Digestion

Overall Process

• Ingestion
• Digestion
• Absorption
• Egestion

Basic System

• Gastrointestinal tract (GI) and accessory organs to help with digestion

Mouth

• Food is taken into the mouth where physical (mechanical) and some chemical digestion begins
• Teeth aide in physical digestion
• Different structure depending on feeding habits
• Tongue mixes food with saliva which is continuously secreted, stimulated by presence of food
• Digestion of carbohydrates begins in the mouth
• Amylase enzyme
• Food travels into esophagus with the help of the tongue, via peristalsis and into the stomach
• Epiglottis blocks passage to trachea

Stomach

· Chyme – partially digested food and gastric juices

· For food storage (1.5 L) and initial protein digestion

· Sphincter muscles

o Cardiac sphincter

o Pyloric sphincter

· Mucous cells

o Protective covering

· Parietal cells

o Secretes HCL

· Peptic cells

o Secretes pepsinogen

· Pepsinogen

o Moves thru cell membrane and mucous lining

o Enters lumen, activated by HCl

· Renin

o Slows movement of milk thru the GI tract

· Ulcers formed when protective mucous layer breaks down

Small Intestine (SI)

• 7m in length, 2.5 cm in diameter
• 3 Sections
1. Duodenum
• first 25-30 cm
• majority of digestion
2. Jejunum
3. Ileum

Secretions

• Prosecretin is converted into secretin
• Absorbed into bloodstream and carried to pancreas
• Pancreas releases bicarbonate ions (buffers the acid)
• Pancreatic secretions
• Trypsinogen
• Protein digesting enzyme
• Once it reaches the SI, the enzyme enterokinase converts it to active trypsin
• Trypsin breaks down long chain peptones into shorter chains
• Erepsins
• Completes protein digestion
• Discharges from SI and pancreas
• Cleaves bonds between peptones, leaving individual amino acids
• Amylase enzymes
• Released from pancreas
• Breaks carbohydrate chains into disaccharides
• Disaccharide enzymes from the SI complete digestion
• Lipase
• Lipid digesting enzymes
• Pancreatic lipase, cholesterol lipase and phospholipase
• Lactose Intolerance

§ Normally lactase breaks the disaccharide lactose into 2 monosaccharides

§ Body does not produce enough lactase therefore they cannot absorb lactose in the SI

§ Disaccharide moves into the LI, water is drawn in, causing diarrhea.

Liver and Gallbladder (GB)

• Fat in SI
• Causes a hormone to be released and carried to GB
• Triggers release of bile salts
• Bile salts
• Produced in liver, stored in gallbladder, carried by bile duct to SI
• Act as detergents, emulsifies large fat globules
• Gallstones
• Cholesterol binds with salt crystals in bile
• Forms large crystals
• Bile Pigments
• Composed of the hemoglobin from aged red blood cells
• Gives feces its characteristic brown colour
• Obstruction of the bile duct, or too much destruction of red blood cells results in jaundice
• Detoxifies
• Harmful chemicals are converted into more soluble chemicals to be dissolved in blood and eliminated

Absorption

• Stomach
• Water, specific vitamins and alcohol
• SI
• Majority of absorption
• Villi
• Fingerlike tubes used to increase SA (10X)
• Microvilli
• Folded outer membrane of villi
• Some nutrients are absorbed by diffusion
• Cells of the SI spend energy to absorb materials
• Carbohydrates and amino acids are absorbed into the capillary network

· Fats are absorbed into lacteals (lymph vessels)

Colon/Large Intestine (LI)

• 1.5 m long, 7.6 cm in diameter
• Chemical digestion already complete
• Stores wastes so water can be reabsorbed
• Houses bacteria
• Use waste materials to make Vit. B and K
• E. coli causes gas from fermentation
• Cellulose found in the LI
• Provides bulk
• Last section of the LI is the rectum
• Receptors in the wall send info
• Feces passes by peristaltic contractions
• Exited thru the anal sphincter
• If this does not occur causes problems
• Feces
• 75% water, 25% solids
• 1/3 of solid is bacteria, 2/3 is undigested material