The Digestive System

Digestion: breakdown of food into simpler molecules small enough to be absorbed into the blood stream. In most animals, the digestive system is a long, hollow tube (gastrointestinal (GI) tract) with two openings, one for food to enter, the other for wastes to leave. It also includes several sets of glands to aid digestion.

Food is:

a. Mechanically broken down (smaller pieces)

b. Chemically broken down into small molecules.

c. The small molecules are absorbed into the blood stream for distribution.

Route of Digestion:

1. Mouth- Food entry pt.. Mechanical breakdown via teeth and tongue. Chemical breakdown via saliva. (Salivary glands release amylase (saliva) to convert starch to simple sugars.) Release of saliva is reflex response to sight, smell, or taste of food. Saliva also contains mucus to speed movement of food, and other enzymes to attack germs.

2. Pharynx- Throat. Tongue pushes food into a ball (bolus) & into pharynx. As food moves down pharynx, it presses down on epiglottis (cartilage flap covering wind pipe) preventing food entering respiratory system.

3. Esophagus- Tube connecting pharynx to stomach. Muscles in esophagus wall contract & force bolus downward (an action called peristalsis). The bolus then passes the sphincter (thick ring of muscle between esophagus and stomach) which keeps digested food from moving back up into esophagus!

4. Stomach- Thick, muscle enclosed sac contracts & churns to continue mechanical breakdown. 3 glands secrete the following:

a. more mucus.

b. Hydrochloric Acid (HCl): Breaks large molecules to smaller ones.

c. Pepsin: Breaks proteins into smaller polypeptides.

2-3 hours later, food is now a pasty mix called chyme. Pyloric valve between stomach and small intestine opens. Peristalsis forces chyme to S.I..

5. Small Intestine- ~21 ft long tube for further digestion.

Duodenum: 1st part of S.I.. Glands on S.I. release enzymes to finish digestion of proteins, carbos, and lipids.

The duodenum is the site of: A. all digestion completed (duodenum). Enzymes from pancreas, S. I., and bile from liver/gall bladder finish digestion.

B. nutrients absorbed thru villi into blood. Villi are finger-like folds in s.i. wall to increase are where food can be absorbed (diffuse through). (see fig. 39-22)

C. Undigested food & water move to Large Intestine.

5. Large Intestine/ Colon

A. Water removed from undigested food, returned to blood.

B. Peristalsis moves wastes to rectum for storage, until it leaves via the anus.

C. Bacteria in L.I. help finish digestion & produce Vitamin K.

All nutrients in blood first go to the liver, which regulates how much of each nutrient is in the blood. Extra molecules are stored in the liver (except some fats).