A Digestive Journey:

1. A Digestive Journey: Part I

2. Mechanical vs. Chemical Digestion • Physical Digestion = chewing, mashing, chopping, breaking food into smaller pieces and the physical movement of food (peristalsis). • Chemical Digestion = chemicals called digestive enzymes and water break down small pieces of food into even smaller molecules that can be absorbed at the cellular level.

3. The Mouth • THE MOUTH • Food gets lubricated and broken down into smaller pieces (increases surface area) • Food is now called a bolus(a “ball of food”) • 2. THE TEETH • FOR MECHANICAL BREAKDOWN: • Incisors – for “cutting”. • Canines – specialized for “tearing”. • Premolars – specialized for “grinding”. • Molars – for “crushing”.

4. The Mouth • 3. THE TONGUE • Muscular organ of mouth • Covered in mucous membranes • Contains projections called papillae (location of taste buds) • Functions: • Assists with the MECHANICAL BREAKDOWN of food by pushing the food around while you chew with your teeth. • - When you're ready to swallow, the tongue pushes bolus toward the back of your throat and into the opening of your esophagus.

5. The Mouth 4. SALIVARY GLANDS • - Saliva is produced here which: • lubricates food • prevents tooth decay • aids in taste bud function - CHEMICAL BREAKDOWN of starch by production of salivary amylase from the salivary glands, an enzyme that begins the breakdown of starch into glucose.

6. Digestion in humans involves mechanical & chemical digestion along each step

7. Some Important Definitions INGESTION – the taking of nutrients. DIGESTION – the breakdown of complex organic molecules into smaller components by enzymes. ABSORPTION – the transport of digested nutrients to tissues of the body. EGESTION – the removal of waste food materials from the body.

8. A series of “stations” along a subway

9. Station #1 – “The Mouth” • Teeth (mechanical) • Tongue (mechanical) • Saliva (chemical)

10. Station #2 The Esophagus • muscular tube, 2 cm in diameter • connects pharynx to stomach • made up of two main muscle layers: I. circular layer • II. longitudinal layer • there is a flap located at the start of the esophagus called the epiglottis, which protects food from entering the windpipe (trachea). • at the endpoints of the esophagus there are sphincters (esophagealand cardiac sphincter, respectively)

11. Station #2 The Esophagus • FUNCTIONS OF ESOPHAGUS • Moves bolus through to the stomach, • through a series of rhythmic contractions • called peristalsis • (MECHANICAL BREAKDOWN) • The circular and longitudinal muscles contract and release NOTE: the purpose of sphincters is to prevent food from going in the reverse direction (i.e. reflux from the stomach into the esophagus).

12. Station #3 The Stomach • - J-shaped stretchable organ • - involves sphinctormuscles (pyloric and esophageal/cardiac) • holds about 2L of food and water • - consists of thick layers of smooth muscle and numerous folds called Rugae. • What’s the purpose of rugae? Enable the stomach to contract & expand.

13. Station #3 The Stomach • Function(s): • - storage of bolus (1.5-2 L) • - MECHANICAL digestion through “churning” • CHEMICAL digestion with acids and enzymes to • convert bolus to chyme • - cardiac and pyloric sphinctors regulate movement of food through digestive tract  keeps food going ONE way. • Gastric pits - secrete hormones and mucus • (mucus forms protective layer) Enzyme Action: - Hydrochloric acid or HCl(secreted by parietal cells) softens food and activates pepsinogen to pepsin - pepsin breaks down PROTEIN into short chains of AA’s Hormonal Action: - Gastrin induces secretion of more mucus