine

Mechanical digestion of the stomach

Gentle mixing waves every 15-25 seconds that mixes bolus w/ 2 quarts/day of gastric juice to turn it into chyme

When are there more vigorous mixing waves of the stomach?

As food moves from body to pyloric region

Where are there intense mixing waves of the stomach?

Near the pylorus, which opens the pylorus and releases 1-2 tsp with each wave

Chemical digestion of the stomach

Where protein & fat digestion begins, HCI kills microbes in food

How does protein digestion begin?

In the stomach, HCl denatures/unfolds protein molecules by transforming pepsinogen into pepsin that breaks peptide bonds between certain amino acids

How does fat digestion begin in the stomach?

Lungual lipase activated & gastric lipase release

When is gastric lipase most effective and what does it break down (in infants)?

at pH 5-6 and it breaks down milk fat in infants

What is the role of mucous cells in the stomach?

Protects stomach walls from being digested with 1-3mm thick layer of mucous

What are the 3 accessory organs in the digestive system?

Pancreas, liver & gall bladder

What is the role of the pancreas?

Pancreatic juice production

What is pancreatic juice composed of?

Water, salts, sodium, bicarbonate, enzymes, proteases

What enzymes are in pancreatic juice?

Pancreatic amylase, pancreatic lipase

What are the pancreatic proteases?

Trypsin, ribonuclease or deoxyribonuclease

When is pancreatic juice used?

In the digestion of all nutrients

What is the heaviest gland in the body (and second largest organ)?

The liver

What are the roles of the liver?

Energy

Production of bile

Storage of glycogen & vitamins (A, B12, D, E, K)

Synthesis of cholesterol

Detoxification of drugs & alcohol

How does the liver produce energy?

Gluconeogenesis

Breakdown of amino acids

Storage of triglycerides & breakdown of fatty acids

Gallbladder

Storage, concentration & delivery of the bile into the duodenum

Bile composition

Water, cholesterol, bile salts (Na & K), CHO, lecithin & electrolytes (Na, bicarbonate & Cl) along with bile pigments

Bile pigments

Bilirrubin from hemoglobin molecule

Globin (bile)

A reusable protein

Heme (bile)

Broken down into iron & bilirubin