BS161: Lecture 5 - Digestive System

What does the Digestive System do?

Acquires nutrients from environment, performs catabolism and decomposes substances to provide energy cells need to function

What is the importance of the Oral Cavity?

• Eating

• Processing

• Liquid

• Mixing

Ingestion, mechanical processing w/ accessory organs, moistening, mixing w/ salivary secretions

What are the accessory organs in the digestive system? (TT)

Teeth and tongue

What is the importance of the Pharynx?

Muscular propulsion of materials into oesophagus

What is the importance of the Oesophagus?

Transport of materials to stomach

What is the importance of the Stomach?

Chemical breakdown of materials by acid and enzymes, mechanical processing through muscle contractions

What is the importance of the Small Intestine? (D/A, OS, V, I)

Enzymatic digestion and absorption of water, organic substrates, vitamins, ions

What is the importance of the Large Intestine?

Dehydration/compaction of indigestible materials in prep for elimination

What is a function of the Digestive System relating to Excretion?

Excretion of waste products by defecation

What is a function of the Digestive System relating to Eating?

Ingesiton

What is a function of the Digestive System relating to processing?

Mechanical process (crushing, shearing)

What is a function of the Digestive System relating to Enzymes?

Digestion - chemical breakdown by enzymes

What is a function of the Digestive System relating to Removal?

Secretion by organs/glands of enzymes/buffers/salts

What is a function of the Digestive System relating to Absorption?

Absorption of nutrients across epithelium into interstitial fluid

What is the role of the Digestive Tract Lining?

Protects surrounding tissues

What is one thing the Digestive Tract Lining protects against relating to enzymes?

Corrosive effects of digestive acids and enzymes

What is one thing the Digestive Tract Lining protects against relating to stresses?

Mechanical stress eg abrasion

What is Abrasion?

Open wound caused by skin rubbing against rough surface

What is one thing the Digestive Tract Lining protects against relating to Bacteria?

Bacteria ingested w/ food or residing in digestive tract

What are the 4 major layers of the Digestive Tract Lining?

• M

• S

• ME

• S

Mucosa, Submucosa, Muscularis Externa, Serosa

How does the Mucosa relate to the other layers of the digestive tract lining?

Innermost layer

How does the Serosa relate to the other layers of the digestive tract lining?

Outermost layer

What does the mucous membrane of the Mucosa consist of?

Epithelium moistened by glandular secretions and lamina propria

What are the different types of Mucosal Epithelium?

• Simple - single layer

• Stratified - multi layer

What are the Oral Cavity, Pharynx, and Oesophagus lined by?

Stratified squamos epithelium

What are the Stomach, Small Intestine and most of the Large Intestine lined by?

Simple columnar epithelium w/ mucous cells

How are Enteroendocrine Cells scattered?

Among columnar cells of digestive epithelium

What is the role of Enteroendocrine Cells?

Secrete hormones that coordinate activities of digestive tract and accessory glads

What is the role of Goblet Cells?

Secrete mucus to lubricate passage of food

How does folding affect the Digestive Tract Lining?

Increases SA for absorption

How is the Digestive Tract Lining folded?

Longitudinal folds, disappear as digestive tract fills

What are Plicae Circulares?

Permanent transverse folds

What is the Submucosa?

Layer of dense irregular connective tissue surrounding muscularis mucosae

Describe the structures of the Submucosa?

Large blood vessels & lymphatic vessels, may contain exocrine glands

What is the importance of Exocrine Glands in the Submucosa?

Secrete buffers and enzymes into digestive tract

What is the Serosa?

Serous membrane covering muscularis externa

What is the Adventitia?

Dense sheath of collagen fibres

What is the role of the Adventitia in the Serosa?

Firmly attaches digestive tract to adjacent strucutres

How does movement occur along the Digestive Tract?

Contractions of muscular layers

What type of tissue does the Digestive Tract consist of?

Visceral smooth muscle tissue

How do Pacesetter cells affect movement of Digestive Tract?

Control rhythmic cycles of activity

How is the wave of contraction triggered (Spontaneous Depolarisation) ?

Wave of contract through entire muscular sheet

What are Pacesetter Cells located?

Muscularis mucosae/externa

Where do Pacesetter Cells surround?

Lumen of digestive tract

What is Peristalsis?

Waves of muscular contracts moves a bolus along digestive tract

Steps of Peristalsis (3)

1. Circular Muscles

2. Longitudinal Muscles

3. Contraction

1. Circular muscles contract behind bolus, circular muscles ahead relax

2. Longitudinal muscles ahead of bolus contract shortening adj segments

3. Wave of contraction in circular muscles forces bolus forward

Where does Digestion begin?

Oral cavity

What is a Function of the Oral Cavity relating to Analysis?

Sensory analysis of material before swallowing

What is a Function of the Oral Cavity relating to Processing

Mechanical processing through actions of teeth, tongue, palatal surfaces

What is a Function of the Oral Cavity relating to Lubrication?

Mixing w/ mucus and salivary gland secretions

What Digestion happens in the Oral Cavity?

Limited digestion of carbs and lipids

What is the job of the Tongue?

Manipulates materials inside mouth

What is a Function of the Tongue relating to Processing?

Mechanical processing by compression, abrasion, distortion

What is a Function of the Tongue relating to Chewing?

Manipulating to assist chewing & prep material for swallowing

What is a Function of the Tongue relating to Analysis?

Sensory analysis by touch, temperature, taste receptors

What is a Function of the Tongue relating to Secretion?

Secretion of mucins and enzyme lingual lipase

What is the role of the Parotid Salivary Glands?

Secrete salivary amylase

What is the role of the Sublingual Salivary Glands?

Mucous secretions - buffers/lubricants

What is the role of the Submandibular Salivary Glands?

Secrete buffers, glycoproteins, salivary amylase

How much saliva is produced by glands each day?

1-1.5 L

What Gland produces majority of Saliva?

Submandibular (70%)

What Gland produces the least Saliva?

Sublingual (5%)

What Gland’s Saliva Production is in the middle?

Parotid (25%)

What is the Composition of Saliva?

99.4% Water, .6% Electrolytes, buffers/glycoproteins, antibodies/waste products/enzymes

What is a role of Saliva relating to the mouth?

Lubricating mouth

What is a role of Saliva relating to materials in the mouth?

Moistening and lubricating materials in mouth

What is a role of Saliva relating to Chemicals?

Dissolve chemicals that stimulate taste buds and provide sensory info

What is a role of Saliva relating to Complex Carbs?

Initiate digestion of complex carbs by salivary amylase

What is Mastification?

Chewing

What is the process of Mastification?

Food forced from oral cavity to vestibule and back, crossing and recrossing occlusal surfaces

What is the Pharynx?

Common passageway for solid food, liquids, air

What are the 3 regions of the Pharynx?

• N

• O

• L

Nasopharynx, Oropharynx, Laryngopharynx

How does food get from the Pharynx to Oesophagus?

Passes through Oropharynx and Laryngopharynx

What is the Oesophagus?

Hollow muscular tube that conveys solid food and liquids to stomach

What does the Mucosa of the Oesophagus Wall contain? (NK, SSE)

Nonkeratinized and stratified squamous epithelium

What do the Submucosa of the Oesophagus Wall do?

Form large folds that extend length of oesophagus

What is the Muscularis Mucosae of the Oesophagus Wall?

Consist of irregular layer of smooth muscle

What is Deglutition?

Swallowing

What are the 3 phases of Deglutition?

• B

• P

• O

Buccal, pharyngeal, oesophageal

What is a function of the Stomach relating to food?

Storage of ingested food

What is a function of the Stomach relating to Mechanical breakdown?

Mechanical breakdown of ingested food

What is a function of the Stomach relating to Chemical Bonds?

Disruption of chemical bonds in food material by acids and enzymes

What is a function of the Stomach relating to Glycoproteins?

Production of intrinsic factor, glycoprotein required for absorption of Vit B12 in small intestine

What are the regions of the stomach?

• C

• F

• B

• P

Cardia, Fundus, Body, Pylorus

What are the 3 layers of smooth muscle in the stomach?

Longitudinal, circular, oblique

What is a role of Simple Columnar Epithelium in lining the Stomach?

Lines all portions of stomach

What is a role of the Simple Columnar Epithelium in Mucus Production in the Stomach?

Produce mucus that covers interior surface of stomach

What are Gastric Pits?

Shallow depressions that open into gastric surface

What is the role of Mucous Cells in the Gastric Pits?

At base/neck of each pit, actively divide replacing superificial cells

Where are Gastric Glands?

Fundus and stomach body

Where do the Gastric Glands reach?

Extend deep into underlying lamina propria

What do Parietal Cells secrete? (IF, H)

Intrinsic factor and HCl

Where are Chief Cells most Abundant?

Near base of gastric gland, secrete pepsinogen

How many sections does the Small Intestine have?

3

How important is the small intestine in absorption?

90% of nutrient absorption occurs there

Where is the Duodenum?

Segment of small intestine closest to stomach

What is the role of the Duodenum?

Mixing bowl that receives chyme from stomach and digestive secretions from pancreas & liver

Where does the Duodenum receive chyme from?

Stomach

How does the Stomach benefit the Duodenum?

Neutralises acids before they damage absorptive surfaces of small intestine

What is the job of Brunner’s Glands?

Secrete alkaline substances