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!