Digestion

1. Digestion

2. The Esophagus: • From the throat , the bolus passes through the esophagus to the stomach. • You do not need gravity to swallow food! • Think about it- If we needed gravity to swallow how would astronauts be able to survive in space? • Contractions of smooth muscles, known as peristalsis, provides the force needed to move food through the esophagus and into the stomach.

3. The Esophagus: • After food passes into the stomach, a thick ring of muscle called the cardiac sphincter closes the esophagus. • This prevents the contents of the stomach from flowing back. • Overeating or drinking excess caffeine can cause a backflow of stomach acid into the esophagus. • The result is a burning sensation known as heart burn.

4. The Stomach: Chemical Digestion • The stomach is a large muscular sac that continues the chemical and mechanical digestion processes. • The lining of the stomach contains millions of microscopic gastric glands which produce hydrochloric acid (HCl). • Why do we need HCl in the stomach? • Other glands in the lining of the intestine release an enzyme known as pepsin. • Pepsin breaks down proteins into smaller polypeptide fragments.

5. The Stomach: Chemical Digestion • Another stomach gland produces mucus, a fluid that lubricates and protects the stomach wall. • If this protective layer is missing the HCl in the stomach would eat away at the stomach lining, causing peptic ulcers. • Most peptic ulcers that occur are caused by a bacteria known as Heliobacter pylori, which can be treated by antibiotics.

6. The Stomach: Mechanical Digestion • Alternating contractions of the stomach’s three smooth muscle layers thoroughly churn and mix the swallowed food. • This churning action causes further breakdown of the food. • Gradually the food becomes a oatmeal-like consistency and is now called chyme.

7. The Stomach: Mechanical Digestion • After an hour or two, the pyloric valve, opens and the chyme enters the small intestine. • (The pyloric valve is located between the stomach and small intestine.)

8. The Small Intestine: • As chyme enters the small intestine, through the pyloric valve, it enters the duodenum. • The duodenum is simply the first part of the small intestine and is where almost all of the digestive enzymes enter the small intestine.

9. The Small Intestine: • Most of the chemical digestion occurs in the small intestine. • As chyme enters the duodenum it mixes with enzymes, digestive fluids from the pancreas, the liver, and from the lining of the duodenum.

10. The Pancreas: • Just behind the stomach is the pancreas, a gland that serves three important functions; • Produce hormones that regulate blood sugar levels. • Produces enzymes that break down carbohydrates, lipids, proteins, and nucleic acids. • Produces sodium bicarbonate (a base that quickly neutralizes stomach acid.

11. The Pancreas: • Unlike the enzymes of the stomach, enzymes in the small intestine do not need a highly acidic environment in order to function. • Which is why the pancreas needs to be able to produce sodium bicarbonate.

12. The Liver & Gallbladder: • The liver assists in the digestion of lipids (fats). • The liver produces bile, which is a fluid that is loaded with lipids and salts. • Fats tend to glob together (Why?), which makes fat digestion by enzymes difficult.

13. The Liver & Gallbladder: • Bile breaks the fats up into smaller globs of fat that disperse in the water environment of the small intestine, making it easier for enzymes to break the fats down. • The gallbladder stores the bile produced by the liver.

14. The Small Intestine: • After leaving the duodenum the chyme moves through the rest of the small intestine. • By this time most of the chemical digestion has been completed by the duodenum. • The chyme is now a mixture of nutrient molecules that are ready to be absorbed.

15. The Small Intestine: • The small intestine has a folded surface and fingerlike projections which provide a large surface area for absorption of nutrients. • The fingerlike projections are called villi. • The villi are covered in microvilli, as the chyme passes the villi the microvilli absorb the nutrients.

16. The Small Intestine: • Nutrient molecules are rapidly absorbed into the cells lining the small intestine. • Most of the products of carbohydrate and protein digestion are absorbed into the capillaries in the villi. • Most of the products of fat digestion are absorbed by lymph vessels.

17. The Small Intestine: • By the time chyme is ready to leave the small intestine it is basically nutrient free. • Only water, cellulose, and other undigestible substances are left. • As chyme moves from the small intestine to the large intestine it passes by the appendix. • In some mammals, the appendix produces cellulose and other materials. • The only time humans notice their appendix is when it becomes clogged and inflamed.

18. The Large Intestine: • The large intestine is must shorter than the small intestine; however, has a much greater diameter. • The primary function of the large intestine is to remove water from the undigested material. • Rich colonies of bacterial in the large intestine produce compounds that the body is able to absorb and use such as potassium (K).

19. Elimination: • The concentrated waste material that remains after most of the water has been removed passes into the rectum and is eliminated from the body through the anus. • The concentrated waste material is now called feces.

20. Vocabulary: • Digestion • Mechanical Digestion • Chemical Digestion • Bolus • Chyme • Amylase • Esophagus • Peristalsis • Stomach • Pepsin • Small intestine • Villi • Large intestine • Feces • Pyloric Sphincter • Gastric glands • Cardiac sphincter • Mucus