PROBLEM A How Does the Digestive System Make 



Foods Usable? 



Food in the digestive tube. The food you 

 swallow enters a long, irregular tube 

 which runs right through your body. 

 This is the digestive tube. It is also called 

 the alimeiitary (al-i-men'tary) cmial. The 

 tube is narrow in some parts, wider in 

 others. In a person of average size it is 

 about thirty feet long; evidently it must 

 be coiled, at least along part of its length. 

 From the diagram, Figure 201, you can 

 learn that food goes from the mouth 

 through the throat into a long straight 

 food pipe (oocp/.ijgz/5 — ee-sof'a-gus). 

 This connects with the stomach. As you 

 continue your meal the food collects in 

 the stomach where it remains for some 

 time. The last of it does not leave until 

 two to four hours after eating. The food 

 does not lie quietly in the stomach. It is 

 being squeezed and moved around until 

 it is a pulpy mass. Then the stomach be- 

 gins to push the food, bit by bit, into the 

 long narrow tube known as the small in- 

 testine. You can readily see that this is 

 coiled. It, too, has muscular walls and by 

 their contraction the food is pushed along 

 the twenty feet of narrow tubing. It 

 takes about another eight hours before 

 the last of the meal has reached the large 

 intestine. The large intestine, or colon, 

 is a much wider and shorter tube into 

 which the small intestine opens at the 

 lower right-hand side of the abdominal 



cavity. Here lies the appendix, a small 

 fingerlike pocket attached to the large in- 

 testine. The large intestine extends up the 

 right side, across the abdomen under the 

 stomach and liver and down the left side, 

 ending in the rectum. The rectum opens 

 to the exterior by an opening known as 

 the anus. It took you a few minutes to 

 trace these parts of the digestive tube. For 

 a meal to travel the length of this tube 

 takes twelve hours or more. If possible 

 examine a model (manikin) or large chart 

 of the digestive tube. 



What happens to food in the digestive 

 tube? Food in the digestive tube is not 

 yet really in your body at all. It is merely 

 in a tube that runs through your body. 

 Yet the billions of cells in your brain, in 

 your hands, in your heart, all over your 

 body need this food to carry on oxidation 

 and other activities. How does the food 

 get to them? Of course, the blood carries 

 it. Our problem, then, is to learn how 

 food gets from the digestive tube into 

 the blood. The walls of the tube are made 

 up of many layers of cells. Blood vessels 

 lie among these cells. By the time the 

 meal you ate reaches the large intestine 

 most of it has diffused through the walls 

 of the tube into the blood stream. 



When you studied diffusion you 

 learned that water, some minerals, and 

 simple sugars pass through cell mem- 



