What is the advantage of a one-way gut? !If you swallow something, is it really inside you?

The Digestive System

Functions and Processes of the Digestive System:

• Move nutrients, water, electrolytes from external to internal environment

• Protective function

• Defense mechanisms

• Mucus, enzymes, acid, lymphoid tissue

The Challenges:

!

• Prevent autodigestion

• Match input with output

Digestive Homeostasis Requires:

1. Digestion: chemical, mechanical breakdown of food

• polymers → monomers

2. Absorption

• From GI lumen to ECF

• Not regulated; "what you eat is what you get."

Digestive Homeostasis Requires:

3. Motility

• Ingestion, mastication, peristalsis, segmentation

• Regulated

Digestive Homeostasis Requires:

4. Secretion

• Exocrine and endocrine (hormones, enzymes, mucus, paracrines)

• Regulated

• Many enzymes are secreted as inactive proenzymes known as zymogens

Layers of the GI Tract: 4 tunics

1. Mucosa

• Absorbs and secretes

• Mostly columnar epithelium

• Modifications increase lumen surface area:

• Rugae in stomach

• Villi and microvilli in intestine

Layers of the GI Tract - 4 tunics

2. Submucosa

• Connective tissue

• Large blood and lymphatic vessels

3. Muscularis

• Inner circular muscle layer

• Outer longitudinal muscle layer

4. Serosa

• Binds and protects

• Lines abdominal cavity

Basolateral side(apical side)

Cellular Orientation

Oral Cavity and Salivary Glands

• Mastication

• Salivary amylase

• Mucus

The Esophagus

• Pharynx to stomach

• Lower esophageal sphincter

• “Should there be an ‘S’ in the LES?”

Open, cut esophagus from a pig

• Stores food, begins protein digestion, churns bolus and forms chyme

• Covered w/ rugae and microscopic gastric pits to increase surface area

Stomach

• Contains (exocrine) gastric glands which secrete:

• mucus

• HCl

• intrinsic factor

• pepsinogen

• gastrin

• Proteases

Stomach

Stomach

• What is the function of such low pH?

• Wait a second... what is the stomach made of?

pepsin

How does the gastric mucosa prevent self-digestion?

• Alkaline mucus

• Tight junctions

• Rapid rate of cell division

• Prostaglandins

Small Intestine

• Secretions of bicarbonate from the intestine itself and the pancreas neutralize the pH

• Secretions of other ions, water, mucus

Small Intestine

• Secretions of digestive enzymes:

• Disaccharidases

• Proteases

• Peptidase

• Phosphatase

• Absorption of carbohydrates, amino acids, lipids, electrolytes, bile, vitamins, water takes place here

• Why so many villi and microvilli?

Large Intestine

• Ascending, transverse, descending, sigmoid colon, rectum, anal canal

• Absorbs water and electrolytes

Large Intestine

• Stores, then passes waste out of the body

• How much? How often?

Large Intestine

• Defecation reflex

• Internal anal sphincter = smooth muscle

• External anal sphincter = skeletal muscle

Nutrient absorbed → hepatic portal vein → liver → hepatic vein → systemic circulation

Accessories: Liver

Liver Functions: Bile production and secretion

• BILE = bile salts, bilirubin, cholesterol, ions and bile acids:

• bile acids are steroid detergents with polar side chains, allowing interactions w/ polar and non-polar molecules

Liver Functions: Detoxification of blood

• 1. Excretion of toxic materials into bile

• 2. Phagocytosis by Kupffer cells

• 3. Chemical alteration by hepatocytes

Liver Functions: Carbohydrate Metabolism

• Liver can decrease blood [glucose]:

• converting it to glycogen

• converting it to lipids

• Liver can increase blood [glucose]:

• production from glycogen

• production from amino acids

Liver Functions: Lipid Metabolism

• Synthesis of triglycerides and cholesterol

• Excretion of cholesterol in bile

• Break down free fatty acids into ketone bodies (ketogenesis)

Liver Functions: Protein Synthesis

• Plasma proteins: albumin, fibronectin, opsonin, globulins

• Hemostasis factors: all factors in hemostasis except for factor VIII

• Coagulation inhibitors, plasminogen, complement proteins

• Carrier proteins: albumin, ceruloplasmin, transcortin, haptoglobin, hemopexin, IGF binding protein, retinol binding protein, sex hormone-binding globulin, thyroxine-binding globulin, transthyretin, transferrin, vitamin D binding protein

• And more...

Accessories: Gallbladder

• Collects and stores bile from the liver and delivers it to the duodenum via the common bile duct.

• Gallstones are cholesterol crystals that collect the precipitation of inorganic salts.

Accessories: Pancreas

• Endocrine fxn: Islets of Langerhans secrete insulin and glucagon into the blood

• Exocrine fxn: acini cells secrete pancreatic juice into the pancreatic duct

Pancreas

• Pancreatic enzymes (like many stomach enzymes) are secreted as zymogens

• Trypsin

• Chymotrypsin

Pancreatic Juice

• Water, bicarbonate

• amylase (digests starch)

• trypsin (digests proteins)

• lipase (digests triglycerides)

• cholesterolesterase (cleaves cholesterol from it's bond w/ other molecules

• ribonucelase (cleaves RNA chains)

• dexoyribonuclease (cleaves DNA chains)

So What? Digestive System Disorders

• Eating disorders

• Ulcers

• Vomiting

• Constipation, Diarrhea

• Lactose intolerance

• Cancers of the digestive system