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The Digestive System
Digestive System
• Also known as the gastrointestinal (GI) tract or alimentary canal
• Long tube open at both ends– Also has accessory organs
• Nutrients absorbed, waste eliminated – Converts food into useable form
Salivary Glands
Esophagus
Stomach
Pancreas
Large Intestine
Anus
Appendix
Small Intestine
Liver
Gallbladder
Mouth
4 Steps to Nutrient Intake
1. Ingestion
2. Digestion- Prepares food for absorption
- Mechanical and chemical breakdown of food
3. Absorption- Small intestine
4. Elimination
Two Types of Digestion
1. Mechanical
2. Chemical
Mechanical Digestion
• Physical breakdown of food
– Mouth (chewing) and stomach (churning)
• Movement of food
– Peristalsis - wavelike involuntary muscle contractions
Chemical Digestion
• In mouth, stomach, small intestine
• Chemicals (e.g., HCl) and enzymes break down complex molecules – e.g., Amylase in saliva - enzyme that
breaks down starch (a carbohydrate)
• Break down substances
• Protein catalysts
• Specific to substrates
Enzymes
Oral Cavity(Mouth)
• Mechanical digestion (teeth) and chemical digestion (saliva with enzymes)
• Bolus of food forms and passes through pharynx into esophagus
• Epiglottis prevents food from entering trachea
Bolus
Esophagus
• Muscular tube that connects pharynx to stomach
• Peristaltic contractions move food to stomach
• Cardiac sphincter at junction prevents food in stomach from going back into esophagus
Stomach• J-shaped muscular organ to churn and squeeze• Acts as a reservoir for food (1.5 L)• Rugae (folds) allow for expansion• Movement of food controlled by two sphincters:
– Cardiac sphincter: Between the stomach & the esophagus
– Pyloric sphincter: Between the stomach & the small intestine
Cardiac sphincter
Stomach
• Digestion turns bolus into chyme – paste-like partially digested food
• Secretes HCl (pH of 2)
– Destroys microbes and breaks down food
• Primarily digests protein
– Pepsin, an enzyme only active at low pH
Small Intestine• Major site of digestion
and absorption• 80% of nutrient
absorption occurs here – diffusion
• About 6 m long, SMALLER diameter than large intestine, 3 sections
• Bile produced by liver breaks down fats• Enzymes secreted by pancreas break down carbs, fats and proteins
Villi and microvilli - increase surface area, maximize absorption
Structure of Small Intestine
Villus
Microvilli
Duodenum
• First section of small intestine
• Receives pancreatic secretions for further chemical digestion
• Basic pH
– Bile from gall bladder and bicarbonate ions from pancreas neutralize acidic chyme from stomach
Jejunum and Ileum
• Make up the other two sections of the small intestine.
• Where most nutrient absorption takes place.
• Peristaltic contractions continue to propel chyme through tract.
Large Intestine• About 1.5 m long and has a LARGER diameter
than the small intestine.• Consists of four sections:
1. Caecum2. Colon3. Rectum4. Anus.
Large Intestine• Main function is to
reabsorb water
• Bacteria produce vitamins which are absorbed
• Compacts unabsorbed material into feces
• Defecation:- Rectal sphincter passes feces into rectum for temporary
storage
- Anal sphincter passes feces from rectum out anus
Graphic Digestion Video!
Animated Digestion Video!